CONDUCTIVITY MEASUREMENT 4 CELL
MADANAGOPAL VIJAYA KUMAR
A thesis submitted in fulfillment of the requirements for the award of the degree of Master of Engineering (Electrical - Electronics and Telecommunication)
Faculty of Electrical Engineering Universiti Teknologi Malaysia
NOVEMBER 2005
PSZ 19:16 (Pind. 1/97)
UNIVERSITI TEKNOLOGI MALAYSIA
BORANG PENGESAHAN STATUS TESISV JUD UL:
-
CONDUCTIVITY MEASUREMENT - 4 CELL SESI PENGAJIAN: 2005/2006/1
Saya
MADANAGOPAL VIJAA YK UMAR (HURUF BESAR)
mengaku membenarkan tesis (PSM/Sarjana/Doktor Falsafah)* ini disimpan di Perpustakaan Universiti Teknologi Malaysia dengan syarat-syarat kegunaan seperti berikut: 1. 2. 3. 4.
Tesis adalah hakmilik Universiti Teknologi Malaysia. Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk tujuan pengajian sahaja. Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran antara institusi pengajian tinggi. **Sila tandakan ( )
¥
SULIT
(Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia seperti yang termaktub di dalam AKTA RAHSIA RASMI 1972)
TERHAD
(Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/badan di mana penyelidikan dijalankan)
TIDAK TERHAD
Disahkan oleh
__________________________________ (TANDATANGAN PENULIS)
(TANDATANGAN PENY E LIA)
Alamat Tetap: NO 13,new N0:3, 10TH Main Road Thiruvalluvar Nagar Chennai -600 118, Tamil Nadu India
Nama Penyelia
Tarikh:
Tarikh:
8 Nov 2005
“I hereby declare that I have read this thesis and in my Opinion this thesis is sufficient in terms of scope and quality for the Award of the degree of Master of Engineering (Electrical-Electronics and Telecommunication)”
Signature : .................................................... Name of Supervisor : PM.Dr.Ruzari Bin Abdul Rahim Date : ....................................................
ii
I declare that this thesis entitled “Conductivity Measurement – 4 cell “ is the result of my own research except as cited in the references. The thesis has not been accepted for any degree and is not concurrently submitted in candidature of any other degree.
Signature : .......................................................... Name
: MADANAGOPAL VIJAYA KUMAR
Date
: .........................................................
iii
To my almighty Sri Sathya SaiBaba
iv
ACKNOWLEDGEMENT
In preparing this thesis, In particular, I wish to express my sincere appreciation to my main thesis supervisor, Professor Dr.Ruzairi Bin Abdul Rahim, for encouragement, Guidance, critics and friendship. I am also very thankful to all colleagues in particular Mr.K.Muralideran R& D Engineer Eutech Instruments Singapore for his guidance, and support in preparation of electronics circuit design. I would also wish to acknowledge my beloved wife Ms.V.Deepa for her continuous support throughout the course and preparation of this Thesis.
v
ABSTRACT
Conductivity measurement is one of the most important and widely used to measure the quality of water in all spread of industries, labs & institutions. The Conductivity or specifically electrolytic conductivity is defined as the ability of a substance to conduct electric current. AC current is passed through two plate and voltage are measured across the plates, the resistance is offered by the liquid is measured in terms of voltage using ohms law principle. The conductivity measurement’s heart is the cell, among them 2 cell is most commonly used but it has few problem such as polarization, field effects, less measurement range etc, answer to solve this problem is 4 cell, this cell has 4 cells it can be of different forms – rings, pins, rectangular and square shaped etc. The 4 cell concept helps to totally remove the field effect, polarization problems, cell constant is the key factor, there are different types of cell constant K=0.01, K=0.1, K= 1.0 and K= 10.0. The cell constant is chosen based on the ranges, cell constant 0.01 is used of low conductance measurement such as ultra pure water. As the conductivity goes higher cell constant will also chosen higher. Frequency lower the conductivity measurement lower is the frequency (32 Hz) as the measurement value goes up the frequency should also go up to get optimum results. The electronics has oscillator which generate required frequency, then high impedance op-amp and frequency dividers are used to measure conductivity. Application of conductivity measurement such as concentration, salinity, resistivity, Total dissolved solids (TDS). All most all industries need this instrument even the drinking water, swimming pool also need this measurement.
vi
ABSTRAK
Pengukuran konduktiviti merupakan satu pengukuran yang amat penting
dan
digunakan dengan meluas untuk mengukur kualiti air dalam kebanayakan industri, makmal dan institusi. “Contucivity” ataupun secara khusus conduktiviti elektrolitik di definasikan sebagfai kebolehan sesuatu “substance” untuk mengkonduksikan arus elektrik. Arus ulang-alik dilalukan melalui 2 plat dan voltan diukur melintasi plat, cecair menyumbang kepada kerintangan dan diukur dalam voltan menggunakan prinsip Hukum Ohm. Cell merupakan elemen paling penting dalam pengukuran konduktiviti, 2 sel biasanya digunakan tetapi mempunyai masalah-masalah seperti polarisasi, field effect(kesan medan)kekurangan julat ukuran dan lain-lain, jawapan untuk menyelesaikan masalah ini adalah dengan menggunakan 4 cell, cell jenis ini mempunyai 4 cell bentuknya segiempat
boleh beraneka-cincin, pin, segiempat tepat dan
dan lain-lain. Konsep 4 cell ini boleh mengahpuskan kesan medan,
masalah polarisasi, konstan cell merupakan factor utama, terdapat pelbagai jenis konstan untuk cell k=0.01, k=0.1, k=1.0 dan k = 10.0 konstan. Untuk cell dipilih berdasarkan julat konstan 0.01 digunakan untuk pengukuran konduksi rendah seperti air semulajadi ultra. Sekiranya konduktiviti bertambah tinggi konstan cell yang dipilih juga tinggi. Frekuensi – sekiranya pengukuran konduktiviti adalah rendah frekuensinya juga turut rendah (32Hz). Andainya nilai pengukuran bertambah frekuensi juga harus bertambah untuk memperolehi kesan optimum. Elektronik ini mempunyai oscillator (pengayun)yang menjana frekuensi seperti yang dikehendaki, op-amp berimpidan tinggi dan pembahagi frekuensi digunakan untuk mengukur konduktiviti. Pengukuran konduktiviti di aplikasikan dalam kepekatan. Kepadatan rintangan (TDS). Kebanyakan industri memerlukan peralatan ini memandangkan air yang diminum, air kolam mandi juga memerlukan penukuran ini.
vii
TABLE OF CONTENTS
CHAPTER
TITLE
PAGE
1.
INTRODUCTION
1
2.
OBJECTIVE
3
3.
SCOPE OF PROJECT
4
4.
RESEARCH METHODOLOGY
5
5.
LITERATURE REVIEW AND THEORY OF
6
ELECTROLYTIC CONDUCTIVITY MEASUREMENT 5.1
Definition of Conductivity
6
5.2
Design of the conductivity cell
9
5.3
Effects of Polarization
10
5.4
Platinization
10
5.5
Temperature effects on conductivity measurement
11
5.5.1
Conductivity Vs PPM
13
5.5.2
Conductivity Vs Resistivity Spectrum
14
5.5.3
Conductivity Vs Concentration
15
5.6
Conductance
16
5.7
Conductivity
17
5.8
Resistivity
17
5.9
Calibration
17
5.10
Standard Solution
18
5.11
Reference Temperature
18
5.12
Automatic Temperature correction
18
viii
6.
7.
5.13
Cable Correction
19
5.14
Total Dissolved Solids (TDS)
19
5.15
TDS Factor
19
5.16
Salinity
20
SENSOR DESIGN
21
6.1
Cell Constant ring type formula
23
6.2
Cell Calculations
23
6.3
Cell Constants
25
6.4
Cell Construction
27
BLOCK DIAGRAM OF THE 4 CELL
29
MEASUREMENT CIRCUIT 7.1
8.
Oscillator Circuit 7.1.1
Waveforms
34
7.1.2
Voltage and Current Circuit
37
7.1.3
Operational Amplifier
39
7.1.4
Multiplexer MC74HC4051
40
SIMULATION RESULTS 8.1
32
Final Simulation of Sensor with electronic
43 47
Results 9.
BILL OF MATERIAL
50
10.
APPLICATIONS
51
11.
10.1
Conductivity Measurements
52
10.2
Resistivity Measurements
52
10.3
TDS Measurements
53
10.4
Salinity Measurements
55
10.5
Concentration Measurements
56
ANALYSIS
57
11.1
Temperature Co-efficient
57
11.2
Polarizations
58
ix 11.3
Geometry
59
11.4
Frequency
59
11.5
Cable resistance and capacitance
60
11.6
Practical Consideration for conductivity/
60
TDS Measurements 11.7
Conductivities of metals can be used for
61
Cell 11.8
4 cell conductivity probe eliminate
62
Polarization and contact coating effetcs 11.9
Comparison between two cells and 4 cells
63
12.
RESULTS
64
13.
CONCLUSIONS
65
14.
13.1
Sensor
65
13.2
Circuit
65
FUTURE WORK
67
REFERENCE
68
APPENDICES
69
x
LIST OF TABLES
TABLE NO.
TITLE
PAGE
5.1
Conductivity Vs Concentration for Common Salts
15
6.1
Cell Constants
25
6.2
Cell Construction
27
7.1
Functional diagram of 74HC4040
34
7.2
Functional diagram of MC74HC4051
42
8.1
Range 0 – 20 micro siemens
43
8.2
Range 0 – 200 micro siemens
44
8.3
Range 0 – 2000 micro siemens
45
8.4
Range 0 – 200 milli siemens
46
8.5
Final simulation of sensor with circuit
48
10.1
Applications
51
10.2
Dissolved salts in sea water
56
11.1
Conductivities of metals
61
11.2
Comparison between two cells and 4 cells
63
xi
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
5.1
Solution Conducts electricity depends on
7
5.2
Conductivity Vs PPM concentration chart
13
5.3
Conductivity/Resistivity Spectrum
14
6.1
Sensor construction
22
6.2
Physical picture of the 4 cell sensor
28
7.1
Block diagram of the 4 cell measurement circuit
29
7.2
Oscillator Circuit
31
7.3
The logic diagram of 74HC4040
32
7.4
The pin diagram of 74HC4040
33
7.5
Logic diagram of 74HC4040
33
7.6
Simulation of Oscillator signal 128Hz
34
7.7
Simulation of Oscillator signal 512Hz
35
7.8
Simulation of Oscillator signal 1.025KHz
35
7.9
Simulation of Oscillator signal 2049Hz
36
7.10
Voltage and Current circuit
37
7.11
Conductivity measurement circuit for 4 cell
38
7.12
Connection Diagram of AD822 Op Amp
39
7.13
Pin diagram of MC74HC4051[14]
41
7.14
Logic diagram of MC74HC4051[14]
41
8.1
Range 0 – 20 micro siemens
44
8.2
Range 0 – 200 micro siemens
45
xii 8.3
Range 0 – 2000 micro siemens
46
8.4
Range 0 – 200 milli siemens
47
8.5
Final simulation of sensor with circuit
49
11.1
Field effects
59
xiii
LIST OF SYMBOLS
A, a
-
Area
AC
-
Alternating current
C
-
Conductivity
D.d
-
Distance
E
-
Volts
G
-
Conductance
HCL
-
Hydrochloric acid
H
-
Hydrogen ions
I
-
Current
K
-
Cell constant
KCL
-
Potassium chloride
mV
-
Milli volts
mg
-
Milligram
NACL
-
Sodium Chloride
NAOH
-
Sodium Hydroxide
ppm
-
Parts per million
ppt
-
Parts per thousand
R0
-
Reverse Osmosis
S
-
Siemens
SS
-
Stainless Steel
T
-
Temperature
Tr
-
Reference Temperature
TDS
-
Total dissolved solids
µS
-
Micro Siemens
Į
-
Temperature Co-efficient
xiv
LIST OF APPENDICES
APPENDIX NO.
A
TITLE
12 – Stage Binary Ripple Counter
PAGE
69
- MC54/74HC4040A B
Hex Schmitt – Trigger Inverter
77
– MC74HC14A C
Dual D Flip – Flop with Set and Reset
86
– MC74HC74A D
Analog Multiplexer Demultiplexer – MC74HC4051A
94
1
CHAPTER 1
INTRODUCTION
Conductivity measurement is one of the most important and widely used to measure the quality of water/solutions in all spread of industries, labs & institutions; for examples drinking water quality in RO plant, feed water for boilers from Demineralization plants. The possibility of using conductance to locate end points in titrations was also recognized early in the development of instrumental methods. Changes in slope of conductance versus titrant volume occur because ionic mobility’s vary and also because of the formation of insoluble or non-ionized materials, accordingly conductometric titration was developed in recent years, high frequencies conductometric titration was developed in recent years. High frequency measurements permit the determination of changes in conductance or dielectric constant with out the introduction of electrodes into direct contact with the solution.[4]
Materials in which current is conducted by ions rather than electrons (as in metal conductors) are called electrolytes. These are divided into two groups strong and weak electrolytes according to their dissociation behavior, i.e. the property of the chemical compounds dissolved in a liquid to totally or partially split into separate
2 groups of ions. The group of strong electrolytes includes all strong acids and bases (e.g., HCL, NaOH). Water is an example of a weak electrolyte. The following applies to conduction in electrolytes: In solutions current is conducted by ions. All ions take part in this process but weak electrolytes only dissociate into ions.
In this project we are addressing 4 cell concepts of design, measurement and applications. The 2 cell in general has problems of polarization, geometry, fieldeffects etc, some this major problem can be overcome with the 4 cell concept.
3
CHAPTER 2
OBJECTIVE
To conduct study and propose the best 4 cell electrode design and electronics used for measurement & also to discuss the application of this electrode. Conductivity measurement is very widely used and there is lot of difficulties in measurement when it comes to wide range of measurement, currently used 2 cell have difficulty in measuring the higher conductivity and also has problem with polarization error which can be corrected with 4 cell, Conductivity meter needs is different from the 2 cell measurement.
4
CHAPTER 3
SCOPE OF PROJECT
To study in detail about the 4 cell –theory, compare with 2 cell concepts, detail design concept of 4 cells, influences of conductivity measurement such as (i)
Polarization,
(ii)
Contamination,
(iii)
Geometry,
(iv)
Frequency used,
(vi)
Temperature effects and temperature co-efficient To discuss the fundamentals of conductivity measurement, detailed
Electronics circuit used in 4 cell conductivity measurement. Conducting study of cell constant and its calculations, cells polarization, cable resistance, cable conductance, temperature effects, understanding calculating temperature co-efficient. To simulate the results of the 4 cell and verify the result with the 2 cell. Application of conductivity measurement and its implications, right choice of cell constant for wide range of water application. One application presentation of 4 cell conductivity.
5
CHAPTER 4
RESEARCH METHODOLOGY
(i)
Understanding of basic theory of conductivity measurement mainly from own filed knowledge, experience, literature, books and journals, which is very important to propose 4 cell and measurement technique.
(ii)
Study and comparison of 2 cell with 4 cell, key issue that is the problem with the 2 cell to find tout the error in the measurement effect of polarization etc and how it can be over come with the 4 cell
(iii)
Study of 4 cell designing concepts and calculation of cell constant, geometry.
(iv)
Study
of
temperature
effect,
temperature
compensation. (v)
Electronics circuit design for 4 cell measurement.
(vi)
Simulating the results of the design.
co-efficient
and
6
CHAPTER 5
LITERATURE REVIEW AND THEORY OF ELECTROLYTIC CONDUCTIVITY MEASUREMENT
5.1
Definition of conductivity
Conductivity or specifically electrolytic conductivity is defined as the ability of a substance to conduct electric current
(i)
Mobility of ions
(ii)
Valence of ions
(iii)
Concentration
(iv)
Temperature Conductivity is the ability of a material to conduct electric current. The
principle by which instruments measure conductivity is simple - two plates are placed in the sample, a potential is applied across the plates (normally a sine wave voltage), and the current is measured. Conductivity (C), the inverse of resistance (R) is determined from the voltage and
7 current values according to Ohm's law. [1] C = I/R = I (amps) / E (volts)
FIGURE 5.1 The charge on ions in any solution has the conductance of electrical current; the conductivity of a solution is proportional to its ion concentration. In some situations, however, conductivity may not correlate directly to concentration. Strong electrolytes are the substances that are fully ionized in the solution. As a result, the concentration of ions in solution is proportional to the concentration of electrolytes added, they include ionic solids and strong acids fro example HCL Week electrolytes are substances that are not fully ionized in solution for example acetic acid partially dissociates into acetate ions and hydrogen ions, acetic acid solution contain both molecules and ions. A solution of a week electrolyte can conduct electricity, but usually not as well as a strong electrolyte because there are fewer ions to carry the charge from one electrode to the other. [1] The term Conductance refers to the readiness of materials to carry an electric current. Liquids which carry an electric current are generally referred to as electrolytic conductors. The flow of current through electrolytic conductors is accomplished by
8 the movement of electric charges (positive and negative ions) when the liquid is under the influence of an electrical field. The conductance of a liquid can be defined by its electrical properties - the ratio of current to voltage between any two points within the liquid. As the two points move closer together or further apart, this value changes. To have useful meaning for analytical purposes, a dimension needs to be given to the measurement; i.e., the physical parameters of the measurement. By defining the physical parameters of the measurement, a standard measure is created. This standard measure is referred to as specific conductance or conductivity. (i)
It is defined as the reciprocal of the resistance in ohms, measured between the opposing faces of 1 cm cube of liquid at a specific temperature.
(ii)
The units used to define conductance are: 1/ohm = 1 mho = 1000 mS = 1,000,000 µS.
(iii)
S.I. units may be used in place of mhos; 1 mho = 1 Siemen (S).
(iv)
Conductivity units are expressed as µS/cm (1.0 dS/m = 1. 0 µS/cm) or mS/cm.
9
5.2
Design of the Conductivity Cell
In theory, a conductivity measuring cell is formed by two 1-cm square surfaces spaced 1-cm apart. Cells of different physical configuration are characterized by their cell constant, K. This cell constant (K) is a function of the electrode areas, the distance between the electrodes and the electrical field pattern between the electrodes. The theoretical cell just described has a cell constant of K = 1.0. Often, for considerations having to do with sample volume or space, a cell's physical configuration is designed differently. Cells with constants of 1.0 cm-1 or greater normally have small, widely spaced electrodes. Cells with constants of K = 0. 1 or less normally have large closely spaced electrodes. Since K (cell constant) is a "factor"which reflects a particular cell's physical configuration, it must be multiplied by the observed conductance to obtain the actual conductivity reading. [6] For example, for an observed conductance reading of 200 µS using a cell with K = 0. 1, the conductivity value is 200 x 0. 1 = 20 µS/cm. In a simplified approach, the cell constant is defined as the ratio of the distance between the electrodes, d, to the electrode area, A. This however neglects the existence of a fringe-field effect, which affects the electrode area by the amount AR. Therefore K = d/(A + AR). Because it is normally impossible to measure the fringefield effect and the amount of AR to calculate the cell constant, K, the actual K of a specific cell is determined by a comparison measurement of a standard solution of known electrolytic conductivity. The most commonly used standard solution for calibration is 0.01 M KCl. This solution has a conductivity of 1412 µS/cm at 25oC
10
5.3
Effects of polarization
When a DC voltage is applied across the electrodes of a conductivity cell, the ions present in solution will be discharged onto the electrodes and by surrendering or accepting electrons, be changed into molecular form. The flow of ions will then virtually cease within a very short time, and consequently, the current will decrease to virtually zero. Therefore, an AC voltage is used for conductivity measurements. Polarization, however, can still actually take place during a half cycle of one polarity, causing a space charge buildup around the electrodes, resulting in a loss of current flow. In addition to polarization effects, conductivity cells with higher cell constants require long, narrow passages to obtain these constants, which make the electrode contacts more susceptible to coating by oils, slurries, or sludges commonly found in streams of high conductivity. [6]
5.4
Platinization
Platinization, or depositing a layer of black platinum on the electrode cells, results in a decreased polarization resistance. The platinum black catalyzes the electrochemical reaction rate, reducing the current density on the electrode cells and the reduction over voltage for H+ ions. [1]
11
5.5
Temperature Effects on Conductivity Measurement
The conductivity process in aqueous solutions is by means of ionic motion, and is different from that of metals. The conductivity invariably increases with increasing temperature, opposite to metals but similar to graphite. It is affected by the nature of the ions, and by viscosity of the water. In low ionic concentrations (very pure water), the ionization of the water furnishes an appreciable part of the conducting ions. All these processes are quite temperature dependent, and as a result, the conductivity has a substantial dependence on temperature. This dependence is usually expressed as a relative change per degree Celsius at a particular temperature, commonly as percent/co at 25 oC , and this is called the slope of the solution. Ultra-pure water has by far the largest slope, 5.2%/oC, while ionic salts run about 2%/oC in the middle ranges. Acids, alkalis, and concentrated salt solutions run somewhat lower, typically 1.5%/oC. Non-aqueous materials such as oleum and conducting organics have quite different temperature dependences. From these figures, it is obvious that a small difference in temperature makes a large difference in conductivity and the effects are very troublesome when a high degree of accuracy is required. In making conductivity readings at high and low temperatures, the data is usually normalized to 25 oC , i.e. it is stated as what the reading is with a 25 oC solution. Fortunately temperature sensors are available which have characteristics similar to those of the solutions to be tested. By the use of supplement resistors and electronic circuitry, the temperature-conductance curves can be shaped to match closely any aqueous solution. The temperature sensor and its associated network are then used as a gain control element in the monitor circuitry, and the conductivity reading is brought to its equivalent value at 25 oC .
12
A modern technique uses a microprocessor and an associated "lookup table" which contains the temperature response data of the solution. The solution temperature is measured and converted to digital form. From this information and the data in the lookup table, temperature compensation can be derived, with accuracy limited only by the number of data points which are placed in the table. Compensation curves are available for all common solutions. Other solutions can be estimated by the examination of similar materials. The conductivity of a solution is critically dependent on temperature. Therefore, the conductivity readings must be referred to a common reference temperature (77 °F/25°C) for comparability. The term “temperature compensation” is used in the sense of a mathematical conversion; i.e. a measured conductivity at any given temperature to the corresponding conductivity value that would be taken at the reference temperature (77 °F/25 °C). The conductivity of most aqueous solutions varies more or less linearly with temperature. In these cases, a linear correction function to compensate for the influence of temperature can be used. For example, the correction coefficient for sewage is approx. 2%/K. If a non-linear relationship exists between temperature and conductivity, (i.e. the coefficient itself varies with temperature) the relationship can as a rule is described in terms of a 4th order polynomial.
13
5.5.1 Conductivity VS PPM
Figure 5.2 Conductivity vs ppm concentration chart [3]
A similar conclusion can be made for all types of dissolved solids. Most preformulated ppm TDS standard solutions are formulated with either sodium chloride (NaCl), potassium chloride (KCl) or the 442 (40% sodium sulfate, 40% sodium bicarbonate and 20% sodium chloride) natural water formulation.[3] In some cases, a KCl solution is made to a specific Conductivity value, and then the ppm values for NaCl, KCl and/or 442 formulations are referenced on the
14 bottle giving the user the option to calibrate to any one of these. A Conductivity value is also usually given.
5.5.2 Conductivity VS Resistivity Spectrum
Below figure 5.3 provide details of Conductivity vs Resistivity spectrum it is exactly reverse of resisitivity, the spectrum provide some detail of certain application of conductivity/Resistivity values.
Figure 5.3 Conductivity/Resistivity spectrums [3]
15
5.5.3
Conductivity Vs Concentration [3]
Table 5.1 Conductivity Vs Concentration
If your test solution's major dissolved solids components are the same as any of these, you may want to choose the pre-made formulation that best approximates your test solution. Generally NaCl is used for brines and the 442 formulation is used for general water and waste water, rinse water, boilers and cooling towers, lakes, streams and wells. Alternatively, if the contents of the ppm standard calibration solution used for calibration are known and if there are figures such as Figure 5.2 tables such as Tables 5.1, you can cross reference the calibration standard solution's "Conductivity to ppm TDS" curve to the curves for other types of dissolved solids solutions. Other curves and tables are available in various reference books. The previous discussion and references are based on standard conditions of temperature (25oC). When measuring Conductivity or TDS in non-standard
16 conditions, corrections for temperature variations must be taken into account before determining the final values of Conductivity and TDS measurements. Otherwise the measurements will not be correct. Meters with temperature compensation overcome this problem, because they incorporate temperature sensing elements and temperature compensating circuitry into the meter so that the value displayed is corrected to a standard temperature. If your meter does not have temperature compensation, you need to use a look-up tables or formulas to correct for the temperature effect, or to calibrate the meter using a calibration standard that has been brought to the same temperature as the test solution.
5.6
Conductance
Conductance (G) is defined as the reciprocal of the electrical resistance (R) Of a solution between two electrodes. G = 1/R (S) The conductivity meter in fact measures the conductance, and displays The reading converted into conductivity Cell constant. This is the ratio of the distance (d) between the electrodes to the area (a) Of the electrodes. K = d/a K = cell constant (cm-1) a = effective area of the electrodes (cm2) d = distance between the electrodes (cm)
17
5.7
Conductivity
Electricity is the flow of electrons. This indicates that ions in solution will Conduct electricity. Conductivity is the ability of a solution to pass current. The conductivity reading of a sample will change with temperature. ț=G•K ț = conductivity (S/cm) G = conductance (S), where G = 1/R K = cell constant (cm-1) -8–
5.8
Resistivity
This is the reciprocal of the conductivity value and is measured in Ohm. It is generally limited to the measurement of ultra pure water, The conductivity of which is very low. .[3]
5.9
Calibration
Determination of the cell constant required to convert conductance readings Into conductivity results. . [3]
18
5.10
Standard solution
A solution of known conductivity that is used to calibrate the Conductivity measuring chain. .[3]
5.11
Reference temperature
Conductivity readings are often referenced to a specific temperature, Typically 20°C or 25°C, for comparative purposes. .[3]
5.12
Automatic temperature correction
Algorithms for automatic conversion of sample conductivity to a reference Temperature. . [3]
19
5.13
Cable correction
The cable correction takes into account the cable resistance and the Cable capacitance. Gm = measured conductance (siemens) Gs = solution conductance (siemens) RC = cable resistance (). [3]
5.14
Total Dissolved Solids (TDS)
This is the measure of the total concentration of ionic species of a sample. Its magnitude is relative to the standard solution used to calibrate The meter. . [3]
5.15
TDS factor
Conductivity readings are converted to TDS readings by multiplication With a known mathematical factor. The factor depends on the reference Material used to prepare the standard. . [3]
20
5.16
Salinity
Salinity is a measurement without unit corresponding to the weight of Dissolved salts in seawater. [3]
21
CHAPTER 6
SENSOR DESIGN
Sensor design is very critical part this project. Key is the cell constant, construction of the cell and material used. This project is using Stainless steel SS316 as this material low and good in conductance. However this can be platinum plated, here in this project it is not used as it may peal off over a period of time. The four-electrode conductivity cell represents an improvement over the twoelectrode cell. The conductivity cell comprises two current electrodes and two voltage electrodes. The electric current enters the solution via the current electrodes, the current intensity is known. The voltage drop in the aqueous solution is determined with very high impedance by the voltage electrodes. The exact geometric form of the electrode arrangement is generally unknown, so that the cell constant is determined by using standard solutions. The electrodes are arranged as rings on a glass form. The outer electrode forms, made of plastic, limit the influences of the measuring vessel and protect the glass form from being damaged. [8] The important point to consider while design the conductivity sensor (i)
Polarization
(ii)
Geometry
(iii)
Cell constant
23
6.1 Cell constant – ring type formula
The sensor design is calculated using following formula R is the radius of the ring= 0.400CM H is the Height of the ring = .315CM Effective area of the ring = 2ʌrh 2*3.142*.315*.40 =0.790cm Cell constant k= L/A K= 0.800/0.790 = 1.01
6.2
Cell calculations
G = (C * L/A )/ 1 + Į( t – Tr) [9] (i)
G = Specific conductivity Conductance (G) is defined as the reciprocal of the electrical resistance (R) of a solution between two electrodes. G = 1/R (S) The conductivity meter in fact measures the conductance, and displays the reading converted into conductivity.
(ii)
C = Absolute Conductivity
Actual conductivity solution used to standardizing the cell.
24
Cell constant
(iii)
This is the ratio of the distance (L) between the electrodes to the area (A) Of the electrodes. K = L/A K = cell constant (cm-1) A = effective area of the electrodes (cm2) L = distance between the electrodes (cm). (iv)
Į = Temperature co-efficient
D
(kT2 kT1 ) u 100 (T2 T2 ) u kT1
kT2 conductivity of reference temperature kT1 conductivity at another temperature The temperature coefficients are as follows, in general 2.1%/deg C is used. Acids: 1.0 - 1.6%/°C Bases: 1.8 - 2.2%/°C Salts: 2.2 - 3.0%/°C Drinking water: 2.0%/°C Ultra pure water: 5.2%/°C (vi)
T = Actual temperature
(vii)
Tr = Reference temperature
25
6.3
Cell constants
Cell constant k = L/A, very critical parameter in the enter design of this project, in this design SS 316 stainless steel is used. The variable L/A is described as cell constant K. Generally it cannot simply be determined from the geometric dimensions, therefore the conductivity cells are calibrated. To do this the conductivity cell is immersed in an aqueous salt solution with a precisely known conductivity and the conductance is then determined. This procedure is repeated with several standard solutions. In this way you obtain in the range of medium and low conductivities, a linear characteristic curve that is extensively independent of the temperature with the rise 1/K. [8] Below table shows the optimum conductivity range for the given cell constant, generally the lower the cell constant, lower the range selection vice versa as the range goes higher, higher cell constant is used, for multipurpose use cell constant K=1.0 is widely used in the lab. TABLE 6.1 Cell constants Cell Constant
Optimum Conductivity Range
0.01
0.055 - 20 µS/cm
0.1
0.5 - 200 µS/cm
1.0
0.01 - 2 mS/cm
10.0
1 - 200 mS/cm
Simple conductivity sensors are constructed of an insulating material embedded with platinum, graphite, stainless steel or other metallic pieces. These metal contacts serve as sensing elements and are placed at a fixed distance apart to
26 make contact with a solution whose conductivity is to be determined. The length between the sensing elements, as well as the surface area of the metallic piece, determine the electrode cell constant, defined as length/area. The cell constant is a critical parameter affecting the conductance value produced by the cell and handled by the electronic circuitry. A cell constant of 1 cm-1 will produce a conductance reading approximately equal to the solution conductivity. For solutions of low conductivity, the sensing plates can be placed closer together, reducing the length between them and producing a cell constant of e.g. 0.1 cm-1. This will raise the conductance reading by a factor of 10 to offset the low solution conductivity and give a better signal to the conductometer. On the other hand the sensing plates can be placed farther apart to create a cell constant of e.g. 10 cm-1 for use in highly conductive solutions. This also produces a conductance acceptable to the meter by reducing the conductance reading, by a factor of 10. [9] In order to produce a measuring signal acceptable to the conductometer, it is highly important that the user chooses a conductivity electrode with a cell constant appropriate for his sample. [9] A conventional electrode consists of two platinum plates (rings) which are coated with a spongy black platinum deposit. This increases greatly the effective surface and reduces polarizing effects. [9] The 4-pole electrode: this design reduces considerably the problems of polarization and fouling. By utilizing four platinum rings, no current flows through the measuring circuit. [9] The AC current (I) is only applied to the outer pair of rings allowing the inner pair of rings to measure the voltage (V) without any polarization effects. Superior to function in almost any type of solution, also the whole conductivity range is covered by a single electrode. [9]
27
6.4
Cell Construction
Cell is constructed using 4 rings made of high grade stainless steel and it is connected using copper wires coated with enamel, the wires are spot welded for better connection and then based on the formula “L” is the distance between the inner rings and it is placed inside a tube and bonding is done using 2 pot epoxy which of high grade and having very high resistances. Cell standardization Cell can be standardized using the standard solution through which real cell constant can be determined, using below table TABLE 6.2 Conductivity standard solutions
Conductivity std solution Micro siemens 26.6 µS 1015 µS 1413 µS 12880 µS 111800 µS
28
FIGURE 6.2 Physical picture of the 4 cell sensor.
29
CHAPTER 7
BLOCK DIAGRAM OF THE 4 CELL MEASUREMENT CIRCUIT
Figure 7.1 Block diagram of the 4 cell measurement circuit Block diagram Figure 7.1 show the basic measurement
technique used,
oscillator will produce ac signal which will be passes through the conductivity cell and then passed to the current source,. Amplifier will further amplify the low level signal to high level signal and then pass it to Rectifier which convert AC signal to dc and then the signal is transmitter or displayed.
30
A typical conductivity meter applies an AC current (I) at an optimal frequency to two active electrodes and measures the potential (v) in another 2 cell for four cell concept. Both the current and potential are used to calculate the conductance (I/V). The conductivity meter then uses the conductance and cell constant to display the conductivity. Conductivity = cell constant x conductance When designing the meter circuit consider capacitance or inductance apart from resistance to balance the capacitive effects in the conductance cell. [1] Frequency – Generally lower frequency should be used for high resistance and high frequency for lower resistance, an oscillator can be used to generate this frequency as desired and changed based on the measured conductivity. [3]
32
7.1 Oscillator Circuit
The oscillator circuit uses mainly quarts crystal oscillator, a crystal oscillator is a timing device that consists of a crystal and an oscillator circuit, providing an output waveform at a specific frequency. When a crystal is placed into an amplifier circuit, a small amount of energy is fed back to the crystal, which causes it to vibrate. These vibrations act to stabilize the frequency of the oscillator circuit. [13] With reference to above diagram The MC74HC14A. The device inputs are compatible with Standard CMOS outputs; with pull up resistors, The HC14A is useful to “square up” slow input rise and fall times. Due to hysteresis voltage of the Schmitt trigger, the HC14A finds applications in noisy environments. The crystal oscillator is used along with the Schmitt Trigger which helps to squares up the slow rise and fall times and also offers low noise immunity. The oscillator signal is further given to frequency divider IC 74HC4040, this device consists of 12 master–slave flip–flops. The output of each flip–flop feeds the next and the frequency at each output is half of that of the preceding one. The state counter advances on the negative–going edge of the Clock input. Reset is asynchronous and active–high.
Figure 7.3 the logic diagram of 74HC4040
33
MC74HC74A device consists of two D flip-flops with individual Set, Reset, and Clock inputs. Information at a Díinput is transferred to the corresponding Q output on the next positive going edge of the clock input. Both Q and Q outputs are available from each flip-flop. The Standard Reset inputs are asynchronous.
Figure 7.4 The pin diagram of 74HC4040
Figure 7.5 Logic diagram of 74HC4040
34
Table 7.1: Functional diagram of 74HC4040
7.1.1 Wave forms
Below pictures show various wave forms for the oscillator outputs from 128 Hz to 2.0 KHz frequency
FIGURE 7.6 Simulation of oscillator signal 128HZ
35
Figure 7.7 Simulation of oscillator signal 512HZ
36 Figure 7.8 Simulation of oscillator signal 1.025 KHz
Figure 7.9 Simulation of oscillator signal 2049 Hz
39
7.1.3
Operational Amplifier
The AD822 is a dual precision, low power FET input op amp what can operate from a single supply of 3.0 V to 36 V or dual supplies of ±1.5 V to ±18 V. It has true single-supply capability with an input voltage range extending below the negative rail, following the AD822 to accommodate input signals below ground in the Single-Supply Mode. Output voltage swing extends to within 10 mV of each rail providing the maximum output dynamic range. Offset voltage of 800 mV maximum, offset voltage drift of 2 mV/C. [13] Input bias currents below 25 pA, and low input voltage noise provide dc precision with source impedances up to a gig ohm. 1.8 MHz unity gain bandwidth, – 93 dB THD at 10 kHz, and 3 V/ms slew rate are provided with a low supply current of 800 mA per amplifier. The AD822 drives up to 350 pF of direct capacitive load as a follower and provides a minimum output current of 15 mA. This allows the amplifier to handle a wide range of load conditions. Its combination of ac and dc performance, plus the outstanding load drive capability, results in an exceptionally versatile amplifier for the single-supply user. The AD822 is available in two performance grades. [13]
Figure 7.12 Connection Diagram of AD822 OP AMP
40
7.1.4 Multiplexer MC74HC4051
The MC74HC4051A, utilize silicon–gate CMOS technology to achieve fast propagation delays, low ON resistances, and low OFF leakage currents. These analog multiplexers control analog voltages that may vary across the complete power supply range (from VCC to VEE). [14] The Channel–Select inputs determine which one of the Analog Inputs/Outputs is to be connected, by means of an analog switch, to the Common Output/Input. When the Enable pin is HIGH, all analog switches are turned off. The Channel–Select and Enable inputs are compatible with standard CMOS outputs; with pull up resistors they are compatible with LSTTL outputs. [14] These devices have been designed so that the ON resistance is more linear over input voltage than Ron of metal–gate CMOS analog switches. [14] For a multiplexer with injection current protection, (i) Fast Switching and Propagation Speeds (ii) Low Crosstalk Between Switches (iii) Diode Protection on All Inputs/Outputs (iv) Analog Power Supply Range (VCC – VEE) = 2.0 to 12.0 V (v) Digital (Control) Power Supply Range (VCC – GND) = 2.0 to 6.0 V (vi) Improved Linearity and Lower ON Resistance Than Metal–Gate (vii) Counterparts (viii) Low Noise
41
Figure 7.13 Pin diagram of MC74HC4051 [14]
Figure 7.5 Logic Diagram of MC74HC4051 [14] Table 7.2
Functional Table of MC74HC4051 [14]
42
The above functional table shows the control selection points used to select frequency of 128HZ, 512HZ, 1028HZ & 2049HZ and also to select feed resistor of 100R, 1K, 10K and 100K.
43
CHAPTER 8
SIMULATION RESULTS
Following table shows the result of range 0-20µS; with selection of 100K the results looks to be linear the formula for calculations is as follows o / p.voltage(mv) u K (Cons tan t ) o / p.current (mv) Following table has the K constant of X100 Table 8.1 Range 0-20 micro siemens feed Resistor 100K I/P Resistance
0/p VoltageMv
Freq =128Hz 0/P CurrentMV
Range 0-20 micro siemens ACTUAL OUT PUT VALUE K ohms
Conductivity Us
open
500
0
0.00
200K
351
177
198.31
5.05
100K
251
250
100.40
10.00
50K
177
350
50.57
20.00
44
Following graph show the input to the output relationship the graph look linear
0-20 Micro Siemens Freq 128Hz & Feed resistor 100k
250
25.00
20.00
198.31
150
20.00
15.00
10.00
100
50
100.40
10.00
5.05
0
50.57
0.00
Condcuctivity Micro Siemens
output Resi stance K ohms
200
5.00
0.00
1
2
3
4
Figure 8.1 Range 0-20 micro siemens simulations Following table shows the result of range 0-200µS; with selection of 10K the results looks to be linear the formula for calculations is as follows o / p.voltage(mv) u K (Cons tan t ) o / p.current (mv) Following table has the K constant of X100 Table 8.2 Range 0-200 micro siemens feed Resistor 10K I/P Resistance open 20K 10K 5K
0/p VoltageMv 500 350 251 177
Freq =512Hz 0/P CurrentMV 0 178 250 350
Rang - 0-200 micro siemens ACTUAL OUT PUT VALUE k ohms 19.66 10.04 5.06
Conductivity Us 0.00 50.86 100.00 1976.28
45
Following graph show the input to the output relationship the graph look linear feed resistor 10k, 0-200 Micro Siemens 25.00
2500.00
1976.28
19.66
15.00
2000.00
1500.00
10.00
10.04
1000.00
5.06
5.00
0.00 1
500.00
100.00
50.86
0.00
Conductivity Micro Siemens
output resistance Kohms
20.00
0.00
2
3
4
Figure 8.2 Range 0-200 micro siemens Following table shows the result of range 0-2000µS; with selection of 1K the results looks to be linear the formula for calculations is as follows o / p.voltage(mv) u K (Cons tan t ) o / p.current (mv) Following table has the K constant of X1
Table 8.3 Range 0-2000micro siemens feed Resistor 1K I/P Resistance open 2k 1k 500ȍ
0/p VoltageMv 500 350 251 175
Freq =1024Hz 0/P CurrentMV 0 177 250 349
Rang - 0-2000 micro siemens ACTUAL OUT PUT VALUE kohms 1.98 1.00 0.50
Conductivity Us 0.00 505.00 996.00 1996.00
46
Feed resistor 1k, 0-2000 Micro Siemens 2500.00
Output Resi stence K ohms
2.00
1996.00
1.98
1.50
2000.00
1500.00 996.00
1.00
1.00
1000.00
505.00
0.50
0.00
0.50
0.00
Conductivity MicroSiemens
2.50
500.00
0.00
1
2
3
4
Figure 8.3 Range 0-2000 micro siemens Following table shows the result of
range 0-200mS; with selection of 100
Ohm the results looks to be linear the formula for calculations is as follows o / p.voltage(mv) u K (Cons tan t ) o / p.current (mv) Following table has the K constant of X100 TABLE 8.4 Range 0-200 milli siemens feed Resistor 100ȍ I/P Resistance open 200ȍ 100ȍ 50ȍ
0/p VoltageMv 500 350 251 177
Freq = 2048Hz 0/P CurrentMV 0 177 250 350
Rang - 0-200 milli siemens ACTUAL OUT PUT VALUE kohms 197.74 100.40 50.57
Conductivity ms 0.00 50.50 99.60 197.60
47
Feed Resistor 100R,Range0 - 200ms 25.00
250.00
19.7740
197.60
15.00
200.00
150.00
99.60 10.00
10.0400
5.00
100.00
5.0571
50.00
0.00
0.00 1
Conductivity milli Siemens
output Resistance ohms
20.00
50.00
0.00 2
3
4
FIGURE 8.4 Range 0-200 milli siemens
8.1
Final simulation of sensor with electronics results
Table below show the results of sensor integrated with the circuit result this sensor has cell constant K= 1.0, and frequencies chosen from 128Hz to 2028Hz, frequencies are chosen such that for range 0-20µS. Lower frequency is chosen the basic reason is that lower frequency will help the current to flow through the solution easily; it is also good to keep the cell constant as low as possible for low conductivities main for very high accuracy requirements such as 18 M ohms requirements of ultra pure water.Same way the frequencies are chosen for 0-200 µS 512 Hz frequency is chosen for better result and for higher micro siemens such as 200ms 2 kHz frequency is chosen.More and more frequencies can be chosen for much better results; in this report 4 frequencies are proven to be better when we check the
48 input versus the output results. The table 8.5 and graph figure 8.6 clearly show the best result obtained, as we go higher and higher frequencies the reading found be bit non linear which is basically due to cell constant which is very for 200 ms range.
Table 8.5 final simulation of sensor with Circuit conductivity std solution
Actual Out Put Value
Micro siemens
Conductivity Us
K Ohms
26.6
26.76
37.3600
1015
1024.00
0.9760
1413
1403.00
0.7120
12880
13060.00
76.5300
111800
119080.00
8.3953
49
Simulation of standard solutions 140000.00
1000000
100000.00
12880
80000.00 1413
1015
1000
60000.00
40000.00 26.6 20000.00 26.76
1403.00
1024.00
13060.00
1
0.00 1
2
3
FIGURE 8.5 final simulation of sensor with Circuit
4
5
observed value M icro siem ens
standard solutions m icro siem ens
119080.00 120000.00 111800
50
CHAPTER 9
BILL OF MATERIAL
Sl no
Description
Total no
1
Stain less steel ring
4
2
Enamel wire
2mtrs
3
Plastic housing for sensor
1
4
Low noise cable 6 core
1 mtrs
5
Epoxy 2part – 3 Bond brand
50ml
6
Crystal oscillator 4.19Mhz
1
7
MC74HC4040 (U18 & U19)
2
8
MC74HC14A (U15)
1
9
MC74HC74A
1
10
MC74HC4051 (U9,U10,U20)
3
11
AD822AR (U8,U13)
2
51
CHAPTER 10 APPLICATIONS Table 10.1 Summary of conductivity applications [10] Summary of conductivity applications [10] Chemicals:[10]
Sulfuric acid and oleum Chlorine-alkali plants Sodium chloride, sodium hydroxide Hydrochloric acid Super phosphate Phosphoric acid Nitric acid Glycerin Fertilizer Detergents Waste water Moisture detection in HF
Foods:[10]
Brines – concentration Sugar: First carbonation Clean-in-place (CIP) applications Saturation control Cooker control Desalting of food products Cheese souring Evaporation control– dried milk, etc. Glucose Lye peeling of fruits and vegetables Rinsing water Waste water; and Pickle making.
Processing: [10] Generation: [10]
Interface monitoring and control Leak detection,HF alkylation; and Scrubbers
. Streams and Lake Water: [10]
Water pollution; and Salt intrusion.
Textiles: [10]
Water quality surveys Scouring baths Rinsing water Carbonizing baths Mercerizing baths Boiler water systems; and Acid washing. Water Treatment:[10]
Ion exchangers Regeneration monitors Reverse osmosis
Flue gas scrubbers Metals and Mining:[10]
Caustic/Alumina ration control Continuous steel pickling Plating solution monitoring/control Alkaline/caustic metal cleaning process Copper flotation; and Heavy metal recovery. Pulp and Paper:[10]
White Liquor Cooking liquor Black liquor Green liquor Weak wash liquor Brown stock washing Steam generation Heat exchangers
52
10.1
Conductivity measurements
Measuring conductivity simply detects the presence of ions in solution and is therefore a non-specific measurement. Conductivity applications encompass for instance monitoring of water purity, drinking water and process water quality. It is also a rapid and inexpensive way of determining the ionic strength of a solution. [1] The conductivity ț is calculated using the conductance G and the cell constant K: ț = G • K (S/cm)
10.2
Resistivity measurements
Resistivity measurements are used as a reliable indicator of ionic water quality, especially for ultra pure water (UPW) and more generally when a resistivity value is preferred to a conductivity value, for example when checking for water contamination in organic solvents.[1] The resistivity of a solution is calculated on the basis of the conductance G compensated for the cable resistance, cable capacitance and cell constant of the conductivity cell used. The resistivity ȡ is calculated as follows: ȡ = 1/ț •cm-1 [1]
53
10.3
TDS measurements
TDS measurements in the pulp and paper industry measure accurately and easily the total organic and inorganic dissolved solids in water. [1] The TDS (Total Dissolved Solids) corresponds to the total weight of cations, anions and the undissociated dissolved species in one litre of water. [1]The standard method, to determine TDS is to evaporate a measured sample of water to dryness at 180°C, under strict laboratory conditions, and carefully weigh the amount of dry solids remaining. The precision of the standard method depends on the nature of the dissolved species. [1] The TDS method in a typical conductivity meter offers a quicker and easier way of determining TDS by measuring the conductivity, then using a conversion factor to give TDS readings. [1] When working with TDS (Total Dissolved Solids) measurements, there is sometimes a need to convert units. Below are a few common measures and conversions to other units. [3] 1 Grain = 0.0648g (grams) = 64.8mg (milligrams) = 0.00228 oz (ounces). [3] Convert grains to grams: multiply grains by 0.0648 gram/grains = grams. [3] Convert ounces to grams: multiply ounces by 28.35 grams/ounce = grams. [3] Convert grams to mg (milligrams): multiply grams by 1000 mg/g = mg Convert mg to ppm in a liquid: divide mg into liters = ppm Convert ppm to ppt: divide ppm by 1000 ppm/ppt = ppt Convert ppt to ppm: multiply ppt by 1000 ppm/ppt = ppm
54 Convert ppm to mg: multiply ppm by liters = mg Convert mg to grams: divide by 1000 mg/g = grams Convert grams to ounces: divide grams by 28.35 grams/ ounce = ounces Convert grams to grains: divide grams by 0.0648 grams/grain = grains From the top, an example of converting grains to ppm and ppt: 5 grains x 0.0648 gram/grains = 0.324 grams 0.324 grams x 1000 mg/g = 324 mg 324 mg ÷ 1 liter = 324 ppm (if in 2 liters . . . 324 mg ÷ 2 liters = 162 ppm) (if in 0.5 liters . . . 324 mg ÷ 0.5 liters = 648 ppm) 324 ppm ÷ 1000 ppm/ppt = 0.324 ppt 0.324 grams in 1 liter = 0.324 ppt (because 1 gram in 1 liter = 1 ppt) For ounces, convert to ppm and ppt: .[3] 5 ounces x 28.35 grams/ounce = 141.75 grams 141.75 grams x 1000 mg/g = 141,750 mg 141,750 mg ÷ 1 liters = 141,750 ppm (If in 2 liters . . . 141,750 ÷ 2 liters = 70,875 ppm) (If in 0.5 liters . . . 141,750 ÷ 0.5 liters = 283,500 ppm) 141,750 ppm ÷ 1000 ppm/ppt = 141.75 ppt. [3]
55
10.4
Salinity measurement
When we measure the salinity of water, we look at how much dissolved salt is in the water, or the concentration of salt in the water. Concentration is the amount (by weight) of salt in water and can be expressed in parts per million (ppm). Here are the classes of water: [11] x
Fresh water - less than 1,000 ppm
x
Slightly saline water - From 1,000 ppm to 3,000 ppm
x
Moderately saline water - From 3,000 ppm to 10,000 ppm
x
Highly saline water - From 10,000 ppm to 35,000 ppm
Ocean water has a salinity that is approximately 35,000 ppm. That's the same as saying ocean water is about 3.5% salts. Sometimes, salinity is measured in different units. Another common unit is the psu (practical salinity units). Ocean water has a salinity of approximately 35 psu. [11] Ocean water is about 3.5% salts. That means that if the oceans dried up completely, enough salt would be left behind to build a 180-mile-tall, one- mile-thick wall around the equator. About 90 percent of that salt would be sodium chloride, or ordinary table salt. Chlorine, sodium and the other major dissolved salts of the ocean are listed in this table: [11]
56
Table 10.2 Dissolved salts in sea water Dissolved salts in sea water (atoms)[11] 55.3 % Chlorine 30.8 % Sodium 3.7 % Magnesium 2.6 % Sulfur 1.2 % Calcium 1.1 % Potassium
10.5
Concentration measurements
Since the charge of the ions in solution facilitates the conductance of electrical current, the conductivity of a solution is highly proportional to its ion concentration. As conductivity is a non-specific technique, concentration calculation using conductivity measurements is valid for samples containing only the species of interest. [1]
57
CHAPTER 11
ANALYSIS
There are many parameter to be taken into consideration such as Cell constant, cell material, frequency used, Temperature co-efficient and electronics components used in the circuit.
11.1
D
Temperature co-efficient
(kT2 kT1 ) u100 (T2 T2 ) u kT1
KT2 Conductivity of reference temperature Kt1 Conductivity at another temperature The temperature coefficients are as follows, in general 2.1%/deg C is used. Acids: 1.0 - 1.6%/°C Bases: 1.8 - 2.2%/°C Salts: 2.2 - 3.0%/°C Drinking water: 2.0%/°C Ultra pure water: 5.2%/°C
58
11.2
polarizations
Electric current to electrode solutions causes an accumulation of ionic species near the electrode surfaces and chemical reactions at the surfaces, producing polarization resistance.
Preventing Polarization
Applying AC current • Optimizing the electrode areas: increasing the active surface area of the electrodes with a layer of platinum black reduces the current density and consequently the polarization effect. • Using a 4-pole conductivity cell: polarization resistance has no influence on the measurement.
59
11.3
•
Geometry
Errors are caused by field effects, the part of the measuring filed falls outside the geometric space of the 2 cell.
•
4 Cell are designed to minimize this effect. Entire measuring field is contained within the body of the cell.
Figure 11.1 field effects
11.4
•
Frequency
Low frequencies are applied at low conductivities, where polarization resistance is lower.
•
High frequencies are applied at High conductivities, where solution resistance is low.
60
11.5
Cable resistance and capacitance
•
Four pole cell has no influence, since the measurement cell different from current cells.
•
Low conductivities need compensation, in general cable capacitance in order of Pico farad.
11.6
Practical Considerations for Conductivity/TDS Measurement
When using a meter to measure either the ppm or Total Dissolved Solids (TDS), or the Conductivity of a liquid, you need to periodically calibrate the meter using a calibration standard calibration solution should contain the same types of dissolved solids known to be given to each type of calibration: Conductivity calibrations are transferable from one type of solution to another ppm TDS calibrations are very specific to one type of dissolved solids solution. These calibrations MUST NOT are transferred from one type of dissolved solids solution to the next. Doing this will result in serious measurement errors. Although the basis for testing ppm of TDS is the Conductivity of the solutions, don't assume that these measurements have the same transferability to different types of solutions. It is always necessary to calibrate all TDS meters with a ppm (parts per
61 million) TDS standard calibration solution that contains the same types of salts or mixture of salts as the solution to be tested. Failure to do this will result in serious errors in the measurement of TDS. This is because TDS meters are calibrated by correlating the Conductivity of the solution to the ppm dissolved solids, and this correlation varies considerably from one type of dissolved solids to the next. In following table there are a number of standard curves which correlate the parts per million of TDS to the Conductivity of these solutions. Note that there is a great deal of variation in the slops of these curves. According to Figure 1, if a meter detects a Conductivity of 6000 µS/cm and is calibrated to read out 1030 ppm of sodium hydroxide (NaOH) as shown in the curve, the meter would not be able to accurately detect ppm contents of sodium chloride (NaCl) in solution. The correct ppm NaCl indication for the detected Conductivity of 6000 µS/cm would be 3200 ppm, as shown in Figure 1, but the meter would only indicate 1030 ppm, which is clearly unacceptable. This shows that it is incorrect to use a meter that has been calibrated for ppm NaOH indications for a ppm NaCl indication.
11.7
Conductivities of Metals can used for cell [15] Table 11.1 Conductivities of Metals Conductivity S/m at 20
Chemical
Deg C
Resistance
Aluminum
3.816 u 10 7
moderate
Low
Copper
5.813 u 10 7
low
low
Gold
4.098 u 10 7
moderate
High
Graphite
7.0 u 10 4
moderate
Very low
Platinum
9.52 u 10 6
high
Very high
Very high
Very high
moderate
moderate
Material
Titanium Stainless steel
1.1 u 10 6
Cost
62
Apart from the conductance property, cost and the chemical resistance also need to be considered among all the Titanium is very high and resistant to almost all chemicals, it is particularly useful when measuring the concentrated solutions, for moderate applications such waste water, paper and pulp, Reverse Osmosis, De ionized water, drinking water, swimming pools, cooling towers and boiler waters etc stain less steel of grade ss316 can be used.
11.8
4 cell conductivity probe eliminates polarization and contact coating effects
The 4 cell conductivity probe consists of 4 bands along a measuring column or sets of concentric rings opposite each other. An AC voltage is applied across the two outermost bands, which causes a current flow through the measuring cell. Located between this pair of electrodes is a second pair of bands. These bands measure the voltage generated across the liquid. The measured voltage across the outer bands is compared with the voltage measured by the inner bands. Any difference between the measured voltages of the two pairs of bands (whether the conductivity of the solution changes, or changes due to polarization or coating effects) initiates correcting action for the voltage across the outer bands. The correcting action remains until the current through the cell will generate a voltage across the outer bands which equal the voltage between the inner bands. Therefore, the four-band conductivity cell can correct for any fouling or polarization which may occur. [6]
63
11.9 Comparison between two cells and 4 cell Table 11.2 Comparison between two cell and 4 cell 4 cell
2 cell
Linear over very large range
non linear over large range
Good for higher conductivities
High conductivities have big errors.
no polarization effects
polarization is a major problem
need more sample
needs less sample
high cost
low cost
highly reliable
low reliability
64
CHAPTER 12
RESULTS
(i) Simulations results shows, that circuits works will in 3 range and 4th range the linearity is not good, it is due to cell constant is not good enough to sense the high range also the frequency should be higher more that 4000hz. (ii) SS 316 material used found to good for normal application; KCL solution is used calibration and checking and found to be ok. (iii) It is also found that when checked in flow or the beaker the value is not drifting since the measuring sensor different from the current. (iv) Low conductivities need compensation, in general cable capacitance in order of Pico farad. (v) 4 cell design is more robust and reading can be depended compare to 2 cell, where reading will drift due external influence and polarization and contamination. (vi) Final conductivity follows formula G = (C * L/A )/ 1 + Į( t – Tr) SS 316 show very good performance in conductivity measurement, most of the KCL standard solution the reading is found well with in the specification, and due to the cell constant 1.0 designed which is not suitable for higher range hence result has some error in the higher range.
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CHAPTER 13
CONCLUSIONS
13.1
•
Sensor
Stainless steel SS 316 4 cell ring type is proposed, as the same can used for lab and with increased dimension can used for Industrial on-line applications and for some critical application Titanium can be used as it is very good in chemical resistance.
•
4 cell is best suitable for entire range and over come the problem of polarization which will happen with 2 cell.
•
4 cell design is more robust and reading can be depended compare to 2 cell, where reading will drift due external influence and polarization and contamination.
13.2
•
Circuit
Crystal oscillator (4.19Mz) is used in this circuit, which is priced reasonable
66 and available easily, micro controller can be used to generate the frequency. •
This circuit uses high impedance op amp to prevent loss in the signals, The AD822 is a dual precision, low power FET input op amp
•
that can operate from a single supply of 3.0 V to 36 V or dual
•
supplies of ±1.5 V to ±18 V.
•
For a multiplexer used i. Fast Switching and Propagation Speeds ii. Low Crosstalk Between Switches iii. Diode Protection on All Inputs/Outputs iv. Analog Power Supply Range (VCC – VEE) = 2.0 to 12.0 V v. Digital (Control) Power Supply Range (VCC – GND) = 2.0 to 6.0 V vi. Improved Linearity and Lower ON Resistance Than Metal– Gate
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CHAPTER 14
FUTURE WORK
(i)
Micro controller shall be used for oscillator, auto ranging and also table store based of Temperature co-efficient auto correction can be done automatic.
(ii)
Different cell constant design.
(iii)
Different materials like platinum, silver, copper and other material can be used and tried for better results.
(iv)
ADC signal can further designed for Transmission purpose for 4-20mA for on-line applications.
(v)
The sensor can be practical check with different applications.
68
REFERENCES
1. Conductivity Theory and Practice – Radio meter Analytical 2. Endresss + Hauser user manual 3. Eutech Instruments Web site 4. Instrumental Methods of Analysis – Willard Merritt & Dean Settle 5. Transducers in mechanical and electronic design – Harry L.Trietley 6. On-Line conductivity measurement -www.WTW.com 7. http://www.wonhitech.co.kr/product04_11.aspleic Conductivity 8. Manual of Electrochemical Analysis, Part 3, electric conductivity – Sartorius 9. Application20% -from Internet 10. Sensorex technical education web site 11. Salinity
-
Dissolved
Salts,
Measuring
Salinity
-
http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research (UCAR). ©1995-1999, 2000 12. Frequency Management - www.fmi-inc.com
13. Analog devices –AD822 data sheet www.analog.com 14. MC74HC4051 data sheet on semiconductor - http://onsemi.com 15. conductivities of materials, Microwave Engineering David M.Pozar 1998.
Conductivity
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APPENDIX - A
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APPENDIX - D
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