Welcome
Basics about Conductivity measurement Dipl.-Ing. Manfred Schleicher
Overview Part 1 • • • • • • • •
General explanations Cell constant Relative cell constant - calibration Calibration with calibration solution ≠ 25 °C Temperature coefficient of liquids and temperature measurement Uncompensated and compensated conductivity Temperature coefficient - calibration Further example for calibration of temperature coefficient
Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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Overview Part 2
• • • • • • • •
Definition reference temperature Different temperature compensations and USP TDS-operation, costumer-specific factor, customer-characteristic curve Repeating Calibration, cleaning of conductivity cells and calibration timer Applications Conductivity in different applications Comparison of conductivity measurement systems Further transmitters for conductive measurement cells
Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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General explanations
Information about this presentation • This presentation imparts basic knowledge about conductivity measurement on example of electrolytic cells and transmitter ecoTRANS Lf 03
JUMO-BlackLine Lf
Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
Transmitter ecoTRANS Lf 03
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General explanation • Conductivity measurement cells measure the conductance in a defined space of liquid (e.g. via two plates): U~ G
I~ • A transmitter supplies a AC voltage • The bigger the current flow, the bigger the measured conductance Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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General explanation • The transmitter calculates the electric conductance with current and voltage Current Conductance = (Siemens) Voltage •
The conductance is the inverse of the ohmic resistance
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E.g. measurement of 0.002S (Siemens) stands for 500 Ohm
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0.002 S stand for 2 mS
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Measurement of conductance in 1 cm3 (cube), the conductance is: Conductivity =
2 mS x 1 cm
1cm²
=2
1 cm² Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
1cm
mS cm 7
Cell constant
Cell constant • If the measurement is not done with the „unit cell“ (distance of the plates / surface of the plates: 1cm / 1cm²) the transmitter must work with a correction factor which is called cell constant (k). Examples:
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• Surface= 1cm² , Distance= 1cm:
k=1
• Surface = 0,25cm², Distance = 1cm:
k=4
• Surface =1 cm², Distance = 0,5cm:
k=0,5
The transmitter calculates: Conductance x 1/cm x k
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Cell constant • Conductivity measurement with the mentioned cells Surface = 1cm², Distance = 1 cm: k = 1 Transmitter measures 2 mS and calculates: 2 x 1 = 2 mS/cm
Surface = 0,25cm², Distance = 1 cm: k = 4 Transmitter measures 0.5 mS and calculates: 0,5 x 4 = 2 mS/cm
Surface = 1cm², Distance = 0,5 cm: k = 0.5 Transmitter measures 4 mS and calculates: 4 x 0.5 = 2 mS/cm Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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Specification of cell constant for transmitter • Generally the cell constant is specified on the cell and must be adjusted at the transmitter
• With ecoTRANS Lf 03 cell constants in the range of k=0.01 till k=10.0 can be adjusted Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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Cell constant and measurement ranges • A cell constant of k=0.01 means an proportion distance / surface of the cell = 0.01cm / 1cm² • Cells with this constant are used for solutions with low conductivity (with the Lf03 the smallest range is 0…1 µS/cm at k=0.01) • A cell constant of k=10.0 means a proportion distance / surface of the cell = 10cm / 1cm² • Cells with this constant are used for solutions with high conductivity (with the Lf03 the biggest range is 0…200 mS/cm at k=10.0)
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Relative cell constant - calibration
Relative cell constant - calibration • The cell constant specified on the cell can vary due to production circumstances by +/- 10% • After input of the cell constant the correction factor is determined by calibration: The relative cell constant • Example: A cell constant of k=1.0 is specified at the cell; at the transmitter this is defined as follows:
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Relative cell constant - calibration • Start of calibration under „Sensor and medium characteristics“
• For the calibration a buffer solution is necessary (solution with specified conductivity)
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Relative cell constant - calibration • The specified conductivity only applies to the specified temperature. So it is essential to temper the solution exactly:
• With the buffer solution the relative cell constant can be defined
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Relative cell constant - calibration • Put the cell into the tempered buffer solution (mostly 25 °C)
• Hint: Heating up to a little bit more than 25°C and then wait until the solution is cooled down to 25°C.
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Relative cell constant - calibration • The conductivity must be specified in the configuration program
• The tolerance due to production process is compensated by calibration Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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Relative cell constant - calibration •
In the example the relative cell constant is 105.4 %
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Now the transmitter works with the constant 1(k) x 105.4 % = 1.054
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The cell has a proportional distance / surface = 1.054 cm / 1 cm²
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Calibration with buffer solution, temperature ≠ 25 °C
Uncompensated conductivity
Temperature of solution Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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Calibration with buffer solution, temperature ≠ 25 °C •
The conductivity of the buffer solution depends on the temperature
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Normally on the bottle of the solution you can read the conductivity at different temperature values, e.g.: 20 °C
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1.278 mS/cm
25 °C
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1.413 mS/cm
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The conductivity between 20 °C and 25 °C can be cal culated by interpolation, e.g. for 20.57 °C:
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The conductivity changes by 0.135 mS/cm due to temperature change of 5K, for each K 0,027 mS/cm
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When the temperature is 20.57 °C the conductivity i s 1.278 mS/cm + 0.57K x 0.027 mS/(cm x K) = 1.29 mS/cm
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Temperature coefficient of solutions and measurement of temperature
Temperature coefficient of solutions • The conductivity of solutions depends on the temperature • The temperature of a buffer solution influences the conductivity as follows (example): Conduct. [mS/cm]
25°C/ 0.4mS/cm 40°C/ 0.52mS/cm 55°C/ 0.64mS/cm
0.4
25°C
40°C
55°C
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Temperature [°C]
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Temperature coefficient of solutions • When the temperature is increased the conductivity of the solutions rises • Comparison of measurement at different temperatures is difficult • Better solution: Back calculation to the reference temperature (mostly 25 °C)
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Temperature measurement • Requirement for the conductivity measurement at the reference temperature is the measurement of the current temperature • The cells normally have a temperature sensor
Resistance thermometer
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Definition Temperature input • Temperature input setting depending on the used sensor
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Uncompensated and compensated conductivity
Uncompensated and compensated conductivity Conduct. [mS/cm]
25°C/ 0.4mS/cm 40°C/ 0.52mS/cm 55°C/ 0.64mS/cm
0.4
Temperature [°C] 25°C
40°C
55°C
• The example shows three different conductivities for the three temperatures, the conductivities are the uncompensated conductivities at the different temperatures • If the conductivity is counted back to the reference temperature, the result is the compensated conductivity Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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Uncompensated and compensated conductivity Conduct. [mS/cm]
25°C/ 0.4mS/cm 40°C/ 0.52mS/cm 55°C/ 0.64mS/cm
0.4
Temperature [°C] 25°C
40°C
55°C
• E.g. at 40 °C the solution has a conductivity of 0. 52 mS/cm (uncompensated conductivity) • The compensated conductivity (conductivity of the solution at 25 °C) is 0.4 mS/cm) • Mostly the compensated conductivity is interesting Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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Temperature coefficient - calibration
Temperature coefficient - explanation
• Many solutions change conductivity linear to the temperature • For the back calculation of the conductivity to 25 °C the straight line must be calculated: conductivity = f (temperature) • The straight line is calculated using the conductivity of the measuring solution at operation temperature (e.g. 55 °C) and the reference temperature (mostly 25 °C)
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Temperature coefficient - explanation
• In the example the conductivity changes by 0.24 mS/cm with a temperature change of 30 K • The relative change is 0.24 / 0.4 = 60 %. Change / Kelvin: 60 % / 30 K =2%/K • Based on 25°C the conductivity rises with each K of temperature by 2 % (2% = temperature coefficient) Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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Calculation of temperature coefficient • The transmitter determines the uncompensated conductivity at the reference temperature and at the operating temperature • When starting with reference temperature, the relevant solution must be tempered to 25°C:
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Start with reference temperature • Calibration under „Sensor and medium characteristics”:
Define operation temperature Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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Tempering to reference temperature (mostly 25 °C) • Hint:
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Tempering to reference temperature (mostly 25 °C) •
If the medium temperature is stable at 25°C confirm measurement
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Essential: consider delay time of temperature sensor
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The medium has to be tempered at operating temperature (e.g.60°C)
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Tempering to reference temperature (mostly 25 °C) • When the operating temperature has reached, the mesurement must be confirmed:
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Explanation for understanding • Due to calibration the temperature coefficient is 2,5 %/Kelvin • The uncompensated conductivity of the liquid at 25 °C is e.g. 0.46 mS/cm
• The transmitter calculates the compensated conductivity from the uncompensated conductivity using the temperature coefficient (conductivity at 25 °C) • At 25 °C the uncompensated and the compensated cond uctivity are the same Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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Further example for calibration of the temperature coefficient
Further example calibration temperature coefficient • The temperature coefficient results from the calibration • The conductivity of the measurement solution are determined at operating and reference temperature (in any order)
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Automatic storage of the calibration point • When reaching the operating and reference temperature the conductivity can be stored automatically:
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Calibration temperature coefficient Measurement during commissioning • Taking a part of the measurement solution out of the process • Put the sensor into the measurement solution • Wait until the shown temperature does not change any more • Define the current temperature as operating temperature
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Calibration temperature coefficient Measurement during commissioning
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Calibration temperature coefficient Measurement during commissioning • Define current temperature as operating temperature • Select „Automatic storage“ of the calibration points
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Calibration temperature coefficient Measurement during commissioning • First measurement value (operating temperature) is automatically stored
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Calibration temperature coefficient Measurement during commissioning • Solution temperature falls… • Storage of the second value when the reference temperature (25 °C) is reached
• End of calibration!
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Manual entry of temperature coefficient • The temperature coefficient can be determined using a second measurement chain or perhaps is already known • The temperature coefficient and the relative cell constant can be manually entered into the transmitter
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Definition reference temperature
Reference temperature • Factory setting: reference temperature 25 °C • The transmitter calculates the conductivity at 25 °C (compensated conductivity) • The reference temperature can be changed • In this example the transmitter calculates the conductivity at 30 °C
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Different temperature compensations and USP
Linear temperature compensation • Many solutions change the conductivity linear to the temperature Conduct. [mS/cm]
Uncompensated conductivity
25°C
Temperature [°C]
• In this case the temperature behaviour is defined using one temperature coefficient • The factory setting of the temperature compensation is linear:
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Natural Water (DIN EN27888 bzw. ISO 7888) • The temperature compensation uses the accordingly defined non-linear temperature compensation • Use in natural groundwater, spring water and surface waters • Defined temperature range: 0°C ≤ T < 36°C.
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ASTM • Use of temperature compensation for measurements in ultrapure water • Considers the extremely nonlinear behavior of the temperature dependence for neutral impurities in accordance to the standard • Defined temperature range : 0°C < T < 100°C
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USP • Monitoring the quality of ultrapure water acc. default USP • Limit for the uncompensated conductivity as a function of temperature, extract from USP :
• If the conductivity is higher than specified in the USP table (not ultrapure water quality), a so called USP contact can be activated Basics about Conductivity measurement | Manfred Schleicher (2012-03-01)
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USP Configuration • Switch off the temperature compensation
• The function is assigned to an output:
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TDS-operation, costumer-specific factor, customer-characteristic curve
*AQUIS 500 CR & dTRANS CR 02 only
TDS-operation • TDS (Total Dissolved Solids) indicates the share of the dissolved solids in a liquid • Common units are mg/ l or ppm • The TDS-value can be generally calculated from the conductivity • Conductivity (µS) x TDS factor = TDS-value • The TDS-factor can be directly input to different JUMO conductivity transmitters
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TDS-operation Configuration • Choosing TDS-operation
• Enter TDS-factor:
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Costumer-specific factor • In the TDS-Operation,the conductivity in µS/ cm will be multiplied with the TDS-factor • Bei using other units (mS/ cm, kΩ*cm, MΩ*cm) for multiplication, the costumer-specific factor can be used
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Costumer-specific factor Configuration • Choosing costumer-specific factor
Unit after multiplication with a factor Input In the example conductivity in mS/cm is factorized with the result in ppm
• The multiplier is given by the TDS-factor:
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Customer-characteristic curve • For linear relation the “TDS-operation“ and “Costumer-specific factor“ allows to calculate a further value from the conductivity • If the increasing of the value is not linear with the increasing of the conductivity, a linearization can be done by the conductivity transmitter
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Customer-characteristic curve Configuration • Choosing costumer characteristic curve and units
µS/ cm will be linearized in ppm
Measured Conductivity in e.g. µS/cm Linearized sign e.g. ppm
• Definition of the calibration points:
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Recommended installation, Repeating Calibration, cleaning and calibration timer
Recommended Installation of Conductivity cells Target - Prevention of air bubbles and depositon
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Not recommended Installation of Conductivity cells Target - Prevention of air bubbles and depositon
Accumulation of air bubbles
Accumulation of air bubbles
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Accumulation of deposition 66
Calibration / cleaning of cells •
The temperature coefficient of a measurement solution which does not change it’s consistence remains the same. There is only one calibration necessary
•
In regular time intervals the calibration of the relative cell constant is executed again during maintenance
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The measurement cell must be cleaned before calibration
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You can use e.g. 2 % salt acid for cleaning (alternatively acetic acid or citric acid)
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Calibration timer •
Select the time for a calibration timer in the transmitter
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Starting with the end of a calibration the time of the calibration timer counts down
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Calibration timer • When the time ran out, the relay output can be activated • The relay can control a signal lamp, the timer end is shown on the display
• The time of the calibration timer is reset after each calibration
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Applications
Applications – Open cooling circuits •
Evaporation causes of water cooling down
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Evaporated water is replaced by new water
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Minerals remain in the water salt water intrusion
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Conductivity is the measuring unit for salt water intrusion (↑ Conductivity ↑ salt water intrusion)
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If a defined conductivity is reached, the water is partially exchanged
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Applications – CIP-Processes •
Plants of the food industry are cleaned with different liquids
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Substances (e.g. water, caustic soda, nitric acid, peracetic acid etc.) are stored in tanks for multiple use
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The medium inside the pipe system is determined by measuring the conductivity
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Applications – CIP-Processes •
Acids and bases must be provided in a determined concentration
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Concentration is determined as well by measurement of the conductivity
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The CTI 500/750 offers the function concentration = f (conductivity) for caustic soda and nitric acid …
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Applications – Bottle cleaning plant •
Returnable bottles are cleaned in cleaning plants in several zones (water – caustic soda solution – water)
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With a conductivity measurement it is determined if the caustic solution is used and/or the water is dirty
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Conductivity in different applications
Conductivity in different applications
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Comparison conductivity measurement systems
Comparison conductivity measurement systems
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Further transmitters for conductive measurement cells
Further transmitters for conductive measurement cells
JUMO dTRANS CR 02 (transmitter / controller)
JUMO AQUIS 500 CR (transmitter / controller)
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Thank you very much for your attention.
