ConApplication Whitepaper_ WP_T_Contacting_Temperature_Measurement

ABB temperature measurement Thermocouples and resistance temperature detectors

Measurement made easy Process temperature measurement practice--contacting

By Gary Freemer, ABB Measurement Products Temperature is one of seven basic values in the current SISystem of units and is probably the most important parameter in measurement technology. Temperature measurements roughly divide into three application categories: - Precision temperature measurements for scientific and basic research - Technical temperature measurements for measurement and control technology - Temperature monitoring using temperature indicators. This white paper will cover typical contacting methods for industrial processing and control. Another available white paper from ABB covers non-contacting methods such as radiation thermometers. Historical background In the mid-18th century Scottish physicist James Clerk Maxwell defined temperature as a property that makes possible the transfer of heat energy to or from one body to another. For Maxwell, temperature was the measure of the average kinetic energy of the molecules within a system. Measurement of temperature provided a way to determine the heat energy content of that system. So to determine the temperature of a system from first principles, you should measure the velocity or motion of its molecules. The system will have no heat content when the molecules have lost all their kinetic energy and are completely at rest (a temperature of absolute zero on the Kelvin scale). Observing and measuring molecular motion is impractical 1 | ABB

Figure 1. Different quantities of electrons accumulate at the cold ends of dissimilar metal conductors, creating a voltage potential. and unrealistic. For practical measurements, we resort to other methods, such as the effects of heat energy on system properties such as geometric expansion and electrical phenomena. The temperature scale must be independent of the special characteristics of the materials used for measurement. The scale must ideally apply to the entire temperature range from the lowest to the highest temperatures. This ensures the transferability of measurement results. The latest is the International Temperature Scale of 1990. ITS-90 defines a temperature scale in the range from 0.65 Kelvin to far above 3000 Kelvin. This scale uses fixed points consisting of phase equilibrium values for extremely pure substances, such as the triple point of water. Mathematical relationships define the temperature values

between points. In industrial practice, electrical temperature contacting sensors--namely thermocouples (TCs) or resistance temperature detectors (RTDs)--heavily dominate measurement and control technology. TCs and RTDs transform the measured temperature value into an electrical signal. A connected field transmitter converts the raw signal to a standard value, usually 4 to 20mA or a corresponding digital output. Thermocouples are more rugged than RTDs and tend to be less expensive. RTDs, on the other hand, are more accurate and can be provided with good thermowell or protection tube design combined with robust installation techniques.

melting point of platinum at 1769 °C (3216 °F), the output voltages for continuous operation are stable only to about 1300 °C (2372 °F). At the higher temperatures grain growth in the wires limits the thermocouple's life span, reducing mechanical strength. Also impurities can diffuse into the wires and change the thermal voltage. The thermocouple is most stable when operated in a clean, oxidizing environment such as air, although short term use in inert, gaseous atmospheres or in a vacuum is possible. Without suitable protection, it should not be used in reducing environments. Metallic protection tubes can be used at the temperatures below 1200 °C (2192 °F). Above these temperatures the ceramics, in particular highly pure aluminum oxide, are most suitable.

Thermocouples As early as 1822 T. J. Seebeck published the observation that a current develops in an electrical circuit comprising two dissimilar metal conductors when each of the two connection points of the conductors is at a different temperature. The thermal current results from a small voltage that's proportional to the temperature difference between the hot and cold ends. It's also a function of the kinds of dissimilar metals. In effect, electrons accumulate on the cold side of one warmed metallic conductor. Fewer or more electrons accumulate on the cold side of the other metallic conductor. The difference in the quantity of electrons at the cold conductor ends accounts for the generated voltage. For identical conductors, the quantity of electrons at the cold ends would be the same, so the effects cancel and no thermal voltage results. The simplest thermocouple designs are those made using insulated thermal wires. The usual insulation materials are glass fibers, mineral fibers, PVC, silicone rubber, Teflon or ceramic. They must be compatible with the installation requirements, such as chemical resistance, temperature resistance, and moisture protection. The color code for the insulation of positive conductor for all thermocouple types is red. The color code for the negative conductor depends on the thermocouple type. Conventional standard thermocouples divide into two groups: precious metal thermocouples Types S, R and B, and the base metal thermocouples Types E, J, K, N and T. Precious metal thermocouples

Figure 2. Thermal voltage characteristics of standard thermocouples.

Type S--Here the positive conductor is platinum alloyed with 10% rhodium while the negative conductor is platinum. While the Type S thermocouple can be used in a temperature range from -50 °C (-58 °F) almost to the

Type R--This thermocouple is similar to type S, having one conductor of platinum alloyed with 13% rhodium and the other of pure platinum. For the most part of its defined temperature range, a Type R thermocouple has a

2 | ABB

temperature gradient about 12% higher than the Type S. The remaining material properties are identical. Type B--Introduced in the 1950s, this thermocouple satisfies requirements for temperature measurements in the range 1200...1800 °C (2192...3272 °F). The positive conductor contains platinum alloyed with 30% rhodium while the negative conductor is platinum alloyed with 6% rhodium. As temperatures approach extreme high end of the range above, operation is measured in hours before appreciable changes in the output thermal voltage occur. Compared to Types S and R, Type B thermocouples offer improved stability, increased mechanical strength, and higher temperature capabilities. Base metal thermocouples Type J--A steep temperature gradient and low material cost have made this one of the most commonly used industrial thermocouples today. The positive conductor is iron and the negative conductor is a copper-nickel alloy called constantan. Although the basic values for Type J in the standard define a temperature range from -210...1200 °C (-346...2192 °F), it's suitable only in a range of 0...750 °C (32...1382 °F) when operating continuously. At higher temperatures the oxidation rate for both conductors increases rapidly. The Type J can be used in vacuum, oxidizing, reducing or inert atmospheres. In sulfur environments, suitable protection should be employed at temperatures above 500 °C (932 °F). Type K--This thermocouple type for middle temperatures better resists oxidation than Types J and E. It's used today in many applications for temperatures over 500 °C (932 °F). Nominally, the thermocouple contains a nickelchromium alloy compared against a nickel-aluminum alloy. While its basic temperature range is -270... 1372 °C (454...2501 °F), at temperatures over 750 °C (1382 °F) the oxidation rate in air for both conductors increases sharply. Suitable protection is necessary at higher temperatures for installations in sulfur and reducing atmospheres. Special considerations also apply for Type K when used in a vacuum or steam. Type N--The newest standard thermocouple, it offers greater thermoelectric stability at temperatures over 870 °C (1598 °F) and less tendency to oxidize compared to thermocouples Types J, K and E. Type N normally consists of a nickel-chromium-silicon alloy paired with a nickel-silicon alloy conductor. Of all the base metal thermocouples, Type N best suits applications with oxidizing, damp, or inert atmospheres. At higher temperatures suitable protection is still necessary in reducing atmospheres and those containing sulfur.

3 | ABB

Type T--One of the oldest thermocouples for low temperature measurements, this is commonly used in the triple point range for neon at -248.5939 °C (-415.4690 °F) up to 370 °C (698 °F). Type T contains a copper conductor paired with a conductor consisting of a coppernickel alloy. It exhibits good thermoelectric homogeneity. The Type T can serve in vacuum, oxidizing, reducing, or inert atmospheres. It is not recommended for use in environments containing hydrogen above 370 °C (698 °F) without suitable protection. Type E--This is the most common thermocouple for low temperature measurements. It consists of a nickelchromium conductor paired with copper-nickel alloyed conductor. For temperatures over 750 °C (1382 °F), the oxidation rate in air for both conductors is high. Protection is necessary in reducing and sulfur containing atmospheres. Interconnection cables The reference junction of the thermocouple may be a great distance from the measurement site for a variety of reasons. Additionally, precious metal thermocouples are costly to route over long distances. In these cases an interconnection cable can connect the actual thermocouple to the reference junction. The interconnection cable must have the same thermoelectrical properties as the corresponding thermocouple over a limited temperature range--usually 25 °C (-13 °F) to 200 °C (392 °F). This range may depend on the temperature resistance of the insulation material. So-called thermal cables have the same nominal composition as the corresponding thermocouple. Compensating cables, onthe other hand, may consist of conductors of different alloys that have the same thermoelectrical properties over a limited temperature range. Standard color codes identify these interconnection cables Resistance temperature detectors The electrical resistivity of all metals increases greatly with increasing temperatures. The electrical resistance of a metal depends on movement of its conduction electrons, which are the surface electrons of the metal's atoms. The atoms of the metal form a dense ion lattice structure that oscillates. As the temperature increases, the oscillation amplitude also increases. This impedes the motion of the conduction electrons. As a result the metal's electrical resistance is temperature dependent. Because of flaws in the metal's crystalline structure, the relationship between temperature and electrical resistance

is slightly nonlinear, but can be approximated by a polynomial. Metals suitable for use as resistance thermometers should have the following properties: - highly sensitive to temperature changes - high chemical resistance - easy workability - availability in a very pure state and - excellent reproducibility of their electrical properties.

temperatures, usually between 0 °C and 100 °C (32 °F and 212 °F). Standard EN 60751 for the platinum sensors specifies the temperature relationship to the resistance, the nominal value, the allowable deviation limits, and the temperature range. For example, in the range from 0...100 °C (32...212 °F) platinum has a temperature coefficient of 0.00385 K-1. This means that a Pt100 measurement resistor at 0 °C (32 °F) has a resistance of 100 ohms and at 100 °C (212 °F) 138.5 ohms. Outside this temperature range other polynomials come into play as correction factors. For small temperature ranges engineers can assume a linear relationship.

Figure 3. Thermal resistance characteristics for various RTDs.

Additionally, the materials should not change their physical and chemical properties in the temperature range of interest. Freedom from hysteresis effects and a high degree of pressure insensitivity are further requirements. Platinum, in spite of its high price, has become dominant as the resistance material of choice for industrial applications. Alternative materials such as nickel, molybdenum, and copper are also used, but play a subordinate role.

Figure 4. WireWire-wound RTD construction.

For platinum the relationship of resistance to temperature is a simple polynomial of the following form: - Rt = R0 (1 + At + Bt2) for t equal or greater than 0 °C - Rt = R0 (1 + At +Bt2 +C(t-100)t3) for t less than 0 °C The value R0 is the resistance of the thermometer at 0 °C. Standard EN 60751 defines the coefficients A, B and C as well as other important properties that platinum resistance thermometers must satisfy. A = 3.9083 x 10-3 K-1 B = -5.775 x 10-7 K-2 C = -4.183 x 10-12 K-4 RTDs with a nominal value of 100 ohms (Pt100) have advantages mentioned above plus a wide application range between -250 °C and 850 °C (-418 °F and 1562 °F). The temperature coefficient of the electrical resistance defines the change in electrical resistance between two

4 | ABB

Figure 5. ThinThin-film RTD construction.

Platinum measurement resistors fall into two categories: thin film and wire-wounded resistors. Ceramic, glass, or plastic serves as the basic carrier materials. Manufacturers can place thin film layers on carriers via vacuum vapor deposition, sputtering, or by sintering a thick platinum paste. They make wire-wound RTD sensing elements by coiling a platinum wire inside or outside a mandrel. For chemical industry applications, most RTDs have internal

coils that are sheathed for protection. The lead wires, especially long wires, from the RTD sensor to the measuring transmitter can add a small amount of resistance, affecting accuracy. To compensate, three-wire and four-wire RTDs have been developed. The temperature transmitter uses these extra wires to offset the lead-wire error. An alternative is to mount the transmitter directly on the thermowell containing the RTD. Protecting temperature sensors Directly installing a temperature sensor without protection can improve the response time. Since the diameters are smaller, the installation length can be made short. Thermocouples, in constrast to RTDs, measure at point locations, allowing short installation lengths.

economical thermowells made from tubing material with a welded plug at the outer end. Specific thermowell designs have been developed for such applications as: - Hot gas measurements in a furnace - Reactors operating with high pressures and temperature - Pipes carrying gases with high particle loads - Flue gas channels - Multi-point sensors in large tanks - Metal melting and salt baths - Plastic extruders - Food and pharmaceutical manufacturing - Surface temperatures - Housing and wall temperatures - Pump bearings.

But few applications in a chemical plants would permit directly installed sensors. Typically plants place temperature sensors within protective thermowells. This increases the life of the sensor under adverse conditions and facilitates a fast sensor exchange without interrupting the process. Thermowells must: - Position the temperature sensitive sensor tip in the process - Protect the temperature sensor - Seal the process areas from the environment. Thermowell selection depends on both the process parameters and on required measurement parameters. Engineers tend to prefer metal thermowells since they assure an absolute seal against the medium and the pressure. But their use is limited to temperatures below 1150...1200 °C (2102...2192 °F). Above these temperatures their mechanical strength and oxidation resistance can shorten operating life. Ceramic thermowells, despite their brittle properties, come into play for very high temperatures or when operating conditions exclude metal. These thermo-wells require special considerations since a single impact could lead to their sudden and complete destruction. In critical installations a second barrier may be necessary to prevent the escape of hazardous material. Thermowells are available in proven and standardized forms with a variety of different process connections. For standardized thermowells, manufacturers publish load diagrams that specify the maximum allowable pressure in air/steam or water at a specific temperature and a specific maximum flow velocity. For processes having less demanding conditions, manufacturers can supply

5 | ABB

Figure 6. Response times increase for sensors installed in thermowells. Coated thermowells increase response times further.

Dynamic Response of Temperature Sensors When the temperature of the measured medium changes, the sensor reacts. Its output signal approaches the new temperature. Finally when the output signal no longer indicates any measurable changes, it has reached equilibrium with the medium's new temperature. Knowledge of the sensor's dynamic response or time constant is important when measuring processes with fast changing temperatures and for sensors operating in control loops. The use of good heat conducting materials and pastes wherever possible will help optimize the sensor's dynamic response. The dynamic response depends on the temperature sensor design, the medium undergoing measurement, and installation parameters. Factors related to the measured medium include its heat capacity, heat transfer coefficient, and flow velocity, but these are given. The sensor's size, weight, material, and internal construction will affect its dynamic response. A thermowell or protection tube will increase the overall response time, as will any protective coatings. Obviously smaller sensors and thermowells provide faster response times. But size must be balanced against the

conditions that the sensor/thermowell must withstand. The chemical medium acts mechanically on the installation through pressure, flow velocity, eddy formation, and vibration. Fatigue failure from vibration is problematical when measuring high velocity streams and when using bare element installations. In the case of thermocouples, grounding the hot junction to the thermowell or sheath improves thermal conductivity and thereby minimizes response time. But this also makes the circuit susceptible to electrical noise and contamination. The reverse is true of ungrounded junctions. Other contacting techniques While thermocouples and RTDs represent virtually all the temperature measurement devices in the chemical processing industries, two other methods are worth noting: semiconductor and silicon measurement resistors. Semiconductors exhibit a characteristic change of their electrical resistance with temperature changes. Cold wire (PTC) and hot wire (NTC or thermistors) types act differently. Semiconductor PTCs are polycrystalline ceramics based on barium titanate. This material generates a very large increase of the electrical resistance in a narrow temperature range. The ideal range is between -50 °C (-58 °F) and 150 °C (302 °F). Also, PTC's have a temperature at which the resistance increases dramatically. For this reason they are specially suitable for use as temperature limit switches for machines and systems. Semiconductor NTCs, on the other hand, consist of a mixture of polycrystalline ceramic oxides, with NiO, CaO, Li2O additives. Their normal temperature range is from 110 °C (-166 °F) to 300 °C (572 °F). Their resistance can change in exponential fashion for changes in temperature. Because of the non-linear curve and the drift when subjected to temperature change stresses, they've experience limited acceptance in industrial measurement technology. Silicon measurement resistors possess a pronounced positive temperature coefficient and can serve for temperature measurements between -70 °C (-94 °F) and 160 °C (320 °F). Over this range the curves deviate only slightly from linear. Silicon measurement resistors have a high temperature coefficient and long term stability. But to date they have not found wide acceptance in the processing industries.

6 | ABB

ABB Inc. 125 East County Line Road Warminster, PA 18974 USA Tel: +1 215 674 6000 Fax: +1 215 674 71 www.abb.com

Notes: Notes We reserve the right to make technical changes or modify the contents of this document without prior notice. With regard to purchase orders, the agreed particulars shall prevail. ABB does not accept any responsibility whatsoever for potential errors or possible lack of information in this document. We reserve all rights in this document and in the subject matter and illustrations contained therein. Any reproduction, disclosure to third parties or utilization of its contents –in whole or in parts – is forbidden without prior written consent of ABB.

Copyright© 2013 ABB All rights reserved

7 | ABB

WP_T_Contacting_Temperature_Measurement_02-2012

Contact us