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Resistance thermometers are slowly replacing thermocouples in many lower temperature industrial applications (below 600°C). Resistance thermometers come in a number of construction forms and offer greater stability, accuracy and repeatability. The resistance tends to be almost linear with temperature. A small power source is required.
No special extension cables or cold junction compensations are required The resistance of a conductor is related to its temperature. Platinum is usually used due to its stability with temperature. The Platinum detecting wire needs to be kept free of contamination to remain stable. A Platinum wire or film is created and supported on a former in such a way that it gets minimal differential expansion or other strains from its former, yet is reasonably resistant to vibration.
Commercial platinum grades are produced which exhibit a change of resistance of 0.385 Ohms/°C (European Fundamental Interval) The sensor is usually made to have 100 Ohms at 0 °C. This is defined in BS EN 60751:1996. The American Fundamental Interval is 0.392 Ohms/°C.
Resistance thermometers require a small current to be passed through in order to determine the resistance. This can cause self heating and manufacturers limits should always be followed along with heat path considerations in design. Care should also be taken to avoid any strains on the resistance thermometer in its application.
Lead wire resistance should be considered and adopting three and four wire connection strategies can result in eliminating connection lead resistance effects from measurements.
Resistance thermometers elements are available in a number of forms. The most common are:
Wire Wound in a ceramic insulator - High temperatures to 850 °C
Wires encapsulated in glass - Resists the highest vibration and offers most protection to the Pt Thin film with Pt film on a ceramic substrate, Inexpensive mass production
Practical Construction
These elements will nearly always require insulated leads attached. At low temperatures PVC, Silicon rubber or PTFE insulators are common to 250 °C. Above this Glass fibre or ceramic are used. The measuring point and usually most of the leads require a housing or protection sleeve. This is often a metal alloy which is inert to a particular process.
Often more consideration goes in to selecting and designing protection sheaths than sensors as this is the layer that must withstand chemical or physical attack along with offering convenient process attachment features.
Standard Resistance Thermometer Data
Temperature sensors are usually supplied with Thin film Elements These are rated as:
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Continuous operation
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-70 to +500 °C
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Tolerance class B
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-70 to +500 °C
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Tolerance class A (1/2B)
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-30 to +350 °C
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Tolerance class 1/3B
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0 to +100 °C
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Resistance Thermometer elements can be supplied which function up to 850 °C Sensor Tolerances are calculated as:
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Class B
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change in t=+/- (0.3+0.005|t|)
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Class A
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change in t=+/- (0.15+0.002|t|)
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1/3 Class B
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change in t=+/- 1/3 x (0.3+0.005|t|)
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1/5 Class B
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change in t=+/- 1/5 x (0.3+0.005|t|)
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1/10 Class B
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change in t=+/- 1/10 x (0.3+0.005|t|)
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Where |t| = absolute temperature in °C. Where elements have a resistance of n x 100 Ohms then the basic values and tolerances also have to be multiplied by n
Resistance Thermometer Wiring Configurations
Two Wire Configuration
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Three Wire Configurationn
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Four Wire Configurationn
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