# Resistance Temperature Detectors or Resistance thermometer

Resistance temperature detectors, or resistance thermometers, employ a sensitive element of extremely pure platinum, copper or nickel wire that provides a definite resistance value at each temperature within its range.

## Thermoresistive Effect

The relationship between temperature and resistance of conductors in the temperature range near 0°C can be calculated from the equation

where

Rt = resistance of the conductor at temperature t (°C)

Rref = resistance at the reference temperature, usually 0°C

α = temperature coefficient of resistance (TCR)

Δt = difference between operating and reference temperature

• Almost all metallic conductors have a positive temperature coefficient (PTC) of resistance so that their resistance increases with an increase in temperature.
• Some materials, such as carbon and germanium, have a Negative Temperature Coefficient (NTC) of resistance that signifies that the resistance decreases with an increase in temperature.
• A high value of α is desirable in a temperature sensing element so that a substantial change in resistance occurs for a relatively small change in temperature.
• This change in resistance (ΔR) can be measured with a Wheatstone bridge, which may be calibrated to indicate the temperature that caused the resistance change rather than the resistance change itself.
Table 1: Temperature Coefficient of Resistance (TCR) for different materials
• Figure 1 indicates the variation of resistance with temperature for several commonly used materials.
• The graph shows that the resistance of platinum and copper increases almost linearly with increasing temperature, while the characteristic for nickel is decidedly nonlinear.

## Construction of RTD

• Resistance thermometers are generally of the probe type for immersion in the medium whose temperature is to be measured or controlled.
• A typical sensing element for a probe type thermometer is constructed by coating a small platinum or silver tube with ceramic material, winding the resistance wire over the coated tube, and coating the finished winding again with ceramic.
• This small assembly is then fired at high temperature to assure annealing of the winding and then it is placed at the tip of the probe. The probe is protected by a sheath to produce the complete sensing elements as shown in Figure 2.
• Practically, all resistance thermometers for industrial applications are mounted in a tube or well to provide protection against mechanical damage and to guard against contamination and eventual failure.
• Metal tubes offer adequate protection to the sensing element at temperatures up to 100°F, although they may become slightly porous at temperatures above 1500°F and then fail to protect against contamination.
• Protecting covers are designed for use in liquid or gases at high pressure such as in pipe lines, steam power plants, pressure tanks, pumping stations, etc. The use of a protecting cover becomes imperative at pressures above three times of atmospheric pressure.
• Protective wells are drilled from solid bar stock, usually carbon steel or stainless steel, and the sensing element is mounted inside. A waterproof junction box with provision for conduit coupling is attached to the top of the tube or well.

## Working of RTD

A typical bridge circuit with resistance thermometer Rt in the unknown position is shown in Figure 5.

• The function switch connects three different resistors in the circuit. RRef is a fixed resistor whose resistance is equal to that of the thermometer element at the reference temperature (say, 0°C).
• With the function switch in the ‘REFERENCE’ position, the zero adjust resistor is varied until the bridge indicator reads zero.
• Rfs is another fixed resistor whose resistance equals that of the thermometer element for full-scale reading of the current indicator.
• With the function switch in the ‘FULL SCALE’ position, the full scale adjust resistor is varied until the indicator reads the full scale. The function switch is then set to the ‘MEASUREMENT’ position, connecting the resistance thermometer Rt in the circuit.
• When the resistance temperature characteristic of the thermometer element is linear, the galvanometer indication can be interpolated linearly between the set of values of reference temperature and full scale temperature.

• The Wheatstone bridge has certain disadvantages when it is used to measure the resistance variations of the resistance thermometer. These are the effects of contact resistances of connections to the bridge terminals, heating of the elements by the unbalance current and heating of the wires connecting the thermometer to the bridge.
• Slight modifications of the Wheatstone bridge, such as the double slide wire bridge eliminates most of these problems. Despite these measurement difficulties, the resistance thermometer method is so accurate that it is one of the standard methods of temperature measurement within the range –150° to 650°C.

## Materials Used in RTD

### platinum

• Used for precision applications
• Chemically stable at high temperatures
• Resists oxidation
• Can be made into thin wires of high chemical purity
• Resists corrosion
• Can withstand severe environmental conditions.
• Useful to about 800 °C and down to below –250°C.
• Very sensitive to strain
• Sensitive to chemical contaminants
• Wire length needed is long (high conductivity)

### Nickel and Copper

• Less expensive
• Reduced temperature range (copper only works up to about 300°C)
• Can be made into thin wires of high chemical purity
• Wire length needed is long (high conductivity)
• Copper is not suitable for corrosive environments (unless properly protected)
• At higher temperatures evaporation increases resistance