Temperature Transducers

About the temperature transducers

The suitability of different instruments in any particular measurement situation depends substantially on whether the medium to be measured is a solid or a fluid. For measuring the temperature of solids, it is essential that good contact is made between the body and the transducer unless a radiation thermometer is used. This restricts the range of suitable transducers to thermocouples, thermopiles, resistance thermometers and thermistors.

The range of instruments working on the thermal expansion principle are mainly used as temperature-indicating devices rather than as components within automatic control schemes. Mercury-in-glass thermometers are used up to +1000C, bimetallic thermometers to +1500C and pressure thermometers to +2000C. The displacement form of output in these last two devices is frequently connected mechanically to the pointer of a chart recorder in process monitoring applications. The usual measurement accuracy is in the range of 0.5% to 1.0%.

The most common instrument used in industry for temperature measurement is the base-metal thermocouple. This is relatively cheap, has a typical accuracy of 0.5% of full scale and can measure temperatures in the range of -250C to +1200C. Noble-metal thermocouples are much more expensive, but are chemically inert and can measure temperatures up to 2300C with an accuracy of 0.2% of full scale.

Resistance thermometers and thermistors are another relatively common class of devices used for measuring temperature. Resistance thermometers are used in the temperature range of -270C to +1100C to give a measurement accuracy of 0.5%. Thermistors are much smaller and cheaper, and give a fast output response to temperature changes, but they have a lower measurement sensitivity than resistance thermometers.

Dual diverse sensors are a new development which include a thermocouple and a resistance thermometer inside the same sheath. Both of these devices are affected by various factors in the operating environment, but each tends to be sensitive to different things in different ways. Thus, comparison of the two outputs means that any change in characteristics is readily detected, and appropriate measures to replace or recalibrate the sensors can be taken.

Pulsed sensors are a further recent development. They consist of a water-cooled thermocouple or resistance thermometer, and enable temperature measurement to be made well above the normal upper temperature limit for these devices. At the measuring instant, the water cooling is temporarily stopped, causing the temperature in the sensor to rise towards the process temperature. Cooling is restarted before the sensor temperature rises to a level where the sensor would be damaged, and the process temperature is then calculated by extrapolating from the measured temperature according to the exposure time.

The quartz thermometer is a relatively new development. It is fairly expensive because of the complex electronics required to analyze the frequency-change form of output. It only operates over the limited temperature range of -40C to +230C, but gives a high measurement accuracy of 0.1% within this range.

Radiation thermometers are of considerable benefit in many measurement situations because of their non-invasive mode of measurement. In some applications, the disturbance imposed by putting a transducer into a medium to measure its temperature can be significant. Radiation thermometers are non-contact devices and so avoid such disturbance. Optical pyrometers are commonly used to monitor temperatures above 600C in industrial furnaces, etc., but only give a measurement accuracy of 5%. Various forms of radiation pyrometer are used over the temperature range between -20C and +1800C and can give measurement accuracies as good as 0.05%. One particular merit of narrow-band radiation pyrometers is their ability to measure fast temperature transients of duration as small as 10s. No other instrument can measure transients anywhere near as fast as this.

The serious difficulty usually encountered in applying radiation thermometers is the need to calibrate the instrument to the body being measured, so correcting for the variable emissivity. This problem is avoided in the two-color pyrometer.

More Temperature Measurements

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