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 +1000°C, bimetallic thermometers to
+1500°C and pressure thermometers to +2000°C. 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 -250°C to +1200°C. Noble-metal
thermocouples are much more expensive, but are chemically inert and
can measure temperatures up to 2300°C 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 -270°C to +1100°C 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 -40°C to +230°C, 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
600°C in industrial furnaces, etc., but only give a measurement
accuracy of ±5%. Various forms of radiation pyrometer are used over
the temperature range between -20°C and +1800°C 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 10µs. 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.
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Temperature Measurements
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