Guide to Measurement and Instrumentation -- Semiconductor Devices and Radiation Thermometers

Semiconductor Devices

Semiconductor devices, consisting of either diodes or integrated circuit transistors, have only been commonly used in industrial applications for a few years, but they were first invented several decades ago. They have the advantage of being relatively inexpensive, but one difficulty that affects their use is the need to provide an external power supply to the sensor.

Integrated circuit transistors produce an output proportional to the absolute temperature.

Different types are con figured to give an output in the form of either a varying current (typically 1 mA_ K) or a varying voltage (typically 10 mV_ K). Current forms are normally used with a digital voltmeter that detects the current output in terms of the voltage drop across a 10-K-ohm resistor. Although the devices have a very low cost (typically a few dollars) and a better linearity than either thermocouples or resistance thermometers, they only have a limited measurement range from _50 to +150_ C. Their inaccuracy is typically _3%, which limits their range of application. However, they are widely used to monitor pipes and cables, where their low cost means that it’s feasible to mount multiple sensors along the length of the pipe/cable to detect hot spots.

In diodes, the forward voltage across the device varies with temperature. Output from a typical diode package is in the micro-amp range. Diodes have a small size, with good output linearity and typical inaccuracy of only _0.5%. Silicon diodes cover the temperature range from _50 to +200_ C and germanium ones from _270 to +40_ C.

Radiation Thermometers

Fgr. 10 Wavelength

All objects emit electromagnetic radiation as a function of their temperature above absolute zero, and radiation thermometers (also known as radiation pyrometers) measure this radiation in order to calculate the temperature of the object. The total rate of radiation emission per second is given by:

E = KT4

The power spectral density of this emission varies with temperature in the manner shown in Fgr. 10. The major part of the frequency spectrum lies within the band of wavelengths between0.3and40 mm, which corresponds to visible (0.3_0.72 mm) and infrared (0.72_1000 mm) ranges. As the magnitude of the radiation varies with temperature, measurement of the emission from a body allows the temperature of the body to be calculated. Choice of the best method of measuring the emitted radiation depends on the temperature of the body. At low temperatures, the peak of the power spectral density function lies in the infrared region, whereas at higher temperatures it moves toward the visible part of the spectrum. This phenomenon is observed as the red glow that a body begins to emit as its temperature is increased beyond 600_ C.

Different versions of radiation thermometers are capable of measuring temperatures between _100 and +10,000_ C with measurement inaccuracy as low as _0.05% in the more expensive versions (although this level of accuracy is not obtained when measuring very high temperatures).

Portable, battery-powered, hand-held versions are also available, and these are particularly easy to use. The important advantage that radiation thermometers have over other types of temperature-measuring instruments is that there is no contact with the hot body while its temperature is being measured. Thus, the measured system is not disturbed in any way.

Furthermore, there is no possibility of contamination, which is particularly important in food, drug, and many other process industries. They are especially suitable for measuring high temperatures beyond the capabilities of contact instruments such as thermocouples, resistance thermometers, and thermistors. They are also capable of measuring moving bodies, for instance, the temperature of steel bars in a rolling mill. Their use is not as straightforward as the discussion so far might have suggested, however, because the radiation from a body varies with the composition and surface condition of the body, as well as with temperature.

This dependence on surface condition is quantified by the emissivity of the body. The use of radiation thermometers is further complicated by absorption and scattering of the energy between the emitting body and the radiation detector. Energy is scattered by atmospheric dust and water droplets and is absorbed by carbon dioxide, ozone, and water vapor molecules.

Therefore, all radiation thermometers have to be calibrated carefully for each particular body whose temperature they are required to monitor.

Various types of radiation thermometers exist, as described next. The optical pyrometer can only be used to measure high temperatures, but various types of radiation pyrometers are available that, between them, cover the whole temperature spectrum. Intelligent versions (see Section 12) also now provide full or partial solutions to many of the problems described later for non-intelligent pyrometers.

Optical Pyrometer

The optical pyrometer is designed to measure temperatures where the peak radiation emission is in the red part of the visible spectrum, that is, where the measured body glows a certain shade of red according to the temperature. This limits the instrument to measuring temperatures above 600_ C. The instrument contains a heated tungsten filament within its optical system. The current in the filament is increased until its color is the same as the hot body: under these conditions the filament apparently disappears when viewed against the background of the hot body. Temperature measurement is therefore obtained in terms of the current flowing in the filament. As the brightness of different materials at any particular temperature varies according to the emissivity of the material, the calibration of the optical pyrometer must be adjusted according to the emissivity of the target. Manufacturers provide tables of standard material emissivities to assist with this.

The inherent measurement inaccuracy of an optical pyrometer is_5_ C. However, in addition to this error, there can be a further operator-induced error of _10_ C arising out of the difficulty in judging the moment when the filament "just" disappears. Measurement accuracy can be improved somewhat by employing an optical filter within the instrument that passes a narrow band of frequencies of wavelength around 0.65 mm corresponding to the red part of the visible spectrum. This also extends the upper temperature measurable from 5000_ C in unfiltered instruments up to 10,000_ C The instrument cannot be used in automatic temperature control schemes because the eye of the human operator is an essential part of the measurement system. The reading is also affected by fumes in the sight path. Because of these difficulties and its low accuracy, hand-held radiation pyrometers are rapidly overtaking the optical pyrometer in popularity, although the instrument is still widely used in industry for measuring temperatures in furnaces and similar applications at present.

Fgr. 11 -- Incoming radiation Filament Eyepiece Varying current

Fgr. 12 --- Radiation from object Detector

Radiation Pyrometers

All the alternative forms of radiation pyrometers described here have an optical system similar to that of the optical pyrometer and focus the energy emitted from the measured body.

However, they differ by omitting the filament and eyepiece and having instead an energy detector in the same focal plane as the eyepiece was. This principle can be used to measure temperature over a range from_100 to 360 _ .C

The radiation detector is either a thermal detector, which measures the temperature rise in a black body at the focal point of the optical system, or a photon detector.

Thermal detectors respond equally to all wavelengths in the frequency spectrum and consist of thermopiles, resistance thermometers, or thermistors. All of these typically have time constants of several milliseconds because of the time taken for the black body to heat up and the temperature sensor to respond to the temperature change.

Photon detectors respond selectively to a particular band within the full spectrum and are usually of the photoconductive or photovoltaic type. They respond to temperature changes much faster than thermal detectors because they involve atomic processes, and typical measurement time constants are a few microseconds.

Fiber-optic technology is used frequently in high temperature measurement applications to collect the incoming radiation and transmit it to a detector and processing electronics that are located remotely. This prevents exposure of the processing electronics to potentially damaging, high temperature. Fiber-optic cables are also used to apply radiation pyrometer principles in very difficult applications, such as measuring the temperature inside jet engines by collecting the radiation from inside the engine and transmitting it outside.

The size of objects measured by a radiation pyrometer is limited by the optical resolution, which is defined as the ratio of target size to distance. A good ratio is 1:300, which would allow temperature measurement of a 1-mm-sized object at a range of 300 mm. With large distance/ target size ratios, accurate aiming and focusing of the pyrometer at the target are essential. It’s now common to find "through the lens" viewing provided in pyrometers, using a principle similar to SLR camera technology, as focusing and orientating the instrument for visible light focuses it automatically for infrared light. Alternatively, dual laser beams are sometimes used to ensure that the instrument is aimed correctly toward the target.

Various forms of electrical output are available from the radiation detector: these are functions of the incident energy on the detector and are therefore functions of the temperature of the measured body. While this therefore makes such instruments of use in automatic control systems, their accuracy is often inferior to optical pyrometers. This reduced accuracy arises first because a radiation pyrometer is sensitive to a wider band of frequencies than the optical instrument and the relationship between emitted energy and temperature is less well defined.

Second, the magnitude of energy emission at low temperatures gets very small, according to Equation (9), increasing the difficulty of accurate measurement.

The forms of radiation pyrometer described here differ mainly in the technique used to measure the emitted radiation. They also differ in the range of energy wavelengths, and hence the temperature range, which each is designed to measure. One further difference is the material used to construct the energy-focusing lens. Outside the visible part of the spectrum, glass becomes almost opaque to infrared wavelengths, and other lens materials such as arsenic trisulfide are used.

Broad-band (unchopped) radiation pyrometers --- The broad-band radiation pyrometer finds wide application in industry and has a measurement inaccuracy that varies from _0.05% of full scale in the best instruments to _0.5% in the least expensive. However, their accuracy deteriorates significantly over a period of time, and an error of 10_ C is common after 1_2 years operation at high temperatures. As its name implies, the instrument measures radiation across the whole frequency spectrum and so uses a thermal detector. This consists of a blackened platinum disc to which a thermopile} is bonded. The temperature of the detector increases until the heat gain from the incident radiation is balanced by the heat loss due to convection and radiation. For high-temperature measurement, a two couple thermopile gives acceptable measurement sensitivity and has a fast time constant of about 0.1 s. At lower measured temperatures, where the level of incident radiation is much less, thermopiles constructed from a greater number of thermocouples must be used to get sufficient measurement sensitivity. This increases the measurement time constant to as much as 2 s.

Standard instruments of this type are available to measure temperatures between _20 and +1800_ C, although much higher temperatures in theory could be measured by this method.

Chopped broad-band radiation pyrometers---Construction of this form of pyrometer is broadly similar to that shown in Fgr. 12 except that a rotary mechanical device is included that periodically interrupts the radiation reaching the detector. The voltage output from the thermal detector thus becomes an alternating quantity that switches between two levels. This form of a.c. output can be amplified much more readily than the d.c. output coming from an unchopped instrument. This is particularly important when amplification is necessary to achieve an acceptable measurement resolution in situations where the level of incident radiation from the measured body is low. For this reason, this form of instrument is the more common when measuring body temperatures associated with peak emission in the infrared part of the frequency spectrum. For such chopped systems, the time constant of thermopiles is too long. Instead, thermistors are generally used, giving a time constant of 0.01 s. Standard instruments of this type are available to measure temperatures between +20 and +130_ C. This form of pyrometer suffers similar accuracy drift to unchopped forms. Its life is also limited to about 2 years because of motor failures.

Narrow-band radiation pyrometers --- Narrow-band radiation pyrometers are highly stable instruments that suffer a drift in accuracy that is typically only 1_ C in 10 years. They are also less sensitive to emissivity changes than other forms of radiation pyrometers. They use photodetectors of either the photoconductive or the photovoltaic form whose performance is unaffected by either carbon dioxide or water vapor in the path between the target object and the instrument.

A photoconductive detector exhibits a change in resistance as the incident radiation level changes, whereas a photovoltaic cell exhibits an induced voltage across its terminals that is also a function of the incident radiation level. All photodetectors are preferentially sensitive to a particular narrow band of wavelengths in the range of 0.5_1.2 mm and all have a form of output that varies in a highly nonlinear fashion with temperature, and thus a microcomputer inside the instrument is highly desirable. Four commonly used materials for photodetectors are cadmium sulfide, lead sulfide, indium antimonide, and lead tin telluride. Each of these is sensitive to a different band of wavelengths and therefore all find application in measuring the particular temperature ranges corresponding to each of these bands.

Output from the narrow band radiation pyrometer is normally chopped into an a.c. signal in the same manner as used in the chopped broad-band pyrometer. This simplifies amplification of the output signal, which is necessary to achieve an acceptable measurement resolution. The typical time constant of a photon detector is only 5 ms, which allows high chopping frequencies up to 20 kHz. This gives such instruments an additional advantage in being able to measure fast transients in temperature as short as 10 ms.

Two-color pyrometer (ratio pyrometer) --- As stated earlier, the emitted radiation-temperature relationship for a body depends on its emissivity. This is very difficult to calculate, and therefore in practice all pyrometers have to be calibrated to the particular body they are measuring. The two-color pyrometer (alternatively known as a ratio pyrometer) is a system that largely overcomes this problem by using the arrangement shown in Fgr. 13. Radiation from the body is split equally into two parts, which are applied to separate narrow-band filters. Outputs from the filters consist of radiation within two narrow bands of wavelengths l1 and l2. Detectors sensitive to these frequencies produce output voltages V1 and V2, respectively. The ratio of these outputs, (V1/V2), can be shown to be a function of temperature and to be independent of emissivity, provided that the two wavelengths, l1 and l2, are close together.

The theoretical basis of the two-color pyrometer is that output is independent of emissivity because emissivities at the two wavelengths l1 and l2 are equal. This is based on the assumption that l1 and l2 are very close together. In practice, this assumption does not hold and therefore the accuracy of the two-color pyrometer tends to be relatively poor. However, the instrument is still of great use in conditions where the target is obscured by fumes or dust, which is a common problem in the cement and mineral processing industries. Two-color pyrometers typically cost 50_100% more than other types of pyrometers.

Selected waveband pyrometer --- The selected waveband pyrometer is sensitive to one waveband only, for example, 5 mm, and is dedicated to particular, special situations where other forms of pyrometers are inaccurate. One example of such a situation is measuring the temperature of steel billets being heated in a furnace. If an ordinary radiation pyrometer is aimed through the furnace door at a hot billet, it receives radiation from the furnace walls (by reflection off the billet) as well as radiation from the billet itself. If the temperature of the furnace walls is measured by a thermocouple, a correction can be made for the reflected radiation, but variations in transmission losses inside the furnace through fumes and so on make this correction inaccurate. However, if a carefully chosen selected waveband pyrometer is used, this transmission loss can be minimized and the measurement accuracy is thereby improved greatly.

Fgr. 13: Beam splitter; Band pass filter

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