Guide to Measurement and Instrumentation -- Thermography and Temperature Metrics -- part 1

On this page (below):

• Thermography (Thermal Imaging)
• Thermal Expansion Methods
• Quartz Thermometers
• Fiber-Optic Temperature Sensors

Thermography (Thermal Imaging)

Thermography, or thermal imaging, involves scanning an infrared radiation detector across an object. The information gathered is then processed and an output in the form of the temperature distribution across the object is produced. Temperature measurement over the range from _20_ Cup to +1500_ C is possible. Elements of the system are shown in Fgr. 14.

Fgr. 14: Control unit; Scanning radiation detector; Processor Display unit; Display out; Light in from scene

The radiation detector uses the same principles of operation as a radiation pyrometer in inferring the temperature of the point that the instrument is focused on from a measurement of the incoming infrared radiation. However, instead of providing a measurement of the temperature of a single point at the focal point of the instrument, the detector is scanned across a body or scene, and thus provides information about temperature distributions.

Because of the scanning mode of operation of the instrument, radiation detectors with a very fast response are required, and only photoconductive or photovoltaic sensors are suitable. These are sensitive to the portion of the infrared spectrum between wavelengths of 2 and 14 mm.

Simpler versions of thermal imaging instruments consist of hand-held viewers that are pointed at the object of interest. The output from an array of infrared detectors is directed onto a matrix of red light-emitting diodes assembled behind a glass screen, and the output display thus consists of different intensities of red on a black background, with the different intensities corresponding to different temperatures. Measurement resolution is high, with temperature differences as small as 0.1_ C being detectable. Such instruments are used in a wide variety of applications, such as monitoring product flows through pipe work, detecting insulation faults, and detecting hot spots in furnace linings, electrical transformers, machines, bearings, etc. The number of applications is extended still further if the instrument is carried in a helicopter, where uses include scanning electrical transmission lines for faults, searching for lost or injured people, and detecting the source and spread pattern of forest fires.

More complex thermal imaging systems comprise a tripod-mounted detector connected to a desktop computer and display system. Multicolor displays are used commonly in such systems, where up to 16 different colors represent different bands of temperature across the measured range. The heat distribution across the measured body or scene is thus displayed graphically as a contoured set of colored bands representing the different temperature levels. Such color thermography systems find many applications, such as inspecting electronic circuit boards and monitoring production processes. There are also medical applications in body scanning.

Thermal Expansion Methods

Thermal expansion methods make use of the fact that the dimensions of all substances, whether solids, liquids, or gases, change with temperature. Instruments operating on this physical principle include the liquid-in-glass thermometer, bimetallic thermometer, and pressure thermometer.

Fgr. 15--Motion of free end (c) Bulb containing fluid

Liquid-in-Glass Thermometers

The liquid-in-glass thermometer is a well-known temperature-measuring instrument used in a wide range of applications. The fluid used is normally either mercury or colored alcohol, which is contained within a bulb and capillary tube. As the temperature rises, the fluid expands along the capillary tube and the meniscus level is read against a calibrated scale etched on the tube. Industrial versions of the liquid-in-glass thermometer are normally used to measure temperature in the range between _200 and +1000_ C, although instruments are available to special order that can measure temperatures up to 1500_ C.

Measurement inaccuracy is typically _1% of full-scale reading, although an inaccuracy of only _0.15% can be achieved in the best industrial instruments. The major source of measurement error arises from the difficulty of correctly estimating the position of the curved meniscus of the fluid against the scale. In the longer term, additional errors are introduced due to volumetric changes in the glass. Such changes occur because of creep-like processes in the glass, but occur only over a timescale of years. Annual calibration checks are therefore advisable.

Bimetallic Thermometer

The bimetallic principle is probably more commonly known in connection with its use in thermostats. It’s based on the fact that if two strips of different metals are bonded together, any temperature change will cause the strip to bend, as this is the only way in which the differing rates of change of length of each metal in the bonded strip can be accommodated. In the bimetallic thermostat, this is used as a switch in control applications. If the magnitude of bending is measured, the bimetallic device becomes a thermometer. For such purposes, the strip is often arranged in a spiral or helical configuration, as shown in Fgr. 15b, as this gives a relatively large displacement of the free end for any given temperature change. The measurement sensitivity is increased further by choosing the pair of materials carefully such that the degree of bending is maximized, with Invar (a nickel-steel alloy) or brass being used commonly.

The system used to measure the displacement of the strip must be designed carefully. Very little resistance must be offered to the end of the strip, as otherwise the spiral or helix will distort and cause a false reading in measurement of the displacement. The device is normally just used as a temperature indicator, where the end of the strip is made to turn a pointer that moves against a calibrated scale. However, some versions produce an electrical output, using either a linear variable differential transformer or a fiber-optic shutter sensor to transduce the output displacement.

Bimetallic thermometers are used to measure temperatures between _75 and +1500_C.

The inaccuracy of the best instruments can be as low as _0.5% but such devices are quite expensive. Many instrument applications don’t require this degree of accuracy in temperature measurements, and in such cases much less expensive bimetallic thermometers with substantially inferior accuracy specifications are used.

All such devices are liable to suffer changes in characteristics due to contamination of the metal components exposed to the operating environment. Further changes are to be expected arising from mechanical damage during use, particularly if they are mishandled or dropped.

As the magnitude of these effects varies with their application, the required calibration interval must be determined by practical experimentation.

Pressure Thermometers

Pressure thermometers have now been superseded by other alternatives in most applications, but they still remain useful in a few applications such as furnace temperature measurement when the level of fumes prevents the use of optical or radiation pyrometers. Examples can also still be found of their use as temperature sensors in pneumatic control systems. The sensing element in a pressure thermometer consists of a stainless-steel bulb containing a liquid or gas. If the fluid were not constrained, temperature rises would cause its volume to increase. However, because it’s constrained in a bulb and cannot expand, its pressure rises instead. As such, the pressure thermometer does not strictly belong to the thermal expansion class of instruments but is included because of the relationship between volume and pressure according to Boyle's law: PV = KT. The change in pressure of the fluid is measured by a suitable pressure transducer, such as the Bourdon tube. This transducer is located remotely from the bulb and is connected to it by a capillary tube as shown in Fgr. 15c.

Pressure thermometers can be used to measure temperatures in the range between _250 and +2000_ C, and their typical inaccuracy is _0.5%of full-scale reading. However, the instrument response has a particularly long time constant.

The need to protect the pressure-measuring instrument from the environment where the temperature is being measured can require the use of capillary tubes up to 5 m long, and the temperature gradient, and hence pressure gradient, along the tube acts as a modifying input that can introduce a significant measurement error. Errors also occur in the short term due to mechanical damage and in the longer term due to small volumetric changes in the glass components. The rate of increase in these errors is mainly use related and therefore the required calibration interval must be determined by practical experimentation.

Quartz Thermometers

The quartz thermometer makes use of the principle that the resonant frequency of a material such as quartz is a function of temperature, and thus enables temperature changes to be translated into frequency changes. The temperature-sensing element consists of a quartz crystal enclosed within a probe (sheath). The probe usually consists of a stainless-steel cylinder, which makes the device physically larger than devices such as thermocouples and resistance thermometers. The crystal is connected electrically so as to form the resonant element within an electronic oscillator. Measurement of the oscillator frequency therefore allows the measured temperature to be calculated.

The instrument has a very linear output characteristic over the temperature range between _40 and +230_ C, with a typical inaccuracy of _0.1%. Measurement resolution is typically 0.1_ C but versions can be obtained with resolutions as small as 0.0003_ C. The characteristics of the instrument are generally very stable over long periods of time and therefore only infrequent calibration is necessary. The frequency change form of output means that the device is insensitive to noise. However, it’s very expensive and only available from a small number of manufacturers.

Fiber-Optic Temperature Sensors

Fiber-optic cables can be used as either intrinsic or extrinsic temperature sensors, as discussed in Section 13, although special attention has to be paid to providing a suitable protective coating when high temperatures are measured. Cost varies from $1000 to $5000, according to type, and the normal temperature range covered is 250 to 3000_ C, although special devices can detect down to 100_ C and others can detect up to 3600_ C. Their main application is measuring temperatures in hard-to-reach locations, although they are also used when very high measurement accuracy is required. Some laboratory versions have an inaccuracy as low as _0.01%, which is better than a type S thermocouple, although versions used in industry have a more typical inaccuracy of _1.0%.

While it’s often assumed that fiber-optic sensors are intrinsically safe, it has been shown that flammable gas may be ignited by the optical power levels available from some laser diodes. Thus, the power level used with optical fibers must be chosen carefully, and certification of intrinsic safety is necessary if such sensors are to be used in hazardous environments.

One type of intrinsic sensor uses cable where the core and cladding have similar refractive indices but different temperature coefficients. Temperature rises cause the refractive indices to become even closer together and losses from the core to increase, thus reducing the quantity of light transmitted. Other types of intrinsic temperature sensors include the cross-talk sensor, phase-modulating sensor, and optical resonator, as described in Section 13. Research into the use of distributed temperature sensing using fiber-optic cable has also been reported.

This can be used to measure things such as the temperature distribution along an electricity supply cable. It works by measuring the reflection characteristics of light transmitted down a fiber-optic cable bonded to the electrical cable. By analyzing back-scattered radiation, a table of temperature versus distance along the cable can be produced, with a measurement inaccuracy of only _0.5_ C.

A common form of extrinsic sensor uses fiber-optic cables to transmit light from a remote targeting lens into a standard radiation pyrometer. This technique can be used with all types of radiation pyrometers, including the two-color version, and a particular advantage is that this method of measurement is intrinsically safe. However, it’s not possible to measure very low temperatures because the very small radiation levels that exist at low temperatures are badly attenuated during transmission along the fiber-optic cable. Therefore, the minimum temperature that can be measured is about 50_ C, and the light guide for this must not exceed 600 mm in length. At temperatures exceeding 1000_ C, lengths of fiber up to 20 m long can be used successfully as a light guide.

One extremely accurate device that uses this technique is known as the Accufibre sensor. This is a form of radiation pyrometer that has a black box cavity at the focal point of the lens system.

A fiber-optic cable is used to transmit radiation from the black box cavity to a spectrometric device that computes the temperature. This has a measurement range of 500 to 2000_ C, a resolution of 10_5_ C, and an inaccuracy of only _0.0025% of full scale.

Several other types of devices marketed as extrinsic fiber-optic temperature sensors consist of a conventional temperature sensor (e.g., a resistance thermometer) connected to a fiber-optic cable so that transmission of the signal from the measurement point is free of noise. Such devices must include an electricity supply for the electronic circuit needed to convert the sensor output into light variations in the cable. Thus, low-voltage power cables must be routed with the fiber-optic cable, and the device is therefore not intrinsically safe.

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