Guide to Measurement and Instrumentation -- Thermography and Temperature Metrics -- part 2

Color Indicators

The color of various substances and objects changes as a function of temperature. One use of this is in the optical pyrometer as discussed earlier. The other main use of color change is in special color indicators that are widely used in industry to determine whether objects placed in furnaces have reached the required temperature. Such color indicators consist of special paints or crayons that are applied to an object before it’s placed in a furnace. The color-sensitive component within these is some form of metal salt (usually of chromium, cobalt, or nickel).

At a certain temperature, a chemical reaction takes place and a permanent color change occurs in the paint or crayon, although this change does not occur instantaneously but only happens over a period of time.

Hence, the color change mechanism is complicated by the fact that the time of exposure as well as the temperature is important. Such crayons or paints usually have a dual rating that specifies the temperature and length of exposure time required for the color change to occur. If the temperature rises above the rated temperature, then the color change will occur in less than the rated exposure time. This causes little problem if the rate of temperature rise is slow with respect to the specified exposure time required for color change to occur. However, if the rate of rise of temperature is high, the object will be significantly above the rated change temperature of the paint/crayon by the time that the color change happens. In addition to wasting energy by leaving the object in the furnace longer than necessary, this can also cause difficulty if excess temperature can affect the required metallurgical properties of the heated object.

Paints and crayons are available to indicate temperatures between 50 and 1250_ C. A typical exposure time rating is 30 minutes, that is, the color change will occur if the paint/crayon is exposed to the rated temperature for this length of time. They have the advantage of low cost, typically a few dollars per application. However, they adhere strongly to the heated object, which can cause difficulty if they have to be cleaned off the object later.

Some liquid crystals also change color at a certain temperature. According to the design of sensors using such liquid crystals, the color change can either occur gradually during a temperature rise of perhaps 50_ C or change abruptly at some specified temperature. The latter kinds of sensors are able to resolve temperature changes as small as 0.1_ C and, according to type, are used over the temperature range from _20 to +100_ C.

Change of State of Materials

Temperature-indicating devices known as Seger cones or pyrometric cones are used commonly in the ceramics industry. They consist of a fused oxide and glass material that is formed into a cone shape. The tip of the cone softens and bends over when a particular temperature is reached. Cones are available that indicate temperatures over the range from 600 to +2000_ C.

Intelligent Temperature-Measuring Instruments

Intelligent temperature transmitters have now been introduced into the catalogues of almost all instrument manufacturers, and they bring about the usual benefits associated with intelligent instruments. Such transmitters are separate boxes designed for use with transducers that have either a d.c. voltage output in the millivolts range or an output in the form of a resistance change. They are therefore suitable for use in conjunction with thermocouples, thermopiles, resistance thermometers, thermistors, and broad-band radiation pyrometers. Transmitters normally have nonvolatile memories where all constants used in correcting output values for modifying inputs, etc., are stored, thus enabling the instrument to survive power failures without losing such information. Other facilities in intelligent transmitters include adjustable damping, noise rejection, self-adjustment for zero and sensitivity drifts, and expanded measurement range.

These features allow an inaccuracy level of _0.05% of full scale to be specified.

Mention must be made particularly of intelligent pyrometers, as some versions of these are able to measure the emissivity of the target body and automatically provide an emissivity-corrected output. This particular development provides an alternative to the two-color pyrometer when emissivity measurement and calibration for other types of pyrometers pose difficulty.

Digital thermometers also exist in intelligent versions, where inclusion of a microprocessor allows a number of alternative thermocouples and resistance thermometers to be offered as options for the primary sensor.

The cost of intelligent temperature transducers is significantly more than their nonintelligent counterparts, and justification purely on the grounds of their superior accuracy is hard to make. However, their expanded measurement range means immediate savings are made in terms of the reduction in the number of spare instruments needed to cover a number of measurement ranges. Their capability for self-diagnosis and self-adjustment means that they require attention much less frequently, giving additional savings in maintenance costs.

Many transmitters are also largely self-calibrating in respect of their signal processing function, although appropriate calibration routines still have to be applied to each sensor that the transmitter is connected to.

Choice between 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’s 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, thermistors, semiconductor devices, and color indicators. However, fluid temperatures can be measured by any of the instruments described in this section, with the exception of radiation thermometers.

The most commonly used device in industry for temperature measurement is the base metal thermocouple. This is relatively inexpensive, with prices varying widely from a few dollars upward according to the thermocouple type and sheath material used. Typical inaccuracy is _0.5% of full scale over the temperature range of _250 to +1200_ C. Noble metal thermocouples are much more expensive, but are chemically inert and can measure temperatures up to 2300_ C with an inaccuracy of _0.2% of full scale. However, all types of thermocouples have a low-level output voltage, making them prone to noise and therefore unsuitable for measuring small temperature differences.

Resistance thermometers are also in common use within the temperature range of _270 to +650_ C, with a measurement inaccuracy of _0.5%. While they have a smaller temperature range than thermocouples, they are more stable and can measure small temperature differences.

The platinum resistance thermometer is generally regarded as offering the best ratio of price to performance for measurement in the temperature range of _200 to +500_ C, with prices starting from $20.

Thermistors are another relatively common class of devices. They are small and inexpensive, with a typical cost of around $5. They give a fast output response to temperature changes, with good measurement sensitivity, but their measurement range is quite limited.

Semiconductor devices have a better linearity than thermocouples and resistance thermometers and a similar level of accuracy. Thus, they are a viable alternative to these in many applications. Integrated circuit transistor sensors are particularly inexpensive (from $10 each), although their accuracy is relatively poor and they have a very limited measurement range (_50 to +150_ C). Diode sensors are much more accurate and have a wider temperature range (_270 to +200_ C), although they are also more expensive (typical costs are anywhere from $50 to $800).

A major virtue of radiation thermometers is their noncontact, noninvasive mode of measurement. Costs vary from $300 up to $5000 according to type. Although calibration for the emissivity of the measured object often poses difficulties, some instruments now provide automatic calibration. Optical pyrometers are used to monitor temperatures above 600_ C in industrial furnaces, etc., but their inaccuracy is typically _5%. Various forms of radiation pyrometers are used over the temperature range between _20 and +1800_ C and can give measurement inaccuracies as low as _0.05%. One particular merit of narrow-band radiation pyrometers is their ability to measure fast temperature transients of duration as small as 10 ms. No other instrument can measure transients anywhere near as fast as this.

The range of instruments working on the thermal expansion principle are used mainly as temperature-indicating devices rather than as components within automatic control schemes.

Temperature ranges and costs are: mercury-in-glass thermometers up to +1000_ C (cost from a few dollars), bimetallic thermometers up to +1500_ C (cost $50 to $150), and pressure thermometers up to +2000_ C (cost $100 to $800). The usual measurement inaccuracy is in the range of _0.5 to _1.0%. The bimetallic thermometer is more rugged than liquid-in-glass types but less accurate (however, the greater inherent accuracy of liquid-in-glass types can only be realized if the liquid meniscus level is read carefully).

Fiber-optic devices are more expensive than most other forms of temperature sensors (costing up to $6000) but provide a means of measuring temperature in very inaccessible locations.

Inaccuracy varies from _1% down to _0.01% in some laboratory versions. Measurement range also varies with type, but up to +3600_ C is possible.

The quartz thermometer provides very high resolution (0.0003_ C is possible with special versions) but is 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 to +230_ C, but gives a low measurement inaccuracy of _0.1% within this range. It’s not used commonly because of its high cost.

Color indicators are used widely to determine when objects in furnaces have reached the required temperature. These indicators work well if the rate of rise of temperature of the object in the furnace is relatively slow but, because temperature indicators only change color over a period of time, the object will be above the required temperature by the time that the indicator responds if the rate of rise of temperature is large. Cost is low; for example, a crayon typically costs $5.

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