14 Temperature measurement

 

14.8 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 tempera[1]ture changes to be translated into frequency changes. The temperature-sensing element consists of a quartz crystal enclosed within a probe (sheath). The probe commonly consists of a stainless steel cylinder, which makes the device physically larger than devices like thermocouples and resistance thermometers. The crystal is connected electrically so as to form the resonant element within an electronic oscillator. Measure[1]ment 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°C 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[1]change form of output means that the device is insensitive to noise. However, it is very expensive, with a typical cost of £3000 ($5000).

 

14.9 Fibre-optic temperature sensors

Fibre-optic cables can be used as either intrinsic or extrinsic temperature sensors, as discussed in Chapter 13, though special attention has to be paid to providing a suitable protective coating when high temperatures are measured. Cost varies from £1000 to £4000, according to type, and the normal temperature range covered is 250°C to 3000°C, though 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, though 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%. Whilst it is often assumed that fibre-optic sensors are intrinsically safe, it has been shown (Johnson, 1994) that flammable gas might be ignited by the optical power levels available from some laser diodes. Thus, the power level used with optical fibres must be carefully chosen, 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 sensor include the cross-talk sensor, phase modulating sensor and optical resonator, as described in Chapter 13. Research into the use of distributed temperature sensing using fibre-optic cable has also been reported. This can be used to measure things like the temperature distribution along an electricity supply cable. It works by measuring the reflection characteristics of light transmitted down a fibre-optic cable that is bonded to the electrical cable. By analysing the 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 fibre-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 pyrometer, including the two-colour version, and a particular advantage is that this method of measurement is intrinsically safe. However, it is 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 fibre-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 fibre up to 20 m long can be successfully used 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 fibre-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 500°C to 2000°C, a resolution of 10^-5°C and an inaccuracy of only ±0.0025% of full scale.

Several other types of device that are marketed as extrinsic fibre-optic temperature sensors consist of a conventional temperature sensor (e.g. a resistance thermometer) connected to a fibre-optic cable so that the transmission of the signal from the measure[1]ment point is free of noise. Such devices must include an electricity supply for the electronic circuit that is needed to convert the sensor output into light variations in the cable. Thus, low-voltage power cables must be routed with the fibre-optic cable, and the device is therefore not intrinsically safe.

 

14.10 Acoustic thermometers

The principle of acoustic thermometry was discovered as long ago as 1873 and uses the fact that the velocity of sound through a gas varies with temperature according to the equation:

where v is the sound velocity, T is the gas temperature, M is the molecular weight of the gas and both R and ˛ are constants. Until very recently, it had only been used for measuring cryogenic (very low) temperatures, but it is now also used for measuring higher temperatures and can potentially measure right up to 20 000°C. However, typical inaccuracy is ±5%, and the devices are expensive (typically £6000 or $10 000). The various versions of acoustic thermometer that are available differ according to the technique used for generating sound and measuring its velocity in the gas. If ultrasonic generation is used, the instrument is often known as an ultrasonic thermometer. Further information can be found in Michalski, (1991).

 

14.11 Colour indicators

The colour 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 colour change is in special colour indicators that are widely used in industry to determine whether objects placed in furnaces have reached the required temperature. Such colour indicators consist of special paints or crayons that are applied to an object before it is placed in a furnace. The colour-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 colour change occurs in the paint or crayon, although this change does not occur instantaneously but only happens over a period of time.

Hence, the colour 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 colour change to occur. If the temperature rises above the rated temperature, then the colour 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 colour 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 colour change happens. Besides 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°C and 1250°C. A typical exposure time rating is 30 minutes, i.e. the colour 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 pounds 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 colour at a certain temperature. According to the design of sensors using such liquid crystals, the colour change can either occur gradu[1]ally during a temperature rise of perhaps 50°C or else change abruptly at some specified temperature. The latter kind 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°C to +100°C.

 

14.12 Change of state of materials

Temperature-indicating devices known as Seger cones or pyrometric cones are commonly used 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°C to +2000°C.