Transducer have been devised which produce either changes in voltage or change in impedance whenever the temperature changes. Temperature measurement sensors can be divided into two categories. They are –
Measurement using change in resistance and measurement using change in voltage.
The temperature sensor that uses change in resistance to measure
temperature is called a resistance thermometer. Resistance thermometer can be further classified into Resistance Temperature Detectors (RTD)and
Temperature Parameters
Before going into detail, it is important to know some of the basic temperature parameters and instrumentation systems.
Span: The difference between the maximum and minimum values of
temperature in the calibrated range is called span. It is always good to have very low values of span. The minimum span is the smallest range that the manufacturer can accurately calibrate within the device’s range.
Turndown: It is the ratio of maximum measurable parameter to minimum
measurable parameter.
Immersion Length: The immersion length of a Thermowell is the distance
Temperature
The term ‘temperature’ can be defined in terms of heat. Heat is a measure of
the energy contained in a body, which is due to the irregular motion of its molecules or atoms. The internal energy of body or gas increases with increasing temperature. To measure the value of temperature, some of the following phenomenon are needed:
Change in physical dimensions or characteristics of liquids, metals, or gases Changes in electrical resistance
Bimetallic Thermometer
In an industry, there is always a need to measure and monitor temperature of
a particular spot, field or locality. The industrial names given to such temperature sensors are Temperature Indicators (TI) or Temperature Gauges (TG). All these temperature gauges belong to the class of instruments that are known as bimetallic sensors.
Two basic principles of operation is to be followed in the case of a bimetallic
sensor. They are :
A metal tends to undergo a volumetric dimensional change
(expansion/contraction), according to the change in temperature.
Different metals have different co-efficient of temperatures. The rate of
Working
A change in temperature causes the free end of the strip to expand or
contract due to the different co-efficients of expansion of the two metals. This movement is linear to the change in temperature and the deflection of the free end can be read out by attaching a pointer to it. This reading will indicate the value of temperature. Bimetallic strips are available in different forms like helix type, cantilever, spiral, and also flat type.
The figure below shows the working of a bimetallic sensor. Two metals, blue
Bimetallic thermometers are not recommended for continued use above 420
degree Celsius. The thermal stability of the bimetallic thermometer is an
Selection of Temperature Measurement Devices
Usually a given temperature measurement can be satisfactorily made by several different types of temperature sensing devices. Ranges and capabilities of the various temperature measuring devices overlap in many instances. The following shall be used as guidelines for selection of Temperature Elements.
1. Bimetallic Thermometers
If the temperature to be measured is not required for automatic control,
2. Thermocouples
Thermocouples are generally used for high temperature applications. At high temperatures there is a clear preference for thermocouples over RTDs. Project specific requirement should be checked before selection of Thermocouple or RTD for specific application.
Thermocouples are one of the most common sensors used to measure
temperature because they are relatively inexpensive yet accurate sensors that can operate over a wide range of temperatures.
It consists of two wires of different metals twisted and brazed or welded
together with each wire covered with insulation which may be either
1. Mineral (magnesium oxide) insulation for normal duty 2. Ceramic insulation for heavy duty.
The working principle of thermocouple is based on three effects, discovered by Seebeck, Peltier and Thomson. They are as follows:
1) Seebeck effect
2) Peltier effect: Heat is generated at the cold junction and is absorbed from the hot junction thereby heating the cold junction slightly and cooling the hot junction slightly.
3) Thomson effect: The junction emf may be slightly altered if a temperature gradient exists along either or both materials.
Law of Intermediate Temperatures
Law of Intermediate Metals
Thermocouple Materials
The choice of materials for thermocouples in governed by the following factors :
1. Ability to withstand the temperature at which they are used.
2. immunity from contamination/oxidation, etc which ensures maintenance of the precise thermoelectric properties with continuous use
Thermocouple can be broadly classified in two categories: 1. Base – metal thermocouples
2. Rare metal thermocouple
Base metal thermocouples uses the combination of pure metals and alloys of iron, copper and nickel and are used in temperature up to 1450K. Their chief limitation is the lower operating range because of their low melting point and vulnerability to oxidation.
Rare metal thermocouple uses the combination of pure metals and alloys of platinum for temperatures up to 1600oC and tungsten, rhodium and
Applications and type of Thermocouple
Sensors
Type K (-270 to +1370°C / -454 to +2498°F)
The Type K is a 'general purpose' thermocouple with a wide temperature range. With a variety of probe types available, it is suitable for use across many industries and processes.
Testing temperatures associated with process plants e.g. chemical production and petroleum refineries
Type J (-210 to +1200°C / -346 to +2192°F)
The Type J is a popular thermocouple that is commonly used to monitor temperatures of inert materials and in vacuum applications. This
thermocouple is susceptible to oxidisation so is not recommended for damp conditions or low temperature monitoring. (Note the accuracy of this sensor may be permanently impaired if used above 760°C.)
Monitoring in a vacuum and for inert metals
Type T (-270 to +400°C / -454 to +752°F)
The Type T is used widely in the food industry, mainly due to the high level of accuracy it provides and because it performs well in the presence of moisture without oxidising. If in general a lower range temperature measurement is required, the Type T is popular choice.
Monitoring in food processing and production to identify potential food safety hazards and comply with HACCP regulations
Type N (-270 to +1300°C / -454 to +2372°F)
The Type N also has a wide temperature range, but is better suited to high temperature monitoring than the Type K because it is more stable and resists oxidisation.
Temperature profiling in ovens, furnaces and kilns
Temperature measurement of gas turbine and engine exhausts
RTD (Resistance Temperature
Device/Detector)
RTD is a type of electrical resistance thermometer
The electrical resistance of many metals (e.g. copper, silver, aluminium, platinum) increases approximately linearly with absolute temperature and this feature makes them useful as temperature sensors. The resistance of a wire of the material is measured by passing a current (AC or DC) through it and measuring the voltage with a suitable bridge or voltmeter, and the reading is converted to temperature using a calibration equation.
Because RTDs are simply resistors, they can be fabricated in all shapes and sizes, or embedded in all sorts of probes. For industrial use, you usually purchase them mounted in probes that include connectors, sheaths, and handles for convenient mechanical placement.
Radiation Methods/ Pyrometry
Thermistor or thermocouple etc. are require the thermometer to be brought into physical contact with the body whose temperature to be measured.
In case of very hot bodies , thermometer may melt at the high temperature.
Also for bodies that are moving , a non-contacting device for measuring the temperature is most convenient
If the distribution of temperature over the surface of an object is required, a non-contacting device readily scan the surface.
For temperatures above 650 o C, the heat radiations emitted from the body
are of sufficient intensity to be used for measuring the temperature.
The main theory behind a radiation pyrometer is that the temperature is measured through the naturally emitted heat radiation by the body. This heat is known to be a function of its temperature. According to the application of the device, the way in which the heat is measured can be summarized into two:
Total Radiation Pyrometer – In this method, the total heat emitted from the hot source is measured at all wavelengths.
