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 Thermistor.
Some other types of temperature sensors are
- Optical Pyrometer
- Radiation Pyrometer
- Bimetallic Thermometer
- Filled System Temperature Measurement
Before going into detail, it is important to know some of the basic temperature parameters and instrumentation systems.
- Range: The range of a temperature measuring device is the maximum and minimum temperature it can indicate, record, measure or transmit. The range should be decided in such a manner that the normal operating temperature is almost (50-70)% of the full scale with the maximum temperature range close to, but more than the upper range of scale.
- 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 between the free end/tip of the Thermowell and the point of immersion in the medium that is being measured. The standard symbol for the immersion length of a Thermowell is “R”.
- Insertion Length: The insertion length of a Thermowell is the distance between the free end/tip of a Thermowell and (but not including) the external threads of other means of attachment to a vessel. The standard symbol for the insertion length of a Thermowell is “U”.
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. Temperature is a variable which together with other parameters such as mass, specific heat etc. describe the energy content of a body. When energy in the form of heat is introduced to or extracted from a body, altered molecular activity will be made apparent as a temperature change.
To measure the value of temperature, some of the following phenomenon is needed.
- Change in physical dimensions or characteristics of liquids, metals, or gases
- Changes in electrical resistance
- Thermoelectric effect
- Radiant energy
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 volumetric change depends on this co-efficient of temperature.
The device consists of a bimetallic strip of two different metals and they are bonded together to form a spiral or a twisted helix. Both these metals are joined together at one end by either welding or riveting. It is bonded so strong that there will not be any relative motion between the two. The image of a bimetallic strip is shown below.
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 and red are riveted together. If it is used in an oven, the red metal would expand faster than the blue metal. If it is used in a refrigerator, a rise in temperature causes the blue metal to expand faster than the red one. As a result the strip will bend upward and short circuits with a metal wore so that current begins to flow. If the size of the gap between the strip and the wire is adjusted, you can control the temperature.
Bimetallic thermometers are generally available with 2, 3, 4, 5 or 6-inch concentric dials, preferably of the non-parallax type (i.e. not visually misaligned or displaced), with external zero adjustment, and 1/2-inch mounting thread. The stem should be of Stainless Steel SS316), having a 1/4-inch diameter, and of a customized length to suit process requirements. The dial orientation may be bottom or back, known as “straight” or “angle”, respectively. However, an all angle adjustable swivel connection is preferred in order to enable the dial to be read from the most convenient location.
Bimetallic thermometers are not recommended for continued use above 420 degree Celsius. The thermal stability of the bimetallic thermometer is an inherent characteristic of the metals used and continued operation cannot be assured above 471°C.
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, recording, or indication in the control room, a bimetallic thermometer should be used.
2. Filled System Thermometers
Occasionally a temperature transmitter, recorder and/or controller has to be installed in a location or under circumstances where electrical power is not available or inconvenient to use, while instrument air (at 20 psig or above) is present. Under these circumstances, a filled system thermometer is an ideal choice because it can be combined with a commercially available pneumatic transmitter, recorder and/or controller. If a local temperature measurement is in an inaccessible location so that a bimetallic thermometer cannot be easily read, a filled system thermometer is recommended because its capillary tubing can be led to an indicator that can be installed/located in a convenient place.
A liquid filled thermometer should be considered for corrosive areas or where vibration is a problem.
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.
4. Resistance Thermometers
Listed below is some temperature measurement applications for which the use of a RTD is normally preferred over a thermocouple:
- Temperature of turbine inlet steam whose design temperature is close to the maximum allowable temperature for piping and equipment
- Temperature of permanent turbine test points
- Average temperature of nuclear reactor coolant
- Average combustion turbine inlet air temperature
- Condenser cooling water inlet to outlet temperature gain
- Motor stator winding and bearing temperatures
Thermocouples are not suitable for narrow range temperature measurements. This is because the change in the EMF developed by a thermocouple over a narrow temperature range is very small and difficult to measure. Therefore, for narrow spans or small temperature differences a RTD is recommended.