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CATEGORIES OF TEMPERATURE MEASUREMENT DEVICES
Filled Systems For Temperature measurement
Filled systems are so named because they use the thermal expansion of a fill fluid to provide temperature sensing and measurement. Filled thermal systems can provide local or remote temperature indication.
A filled system consists of a bulb immersed in the measuring fluid, a long capillary or fine bore tube, a measuring unit that may be a Bourdon tube or bellows, and a filling fluid which may be liquid or gas. Figure shows an example filled system. The whole system is gas tight and filled completely with an appropriate liquid or gas.
Figure Filled system
Filled Thermal system For Temperature measurement
The purpose of a filled thermal system is to provide a remote temperature indication and/or temperature recording. The filled thermal system is similar to a pressure gauge; instead of the sensing tube responding to changes in pressure, it responds to temperature changes. The filled thermal system (Figure) consists of a temperature sensor bulb connected by a capillary tube to a readout instrument.
Figure Filled Thermal System
Temperature Switch For Temperature measurement
Temperature switches provide high or low temperature detection, backup existing alarms, actuate additional equipment for a safety interlock system. Some temperature switches (Figure) consist of a bulb, capillary tubing, and electrical switch elements. The temperature switch’s bulb and capillary design is classified into one of the filled systems. Other temperature switches can use bimetallic elements as the temperature element.
Figure Filled System Temperature Switch
Bimetallic Thermometer For Temperature measurement
A bimetallic thermometer (figure 10) is so named because the temperature-sensing element consists of two different metals that have different coefficients of expansion.
The two different metals are bonded or brazed together ( Figure 11) . When the temperature element is exposed to heat, it bends towards the metal with the lower expansion rate.
Figure Bimetallic Thermometer
Figure Bimetal strip
Because the bending or deflection is small, the bimetallic strip is usually in a helix or spiral form. The helix form ( Figure ) is the more common type of bimetallic thermometer sensing element. The bimetallic thermometer’s stem containing the helix is sometimes filled with a silicone fluid that provides better heat transfer and vibration resistance.
Figure Helix Strip
Thermistor For Temperature measurement
Thermistors are temperature sensors made up of semiconductor material that gives an extremely high change in resistance for a small change in temperature. Unlike an RTD, a thermistor’s resistance decreases in response to a temperature ( Negative Temperature Coefficient ) increase.
The extremely high nonlinear response (figure) in resistance to a small temperature change means that the thermistor has an extremely high temperature-resistance coefficient. Because the temperature-resistance coefficient is extremely high, the thermistor is useful for narrow spans allowing the thermistor circuit to detect minute changes in temperature which could not be observed with an RTD or thermocouple circuit. Most thermistors are in the shape of a disc, although some look like a standard resistor.
Figure
Although thermistors are small, have high sensitivity and fast response, they are not widely applied in process measurements for several reasons. The nonlinearity characteristics of thermistors make them impractical to use over a wide temperature range; the thermistor’s operating range must be narrow. Thermistors also have limitations on interchangeability, accuracy, calibration, and stability that makes their use in process control applications impractical because more suitable RTD and thermocouple sensors are available.
For that reason, the remainder of this section will focus on RTDs and thermocouples as the preferred temperature sensing methods to use in process measurements and control.
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