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The temperature sensor for an electronic controller may be a length of wire or a thin metallic film (called a resistance temperature device or RTD) or a thermistor. Both types of resistance elements change electrical resistance as temperature changes. The wire increases resistance as its temperature increases. The thermistor is a semiconductor that decreases in resistance as the temperature increases. Because electronic sensors use extremely low mass, they respond to temperature changes more rapidly than bimetal or sealed-fluid sensors. The resistance change is detected by a bridge circuit. Nickel “A”, BALCO, and platinum are typical materials used for this type. | CONTROL FUNDAMENTALS Fig. 50. Rod-and-Tube Element. In a remote-bulb controller Fig. 51 a remote capsule or bulb is attached to a bellows housing by a capillary. The remote bulb is placed in the controlled medium where changes in temperature cause changes in pressure of the fill. The capillary transmits changes in fill pressure to the bellows housing and the bellows expands or contracts to operate the mechanical output to the controller. The bellows and capillary also sense temperature but because of their small volume compared to the bulb the bulb provides the control. MECHANICAL OUTPUT Fig. 51. Typical Remote-Bulb Element. Two specialized versions of the remote bulb controller are available. They both have no bulb and use a long capillary 15 to 28 feet as the sensor. One uses an averaging sensor that is liquid filled and averages the temperature over the full length of the capillary. The other uses a cold spot or low temperature sensor and is vapor filled and senses the coldest spot 12 inches or more along its length. Electronic temperature controllers use low-mass sensing elements that respond quickly to changes in the controlled condition. A signal sent by the sensor is relatively weak but is amplified to a usable strength by an electronic circuit. The temperature sensor for an electronic controller may be a length of wire or a thin metallic film called a resistance temperature device or RTD or a thermistor. Both types of resistance elements change electrical resistance as temperature changes. The wire increases resistance as its temperature increases. The thermistor is a semiconductor that decreases in resistance as the temperature increases. Because electronic sensors use extremely low mass they respond to temperature changes more rapidly than bimetal or sealed-fluid sensors. The resistance change is detected by a bridge circuit. Nickel A BALCO and platinum are typical materials used for this type of sensor. In thermocouple temperature-sensing elements two .