The former are prone to lose calibration. Humidity Humidity sensors may be animal or plastic skins varying in length with changes in rh or lithium chloride coating changing in electrical resistance. An important feature is the proportional band of temperature over which the controls call for up to full plant capacity. The signals are transmitted to amplifiers which respond to the degree of error. The mechanical items are used to alter the value of potentiometers or make-or-break contacts. These, in turn, provide an electrical or mechanical signal. Dry bulb sensors may be bi-metal strips, thermistors or refrigerant-filled phials or bellows responding to pressure differences caused by temperature change. In large rooms separate thermostats can be arranged to give an average signal but individual zones of control each separately treated are much better. Where underfloor air return is practised it is strongly recommended that the sensor be placed close under a return grille to prevent changes of condition occurring between the room and the sensing position. Bevan IEng, MIPIantE, Michael Farrell CEng, FCIBSE, FIMechE, in Plant Engineer's Reference Book (Second Edition), 2002 18.1.6.1 Room conditionÄry bulb temperature This is sensed by a thermostat in the conditioned space or in the return duct.
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