Detectors and sensors
In principle detectors two-position (on-off); however, in many cases they have some hysteresis. This means that there are three levels on which they operate. They move in one direction, when their contribution is above a certain level and switch to the other side, where their contribution to below the lower level. In between these two levels, they retain their earlier arrived in the state. Using a simple detector as a mechanism of control will cause a manageable switch to some extent because all devices have some time lag in operation. The excess amount depends on the delay time of the detector and the extent to which the rate of supply of energy to the process exceeds the load (see Fig. 28.2). Control range, thus, will differential detector together with upper and lower slips under load.
Where two position sensor and controller, it provides the only two ways out: either the maximum to zero. Narrow dead zone gives more control, but leads to more rapid Cycling, which may be undesirable, especially if the compressors be switched on and off.
Two positions detector can be used for floating control. At the upper limit it will work control in one direction, and if it reaches the lower limit, he will work control in the other direction. Between the two limits the control is not performed.
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Air-Conditioning 