![]() ![]() This proportioning action occurs within a selected bandwidth around the set point the set point being the bandwidth mid point. However, controllers are available that provide a proportioning time control, in which it is possible to alter the ratio of the ‘on’ time to the ‘off’ time to control the controlled condition. The operating differential depends on the characteristics of the application such as the tank, its contents, the heat transfer characteristics of the coil, the rate at which heat is transferred to the thermostat, and so on.Įssentially, with on / off control, there are upper and lower switching limits, and the valve is either fully open or fully closed - there is no intermediate state. The switching differential of the thermostat depends on the type of thermostat used. The difference between the peak and the trough is known as the operating differential. Steam is admitted through the coil but again, it takes time to have an effect and the water temperature continues to fall for a while, reaching its trough of undershoot at point E. The temperature of the tank contents will fall to 59☌ before the valve is asked to open and will rise to 61☌ before the valve is instructed to close.įrom this point onwards, the water temperature in the tank continues to fall until, at point D (59☌), the thermostat tells the valve to open. A diagram of the switching action of the thermostat would look like the graph shown in Figure 5.2.2. This 2☌ (☑☌) is known as the switching differential. Thus there is an in-built switching difference in the thermostat of ☑☌ about the 60☌ set point. In this case the upper switching point might be 61☌ (the point at which the thermostat tells the valve to shut) and the lower switching point might be 59☌ (the point when the valve is told to open). This is essential to prevent over-rapid cycling. From then on it could open and shut rapidly, causing wear.įor this reason, the thermostat would have an upper and lower switching point. ![]() Logic dictates that if the switching point were actually at 60☌ the system would never operate properly, because the valve would not know whether to be open or closed at 60☌. The thermostat is set to 60☌, which is the required temperature of the water in the tank. ![]()
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