Pressure Control as Operating Control
A Pressure control can also be used as an operating control. The electrical circuitry is the same as with a temperature controller as can be seen by clicking the Buttons. The LPC makes on a rise of pressure.
The graph below shows what is happening. The control is set to start refrigeration if the box warms up to 40 °F. However when using a pressure control the Cut In, Cut Out and differential are are in units of pressure, not temperature. The corresponding pressures must be determined for the refrigerant being utilized and the differential must be wide enough so that the equipment does not short cycle.
Cut In and Cut Out set points can be adjusted by screws on the top of an LPC. Some LPC's provide Cut In and Differential adjustment screws instead. The sensing line of the control must sense constant low side pressure and the evap fan must be left running continuously. The recirculating air imposes a load on the evaporator which is sensed by the LPC.