The range of a pressure switch is the minimum to maximum pressure at which the switch can be set. Because refrigeration systems have two basic pressure areas, high side and low side, pressure switches are often described as either high or low pressure switches, based on which side of the refrigeration system the switch is designed to operate. However, this is a bit of an oversimplification. It is possible for two pressure switches with identical ranges to behave very differently if they have different switching actions.
The switching action describes what occurs when pressure rises above the switch set-point. Basically, only two things can happen: the switch either closes or it opens. So, pressure switches are classified as either close on rise, or open on rise. For refrigeration system safety applications, close on rise switches are used to protect against low system pressure, and open on rise switches are used to protect against high system pressure. But there are many other applications for pressure switches. For example, a close on rise pressure switch can be used for condenser fan cycling to maintain head pressure. The condenser fan is energized when the condenser pressure rises to the switch cut-in point. Similarly, an open on rise pressure switch can be used to control the harvest cycles on an ice machine. When the evaporator pressure drops to the switch cut-in point, the harvest cycle is initiated. The safest way to describe pressure switches is by both their switching action and their pressure range.
Cut-in, Cut-out, and Differential
It is important to understand that the switch contacts cannot both open and close at exactly the same pressure – there has to be a difference between the pressure when the switch closes and the pressure when the switch opens. This difference is called differential. Three terms are used when describing pressure switch settings: cut-in, cut-out, and differential. Cut-in is the pressure when the switch closes, cut-out is the pressure when the switch opens, and differential is the difference between the two.
High Event and Low Event
Sometimes the terms high event, low event, and differential are used. In the case of a close on rise pressure switch, the high event would be the cut-in and the low event would be the cut-out. In the case of an open on rise switch, the high event would be the cut-out and the low event would be the cut-in.
What Difference Does it Make?
There are both physical and practical reasons for having a differential. Physically, pressure switches are mechanical devices which use levers that are controlled by springs and pressure bellows. There must be some mechanical motion to open or close the electrical switch contained within the pressure switch. Since this motion is created by a change in pressure, the opening and closing point cannot be the same point. Practically, you really would not want the control system to respond so quickly. For example, once the compressor stops running, the high side pressure almost immediately drops a little, even on systems with hard shut-off expansion valves. If a small drop caused the high pressure switch to close again, the compressor would quickly cycle on and off repeatedly, which would cause more damage than the high pressure alone. I hope this helps explain why there are multiple settings on a pressure switch and what they all mean.