Many of the older fractional horsepower refrigeration systems, such as reach-in coolers or prep table coolers, use a mechanical constant cut-in control to regulate the air temperature within the case. This style of control directly measures the temperature of the evaporator coil to control the case temperature and provides a method of defrosting the coil during each off cycle. Each time the control cycles the compressor off, the control will not cycle the compressor back on until the evaporator is void of frost and reaches the desired cut-in temperature of the case.

Today’s newer systems do not use this style of temperature control. Most manufacturers today are designing systems using an electronic temperature controller, which directly senses air temperature. These controllers have many advantages over the older mechanical controls, including the following:

• They allow for a more precise control of the case temperature;
• They typically use an adjustable, timed initiated/temperature terminated defrost cycle, with an additional temperature sensor measuring the coil temperature;
• They display the case temperature, so owners can observe the actual case temperature more easily;
• Some incorporate an alarm function to alert the owner when a case temperature is too high; and
• Many have short cycling protection for the compressor, using a minimum off time delay.

Since these controls also measure air temperature instead of coil temperature, they allow the system to operate with a slightly low refrigerant charge and potentially still maintain the case temperature. The older mechanical constant cut-in controls are more affected with a slight loss of refrigerant since the coil temperature at the sensing bulb will be different.