Refrigerant gauges are nothing more than simple readout dials that show the amount of pressure contained within an air-conditioned system. They are normally composed of a small, rounded metallic frame that holds a register dial beneath a sheet of glass or plastic, with a sole opening compatible to connect with refrigerant lines. Very few appliances actually have built-in refrigerant gauges; instead, the tool is carried by heating ventilation and air conditioning (HVAC) repairmen as they service various pieces of appliances and machinery. When the reading on the gauge shows that the pressure is lower than expected, the technician can determine how much coolant needs to be added to the appliance or air conditioning system for it to function properly.
A similar example of refrigerant gauges can be found on many different types of air pumps. They function exactly in the same way, the only difference being that refrigerant gauges are registering coolant while the gauge on the pump is measuring the outward force of air. Since the overall pressure between these two types of gas would be different, the gauges would not be considered interchangeable, but each of them look virtually identical in their actual composition.
There are a growing number of commercial appliances being constructed with incorporated refrigerant gauges as new forms of coolant are becoming available to the general public, and these are installed so that consumers can maintain their systems without calling a certified repairman. The sale of gasses like freon have only been available to licensed HVAC dealers since these contaminants are harmful to the environment—as more environmentally-friendly replacements become available, the number of pre-installed refrigerant gauges should continue to grow in popularity. Eventually, all residential and commercial appliances that require coolant should have a refrigerant gauge built into the system so that consumers can monitor them without assistance.
Of course, each type of refrigerant gauge is programmed to recognize the inert pressure from one specific type of gas, so current appliances would still have to be serviced by a licensed dealer. That is why servicemen have multiple types of refrigerant gauges within their vehicle whenever they respond to a call. Until a completely safe coolant is discovered, this will likely not change. It is also important to note that most appliances and air-conditioning units are designed to only accept one specific type of refrigerant, so if freon or any other dangerous coolants are ever discontinued, consumers will be forced to upgrade to newer models.