 Thermal expansion valves are often referred to generically as "metering devices" although this may also refer to any other device that releases liquid refrigerant into the low-pressure section but does not react to temperature such as a capillary tube or a pressure-controlled valve.Flow control, or metering, of the refrigerant is accomplished by use of a temperature sensing bulb, filled with a gas or liquid charge similar to the one inside the system, that causes the orifice in the valve to open against the spring pressure in the valve body as the temperature on the bulb increases.As the suction line temperature decreases, so does the pressure in the bulb and therefore on the spring, causing the valve to close.