Typically a part of thermostatic expansion valves or other A/C components that work by sensing temperatures of various other components of the A/C system, the capillary tube is typically a very small diameter tube that is filled with a gas and is made with a precisely calibrated length and inside diameter. That capillary tube is used to connect the remote bulb or coil to the expansion valve.

The remote bulb or coil is what is used to sense the temperature of another part of the A/C system. In the case of a typical thermostatic expansion valve or TXV, the sensing bulb is located at the evaporator outlet and is firmly attached or clamped to the outlet tube and covered with an insulation. The bulb senses the outlet temperature and that information is relayed back to the expansion valve via the capillary tube. As the evaporator outlet becomes too cool, the expansion valve will open to allow more refrigerant into the evaporator. As the temperature sensing bulb senses warmer evaporator outlet temperatures, the expansion valve will close to allow refrigerant pressures and temperatures to drop.

Similar capillary tubes are used on some compressor cycling switches. In some cases, that capillary tube is secured in the fins of the evaporator to sense the temperature of the evaporator. When the evaporator becomes too cold, the compressor will be turned off. When the temperature of the evaporator increases, the compressor will be cycled back on so that compressor suction will pull the pressure and temperature lower. Overall, the compressor is able to maintain a mean temperature to provide maximum cooling performance without freezing the condensation on the evaporator.