Capillary tubes are generally used as the metering device on small, fractional-horsepower refrigeration systems.

Charging these systems with refrigerant requires a different procedure than those systems using a thermostatic expansion valve.

A capillary tube system can be charged two ways:

1. Weighing in the exact amount of refrigerant charge as stated by the equipment manufacturer; or

2. Measuring the amount of superheat at the outlet of the evaporator.


Weighing in the refrigerant charge is a more exact method and is preferred by most manufacturers. This requires determining the amount of refrigerant originally charged into the system. This information is normally stated on the equipment’s data plate.

With the old refrigerant removed from the system and the system dehydrated, weigh in the refrigerant charge using an electronic scale or a Dial-a-Charge©. This will ensure that the proper amount of refrigerant has been added to the system.

If any refrigerant has been added or changed on the system, the amount of refrigerant may need to be adjusted to compensate for these new or different components. An example of this might be if a new, larger, liquid-line filter-drier was installed — more refrigerant may need to be added to compensate for this component. A technician should consult with the manufacturer of the equipment or the component to determine the change in the refrigerant charge.


Many times a service technician may not be able to find the correct refrigerant charge for the system, or may find a system that is only slightly short of refrigerant. Weighing in the refrigerant charge may not be practical if either the original charge is unknown, or the customer is unwilling to pay for the extra time required to remove the original charge. An alternate charging method is for the technician to measure the amount of superheat at the outlet of the evaporator and before a suction line heat exchanger, if one is installed.

A capillary tube, which is soldered to the suction line, will act as a heat exchanger. The superheat reading needs to be taken before this junction. A technician can then slowly add refrigerant to the system until a superheat reading of 8 to 12 degrees F is observed.

Note: Take care to slowly add refrigerant to the system. It may take up to 10 min for the system to balance with the new charge.

Many times it is difficult to measure superheat at the proper location. An alternative method for medium-temperature applications may be to look for the frost line on the suction line just as it exits the case. A technician can slowly add refrigerant to the system until frost is observed on the suction line where it exits the case.

If the frost starts to develop further along the suction line toward the compressor, this is an indication that too much refrigerant was added to the system and should be removed. In order to use this method, the case needs to be at or close to its normal operating temperature. Once the case has reached this temperature, the technician can observe the frost pattern.

Although this is not a very accurate method, it should assure the technician that the system has a sufficient charge of refrigerant.

Marchese is owner of Coldtronics, Pittsburgh, PA. He can be reached at 412-734-4433; (website); or (e-mail).

Publication date: 04/01/2002