Refrigeration gauge set (C) D FriedmanGuide to Refrigeration Gas Test Gauge Use

  • GAUGE, REFRIGERATION PRESSURE TEST - CONTENTS: Recommendations for refrigeration gas pressure measurement using a test gauge set. Proper Use of the Refrigeration Charging / Testing Gauge Set to Keep Moisture & Dirt out of an Air Conditioner, Heat Pump, Refrigerator, Freezer, etc. - How to hook up a refrigerant test / charging gauge set to the test ports - Attaching the Refrigeration Gauge Set to the Air Conditioner, Heat Pump, or other refrigeration equipment - Procedures for Reading the Gauges on a Refrigeration Gauge Set. Procedure for Charging the HVAC System or Appliance with Refrigerant Using a Gauge Set
  • POST a QUESTION or READ FAQs about how to hook up and use an air conditioner, heat pump, or other refrigeration equipment refrigerant gas pressure test gauge

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Air conditioner & heat pump refrigerant test gauges: this article describes the connections, use, and reading of a refrigerant gas pressure test gauge set. We describe the procedure for using test gauges when adding or replacing refrigerant: charging an air conditioner, heat pump, refrigerator with refrigerant gas.

We explain how a refrigeration gauge set should be connected to HVAC equipment to avoid contamination damage and we review the refrigeration system evacuation and cleaning procedure.

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Details on Proper Use of the Refrigeration Charging / Testing Gauge Set to Keep Moisture & Dirt out of an Air Conditioner, Heat Pump, Refrigerator, Freezer, etc.

R-22 refrigerant and air conditioning test gauges (C) Daniel Friedman

To keep moisture out of a refrigeration system, in addition to finding and fixing leaks, we need to know how to properly use a refrigerant gauge set with charging lines, and how to use cap off plugs on the charging fittings.

To be clear, when connecting an HVAC refrigeration gauge set to test fittings on an air conditioner or heat pump we must:

  1. Connect the gauge set center supply tube to a canister of the proper refrigerant gas matching the refrigerant in the system being tested
  2. Leave some positive pressure of refrigerant gases in each of the two gauge test connection hoses - the high pressure side and the low pressure side, so that when the gauge hose fitting is connected to the service port on the HVAC equipment no outside air or moisture are pushed into the system piping.

Refrigeration Servicing Gauge Set Installation & Use

Reading the Gauges on a Refrigeration Gauge Set

We use (and illustrate) a traditional Imperial System Analyzer gauge set that provides three charging lines (refrigerant canister, high side, low side). This is a diaphragm type gauge but other methods of measuring and charging systems are available.

How to Read the Low Pressure Side Refrigerant Gauge

Photograph of a
commercial air conditioning compressor charging gauge set (C) InspectAPedia.comThe compound gauge at left is used on the low pressure side of the system and shows pressure readings in black, from 0 to 300 psi gauge pressure. [Click any image to see an enlarged, detailed version.]

Temperature corresponding to pressure is shown in red on this gauge for R12 and R22, or for newer refrigerants on newer gauges.

Vacuum is also shown on this gauge on a scale from 0-30 in. Hg. in green..

Reminder: as we discuss at REFRIGERANT PRESSURE READINGS if you use pressure test gauges (GAUGE, REFRIGERATION PRESSURE TEST) to measure the refrigerant pressure in the static or equalized air conditioning or heat pump system, the gauges only tell you the refrigerant pressure, not the quantity of refrigerant that is present in the system.

For example at 70F ambient temperature and with R12 refrigerant, the static system pressure would be at 70 psi as long as there is enough refrigerant in the system to have at least some in liquid state.

Photograph of a
commercial air conditioning compressor charging gauge set (C)

Reading the High Pressure Side Refrigerant / HVAC Test Gauge

The compound gauge on the right of this gauge set is used on the high pressure side of the refrigeration system and shows refrigerant pressures, typically from 0 - 500 psi on the black scale or 0 - 35 KG/CM3 on the outermost red scale.

In the center of the refrigerant gauge the red scales give temperature readings for three older refrigerants (this is an old gauge): R502, R12, and R22.




Attaching the Refrigeration Gauge Set to the Air Conditioner, Heat Pump, or other refrigeration equipment

AC system refrigerant access ports (C) Daniel Friedman

Find the service ports

Central air conditioning systems, heat pumps, and split systems typically have service ports installed specifically for the attachment of test gauges for system inspection, evacuation, and charging.

Residential refrigerators, freezers, and window or portable air conditioners typically will not have these service ports. To service one of those latter devices you'll need to cut the refrigerant line and install (solder in place) a tee and a service port.

Our photo (left) shows four covered service ports on this split system compressor/condenser unit. That's because this unit supports two indoor wall-mounted cooling units.

If its not obvious to you that the larger diameter line is the low pressure or suction side and the smaller diameter refrigerant line is the high pressure side, then really you should not be messing with this equipment before taking a refresher class in HVAC servicing and repair.


Using Temporary Access Valves for HVAC or Refrigeration Testing & Diagnosis

Tapaline® and other piercing valves are available in various sizes to allow the HVAC technician to tap into the refrigerant lines on a system in order to perform diagnosis where there are not already service valves installed. Smaller bullet-type valves are installed using an allen wrench.

Use these valves as a temporary service tool, preferably attached on the process tube. [See Types of air conditioner or heat pump compressors & compressor designs if you don't know what a process tube is.]

Watch out: do not leave these temporary test valves on the refrigerant piping - you're asking for a future leak. These valves are suitable to aid diagnosis of a refrigeration system by avoiding disturbing the troubled state of the system (as you'd do if you had to cut lines and solder in service valves just to do a test).

After using a temporary tap-in valve on the process tube at a compressor unit, you should solder off the line puncture and add a permanent service valve fitting - the type that uses a schrader valve and cap - also referred to as an access valve or line valve.

Connect the Test Gauges to the Air Conditioner, Heat Pump, etc

Refrigeration gauge set (C) D FriedmanIn the shop we connect both high and low pressure gauges to the equipment being tested. In the field technicians often use only the low pressure side of the HVAC test gauge set, since high side problems also show up on the low side.

On test gauge sets such as ours shown here, the service lines always open or close a connection between the HVAC equipment refrigeration test ports and the gauges. The end-valves (silver handles) on either side of the gauge set then open or close a further connection between that valve and either high or low side connection and the yellow hose that will in turn connect to a refrigerant source.

You'll notice in our photo that both ends of the refrigerant hoses are attached to the gauge set. On the front of this gauge (and more easily seen in the photograph) are three blind connector plugs to which we connect the hoses when the gauge is not in use - this step is to help keep the gauge hoses clean of debris.

In the photo at left, the "live" hose connection ports are somewhat hidden behind the gang of hoses and the "blind" connectors used to keep the hoses and their end connectors clean when not in use.

The gauge set has stop valve handles that you see on either side of the gauge. These controls open or close the high or low side service lines once they have been properly connected to the high or low side service port.

As we've mentioned before, we would not normally connect our gauge set to the service ports without first attaching a can of the proper refrigerant to the gauge set service port and then using that refrigerant to purge any air that may be in the refrigerant hoses. That's to avoid blowing air and contaminants into the HVAC system.

On our air conditioner or heat pump test gauges we use a

  • A blue flexible hose on the low-side gauge. In the photo above the blue hose is connected to the left-hand low pressure gauge port, while its free end is connected to a right-hand blind port. When in use the free end of the low pressure gauge is removed from its "blind port" and is then connected to the low side of the system at that service port.

    [Click to enlarge any photo to see more detail]
  • A red hose on the high side gauge. In the photo the red hose is connected to the right-hand high pressure gauge and its free end - the end that will connect to the HVAC equipment test gauge port - is connected to a left-hand blind port to keep the fitting clean when not in use. The high pressure gauge (red hose) will be connected to the high side of the system at that service port.
  • A yellow hose on the center gauge service port just to help avoid any confusion in the field. The free end of the yellow hose is also connected to a center blind port to keep out debris. The center service port on the gauge set will connected to a refrigerant canister, charging device, or to an evacuator pump, depending on what the HVAC technician needs to do.

Depending on which valves you open or close, this gauge set permits charging the refrigerant system on either the high side or the low side.

Refrigerant Test Gauge Hose Hookup Details

Refrigeration gauge hookup sketch (C) D FriedmanIn our sketch at left

  • (A) is the low side pressure gauge,
  • (B) the high side pressure gauge,
  • (C) is the service line at the gauge center, connected to a refrigerant gas source or perhaps to an evacuator pump.
  • (D) is the location of the high side gauge control valve and
  • (E) is the functioning location of the low side control valve.

Watch out: not all HVAC test gauge sets may have their high pressure and low pressure gauges in the same left and right positions.

What is critical is that the low pressure gauge is connected to the low pressure port (by convention using a blue hose) and the high pressure gauge is connected to the high pressure port (by convention using a red hose).

Watch out: Valves (D) and (E) are used to isolate their sides from the service port.

Open only one control valve at a time, then close it before opening the other.

  1. Attach the refrigerant canister to the gauge set
  2. Loosely attach the low side gauge (A) line to the low side service port. Assure that the service valve is turned all the way OUT
  3. Open the refrigerant canister (C) and by opening the gauge valve (E - low side) purge the lines (do not use the system's refrigerant to purge the gauge lines)
  4. Tighten the service line to the service port
  5. Turn off the refrigerant gas canister supply
  6. Turn the equipment service valve in slightly and run the system. The gauge(s) will indicate what the system is doing.

Watch out: as we warn at many places in this article series, do not send liquid refrigerant into the low side of a refrigeration system. Liquid refrigerant will enter the bottom of the compressor motor and can damage the compressor, or even if the compressor tolerates and passes the refrigerant through its pumping system, the refrigerant can carry away the lubricating oil from the compressor, and/or cause an air trap in the system.

We use the same procedure for attaching the gauges to the high side of the system.

Restating a bit:

For the gauge set shown above, turn the valves all the way out to attach the gauges, since service ports are stopped OFF.

  • Turn the gauge control valve all the way in (closed) to stop off the line to the evaporator/condenser (depending on which valve you are using).
  • Attach the gauge hose test line loosely to the test port;
  • Feed a small amount of refrigerant through the gauge test line and the charging line to purge any air. [NOTE that newer test equipment may provide other solutions for this step to avoid releasing any refrigerant to the atmosphere.]
  • Then tighten the connection of the test hose to the equipment service port.

To Remove the Service Test Gauges on Refrigeration Equipment

Back-seat (all the way out) the equipment service valve to close off the service port, then remove the gauge and cap both the service port and the test gauge hose ends.


or at OVER CHARGED of REFRIGERANT, EFFECTS or select a topic from the More Reading links shown below.

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