Refrigeration gauge set © D Friedman at Guide to Refrigeration Gas Test Gauge Use

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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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Details on Proper Use of the Refrigeration Charging / Testing Gauge

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

To keep moisture & dirt out of an air conditioner, heat pump, refrigerator, freezer, etc., 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


if you use pressure test gauges 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.

Reading the High Pressure Side Refrigerant / HVAC Test Gauge

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

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/CM3On 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 COMPRESSOR / CONDENSER TYPES 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 © D Friedman at In 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.

[Click to enlarge any photo to see more detail]

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

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

Watch out: Even all three hoses of your gauge set is connected to high side, refrigeration canister or charging cylinder, and low side, never open both high side and low side gauge connection valves at the same time.

Exposing the low side and gauge to high pressure could certainly blow the gauge, damage the HVACR system, and cause serious injury as well. - Thanks to reader NJSnowman for suggesting this clarification, via comments. 2016/08/16

Refrigerant Test Gauge Hose Hookup Details

Refrigeration gauge hookup sketch © D Friedman at In our sketch at left

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.

Watch out: Never open both high side and low side valves on the gauge set at the same time.

Exposing the low side and gauge to high pressure could certainly blow the gauge, damage the HVACR system, and cause serious injury as well. - Thanks to reader NJSnowman for suggesting this clarification, via comments. 2016/08/16

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.

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.

What is the Process Port on an HVAC Compressor Motor & How is it Used?

Question: OK to use the process port or charging port on the compressor itself to charge the system?

2017/04/06 John said:

I am charging a Commercial kitchen fridge with the compressor and condenser located externally from the fridge itself.

The charging port on the service valve is leaking, there is a charging port on the compressor itself, can I use this to charge the system?

Reply: Yes

Yes, if you're charging with gas not liquid and you're on the suction side. But I worry that the leaky port is going to continue to leak - I'm not an expert but in my experiences the valve cap doesn't make a reliable long term seal.

John said:

Thanks, I check somewhere else and it tells me that the port I was talking about is a process port. There is the suction and discharge line, but this particular one says it's a process port. What worries me is the uncertainty of this process port. if it is on the low or high side of the compressor.

(Mod) said:


Doing a little research about process ports I read that it's an "alternative suction line" - and on smaller HVAC compressor motors both ports are open to the same compressor chamber - so either could be used for charging.

In some HVAC texts the process port or tube is used to evacuate the system and would still be in common with the compressor suction side interior.

I have certainly read that HVAC techs charge (gas only ie low side) through a process port and even prefer that.

On some higher-efficiency or larger compressors that's not so clear as I note below though I think the principal difference is the port diamater and its intended use. On some larger compressors the process tube may be a smaller-diameter tube used for an auxiliary function such as an ice maker - still charging through that port could be OK.

A definitive answer would come from the manufacturer of the specific HVAC compressor motor.

The Tecumseh company warns:

Important! The AE2 is a high efficiency compressor and uses directed suction intake. It is necessary (with the exception of ice cube machine applications) to use the designated suction tube. For ice cube machines: Tecumseh recommends to use the process (alternate suction tube) as the suction tube.

And this PDF from Manitowoc

Here is another example of using the process port on a compressor motor, from Embraaco

How to remove a used compressor

We recommend that the used refrigerant fluid is collected for later recycling or incineration, according to the following procedure: First, install a perforating valve in the compressor process tube.

Connect the perforating valve to the recovery equipment that is then connected to the receiving cylinder. Now just connect the recovery equipment. Open the receiving cylinder valve and then the perforating valve.

It is very important to keep the recovery equipment operating as long as it is necessary to collect all refrigerant. The duration of this process will depend on the equipment used and refrigeration system. -


or at OVER CHARGED of REFRIGERANT, EFFECTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

How to Make Air Conditioner Pressure Gauge Readings

Photograph of a
commercial air conditioning compressor charging gauge set (C) InspectAPedia.comIn our illustration of air conditioner service equipment pressure test gauges at page top and at left, you notice that there are two gauges and two sets of connectors and control valves. "

Gauge pressure" can read either the pressure inside the condenser unit (the "high side" of the system) or the pressure inside the evaporator (cooling coil) or "low side" of the system.

The gauge set accepts three connecting hoses:

  1. Low pressure side (blue hose on my gauges)
  2. High pressure side (red hose)
  3. Refrigerant gas source (center fitting)

Note that gauge pressure is an absolute pressure reading before any correction for ambient temperatures around the unit. When comparing measured or gauge pressure with recommended refrigerant pressures it is necessary to correct gauge pressure for ambient temperature variations. A/C equipment, gauges, refrigerant charging manuals etc. include pressure charts to aid in this correction.

Low-side pressure calculation example

Using now-obsolete R-12 refrigerant gas as an example, looking at the low-side refrigerant pressure (the low side is the side at which the refrigerant liquid has boiled into a gaseous state), we can calculate the temperature at which the refrigerant should boil for given ambient conditions:

(38-45 degF example) - (18 degF temperature difference between inside the condenser and ambient) = 20 degF = the temperature at which the refrigerant must boil, i.e. the state change from liquid to vapor. Looking at 20 degF. in the table for R12 state changes shows us that we should see 21 pounds of pressure.

High-side pressure calculation example

At 80 to 100 psi pressure on the high side, if ambient temperature is 72 degF, heat will be transferred successfully to outdoor air at temperatures of 84 degF to 117 degF at the condenser coil.


Continue reading at REFRIGERANT PRESSURE READINGS & CHARTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see REFRIGERANT TEST GAUGE FAQs posted originally at this article

Or see these

Air Conditioning & Heat Pump Refrigerant Articles

Suggested citation for this web page

GAUGE, REFRIGERATION PRESSURE TEST at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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