R134a Pressure Gauge Readings

R134a pressure in car air conditioning

Car Air Conditioning / October 28, 2017

A visitor comes to us with questions about what might be wrong with the a/c in their car. It goes something like this. My air conditioning was working last week, and then it quit working. What do you think is wrong? Or, maybe something like this... I added some refrigerant to my car, and it still won't get cold. What could be wrong? Well, unfortunately, questions like that are almost impossible to answer without detailed information. That's where the manifold gauge set comes into play. We use a manifold gauge set to get more information about what's happening inside the system. We created this FAQ to help answer some of the most common questions we receive on using a gauge set and hopefully provide some explanation on pressure readings.

As a prerequisite, anyone interested in learning how to use a gauge set should first take a few moments to learn how the refrigeration cycle works and understand how refrigerant flows through the system. This in itself will help you immensely when it comes time to identify the high and low side of the system. You'll also begin to understand the role pressure plays on different parts of the system. That's important, since the gauge set is used to measure pressure within the system.

The basic manifold gauge set usually has three hoses. Two hose will be attached to the service ports on the vehicle during service. Each hose has its own identifying color. In most cases, the hose intended for the low pressure service port is blue, and the hose intended for the high pressure service port is red. The middle hose should be yellow. That yellow hose will be attached to the refrigerant cylinder while charging or the vacuum pump when the system is being evacuated of air and moisture. Your manifold gauge set should have a corresponding gauge and control knob for each of the two service hoses. Like the color of the hose, the gauges and control knobs will usually be colored to indicate high or low pressure.

What are all the numbers on the high and low side gauge?

The low side pressure gauge is called a compound gauge. That means it can be used to measure pressure or vacuum. The numbers around the outside of this gauge indicate pressure in pounds per square inch (PSIG), and the numbers near the bottom indicate vacuum in inches of mercury. The smaller scales near the middle of the gauge list the temperature relationship of different refrigerants. The gauges pictured here lists the temperature of R12, R22, and R502. Regardless of which refrigerant is being used, the scale designated as PSI is the one used to read system pressures when charging and diagnosing an a/c system. The working pressure of this gauge is from 0 to 120 PSI.

The red, high side gauge is used to measure the high pressure side of the a/c system. This gauge has no markings that indicate vacuum. It reads positive pressure only. The working pressure of this gauge is also much higher than the low side gauge. Notice the scale on this high side gauge reads from 0 to 500 PSI

I hooked up the service hoses with the car turned off. Both gauges show pressure. What does this mean?

The pressure readings you see when the a/c system is not operating is called static pressure. When the system is off, and temperature is stable, the pressure you see on both the high and low side gauges should be the same, or very close. Both the high and low side of the system have equalized.

What static pressure should I expect to see when I hook up my gauge set.

Each refrigerant has it's own static pressure at every corresponding degree in temperature. The important thing to keep in mind is static pressure changes based on temperature. Any change of temperature brings with it a change of pressure. The greater the temperature, the greater the pressure. You can use a refrigerant pressure chart to find static pressures at various temperatures. Static pressure will not be used to determine if a system is fully charged. Using the chart below, if the R-134a system has a static pressure of 88 psi at 80 degrees F., we can then assume the system has some amount of liquid refrigerant. The system may be full -or - may not be. At the same temperature, if the system showed only 75 psi, we could say with confidence, the system is low. This is because static pressures shown on a temperature chart would show inadequate pressure for the presence of any liquid refrigerant.

- Coming soon

Can I tell if the system is full with a static pressure reading?

No. We might determine if there is liquid refrigerant in the system, but we won't be able to tell how much liquid it contains. For example, a thirty pound can of refrigerant will show the same pressure whether it has thirty pounds in it or if it only has 1 ounce. With static pressure, you will only know if the system has some amount of liquid refrigerant present.

What good is a static pressure reading then?

With our initial pressure reading, we can tell if the system has enough pressure to satisfy the low pressure switch and enable the to operate. Static pressure is used to determine if a jug of refrigerant is contaminated with air. Static pressure can also be used to determine if a system has enough pressure to begin leak testing. Your static pressure should be no lower than 50 psi when leak testing.

What's the minimum static pressure I need for the compressor to operate?

Most systems will have a low pressure cut off switch that turns the system off at approximately 20 psi. The compressor will not function again until the pressure reaches approximately 45 psi. So, In most cases, you will need a static pressure of at least 45 psi before you begin to see the compressor operate.

Can I test the system with only 45 psi?

You can begin testing with only 45 psi. You won't get any cold air, but you should should start to see some compressor engagement. As soon as the compressor engages, it will cycle off rather quickly when the suction side of the compressor draws the pressure on...

Source: aircondition.com