Automotive Air Conditioning Information Forum

On my Mastercool gauges, there are three separate scales. An outer one for PSI readings and two inner ones, one labeled R-134a and the other for R-12 along with various numbers which I assume represent degrees Fahrenheit. How are these inner scales used? I'm asking specifically about the blue scale on the low side gauge and the red one on the high side, which is indicated with R-134a.
One guy I spoke with suggested that one reads the scale that corresponds to the type of refrigerant one is using and ignores the outer PSI scale.

I'm sorry if this seems like a dumb question, but what's the story here?

Thanks.

That is basically a P/T chart for both R12 (white scale) and R134a (Red / blue scales)

It is used as a quick reference.

I'm not an AC expert, but have vehicles with R134a and one still with R-12. I only use the psi scale.

bohica2xo wrote:That is basically a P/T chart for both R12 (white scale) and R134a (Red / blue scales)

It is used as a quick reference.

So what you're saying is that the red/blue scales used as a quick reference when the system has been equalized after a few hours rest as a quick test and not used when charging/verifying charge while the system is running, correct?

The temperature scales are the saturated temp of the refrigerant.

Dougflas wrote:The temperature scales are the saturated temp of the refrigerant.

What does that mean and why is it relevant and/or useful?

Google is your friend.

Dougflas wrote:Google is your friend.

I'm sorry, but that doesn't help me to understand what's going on.

Wren wrote:

Dougflas wrote:The temperature scales are the saturated temp of the refrigerant.

What does that mean and why is it relevant and/or useful?

It lets you know the temperature of the refrigerant in the evaporator and condenser.

It's a fundamental concept in understanding how AC works, and something you need to know if you are going to understand what you are working on.

I will explain it as briefly as I can, but you would be well served to search out and read up other sources on the subject.


Saturation temperature means boiling point. Remember, the saturation temperature changes according to pressure. This is how an AC system uses a compressor to manipulate pressures to allow evaporation in the evaporator (absorbing heat) and then condensation in the condenser (shedding heat).

It is also useful for determining temperature at your heat exchangers. A functioning condenser cannot be warmer than it's saturation temperature, and a functioning evaporator cannot be colder than it's saturation temperature.