help reading my gauges

[Awesome67mustang]

I got myself a set of AC gauges.. but having a hard time figuring out how to read it. I hooked it up to my 2009 nissan rogue, using 134a. Should I be reading the psi value or the R134a value? I took a picture of the guage, it was a nice day at 80f.

The low side is looks like 110psi but if I look at the R134a row, it reads at about 93.
The high side reads at 350 psi, which seems crazy high, but on the r134a row the needle is around 170.

the pressure charts indicate that the desired pressure should be
low 40-50 psi at 80f
high 175-210 psi at 80f

the reading in my gauge are certainly way off, which would explain why my ac is not working
but in trying to diagnose it I want to make sure i'm using the correct values. should I use the psi or the row marked 134a. isn't psi the same regardless of type of refrigerant you use? why do these gauges have different values for different refrigerants?

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[JohnHere]

I got myself a set of AC gauges.. but having a hard time figuring out how to read it. I hooked it up to my 2009 nissan rogue, using 134a. Should I be reading the psi value or the R134a value? I took a picture of the guage, it was a nice day at 80f.

The low side is looks like 110psi but if I look at the R134a row, it reads at about 93.
The high side reads at 350 psi, which seems crazy high, but on the r134a row the needle is around 170.

The main (outermost, black-on-white) numeric scales on your gauges calibrate in "bar," which is the unit of pressure measurement used in European and other countries. The second numeric scale toward the inside of your gauges (also black-on-white) calibrate in PSI, which are the scales you want. Normally, Manifold Gauge Sets intended for the U. S. market have PSI as the outermost scales, and they are more granular, but your gauges will still work as long as you pay careful attention and read the correct scales.

The R-134a numeric scales (low side is white-on-blue, high side is white-on-red) calibrate in "degrees Fahrenheit (shown as °F in the center of the gauges) and give the temperature of the refrigerant. In addition to the R-134a scales, you also have temperature scales for R-410a, R-22, and R-12, none of which you'll use for your vehicles.

the pressure charts indicate that the desired pressure should be
low 40-50 psi at 80f
high 175-210 psi at 80f
the reading in my gauge are certainly way off, which would explain why my ac is not working
but in trying to diagnose it I want to make sure i'm using the correct values. should I use the psi or the row marked 134a. isn't psi the same regardless of type of refrigerant you use? why do these gauges have different values for different refrigerants?

I don't use pressure charts, which can often throw you off. For example, at 40-50 PSI on the low side, the evaporator would be from ~45°F to ~54°F, too warm for much of any cabin cooling.

See above for an explanation of the various scales on your gauges.

PSI (pressure) is the same regardless of refrigerant, but changing the temperature of a refrigerant will also change a refrigerant's pressure.