# What's going on with the "temperature" part of this pressure gauge?

Ok, I think I finally understand the pressure bit of this gauge that came with this set of poorly documented hoses I got off Amazon. From 0 to 350, it's actually measuring pounds per square inch (PSI). From 0 to -30, it's not using psi at all but inches of mercury (") roughly equivalent to a psi value about half what's on the gauge so that -30" is actually about -15 psi.

What's going on with the middle part of the gauge? There's no separate dial. Is it really giving me a Fahrenheit temperature reading (°F) of the refrigerant (R-134a/R-502/R-12/R-22) under the stated pressure so at 0 psi the temperatures should respectively be -15°/-55°/-20°/-40°F? Is this really a one-for-one thing for each of these refrigerants that I should be able to trust? or should I just completely ignore that bit and use a separate gauge if I need to check the coolant temperature?

• This isn't a definitive answer as I'm not expert enough in the subject, my understanding is that each refrigerant needs to be at a specific pressure to achieve a coolant temperature. If you want your R-22 refrigerant to be 0 degrees F you'd want it at 24psi, 24psi for R-134a gives you 28F. So it allows you to aim for the desired coolant temperature instead of having to figure out the pressure you need from a chart.
– GdD
Commented Sep 14, 2022 at 8:16
• Welcome to Motor Vehicle Maintenance & Repair! One thing to point out, do not confuse vacuum with negative pressure. While it may be read on the same gauge, it's not the same thing. You cannot compare a vacuum to a pressure and have it equate. Hard to explain, but a pressure medium continues to apply force on the objects in it, while a vacuum doesn't. We need a way to be able to measure vacuum and thus we have the scale, but don't try to equate the two, because it doesn't work. Commented Sep 14, 2022 at 9:56
• @Pᴀᴜʟsᴛᴇʀ2 Which part of the question was that in response to? Did I somehow misunderstand how the negative part of the scale works or should be used? I do know that if I'm draining the system I need to be using a micron gauge instead fwiw, but I don't quite know what to make of your warning.
– lly
Commented Sep 14, 2022 at 10:32
• At the end of your first paragraph, you state, "... roughly equivalent to a psi value about half what's on the gauge so that -30" is actually about -15 psi." You shouldn't look at it that way because it doesn't work that way. Vacuum and pressure are way too different things. Commented Sep 14, 2022 at 11:23