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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?

Gauge

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    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

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Pressure can be measured in pounds per square inch (psi), inches of mercury (inHg), millimeters of mercury (mmHg), kilograms per square centimeter (kg/cm²), kilopascals (kPa), bars (100 kPa), megapascals (MPa) — there are several different scientific, professional, and national conventions.

I think you need to review basic refrigeration fundamentals, measurements using psi and inHg and refrigeration temperature/pressure charts. The t/p charts show chemical characteristics relative to temperatures. Find a t/p chart for r134a. At specific temperatures or pressures in a sealed volume, refrigerant reacts with repeatable results. The vacuum gauge shows the t/p relationship of each refrigerant as pressure varies. You'll need to know service manual lists of normal operating pressures for the low and high sides and whether that's measured at an idle rpm or a higher specified rpm to correlate values in determining if a system is operating correctly.

Sea level pressure is 14.7 psi, 29.9 inHg, etc. Zero on gauges use it as a set reference point for easier use. When a vacuum occurs, measurements are in inHg, not psi. There is no such thing as negative pressure but the term is used loosely sometimes for expedience. In refrigeration, evacuating a system should show the vacuum gauge needle move ccw from '0' towards 29.99 (the '-' connotation is left out) indicating lower than sea level pressure approaching a vacuum. Search for descriptions of how a tube filled with mercury is inverted into a container of mercury; as the heavy mercury tries to empty from the glass tube, a vacuum is created but the column stabilizes as atmospheric pressure pushes down on the container of mercury to equalize liquid in the glass tube trying to flow out. At sea level, that's 29.99 inches of mercury. Measuring vacuum in inHg is much easier than referring to psi.

Low side pressure can be anywhere from 25–40 psi, correlating to whatever temperature is shown on the gauge. Actual vent temps would be some degrees higher and above freezing to prevent creating an ice box blocking airflow. Refrigeration gauges may have service instructions for replacing miscellaneous parts with diagrams. At one time long ago, instructions described how each three-way valve operated. Understanding refrigeration gauges are a must before connecting to any system.

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