Almost right. Because of how vacuum gauges work, the formula would be 30 - 30*(10/15). But the temperatures must be from absolute zero, i.e Kelvin. The relative temperature drop in Kelvin from room temp to freezer temp will not be anywhere near as high as you depicted and would not move the needle on the vacuum gauge very much. Certainly not enough to call it a calibration.
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Good point. I concur. So how would a diy’er check their own vacuum gauge? …hmmm … Here’s an idea, connect the gauge in question to the intake manifold of an OBD II car that uses a MAP sensor. Warm idle. A scan tool should provide a pretty accurate MAP reading of the intake manifold vacuum, to compare against the vacuum gauge.
A better method would be one that doesn’t rely on any other measurements except something that is almost certainly accurate, like physical dimensions (inches, mm, etc). A corporation that manufactures and tests their products using vacuum gauges of course would need to obtain some sort of irrefutable method or reference chamber from a gov’t standards agency.
I still think, in my simple scientifically uneducated mind anyway, that hooking up a hand vacuum pump w/gauge to a vacuum gauge and pumping up to a few different #'s, if they are showing the same # then it would be odd that they are off by the same amount and I would call that pretty accurate…
You are just checking vacuum on an engine, not building the Space Shuttle… lol
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I am going to check vacuum on this engine to get an idea of how healthy it is no matter what. I don’t need the accuracy and precision used when building a space ship for sure but want to at least get a rough idea. I will also get the parking brake adjusted.
I plan to keep driving it until something major gives it up. I had planned to get rid of this truck before the pandemic but the one I bought to replace it with has never quite gotten all the kinks worked out and has overall been a disappointment. It is sitting at the shop now. While it was giving me trouble, the 1997 needed a bunch of work and I had that done so that I have one good running fullsize 4WD truck on hand. Used trucks of unknown quality were selling at a premium and I at least know the good and bad about this one.
True enough, but its still an important principle. In the late 1880’s/early 1900’s scientists were very confused why that eqn worked no matter what the gas was. Air produced the same pressure effect as hydrogen, carbon dioxide, etc. They were thinking (correctly) that the cause of the pressure force was the molecules banging against the side of the container. The puzzle was why the pressure is the same with either a light weight molecule like hydrogen or a heavy weight molecule like carbon dioxide. Eventually they figured it out. The heavy weight molecules bang with the same force as the light weights b/c the heavier ones are moving more slowly.
This principle was used to create the first water-condensing steam engines. There’s more force when the container holds a gas that changes to a liquid upon cooling, so it works pretty good for engines. The same basic principle is used for a car’s air conditioning system, and to create the vacuum that pulls the air/fuel mixture into the cylinder.
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Procrastination: the parking brake doesn’t work so you guys are searching for the ideal vacuum gauge. Nobody wants to get their hands dirty.
The OP invoked the vacuum gauge calibration issue. It’s their thread. The rest of us are just offering opinions.