Vacuum Gauges not zero'd?

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PA Dodger

PA Dodger

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I have a hand me down name brand vacuum gauge. It reads 2" mercury when not hooked up. So, I bought a name brand new one. Same thing. I know altitude plays into the gauge's reading but I'm at 675' above sea level so the error should only be 1/2" maybe. I researched and don't see any way to zero them out. Am I missing something? I know I can do a little math-a-ma-cating but I like it when things are what they are supposed to be. Am I missing something here?
 
I have a 80s vacuum gauge and a new one. I'm at 5000 ft. Both read 0 when not attached.
 
If it is a bourdon tube style gauge, it should be zero when not connected to a measuring port because the pressure inside and outside the curved tube would be equal and it should have been calibrated when manufactured. My Sun diagnostic vacuum gauge was manufactured in 1979 and reads about 1/4" hg positive, or about the width of the needle. Expensive gauges will sometimes have a cap in the lens that can be removed so you can put a small screwdriver on a calibration screw but consumer gauges don't usually have that feature.
 
Sometimes there will be a small flathead screw on the back to calibrate them.
 
ACME SS said:
If it is a bourdon tube style gauge, it should be zero when not connected to a measuring port because the pressure inside and outside the curved tube would be equal and it should have been calibrated when manufactured. Expensive gauges will sometimes have a cap in the lens that can be removed so you can put a small screwdriver on a calibration screw but consumer gauges don't usually have that feature.
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They are both Bourdin style. They don't have calibration screws. I did see where you can disassemble and pull the needle off and reset to zero. I just find it odd that two gauges made 50 years apart read exactly the same error.
 
PA Dodger said:
They are both Bourdin style. They don't have calibration screws. I did see where you can disassemble and pull the needle off and reset to zero. I just find it odd that two gauges made 50 years apart read exactly the same error.
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That is really odd. Yeah if you can do that, go for it.
 
I've bought 2 vacuum gauges off ebay. One is liquid filled. The simple answer is you need a decent one with a zero adjust device. I have something like ? 3? used vacuum gauges I've picked up over the years and it turns out that NONE of them are anywhere near accurate.

I found this out when playing with "all this stuff" recently when getting my old tools together for an HVAC project

It is true that atmospheric changes will NOT affect the zero, but it WILL affect the reading during operation. There are charts you can find online for compensation
 
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