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One gauge measures pressure in PSIs = pounds per square inch, the other one measures vacuum in IN.HG = inches of mercury. Being different scales I don't have an idea how one can measure pressure with a vac gauge.
Just my 2 cents.

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most of the vacuum gauges can be used to read pressure- no different than a boost gauge for a turbo or blower. vacuum or pressure, you are just taking a reading of how it compared to atmospheric pressure.
just unhook the fuel line at the carb, and hook it up to the vacuum gauge. start the car, and read the pressure. the engine will run this way until the fuel in the bowl is gone.

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novaderrik, are you talking about the press/vac gauges which come with two scales? I have one on my Turbo car that can read press and vac, it has a "0" in between the two scales. If the needle moves up you are in boost, if it moves back then you are in vacuum. The ones I though before are the ones that just read vacuum, this ones have the needle in a position that can't move forward to indicate pressure. You have to suck from them in order to see the needle moving in the correct direction, they just move backwards. I have several of both ones at hand, in my shop or at the factory I work for.

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novaderrik,

Could you provide a way to convert the reading on a vaccum gauge to PSI? If you can't convert the scale what good is the reading?

Maybe an old oil pressure gauge would be better if it had a readable scale below 10 psi... or even a compressor gauge!!

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...Dennis
"The '69, the '96 & the club"
 

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Carl you are 100% on that... I had to look for myself and the needle goes both ways!! novaderrik thanks for the suggestion. As often as I have used my vacuum gauge lately I never noticed the psi scale...

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...Dennis
"The '69, the '96 & the club"
 

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Carl, just to clarify one doubt. By using the conversion you mentioned, for example, 15in.Hg = 7.365psi. Those converted PSIs keep being vacuum, not pressure, right?

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Orench if you want to get technical there is really no 'true vacuum' in nature only 'pressures' that are lower than atmosheric.Yea I know you 'feel' it sucking on your finger when you pull off a hose.At sea level the atmospheric pressure is 14.7 psi or 29.92hg inches of mercury.Lets round those numbers up for easy math and say 15 psi and 30 hg.1 psi is equal to 2 inches of mercury.The vacuum gauge is reading in inches of mercury.If you hook it up to your idling car and the gauge says 20hg you really have 5 psi in the manifold.30hg(normal atmoshere)- 20(reading on gauge)=10hg.10hg is equal to 5psi.
 

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actually, if you want to get really technical, vacuum doesn't "suck" air in- the lower pressure causes air to get blown in from the higher pressure area around it. the one constant thing in the universe is that everything always tries to balance out. eventually, one day billions of years from now, the whole universe will reach a state of balance, and everything will cease to be. the energy created by the unbalanced energies in the universe is what makes everything happen-even makes our cars run. crazy, huh?

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1971 Nova(looks like 69 camaro from underneath!)
355sb, vortec heads, HOT cam,T-10 tranny, 3.70 gears 16X8" IROC wheels. 12" Corvette brakes on the way.
see pics here http://community.webshots.com/user/novaderrik
 

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ORENCH,

It depends on your point of referance.

For us, we consider atmospheric pressure (14.7 psi @ sea level) to be zero. The "vacuum" reading being taken is showing a pressure drop, measured in inHg, in relation to atmospheric. This pressure drop can be considered negative for calculations, but what we really care about is the magnitude of the pressure drop.

In other cases, gauge pressure (zero psi) is used. In this case there can never be a negative condition. It's also difficult to deal with for everyday guys like us.

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Much clear now, if 14.7psi at seal level is considered "0" then the 7.365 obtained in the conversion is negative (-7.365). That for our purposes.Thanks.

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<BLOCKQUOTE>quote:</font><HR>Originally posted by CarlC:
ORENCH,

It depends on your point of referance.

For us, we consider atmospheric pressure (14.7 psi @ sea level) to be zero. The "vacuum" reading being taken is showing a pressure drop, measured in inHg, in relation to atmospheric. This pressure drop can be considered negative for calculations, but what we really care about is the magnitude of the pressure drop.

In other cases, gauge pressure (zero psi) is used. In this case there can never be a negative condition. It's also difficult to deal with for everyday guys like us.
<HR></BLOCKQUOTE>

Absolute pressure references absolute zero pressure, as found in a perfect vacuum. The units are followed by an 'a', as in psia ( pounds per square inch absolute ). At sea level an absolute pressure indicator will show about 14.7 psia.
Gage or Gauge pressure references the surrounding atmosphere, so it shows the difference in pressure between atmospheric pressure and applied pressure. A standard bourdon tube instrument reads gage pressure. The pressure readings are followed by a 'g' to indicate that this is a gage pressure reading ( i.e. psig ). 99.9% of all pressure gauges and sensors read gage pressure.

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onovakind67,

You are correct. It's a matter of reference points. Generally, when expressing "vacuum", it is a negative unit referenced from atmospheric pressure. On the other hand, "pressure" is always a positive unit when referenced from "absolute zero" (~30 in.hg)

It's been a long time since I've had to use these terms.

The gauges capable of reading both vacuum and gauge pressure are referred to as compound gauges.

H-dog

[This message has been edited by H-dog (edited 02-26-2002).]
 

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My wife used to work at Sears and hated it!

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