in.Hg MEASURED in PSI??

[96eclipsex]

This may be a Stupid question but after looking at my boost gauge it poped into my head.

If you were to measure in.hg in PSI would -10in.Hg be -5 PSI???

 

[Auto RS T]

PSI is pressure per square inch, in.hg is a vacuum measurement.

 

[2gGSX]

If you really want the conversion

1inHg x 25.4mm/in x atm/760mmHg x 14.7psi/atm = .49128 psi

So yes, -10 inHg is roughly -5 psi

 

[brute]

Generally, if you take any Hg measurement and 1/2 it, that's the psi reading. The conversion rate is like 0.4911 or something, so roughly half.

 

[zippyshoe]

Just some things to keep in mind when interpreting your boost/vacuum gauge reading - these gauges are calibrated so that the position labeled "0" psi corresponds to an absolute pressure of 1 atmosphere (normal atmospheric pressure = 14.7 psi). The values to the left of "0" indicate absolute pressure values less than 1 atmosphere, which represents a partial vacuum - these values are expressed in inches Hg with 30 inches Hg being equivalent to a perfect vacuum. The values to the right of "0" indicate absolute pressure values greater than 1 atmosphere, which represent "boost" pressures. Therefore, a boost pressure of 20 psi indicated on the gauge represents a pressure of 20 psi above normal atmospheric pressure (which is equivalent to an absolute pressure of 14.7+20 = 34.7 psi). Hope this makes sense.

 

[brute]

Altitude plays a huge role in this kind of thing. For example, Albuquerque has an atmospheric pressure of about 12.08 because it's altitude is like a mile above sea level. This is part of the reason that between cars, vacuum readings vary immensely. Not to mention every car is different.

 

[spyderturbo007]

Just some things to keep in mind when interpreting your boost/vacuum gauge reading - these gauges are calibrated so that the position labeled "0" psi corresponds to an absolute pressure of 1 atmosphere (normal atmospheric pressure = 14.7 psi). The values to the left of "0" indicate absolute pressure values less than 1 atmosphere, which represents a partial vacuum - these values are expressed in inches Hg with 30 inches Hg being equivalent to a perfect vacuum. The values to the right of "0" indicate absolute pressure values greater than 1 atmosphere, which represent "boost" pressures. Therefore, a boost pressure of 20 psi indicated on the gauge represents a pressure of 20 psi above normal atmospheric pressure (which is equivalent to an absolute pressure of 14.7+20 = 34.7 psi). Hope this makes sense.

Excellent post zippyshoe. You get a rep point.

 

[zippyshoe]

Excellent post zippyshoe. You get a rep point.

Thanks a lot, spyderturbo007.

 

[kmoore]

PSI is pressure per square inch, in.hg is a vacuum measurement.

I thought psi was "pounds per square inch", i may be wrong though. I guess its the same thing

 

[DGajre777]

I thought psi was "pounds per square inch", i may be wrong though. I guess its the same thing

PSI is pressure measured in pounds per square inch.

 

[donmagicjuan]

I'm not trying to start a fire, but every mechanical pressure gage I've come across that was designed to read in pounds per square inch gage (psig) or in inches Hg had a little vent on it to reference the low pressure side to current atmospheric pressure. This would make it a variable reference, then, and the gage would read zero whenever the high pressure side was also at that same atmospheric pressure. So, for example, 20 psi of boost would mean your manifold pressure is 20 psi above the current atmospheric pressure.

Absolute pressure sensors and gages, however, function exactly as zippyshoe described. If we had these for boost gages, a guy in Denver could never observe zero psi on his gage with the car off; it would always be at a slight vacuum reading. MAP sensors are a great device for logging boost, but I consistently see a disparity between my boost gage and my logged boost levels, and I believe this difference in design has a lot to do with it.

 

[jumpfroggy]

MAP sensors are a great device for logging boost, but I consistently see a disparity between my boost gage and my logged boost levels, and I believe this difference in design has a lot to do with it.

This is something I've wondered a lot about. From another thread:

Only the 2g NT's have the MAP sensor, because they don't use a MAS.

So my 2G TSI has only a MAF, no MAP? And it seems like MAF would be good for tuning (measure actual airflow), while a MAP would let me log boost. Is this correct? Also, do people add on MAP sensors just to log boost, or are there other reasons? I've been wondering if we could just use MAP's to log boost, but I guess from reading your post that you do. Did you add one on?

 

[donmagicjuan]

Yes, you would have to add a MAP sensor that is compatible with your logging software to permit logging boost. I currently run a GM 3-bar MAP sensor that is primarily for controlling my alcohol injection, but it also gives me this added luxury.

 

[zippyshoe]

I'm not trying to start a fire, but every mechanical pressure gage I've come across that was designed to read in pounds per square inch gage (psig) or in inches Hg had a little vent on it to reference the low pressure side to current atmospheric pressure. This would make it a variable reference, then, and the gage would read zero whenever the high pressure side was also at that same atmospheric pressure. So, for example, 20 psi of boost would mean your manifold pressure is 20 psi above the current atmospheric pressure.

Absolute pressure sensors and gages, however, function exactly as zippyshoe described. If we had these for boost gages, a guy in Denver could never observe zero psi on his gage with the car off; it would always be at a slight vacuum reading. MAP sensors are a great device for logging boost, but I consistently see a disparity between my boost gage and my logged boost levels, and I believe this difference in design has a lot to do with it.

Good info, donmagicjuan! That makes sense that automotive boost gauges would use the current actual atmospheric pressure as its reference instead of simply using a fixed nominal pressure at sea level. That way, it would automatically compensate for changes in pressure due to changes in altitude, barometric pressure, etc.