Bar to PSI @ Altitude

[JordanButter]

Ok my boost gauge reads in bar/hkpa instead of psi now when converting to psi is the altitude really that big of a factor. I found this link also which is what got me started on thinking about the altitude.

dragsource.com/index.php?...s&calctoview

I can't post links so put the http// and www. in yourself.

[cyberslug]

I would guess that it "should" be but it isn't in real life. I don't think boost gauges are really that accurate, for instance mine reads that I'm in vacuum when my car is off. I'm sure that because it calibrated for sea-level and the pressure is lower here. BAR should be atomospheres, but I would bet it's just statically set for 14.5(it that right?) at sea-level. Has anyone seen an boost gauge that lets you calibrate it for altitude?

[Bryan Savage]

Pa = 1.451 x 10^-4 lb/in^2
kPa = 0.1451 lb/in^2
1 Bar = 10^5 Pa
1 Bar = 14.51 lb/in^2

Pressure is pressure. No "compensation", no "correction factors".

[cyberslug]

I agree and disagree, please educate me. Pressure is pressure, but doesn't "zero" change with alt? I.E. Bar =1 atmosphere, but 1 atmosphere isn't 14.5psi here, so "1 Bar = 14.51 lb/in^2" is wrong.... I've been wondering this for a while. Although, this really only matters for zero'ing out.

[Bryan Savage]

By definition, pressure is pressure. 14.51 is not "wrong", but it might not be what you want. You can have "absolute pressure", where zero is zero, or you can have "gauge pressure", which is what you're probably looking for.

If you have the ability to "zero" your gauge, just have the vacuum hose disconnected and set that as "zero". All other readings from that will use that as their baseline, but the definition of pressure as force per unit area doesn't change. If you want to boost one bar of pressure, the "absolute" pressure won't be truly correct, but what your gauge reads (as long as you set ambient pressure as the zero mark) will be one bar above "your" zero.

At this altitude, the atmospheric pressure is a bit less, but as long as that is where you set your meter stick, any readings taken after that will be correct with reference to your baseline. This is what you might be talking about with your "correction" factors.

[JordanButter]

My gauge is set correctly, but since the atmospheric pressure is less would that really affect the conversion from bar to psi

[Jon Sisk]

<Geek Alert>

A bar is a unit defined as 100kPa which roughly equates to 1 atm at sea level (.9869 atm) - which is ~14.5 psi. At this elevation 1 bar on your gauge is not 2x the atmospheric pressure but it is still 14.5 psi. Just like 3 feet is a yard no matter what you are measuring, 1 bar is 14.5 psi no matter where is you are. So whenever your guage reads 1 bar it is 14.5 psi above atmospheric pressure (which does depend on altitude). All that is changing with elevation is the definition of zero :)

</Geek Alert>

[JordanButter]

Ok thanks, I'd better go turn my boost down a wee-bit.

[rlarsen]

Remember, there are two kinds of pressure measurement, absolute and gauge. Unit-wise, pressure is pressure. 1psig is 1psig anywhere, just like 1psia is 1psia anywhere. However, if you're gauge reads 14.7psig at sea level, and 14.7psig at elevation, your turbo is still delivering 14.7psi of pressure to the gauge either way. However, at sea level, the absolute pressure will be 14.7psi + 14.7psi (atmospheric at sea level) = 29.4psia, whereas at 6000ft., the absolute manifold pressure will be 14.7psi + 12.2psi = 26.9psia.

Hope that helps.