Please Help.. How much HG is in one PSI? (Or vice versa..) HELP!!

Undisputed · Apr 28, 2010

Please Help.. How much HG is in one PSI? (Or vice versa..) I know psi and bar for boost, but have no idea what the conversion for HG - Psi is.. and it seems every online conversion is different!?!?!? HELP!!!

Undisputed · Apr 28, 2010

I nver even heard of "HG"... lol..but my friend had the gauge in his car and I was going to use it to check how much boost I'm running (when we dyno tune it) and, lol.. I don't know wtf psi is in HG...haha

RatStang · Apr 28, 2010

Pressure Units Online Converter

Undisputed · Apr 28, 2010

DontGivaRatz said:
Pressure Units Online Converter

lol...am I losing it..or does this mean that 10lbs of boost (psi) is 517 HG?!?!

RatStang · Apr 28, 2010

2.04 Hg

bynumj · Apr 28, 2010

Hg is used to measure a vacuum relative to ATM (atmosphere) and is denoted as “in of Hg” (inches of mercury). The term psi is “pounds-force per square inch” for our application is actually psig “pounds-force per square inch gauge” and is the pressure relative to the surrounding atmosphere (14.7 psia or 1 ATM @ sea level). From our stand point you would not use psi to measure a vacuum or Hg to measure pressure above ATM.

stangdude2000 · Apr 28, 2010

All pressures need to be specified as to whether or not you are looking at Gauge Pressure (The pressure above or below current atmospheric pressure - PSIG is the engineering way to refer to it using English units) or Absolute Pressure (pressure in a vacuum). In anything other than an engineering application or when speaking about barometric pressure, it is safe to assume you are referring to Gauge Pressure.

In this application, you do mean gauge pressure. 10 lbs of boost is approximately 20.36 inches of mercury, giving you DontGivaRatz's conversion ratio of 2.04 in hg/psi.

For future reference, Google is a PHENOMENAL unit converter. When you get your pressure reading in Inches of HG, just type in google "20 inches of mercury to psi" and it'll convert for you.

bynumj · Apr 28, 2010

Undisputed said:
lol...am I losing it..or does this mean that 10lbs of boost (psi) is 517 HG?!?!

If you wanted to make a conversion from “psi” (psia) to “in of Hg” you need to consider ATM in the equation. The conversion is:

1 psi = 2.036 in of Hg
Also consider that 1 ATM is 14.7 psi
Therefore,
If you want to know what 10 psig is equivalent to in “in of Hg” you must convert to “psi”:
10 psig = 24.7 psi
24.7psi*2.036 in of Hg/psi = 50.29 in of Hg (1.68 ATM)

Undisputed · Apr 28, 2010

bynumj said:
If you wanted to make a conversion from “psi” (psia) to “in of Hg” you need to consider ATM in the equation. The conversion is:

1 psi = 2.036 in of Hg
Also consider that 1 ATM is 14.7 psi
Therefore,
If you want to know what 10 psig is equivalent to in “in of Hg” you most convert to “psi”:
10 psig = 24.7 psi
24.7psi*2.036 in of Hg/psi = 50.29 in of Hg (1.68 ATM)

wow...so a boost/vacuum gauge that reads to 10 hgs in boost , and down to 30 in vacuum wont really be of use in my car at all since i'm somewhere above 10psi.... that 10HG gauge would be maxxed out once i passed around 5 lbs of boost i'm guessing....damn..hahah

stangdude2000 · Apr 28, 2010

bynumj said:
If you wanted to make a conversion from “psi” (psia) to “in of Hg” you need to consider ATM in the equation. The conversion is:

1 psi = 2.036 in of Hg
Also consider that 1 ATM is 14.7 psi
Therefore,
If you want to know what 10 psig is equivalent to in “in of Hg” you most convert to “psi”:
10 psig = 24.7 psi
24.7psi*2.036 in of Hg/psi = 50.29 in of Hg (1.68 ATM)

All pressures can be worked independently of atmospheric pressure as long as you are always making the same assumptions. You do NOT need to convert to atmospheric pressure unless the conditions require it, as long as it is understood that you are talking about gauge pressure.

As I said before, unless you are working in a specific engineering application, there is really no need for specifying gauge pressure.

With your example above, it is 50.29 in of Hg in ABSOLUTE pressure. Assuming the supercharger is running 10 PSI (PSIG if you want to be technical), he would read on a gauge for his supercharger 20.36 in HG, NOT 50.29.

stangdude2000 · Apr 28, 2010

Undisputed said:
wow...so a boost/vacuum gauge that reads to 10 hgs in boost , and down to 30 in vacuum wont really be of use in my car at all since i'm somewhere above 10psi.... that 10HG gauge would be maxxed out once i passed around 5 lbs of boost i'm guessing....damn..hahah

That is correct. I'm assuming it is a standard pressure gauge as well and not an absolute pressure gauge. If it reads ZERO when it is disconnected, it is a standard gauge. If it reads -30/+10 for its limits, it can read from a 14.7 psi vacuum (which happens to be approximately equivalent to the pressure in a vacuum) to about 4.9 lbs of boost.

bynumj · Apr 28, 2010

The OP was asking about conversions so the calculation given in my previous post above was in absolute values but a gauge would read relative to atmosphere for our applications. So, if we want to talk about gauges for our cars, we do not measure pressure in “In Hg”, we measure pressure in “psi” and vacuum in “In Hg” all with atmosphere as reference. A gauge reading of 0 is equal to 14.7psia (1 ATM). When the gauge is reading 10 psi, you are actually at 24.7psia (1.68 ATM). A gauge reading of 20 In Hg is a vacuum and is 4.9psia (0.3 ATM). A typical vac/boost gauge will be from 30 In Hg to 15 psi (approx 0 ATM – 2 ATM) or 30 In Hg to 30 psi (approx 0 ATM – 3 ATM). At idle you should be at approx 20 In Hg and will see a peak on the gauge of your boost at 11 psi when you are at peek rpm under load.

Undisputed · Apr 28, 2010

bynumj said:
The OP was asking about conversions so the calculation given in my previous post above was in absolute values but a gauge would read relative to atmosphere for our applications. So, if we want to talk about gauges for our cars, we do not measure pressure in “In Hg”, we measure pressure in “psi” and vacuum in “In Hg” all with atmosphere as reference. A gauge reading of 0 is equal to 14.7psia (1 ATM). When the gauge is reading 10 psi, you are actually at 24.7psia (1.68 ATM). A gauge reading of 20 In Hg is a vacuum and is 4.9psia (0.3 ATM). A typical vac/boost gauge will be from 30 In Hg to 15 psi (approx 0 ATM – 2 ATM) or 30 In Hg to 30 psi (approx 0 ATM – 3 ATM). At idle you should be at approx 20 In Hg and will see a peak on the gauge of your boost at 11 psi when you are at peek rpm under load.

so...a gauge like this....would be only effective if i was runing 5 psig or less (approx)...
http://cgi.ebay.com/ebaymotors/25-S...ccessories&hash=item5ad90b3891#ht_1670wt_1165

bynumj · Apr 28, 2010

Undisputed said:
so...a gauge like this....would be only effective if i was runing 5 psig or less (approx)...
http://cgi.ebay.com/ebaymotors/25-S...ccessories&hash=item5ad90b3891#ht_1670wt_1165

Tha is correct. This gauge does not even go low enough for the Vac. Are you looking for gauges?

Undisputed · Apr 28, 2010

bynumj said:
Tha is correct. This gauge does not even go low enough for the Vac. Are you looking for gauges?

Not really..my friend had ordered one of those, and I looked at it and was trying to figure out WTF that boost measurment was..haha.. Eventually I need to get a good boost and air/fuel gauge..I really prefer not to have the gauges on the pillar, but I'm pretty sure I'm going to end up having them there anyway...haha But yeah, so what is a gauge like that one for?! I was so confused when I looked at it, it looked like a normal 10 lbs psi/ 20 vacuum till i read the measurements!! If he puts that in his car the needles will just be flying back and forth maxing out each side of the gauge as he goes from vac into boost huh>? HAHAHA I'm just gonna stick w/ some normal gauges when its time, I cant wait to see the needle show thats gonna happen when he puts that gauge into his car!!

stangdude2000 · Apr 28, 2010

I think we're arguing the same point here, but I think we both were getting thrown off by the other's explanations. The poster implied that both boost AND vacuum are being measured in Inches of Hg on his particular gauge. You were explaining off a typical boost gauge and I was giving a non-auto specific pressure answer.

Either way, if my interpretation of the posts is correct, I'm fairly surprised the OP came across an automotive boost gauge that has a positive pressure in inches of Hg since, well, almost no one measures anything except atmospheric pressure in inches of mercury...

stangdude2000 · Apr 28, 2010

Undisputed said:
But yeah, so what is a gauge like that one for?!

It was likely built for some type of non-automotive application involving small engines with small turbos/superchargers. It is possible its even a gauge for something not related to an engine. No idea what though, since like I said in my previous post, almost no one uses in Hg.

Undisputed · Apr 28, 2010

stangdude2000 said:
I think we're arguing the same point here, but I think we both were getting thrown off by the other's explanations. The poster implied that both boost AND vacuum are being measured in Inches of Hg on his particular gauge. You were explaining off a typical boost gauge and I was giving a non-auto specific pressure answer.

Either way, if my interpretation of the posts is correct, I'm fairly surprised the OP came across an automotive boost gauge that has a positive pressure in inches of Hg since, well, almost no one measures anything except atmospheric pressure in inches of mercury...

haha....weird that its even for sale billed as what it is!! OH WELL, I GUESS HE'S NOT GONNA USE IT, GOOD THING I DIDN'T BUY ONE!! THANKS FOR ALL YOUR INPUT ON THIS!!

GOLDENPONY · Apr 28, 2010

1 inch of HG is equal to 0.4911541 PSI. OR, 1 PSI is equal to 2.036021 inches of Hg. Hope this helps.

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