Converting mm Hg to psia 1

[enviroq]

Hopefully I am not missing the boat here, but I would love it if someone could help me out.

I'm doing some emission factor calculations using API-42. I'm in the middle of converting the Vapor Pressure of toluene and heptane from mm Hg to psia for one of my calculations.

Am I correct in stating that whatever value I get from my conversion to psi, I should add 14.7 psi to that number to arrive at psia?

However, the numbers that are used from my predecessor at my work was 0.58 psia for toluene, and 0.90 psia for heptane. Is that correct?

Toluene's VP is 22 mm of Hg @ 20C
Heptane's VP is 36 mm of Hg @ 20C

I get numbers close to 0.58 and 0.90 when I convert to PSI, but not exact. Is that correct or am I sweating too much over the details?

Let me know if you can help!

Thanks!

 

[dicksewerrat]

Look up the definition of vapor pressure. It will explain it to you. vapor pressure is measured after the nominal air pressure.

 

[enviroq]

So the absolute is already taken into account and thus cancel each other out?

 

[rmw]

Go to

http://www.graham-mfg.com

and request a soft ware called Vac-works II, which has an excellent conversion function in it.

Faster than that, go to

http://www.sugartech.co.za

and find a link to download a freeware from Katmar, I think, if my memory is correct, called Uconeer.

It is an excellent conversion tool.

Any measurement has to be referenced to gage or absolute. So are your vapor pressures in mm of hg absolute, or gage. Once you determine that, you can do the conversions.

rmw

 

[katmar]

Hi Enviroq,

First of all, I am confused by dicksewerrat's comment that "vapor pressure is measured after the nominal air pressure". To a chemical engineer, vapor pressure is the pressure exerted at a particular temperature by the pure substance - by our definition air has nothing to do with it. Unless Dick meant that vapor pressures are measured in absolute terms - I would agree with that. It would be very unusual to express vapor pressure in gage pressure.

The figures used by your predecessor are a bit higher than yours. I presume you have got hold of a good units conversion program (hopefully Uconeer!) by now. The reason is more likely to be with the temperature that he used than a conversion error. The VP of hydrocarbons increases rapidly with temperature.

BTW, the direct link to Uconeer is

http://www.katmarsoftware.com

regards
Katmar

 

[mbeychok]

Enviroq:

I agree with Katmar that your vapor pressure values in mmHg are probably absolute values rather than gage values. However, the source reference where you obtained those values should have explicitly stated that the values are absolute pressures.

Be that as it may, I would point out that 1 atmosphere of pressure is equal to 14.696 psia and also equal to 760 mmHg. Therefore, 1 mmHg = 0.019337 psi. You really don't need to use an online calculator for that, do you? Depending on online calculators to be correct can be a serious mistake in some cases.

Milton Beychok
mbeychok@xxx.net (replace xxx with cox)
(Visit me at

http://www.air-dispersion.com

 

[enviroq]

Thanks everyone.

Katmar & beychok,

I figured that is what was going on with the calculations that I have. Thanks for your help.

I currently use a program called CONVERT ver 4.08. It's a little older now, but it always gave me good numbers from it. I'll have to check out the other one you've mentioned.

Once you have a conversation in place it should be good unless the people that wrote the program messed up!