PSIG to Torr Conversion

skyfox · Sep 26, 2011

Does anyone know the formulae for converting PSIG to Torr?

I have a gauge that reads -14.7 to 100 PSIG. I need to convert the reading from this gauge (4-20ma) input into a micrologix PLC and convert the Gauge pressure to it's Torr equivalent.

Thanks

bce123 · Sep 26, 2011

760 torr = 14.7 psi

TConnolly · Sep 26, 2011

Torr is an absolute scale while PSIG is gauge pressure with respect to atmospheric pressure (which changes locally). Standard atmospheric pressure is 760 torr which is 14.696 PSI. If you want to convert to TORR then you have to either
1)Assume that your atmospheric pressure is at standard pressure and scale your gauge 0 to 114.7 PSI = 0 to 5931.7 torr

or

2)Supply a second gauge that measure the current absolute atmospheric pressure and applies the necessary offset.

What exactly are you trying to measure? I ask because torr is normally used in high vacuum applications so I find it a bit odd that you would use a 100PSI gauge.

skyfox · Sep 26, 2011

Bce123/Alaric,

Thanks for your response.

The gauge is installed on to a cyclinder that gets pressurized by an inert gas and then purged pulling a vacuum of around 1 Torr. I cannot replace the gauge and it's label states its a -14.7 to 100 PSIG (4-20ma)device with a digital display with local display units selection of PSIG & kPA. I have seen lot of conflicting conversion info around the web and wanted to find out the exact conversion. By the way, isn't psi and PSIG are equal? I thought PSIA has to be explicitly stated.

Is Torr and mmHG are the same?

Thanks

leitmotif · Sep 26, 2011

bce123 said:
760 torr = 14.7 psi

More accurately 0 Torr = 0 psi ABSOLUTE
760 Torr = 14.7 psi absolute
At sea level pressure is 760 Torr or 14.7 psia or 0 psig (psi gage).

Dan Bentler

TConnolly · Sep 26, 2011

Torr is by definition 1/760th of one atmosphere. This makes 1 torr about equal to 1 mmHg, but not exactly equal. Unless you're a CERN physicist its close enough.

Scale 4mA to 0 and 20mA to 5932 Torr.

skyfox · Sep 27, 2011

Alaric said:
Torr is by definition 1/760th of one atmosphere. This makes 1 torr about equal to 1 mmHg, but not exactly equal. Unless you're a CERN physicist its close enough.

Scale 4mA to 0 and 20mA to 5932 Torr.

Alaric,

I am a bit confused by this.

Gauge Spec is -14.7 to +100 PSIG (4 - 20 MA)

So if I scale as you stated (4ma to 0 Torr) wouldn't that reading be incorrect as it would be - 760 Torr (-14.7 PSIG at 4mA)insted of zero?

At 20mA's (100 PSIG) woouldn't Torr be equal to 5171 Torr?

Am I missing /overlooking something here?

TConnolly · Sep 27, 2011

Torr is an absolute pressure scale. It does not go negative.

Your gauge pressure spec of -14.7 PSI means 14.7 PSI below atmospheric pressure, and standard atmospheric pressure is 14.7 PSI. 14.7 minus 14.7 is 0.

The full span of your pressure gauge is Max - Min, or 100PSI - (-14.7) which is 114.7 PSI.

So to use Torr, -14.7 PSIG is 0 psi absolute which is 0 Torr and the span is 5932 Torr.

TConnolly · Sep 27, 2011

You mentioned you are evacuating a cylinder to 1 Torr. If you reference -14.7 PSIG to -760 Torr then 1 TORR will be slightly pressureized, not vacuum.

If you really want to double check your scaling, what is the range of the instrument in kpa?

skyfox · Sep 27, 2011

Thanks Alaric,

Much appreciated.

In kpa I would assume it's -101 kpa to 689.47 kpa. (based on a calculated value). Spec printed on the Gauge is -14.7 to +100 PSIG and the gauge is currently set to display in PSIG.

James Mcquade · Sep 28, 2011

Skyfox,

"Torrs (or Tors) Same as Millimeters of Hg (0 degrees C)"
page 260 - Conversion Factors and Tables - 3rd edition by
Zimmerman and Lavine
Also, to convert from CM of Hg at 0 degrees C to psi you multiply by 0.193368 page 60.

regards,
james

PSIG to Torr Conversion

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