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Absolute Pressure

10/26/2012 9:30 AM

Can we use differential pressure transmitter for measuring absolute pressure?

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#1

Re: Absolute Pressure

10/26/2012 9:42 AM

Yes - provided one leg is connected to a (hard-ish) vacuum.

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#2

Re: Absolute Pressure

10/26/2012 10:36 PM

Why not manually add the Atmospheric Pressure of 14.7 psi to the Gauge Pressure reading to obtain the Absolute Pressure?

Why go through all of the extra expense with a specialized pressure gauge??????

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#3
In reply to #2

Re: Absolute Pressure

10/26/2012 11:14 PM

Lest it pin to the left in partial vacuum?

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#4

Re: Absolute Pressure

10/27/2012 3:40 AM

You have already some ideas in previous answers, however is important to know the accuracy of the measurement that you need in order to choose the right method.

Anyway the cheapest way is to add the atmospheric pressure ( if you measure the differential pressure against it ) .

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#6
In reply to #4

Re: Absolute Pressure

10/27/2012 9:21 AM

"Anyway the cheapest way is to add the atmospheric pressure ( if you measure the differential pressure against it )"

The only problem with that concept is that the atmospheric pressure isn't constant and varies from about 87 to 108 kPa (12.62 to 15.66 PSI) so if you are leaving one side of the differential pressure transmitter open to the atmosphere then the indicated pressure could vary by some 21 kPa (3.05 PSI).

That also doesn't allow for altitude either so if your application is mobile then you have to take into account the drop in pressure with altitude.

What you need is to compare it with a constant pressure. It doesn't really matter what that pressure is, however, the pressure has to remain constant and you can't just use a sealed container because the pressure in the container will vary with temperature unless the constant pressure in the container is zero.

To summarise the answer is yes it may be possibly to use a differential pressure transmitter to give you absolute pressure, but it depends on the application and how accurately you need to know the pressure.

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#7
In reply to #6

Re: Absolute Pressure

10/27/2012 5:34 PM

The problem is not with the concept itself, but with the meaning of atmospheric pressure. The meaning of "atmospheric pressure" for me was related to the place and the time of the measurement. Obvious it is not constant and that's why I did not mentioned any value for it and I let the OP to choose the right one for him.

If he is not bother too much with accuracy he may use the standard value of 760mmHg (101.3 kPa) for the pressure at sea level, keeping in mind that it may vary by approx + or - 10% ( this is not taking in consideration abnormal weather conditions as hurricanes or similar events when the variation may be worst than that).

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#8
In reply to #7

Re: Absolute Pressure

10/28/2012 2:55 AM

Can't remember where I read it, but I think in the controls industry for simplicity sake they decided to define the standard atmospheric pressure as 100 kPa. I found this reference in Wikipedia:

"In 1971, the International Union of Pure and Applied Chemistry (IUPAC) said that for the purposes of specifying the properties of substances, "the standard pressure" should be defined as precisely 100 kPa (≈750.01 torr) or 29.53 inHg rather than the 101.325 kPa value of "one standard atmosphere". This value is used as the standard pressure for the compressor and the pneumatic tool industries (ISO 2787)."

However, when it comes to aviation and meteorology it's 101.325 kPa that everything is calibrated to.

Actually going back to the days of mechanical aneroid pressure gauges in most cases if they were calibrated to read absolute pressure they were actually just calibrated to read zero and the pressure they measured was usually in reference to the atmospheric pressure. Of course there were really good expensive gauges that had a vacuum chamber that they measured pressure against but they were pretty rare.

So, I guess that means that it's probably ok for tahir mumtaz to use his differential pressure gauge with one side open to the atmosphere but since it sound like he's using it in the control system application he should add 100 kPa to the reading rather than 101.325 kPa.

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#5

Re: Absolute Pressure

10/27/2012 9:20 AM

Depending on the accuracy you need, you would need to apply a hard vacuum to the low side, or leave the low side open to atmosphere and use a barometer to add the correct atmospheric pressure to your readings.

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#9

Re: Absolute Pressure

10/29/2012 4:29 AM

...which means in summary that although one can, it isn't the best tool for the job.

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#10

Re: Absolute Pressure

10/29/2012 9:24 AM

It is apparent that there are differences in what is the ideal base or zero vacumm/pressure under which to conduct experiments (or operate machinery). So I would offer this; with a focus on the object - maintain a quality of precision of all the instruments utilized, maintain a constant baseline value to qualify the end results & return regularly to the object (material product output, values results & consistency of each/either). The object again is all about results consistency & the method really is based on a sort of reverse engineering. Don't worry about the starting point(s) too much. Carlos PS To parphrase Tommy Jefferson (below) - 'Appreciating splendor requires a tranquil mind & real wealth is someone in the their ideal occupation.'

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#11
In reply to #10

Re: Absolute Pressure

10/30/2012 4:09 AM

The first part of that post sort of made some sense but I have no idea how the second part relates to the problem under discussion, therefore, I believe it deserves an off topic vote.

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#12

Re: Absolute Pressure

10/30/2012 2:17 PM

Yes you can but you must be aware that the low pressure side must be against a high vacuum which is steady, else the pressure will fluctuate.

So it is preferable to have a absolute pressure transmitter which is set up ti measure the high side against the sealed side.

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