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Join Date: Apr 2007
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Calibration of Pressure Gauges

04/03/2007 12:26 PM

Dear members,

Recently one auditor inspected our process parameters in our installations and gave the observation that the pressure gauges should be calibrated. We explained that in our territory/region there is no facility to calibrate so we replace the gauges which damaged or not functioning well. Then he suggested we can develop our own calibration mehtod ON FIRST PRINCIPLE. Can any member explain the possibility and reliability of the above suggestion.

Thanks,

Gunasekaran.

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#1

Re: Calibration of Pressure Gauges

04/03/2007 4:04 PM

Do some research on dead weight testers. Using dead weights is a very common method for calibrating pressure measuring devices.

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#2

Re: Calibration of Pressure Gauges

04/04/2007 12:18 AM

what pressure does the process operate at? Is the process pressure sensitive? and if so, how sensitive.

You may be able to buy a standard pressure gage to the pressure tolerance needed and periodically check the working gages against it. Make a tee with one gage on each end and pressurize it to compare. Never use that gage for work.

Another way is to buy some spare gages and use them as check gages and swap them into the process and toss the old nes and get replacement gages as needed.

Dead weight testers tend to be costly

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Associate

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#3

Re: Calibration of Pressure Gauges

04/04/2007 1:02 AM

Hi !!!

You have not mentioned where are you located in India.

For your information there are several Calibration Laboratory in each state of India and many have NABL accriditation complying ISO-17025 -2005 standard. You can find the list of Laboratories with their scope on Website of NABL

If you have more no of presurre gauges you can have your in house facility by purchasing a Dead Weight pressure gauge calibrator.

Having located at a remote place is not the excuse to avoid periodic calibration of Pressure gauges. I appriciate the auditor who has insisted to get your gauges calibrated. Replacing Pressure Gauge would be a costly proposition.

If you wish we can also calibrate your Pressure Gauges subject to your sending them to us .

Udayan

udayan121049@yahoo.co.in

Cell 092271 46210

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Participant

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#4

Re: Calibration of Pressure Gauges

04/04/2007 1:55 AM

The auditor recommendation is the right one for any process integrity. This recommendation is more critical in any food industry. Please categorise your instruments/pressure gauges in Critical process, Laboratory, and just normal process.

Calibration schedules must be triggered from the computerised management maintenance system - if you have one. The schedule frequency is determined by the criticality of the process and instrument usage. for example, lab instruments can be calibrated every day,week or month, but not a year. Put tolerance limits for each pressure gauge, also depending on manufacturer recommendation and process criticality.

regards,

Francis Kimera

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#5

Re: Calibration of Pressure Gauges

04/04/2007 2:33 AM

A principle is a relationship that is always true under appropriate conditions regardless of program or practice...M. David Merrill, Utah State University

Developing your own pressure calibration method could involve using a water column. This is the method we use to calibrate our pressure-type level transmitters.

You get a pipe of a certain length, say 2 meters. Attach your gauge to the bottom and fill it with water until it overflows. When the overflow stops, your water column will be constant (enough to complete the test at least). The pressure at the gauge will be equal to the head of the water at that height. In this case, your pressure is 2 meters of water / 1.21 bar / 1.23 kg/m2/ 120.93 kilo-pascals / 1209 mbar / 17.54 psi.

For more accurate results, you could read the temperature of the water and obtain the density of water at that temperature. You then correct your pressure calculations as needed.

For higher pressures, you'd need a taller pipe. Of course, for even higher pressures, this method becomes ridiculous.

The other advice to buy a dead weight tester would be best. They're simple to use and fairly accurate.

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#8
In reply to #5

Re: Calibration of Pressure Gauges

04/04/2007 8:23 AM

Are you sure your calculations are right? 14.7 PSI is approximate atmosphereic pressure, which translates roughly to 34 feet (water column) at sea level . . .

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#10
In reply to #8

Re: Calibration of Pressure Gauges

04/04/2007 9:58 AM

Oops!

You're right! 'Should've caught that. The correct figure should be 2.84 psi. Must've pushed a wrong key.

Thanks Bill!

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#13
In reply to #5

Re: Calibration of Pressure Gauges

04/05/2007 12:12 AM

Double oops!

I really messed up here. I failed to make a proper selection on my converter program. The correct figures should be:

2 meters of water (at 4°C) = 0.196 bar / 0.2 kg/cm² / 19.6 kilopascals, 196 mbar / 2.84 psi

I'm using Master Converter 2.7. The pressure conversion has a selector for absolute pressure and gauge pressure. I was converting gauge to absolute when it should be gauge to gauge.

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#6

Re: Calibration of Pressure Gauges

04/04/2007 3:38 AM

We use the dead weight testers in our facility and we have some of the toughest auditors in the U.S. Don't forget to spin the weight when you are adjusting it.

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#7

Re: Calibration of Pressure Gauges

04/04/2007 6:36 AM

A lot depends on the 'accuracy' you are looking for.

First principles means you need a 'known' weight acting on a fluid (on a piston or diaphragm) of a 'known' area. A 100 lb weight acting on 1 sq.in gives 100psi

You choose a combination to suit the range of pressures you are interested in. Or you can build a versatile unit for all pressures - but beware of re-inventing the wheel - you could end inventing a 'dead weight tester

Or you get yourself a good quality gauge that is accurate (pre-calibrated and certified if necessary) that you use to compare with your system gauges.

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#11
In reply to #7

Re: Calibration of Pressure Gauges

04/04/2007 12:10 PM

For really tight accuracy requirements one needs to make allowance for the buoyancy of the weight used on the deadweight tester[DWT]: each weight displaces its own volume of air, which has weight. Thus a weight which is a 'standard' in air will show a slightly different weight in a vacuum, which is really interesting, though probably way beyond the scope of the accuracy required from commercial pressure gauges. An engineer went down this path when producing a calibration standard of weights, tracable to national standards, for calibrating locally some sets of weighing equipment many years ago and was surprised to learn this valuable lesson.

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#9

Re: Calibration of Pressure Gauges

04/04/2007 9:51 AM

There are a number of digital certified pressure comparators on the market for the last 20 years that replace the size and bulk of dead weight testers with relatively small 'pressure producing' mechanisms used for the comparing process. A 2 min Google investigation produced many hits like below.

http://www.crystalengineering.net/index.html

http://www.coleparmer.com/Catalog/product_view.asp?sku=6897416

George

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#12
In reply to #9

Re: Calibration of Pressure Gauges

04/04/2007 1:07 PM

I was going to recommend one those too.

There are many certified devices to calibrate different types of sensors. Some of them come with re-calibration device so you don't need to sent them out for re-cal.

Pineapple

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