Base Pressure Calculation

Discussion in 'Physics' started by tgotwalt1158, Mar 3, 2011.

Feb 28, 2011
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Hi friends!
I have a differential pressure switch but the base pressure value is missing. Could any body tell how to calculate maximum value of base pressure this switch can withstand.
Regards.

Jul 7, 2009
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AFAIK, there is no way to determine it without either a) contacting the people who designed it or b) testing it to destruction. The usual design is a diaphragm that is deflected by the pressure difference and the deflection is measured by various means (capacitance, LVDT, etc.). Since it is a differential sensor, the difference in pressure between the two ports is what's important, not the absolute pressure (of course, the absolute pressure is important when considering the breaking strength of the packaging).

Feb 28, 2011
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Thanks someonesdad! I was thinking on the same lines which you just have mentioned but had little apprehension. Let me further confirm that, is that the same pressure which when applied to a soft drink can which is somehow vacuumed inside will be collapsed due to difference b/w internal & external pressure(i.e. atmospheric pressure)?
Regards

Jul 7, 2009
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141
Here's my guess as to how the thing is constructed (see attached bitmap). You connect side A to a pressure Pa and side B to a pressure Pb. Outside the device is, usually, normal atmospheric pressure.

Clearly, the pressure difference abs(Pa - Pb) (abs means absolute value) is what's important in determining the deflection of the diaphragm separating the two chambers.L eaving Pb exposed to atmospheric pressure means the pressure read on Pa will be a gauge pressure, i.e., with respect to atmospheric pressure. If Pb was pumped out to be a good vacuum, then the pressure on Pa would be read as absolute pressure.

The other thing I'd worry about is the following. Suppose Pa and Pb are connected to the same pressure source which is at gauge pressure P. If P gets too high, it will cause the case of the gauge to rupture, even if the diaphragm isn't moving. Thus, to use the device as it was intended, you need to know the maximum allowed differential pressure and the maximum pressure difference between inside and outside the case. You'll also want to know what materials are exposed to the fluid whose pressure you're measuring (here, I use fluid to mean both liquids and gases). That's because you want to make sure the wetted parts in the gauge are chemically compatible with the fluid (i.e., they won't be corroded or chemically attacked by it).

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Feb 28, 2011
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Ok thanks! that explains a lot. But my concern was only the "Base Pressure" which is also known as "Standard Pressure", since range of differential pressure to be measured by the switch are clearly mentioned with the specs of the switch. Having said that, is base/standard pressure is that pressure which will cause the casing of switch to rupture if is brought beyond to its withstanding limit? Just I want to confirm that point.
Also, another interesting thing I would like to discuss about the base pressure of the bodies of aeroplanes and submarines, means how it is determined, calculated and designed before construction. Hope you will find it interesting otherwise just skip it.
Kind regards

Jul 7, 2009
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I'm an ex-vacuum engineer, so "base pressure" and "standard pressure" mean something entirely different to me. Note that many subfields define their own terms, so don't assume someone in another field knows what you mean -- define it exactly if you want to avoid mix-ups.

Your best bet would be to call an applications engineer of the company that makes the gauge you're using -- they'll be able to tell you exactly what you want to know.

I don't know anything about the engineering of the pressure shells used in airplanes or submarines, but there was an interesting tidbit about it in "Clear the Bridge" by O'Kane, a story about the US submarine Tang in World War 2 (a great read, by the way). He mentioned how they did their submarine dives and the workmen from Mare Island used micrometers to measure the deflection of the hull to check the design as the sub descended to its maximum design depth. They went down in depth until something broke. This is similar to stuff we did 25 years ago called "strife testing". You'd test something under its operational stress until it failed. You'd analyze the failure, fix the design if necessary, then continue the testing. This was done on printers and I still remember the forklifts bringing in the large amounts of paper so these tests could be done -- and they ran them around the clock continuously.

I bought one of those printers on employee purchase and my sister is still using it in her office. That type of testing is one of the secrets to designing reliable products.