Problems with Low Pressure Measurements

[obruno]

We are currently using a manometer (DWYER SERIES 475 MARK III) to measure pressure loss in our current product. We’re measuring in ranges of 2(in. WC) thru 3.5(in. WC) and not loose more than 0.35(in. WC) /min. We’re have a so-called sealed unit which we use to evaluate are measuring system. But have had issues, different readings, when measuring in different rooms. Currently we make measurements in an environmental chamber where there is limited air flow. And our repeatability as far as leak measurement has been fairly good, but there is day to day variation. Currently we estimate +/-0.1(in. WC) in our reading. This is a problem when we have a leak at 0.3(in. WC). Any suggestions on having more accurate results?

 

[StoneCold]

Try reading this page, specifically the bottom section talks about high tech manometers to get more accurate readings. One thing that a lot of pressure vessel people do is measure the temperature of the vessel (air) Temperature changes can show up as losses or gains in pressure in a closed system. You might need a very accurate temperature meausurement to compensate the pressure readings you are getting.

http://www.dwyer-inst.com/htdocs/pressure/ManometerIntroduction.cfm

Regards
StoneCold

 

[obruno]

Thanks, we'll try to get an accurate temp measurement next, to hopefully get a correlation to the changes. I would like to get an idea as to how sensitive manometer is to normal room environment (gusts of air, open and close room door). Being that my expertise is in electrical, air pressure is a new subject for me so I’m trying to learn as much as I can in a short period of time and this subject is turning out to be more complicated than I originally anticipated.

 

[Compositepro]

A mannometer measures differential pressure between the two ports on the instrument. It seems you are trying measuring leakage of a sealed box by loss of pressure in the box. However, anything that affects the pressure inside or outside the box will affect your measurement. 0.1" wc is a very low pressure and temperature changes of your box will cause pressure changes that large. If you have ventilation you will have more than 0.1" variation from room to room. Barometric pressure changes will directly affect your reading. Barometric pressure can vary by 3 or 4 inches of mercury (density 13.5 times that of water).

 

[obruno]

It’s sounding more and more that temperature is the likely issue now. We have yet to begin the experiments to correlate the changes to temperature. Again, I have yet to really grasp the understanding of how low the .1 (in. w/c) measurement is and the amount of change (temp, air gust, etc.) needed for a change like 0.1 or 0.2 (in. w/c). Thanks for your comment.

 

[btrueblood]

I think you will find that the flucutations in temperature can also affect the manometer (electronics) readings independent of the physical pressure change; at least, that has been my past experience with handheld devices.

But your question is how to estimate what the pressure change would be...

Start with PV = mRT (ideal gas law)

For a leak test, you put a fixed amount of gas (mass = m) into a device of fixed volume (V), and those two parameters stay (pretty much) constant, i.e. the density does not change significantly (m/V). So, the equation

P = [ρ]RT

is what you want to use. Here, P is absolute pressure in Pascal (N/m^2), rho is in kg/m^3, R = 287 J/kg-K for air, and T is absolute temperature in Kelvins.

To convert from inH20 to Pa, multiply by 247; i.e.

a 0.1 inH20 change = 24.7 Pa.

A little calculus: take dP/dT = [ρ]R = 344.4. To get a 24.7 Pa change in pressure takes a temperature change of dT = 24.7/344.4 or about 0.07 degrees C. Note that this value is tied to rho, which will change (a little) depending on your actual absolute pressure and temperature during the test.

 

[obruno]

Thanks, looks like we're going to need to have good environmental control. Looks like temp is the biggest factor.