How do I calculate flow rate from pressure drop?

[montyq]

Hi. I have a digital manometer measuring pressure in units of inch h20. I'm trying to measure flow rate of air (in cc/min) over 60 seconds. I can record the pressure drop in inch h20 over that time period, how can I calculate the flow rate in cc/min? The manometer I'm using only has a pressure display, so my only knowns are pressure drop and time period. What other data do I need? I don't work with manometers much... I was trying to use PV=nRT but I'm not sure how to find n. Not sure if that's the best formula to use or not. Please help!

thanks!

 

[montyq]

A little more info on this. I'm trying to measure leakage through a seal, a press fit. I'm finding a lot of formulas using pipe diameters, but in this case my "pipe" is sealed with an imperfect seal, hence the leakage. Do I need to know the gap around the seal? I would assume that to be microns... To the naked eye it looks sealed. I don't think I'll be able to measure that. How else can I calculate this flow rate? thanks!

 

[BigInch]

Get rid of the manometer and install a pressure gage. Now with readings taken over time intervals, you can calculate the gas volume lost during each time interval.

Assuming the ideal gas law applies to your pressure and temperature range,

V2 = P1 * V1 / T1 * T2 / P2

Volume lost = V1 - V2

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[katmar]

I thought a manometer was a pressure gauge? Anyway, this is how I would do it:

I guess the volume is constant and known. You can measure the pressure and temperature. Re-cast your PV = nRT formula to be n = (PV)/(RT). Everything on the right hand side is known or measurable. This will tell you the change in n (the number of moles) from one set of measurements to the next. Convert the change in moles to whatever units you want to report in.

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[BigInch]

In a way. The manometer is the U shaped differential pressure gage, that usually spans across an orifice plate fitting.

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[BigInch]

for which, if you have a typical flow equation as a function of pressure drop for a known geometry of a partifular orifice, you're in business, but he doesn't have one of the customary standard orifice shapes.

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[vzeos]

montyq:

I don’t think you can calculate the flow rate based on pressure drop. I think your best bet is to use the capture method.

You would need to encapsulate the leaking seal such that the leaking air is captured by a capsule of known volume. You would need to measure the temperature and the pressure (use a manometer) in the capsule, and the time duration to collect the leaking air. Now, since the capsule volume is known and you measured the temperature and pressure of the air, you can calculate the standard volume of the air. The standard air volume divided by the time duration to collect the air will give you the average leakage rate.