Volumetric Flow Rate

That's why you're talking about compressed air, not a liquid.

Please note that this measurement will give you an average. Compressors, as pumps, have a characteristic working curve of pressure X flow. The more flow you bleed, the less pressure you get.

In your case, I would first take the reservoir volume, and initial pressure, and calculate the amount of air, in terms of mass, inside. Then, consider the final pressure and time, allow it to cool down, re-measure pressure, and calculate the air mass inside the reservoir again. So, you have a mean mass flow over time. With this information, and knowing the compressor output pressure, you can calculate back the volumetric flow. If the compressor is connected directly to the reservoir, so the air density at the output will have been varied along time, and it must be considered as a function of it. Maybe another intermediary measurement point is required.

The tank is open to atmosphere, so the initial pressure is zero, how do I calculate mass of air? I used a DAQ to measure the test, so I have output data(pressure vs time) from 0-130 psi and then some data after as the pressure was dropping due to cooling of the air. Do I have enough info to make an accurate calculation? Also, how do I get compressor output pressure...is this the same as max pressure?

Q=FLOW RATE (LITERS/MINUTE)

V= VOLUME (LITERS)

T=TIME(MIN)

Q=V/T

SO...

42.8/80 = 32.1/60

Q=32.1 (L/MIN) @ 130PSIG

THIS IS ONLY GENERAL FOR A FLUID PRESSURE. YOU HAVE OTHER VARIABLES WHEN WORKING WITH A GAS. (PRESSURE AND TEMPERATURE)

• 1-stage compressors: motor HP/0.179 = SCFM @ 100 PSIG
• 2-stage compressors: motor HP/0.164 = SCFM @ 100 PSIG
• 3-stage compressors: motor HP/0.159 = SCFM @ 100 PSIG

SCFM is the volumetric gas flow rate (Standard Cubic Feet Per Minute)at standard atmospheric conditions (i.e.. barometric pressure at sea level, 68 F. and 36% relative humidity).

AND OF COURSE YOU CAN ALWAYS PUT A FLOWMETER IN LINE.

It would work only for incompressible fluids, not for air.

The tank is open to atmosphere, so the initial pressure is zero, how do I calculate mass of air? I used a DAQ to measure the test, so I have output data(pressure vs time) from 0-130 psi and then some data after as the pressure was dropping due to cooling of the air. Do I have enough info to make an accurate calculation? Also, how do I get compressor output pressure...is this the same as max pressure?

You can calculate Free Air Delivery (FAD) with the following:

(assuming compressed air)

http://www.em-ea.org/Guide%20Books/book-4/4.8.Compressors.pdf

Or the much simpler approximation on Page 72:

http://www.em-ea.org/Guide%20Books/book-3/Chapter%203.3%20Compressed%20Air%20System.pdf

Good handy formula on page 72.

Thats a great rescource. thanks for the info

1 atmosphere = 14.7 psi

130 psi/14.7 psi = 8.84 atm.

8.84 atm. X 42.8 liters = 378.4 liters at 1 atm.

80 secs/60 secs = 1.33 minutes

378.4 liters/1.33 minutes =284.5 liters/minute at atmospheric pressure.

This assumes that you are at 1 atm. atmospheric pressure, that the gauge reading the pressure is relative to atmosphere (psig) and that the final temperature of the vessel is 60F. To be more accurate, you would need to correct for temperature and pressure using the ideal gas law.

This is equivalent to RVZ717's answer.

However, as others have pointed out, flow from the compressor is dependent on the pressure it is pumping against. The actual flow is probably higher at lower pressures. In other words, if you only filled the vessel to 65 psi, it would probably take less than 40 seconds, and the flow would be higher.

Tad

The answer you gave was at 1 atm. Should I be working in absolute pressure (Ex: 130/14.7 +1) or in atmospheres? Also, in your opinion should I simply graph the first and last data points to show linearity or do you think it would be a better representation if I graphed all 2500 points? When I do it this way the graph is all over the place.