differential pressure gauge, ACFM, SCFM

[howardoark]

Hi, Civil Engineer here, so sorry to ask a really basic question.

We have a differential pressure gauge to measure the difference in total head and static head in a pipe and hence get the velocity head - we're extracting polluted air from the subsurface. The gauge is calibrated for standard conditions (1 atmosphere, 70 degrees F). The vacuum is very small and relative humidity is pretty close to 36% so we're ignoring those factors. To convert the gauge reading to actual velocity (in feet per minute) we use 1096.7*SQRT(velocity head in inches WC/density of air in pcf). To convert that back to SCFM we multiply actual velocity by Ta/530 times the pipe cross-sectional area where Ta is the absolute temperature of the air in degrees Rankine. At least that's what I'm doing.

There is some confusion because the gauge is calibrated for standard conditions so some folk think it is giving them SCFM directly.

Thanks

Jeff

 

[danw2]

1) For actual velocity you're using
actual velocity (in feet per minute) = 1096.7*SQRT(velocity head in inches WC/density of air in pcf)

I can't remember these formulae so I use Fluke's air flow cheat sheet:

Fluke says:
actual air velocity with density correction is V = 1096.7 x √VP/D
V = Velocity, ft/minute (fpm)
VP = Velocity pressure, in. wc
D = Density, lbs/ft3, 0.075 x (530/460+Tact) x (Pact / 29.92)
Tact = measured dry bulb temperature of the actual airstream, °F
Pact = absolute pressure of the actual airstream, in. Hg

Assuming you're calculating density using the absolute temperature and absolute pressure corrections, then your actual velocity looks good.

2) For SCFM we multiply actual velocity by Ta/530 times the pipe cross-sectional area where Ta is the absolute temperature of the air in degrees Rankine,
or SCFM = V(actual) * pipe area * Ta/530

Fluke says: SCFM = ACFM(530/(460+Tact))(Pact/29.92)

So you've inverted the temperature correction factors. It should be 530/Tact[460+T] and the absolute pressure correction needs to be applied as well (note the use of inHgAbsolute units for the static pressure which could be changed to inwc units by using the standard atmospheric pressure of 407.2 inH2O).

 

[howardoark]

Thank you very much, appreciate the help/