# Pound-mass vs pound-force

Mentor
I'm having a slow day (pun intended) today, and I need a little help. I'm getting pound-mass and pound-force mixed up in my head. On a psychrometric chart, it just says "pounds", but is 14 lb/ft^3 the mass or weight density of air? I've been using cheater equations for the past few years, but now I need to do derive the typical hvac cheater equation for pressure vs velocity:

VP=4005 V^1/2

VP = velocity pressure in inchess of water
V = Velocity of air in feet per minute

So where does the 4005 come from...?

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Mentor
Ok, so it would appear that I'm not a complete idiot. See, the problem my boss and I were having is that we were not able to reconcile equation 1 on PDF page 55 http://www.alnor.com/downloads/manuals/1980497B.pdf [Broken] with our "cheater" equation. The problem -- the equation is wrong in our flow-hood manual. Still, I'd like to finish the derivation...

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(P1-P2)/Rho = V^2/2g (P1 is the total pressure, against the flow, and P2 is the static pressure, perpendicular to the flow, of the measurement)

Density of air at STP is, approximately, 0.075lbm/cu.ft.
1lbf/sq.ft = 0.1922 inches wg
g = 115820 ft/min^2

So, V = [2g(P1-P2)/Rho]^1/2 = [2x115820(VP)/(0.075x0.1922]^1/2 = 4008.66(VP)^1/2

Better conversion factors yield the result close to the standard constant 4005.

(eh.. with all hatred towards IP system) Change that g to gc (32.2 lbm.ft/lbf.sec^2)

By the way, the constant 4005 is derived not because better unit conversions are used but the decimal value is chopped off (courtesy : ASHRAE handbook of fundamentals 2005)

Only correction required to the eq. given in the above reference is to put the density under the square root.

Mentor
Thanks - I must've worked on that for an hour and couldn't get my units straight. I've only been in "the real world" for 4 years and already I'm starting to get rusty...

I know the feeling! Possibly the best thing about the ISO/SI system is that it forces a clear distinction between mass and weight?

Astronuc
Staff Emeritus
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