Measuring Bag Volume
Measuring Bag Volume
(OP)
I have a bag of irregular shape connected to an air compressor. I put an initial pressure in the compressor. I release the air into the bag, where the initial pressure drops/stabilizes to an equilibrium pressure. How would I measure the volume of the bag using the Ideal Gas Law, i.e. pV = constant? I think this is harder than you think.





RE: Measuring Bag Volume
Can we assume the bag expands and that this is over a short period of time?
Does the air compressor discharge pressure and temperature stay constant?
Can you measure the equilibrium pressure and temperature within the bag?
With the above questions answered, a model may be set up.
Regards
RE: Measuring Bag Volume
Temp. of gas should be constant. The bag is definitely cold after bag is filled. I assume that means the air from the hose that I'm using to fill the compressor with is cold. I don't think the air could cool that fast in a second.
I can get the equilibrium pressure from the compressor's gauge, which I know is accurate. I can't exactly get the temp. of the gas in the bag.
RE: Measuring Bag Volume
DB
RE: Measuring Bag Volume
Someone check me here.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Measuring Bag Volume
RE: Measuring Bag Volume
U = internal inergy Kj
m = mass kg
H = enthalpy kj
p= pressure N/sq meter
V = volume cu. meters
energy balance of the air bag yields
dU = ho dm - pdV
dH - pdV -Vdp = ho dm -p dV ho is stagnation enthalpy of source (tank, compressor) and next will be assumed constant.
integration yields H - Hi - integ Vdp = ho (m-mi)
where subscript i relates to initail conditions in bag.
mh - mi* hi - integ Vdp = ho (m-mi)
mi, hi, pi, should be known. Problem is to integrate Vdp
Let us assuume mi in bag is negligible
mh - integ Vdp = ho* m
or m(h-ho) = integ Vdp
m and integ Vdp will be positive numbers.
Therefore h - ho = positive number and final specific enthalpy will increase within bag.
For a perfect gas, the temp within bag will increase.
Regards
RE: Measuring Bag Volume
Furthermore on opening the tank to the bag there will be cooling of the tank air! So again you must let it sit. This all requires a leak free system BTW.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Measuring Bag Volume
Using absolute pressure units, if P1 is the pressure of receiver initially, P2 is final pressure and V is water volume, the total volume of air transferred to bag from the receiver (expanded to atmospheric pressure)Vtrans is (P1-P2)*V/Patm.
Now the bag reached a pressure of P2 and contains air Vtrans at atmospheric pressure. So water volume of the bag is PatmxVtrans/P2. (considering unflattened and inelastic bag)
As for the cooing of air, search for Joule Thomson Effect.