In article ,
rangerssuck wrote:

So, given that I have a system that's shut down and pumped out, and that I
can isolate the rink floor pipes from the rest of the system and there are
service valves accessible, does anyone know how I might determine the volume
of these pipes?

Nitrogen is cheap and cheerful. CO2 might be another option, not sure if
it plays havoc with refrigerants (but you'd be pumping it back out
anyway - if you can handle the pressure sit's a nice cheap refrigerant
itself, IIRC.)

If you have the underfloor pipes evacuated/isolated and they are not
where the leaks are (which would seem like "throw in the towel" time to
me, or at least a much more expensive repair) you should be able to
carefully weigh a cylinder, connect it to the system, fill to the
pressure of your choice (using more cylinders if needed, just be sure to
weigh them) and then weigh the depleted cylinders - initial weight minus
final weight is the mass of gas you moved (far more precise than a CFM
meter, AFAIK) and if you know the temperature of the floor, the
pressure, and the mass of gas you should be able to get the volume
(metric units on the weight will make that easier.) Obviously (I hope)
you can't have leaks in your fill rig and have this work, nor will it
work with significant leaks in the pipes.

PV=NRT is the formula that springs to mind from a long time ago, where N
is the number of moles of gas (mass will get you to that, temperture is
in C, R is the gas constant, and Pressure and Volume.

V=(NRT)/P would be the form you want. Or, y'know, cheat:

http://www.webqc.org/ideal_gas_law.html

Nitrogen's molar mass is 28.014 g/mol

If this part of the system is truly leak free you might get better
numbers by waiting a day for the temperature to stabilize, since
expanding from the compressed tank will of course cool it somewhat,
though the mass of the floor will mostly shrug that off. And you
obviously want the most accurate/precise scale you can still weigh a
nitrogen tank on.

So long as pressures are not too high, the deviation between the "ideal
gas law" and real gasses is quite small - should certainly be enough to
get you a good solid estimate.

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