Half Full or Half Empty?

[Neil McDonald]

Can anyone tell me the what the relationship is between the pressure in a gas bottle, CNG or Propane, and how full it is? My gut feel is that when the gas pressure, as measured on the guage, has dropped to half of what it was when the bottle was originally full, the bottle is NOT half full (or half empty!!!), i.e. the relationship is not linear. I asked a chemical engineer and he started talking about the conversion of the contents from liquid to gas phases...... but never did give me a simple reply. Given the scarcity of CNG suppliers it would be nice to have a good idea of how much is left in the bottle at any time.Neil

 

[Terry Cox]

Hi Neil, we use CNG for cooking on our boat...

The bottle holds about 2,500 pounds of gas. At 1,250 pounds I figure it is half full depending upon your viewpoint. The difference between propane and natural gas; propane is a liquid that is heavier than air, natural gas is a gas and is lighter than air.Terry

 

[Don S/V ILLusion]

CNG volume

You may already know this but CNG tanks have a guage pressure indicator from which you can get a good idea how full/empty it is. Your Chem Eng advisor (I am one too) is correct that the relationship isn't linear, however, it is a good approximation which is why tanks typically have a guage on them. This is true only for CNG which is a gas under pressure unlike propane for which the relationship is more complicated.If this helps, a guage pressure of 2000 psi is almost full and 500 psi is the equivalent of about 80% empty.

 

[Mke]

Weight, not pressure

Many times, when a cylinder is filled with a liquified compressed gas, the weight is used, as in CO2 fire extinguishers or propane. The weight of the cylinder is stamped on it, and the filling station knows the weight of the product being pumped, and only the proper amount is pumped. The pressure may vary due to ambient temperatures. Also, as a side note, when your cylinder is low and needs refilled, do not bleed the last bit out to empty the cylinder, and then leave the valve open. You will alow moisture into the cylinder which could cause corrosion inside. Leave the remaining product in the low cylinder.

 

[Ross]

If the contents of the cylinder are entirely

gaseous the volume of the contents will vary directly with the pressure. But if the contents are liquid the pressure will vary directly with the temperature. Example one pound of water is approximately one pint, But one pound of water vapor is approximately one thousand pints at atmospheric pressure. You may compress it until it liquifies at which time one pound of water can be contained in one pint, but if the temperature exceeds 212 degrees F. the pressure necessarily exceeds atmospheric pressure.

 

[Paul Akers]

CNG Tank Lesson

A CNG tank is built just like a SCUBA tank. The stamping on the neck of a CNG tank indicates that it is the same metal alloy and working pressure of 2250 PSI as a SCUBA tank. It also has the mfg date and the date of the last hydrostatic pressure test (4/3 of the working pressure, tested every 5 years). It is tested by submersing it in water then it is pumped to 4/3 of the working capacity. If the tank expands, i.e the water rises, past the 4/3rds mark (the walls of the tank are too flexible), the tank is failed and a hole is drilled into the neck, thereby, rendering it useless. If it passed, the test date is stamped onto the neck beside the mfg date/last test date.  The big difference is the brass neck, or valve, on the tank. THAT is entirely different than a SCUBA tank and requires a trained person to install the CNG fitting. I looked into this in order to use one of my old SCUBA tanks and the cost to purchase a new CNG valve and tank was more costly than just buying a replacement tank. So I spent the $200 to buy a spare tank and the first thing I got back was an old, filled tank from the "pool" of CNG tanks. That is why you swap a CNG tank when you return an empty one. What happened to my $200 is your guess.I am fortunate to have very close access to CNG here in the Northeast, but I am also very happy to be able to keep a backup tank on board. BTW, I usually swap off the tank when it reaches about 100 pounds of pressure (to be safe). By having a spare on board, I usually don'y pay too much attention as to how much remains in the tank at a certain pressure. But when I used to SCUBA dive, I always terminated a dive at 300 pound for the safety factor. Also, by keeping pressure in the tank, it keeps contaminants out of the tank.Also, keep in mind, that CNG and Propane ARE NOT the same when determining how much may be left in the tank (see the next post).

 

[Howard9071]

Propane

Propane weights a little over 4 pounds per gallon and this is how you tell how much gas you have left. The pressure guage on a propane cylinder tell you how much pressure is in the tank not how much gas you have left. A propane tank should only be filled to about 80% to allow for expantion on a hot day. The new valves on the tanks stop the input of gas at that. Opps. i was off some.http://arfarfarf.com/fire/000010.php

 

[Liam]

two tanks

The only really good solution is to have two tanks and not worry about how full they are. When you run out in one switch to the other and go fill the empty.

 

[Funtime]

Simple answer:

P*V=Constant. So the volume and pressure have a linear relation. This should be good enough for your purpose without going any further of scientific detail.

 

[BobD17833]

Paul - CNG supplier?

I am in Boston, although I do not use the CNG quickly, I will eventually run it out. With one tank, I may consider a second if I use it more.Who/Where is the local supplier?BobD Boston

 

[Don S/V ILLusion]

BobD - CNG source

Don't know if Paul found one closer to you - Corp Brothers in Canston, RI which is off Rt10 and can be located more precisely through their web site. I think it was around $15 last season.

 

[Bill Roosa]

Funtime!!!

PV=nRT and you know it. and n ain't constant nor is T.For those inquiring mindsP is the pressure in the tank, this clearly variesV is the volume of the tank and for normal conditions is a constantn is the (gasp) number of moles of gas in the tank. Bet you didn't know CNG would give you a rodent problem ;-) . Seriously, a mole is a very large number representing the number of atoms/molecules in a space. It decreases as you use gas.R is our old friend the universal gas constant and is as you guessed constantT is the temperature of the gas and must be measured in the obscure measure of Kelvins (K) or Rankine (R) degreesSo if we think of n as a measure of how full (number of molecules) the tank is we could right an equations for "fullness"n=PV/RTSince I want a percentage and not the actual number of molecules in the tank I'm going to take some liberties (close your eyes ladies) with the constant R. Lets rename it "r" so we don't get confused and call it the Universal Hunter CNG Gas Constant (UHCNGGC).If we start with a full tank lets say P=the pressure on the gage in psi, lets say 3000 for an example, the tank volume is 3 ft^3 (your tank may be different) the temp is 70 deg F., n is 1.00 (for full all the molecules are there). This leaves r as an unknown.but r=PV/NT or 3000*3/(1.00*(70+460))=16.98 psi*ft^3/UHCNGGC*Rankines) Weird but that is what math does sometimes. The 460 is how you convert F to RankinesSo now I can take my calculated r and plug it into my "fullness" equationso at 1500 psi and 80 deg F (ny 2 varables) my tank would be:n=PV/rT=1500*3/(16.98*(460+80)=0.49% fullA slightly easier equation with the constants V and r combined is n=P/(5.66*(460+T))If you examine a set of examples using your own data you will see that unless your have a fire and the tank gets REALLY HOT the gage does in fact almost exactly tell you how much gas is in the tank. But then you would not look kool consulting your own table on the underside of the nav table that tells you which pressures and temps result in 50%, 25% .....full.

 

[Bill Roosa]

One more thing

Once you calculate r your don't have to recalculate it when you fill up your tank as r is a "universal" constant and will work till your change the tank size.

 

[Don S/V ILLusion]

Proves one thing

It appear there are an almost infinite number of ways to say the same thing

 

[Ross]

George Carlin's answer to that question

is the container is too big. if it weren't so large it would be full.