Question on cylinder pressure

By dee

Date 02-07-2004 20:22

Chevy,
I'm not sure what the question needs for a useful answer... you have 50 cu ft, I presume at 25 cfh you's get about 2 more hours...

Your gauge reads "gauge" pressure, psi"g" (without the quotes) indicates it's the difference between the inside of the tank and the atmospheric pressure outside. Open a theoretical hole in the side of the tank and you would read zero. It kind of automatically corrects for barometric variations and deals with the compressed gas' ability to get out of the tank. Theoretically you'd be able to predict rain by reading the changes if you could be sure the gas inside the tank (and it's temperature) was constant.

Absolute pressure, on the other hand, is independent of outside atmospheric influence- it's "absloute"... at zero it's a total vacuum... you'd have to figure out the pressure the atmosphere is exerting on the tank to determine how much compressed gas can be drawn off before equilibrium is attained...

...in any event I'd start pointing myself toward the weld supply shop.

regards
d

By CHEVYTOWN13 (**)

Date 02-08-2004 07:34

Dang Dee,

You remind me of someone who enters the university as a student and graduates as faculty...You the boss homeboy.

It looks like I'll be filling up every 2 weeks. But that is not an inconvenience as far as I see it. I still can't believe how much more forgiving GMAW is compared to FCAW. As far as clean welds, post clean-up and so on, it pays off in the long run.

Muchas gracias for passing on your knowledge Big D. It is greatly appreciated.

It is true what they say. You learn something new everyday...

Later hometown.

RM

By ssbn727 (*****)

Date 02-08-2004 21:39

Hola Chevytown13!
Let me just ask you this question hermano; How did you get to know that you have 50 cubic feet of gas left in your tank when your cylinder pressure gage will only let you know how many pounds per square inch (gauge) or it's metric equivalent?
Maybe you're reading like Dee mentioned, 50 psig. correct?
You should also have another guage or vertical (clear plastic) tubular meter attached to the regulator, correct?
This is called your flowmeter and what you are reading there is your flow rate in cubic feet per hour which btw, is not the same unit of measurement as is the pressure inside the cylinder so, remember this cardinal!!! Dee explained this pretty good to you also and I would like to add that these variables also change if there is a significant change in relation to how above or below you are with respect to sea-level so, if you're high enough in the mountains or down in Death valley - you'll notice a difference as far as how fast you go through a cylinder of gas hermano...
Hey Caballero! As long as you keep an open mind, you'll always be learning something new everyday just like I and everyone else in this forum does!!!
Que Dios te bendigas!!!

Respectfully,
SSBN727 Run Silent... Run Deep!!!

By CHEVYTOWN13 (**)

Date 02-08-2004 23:46

It's all good hometown.

Wish I could go to Chicago and meet you folks.

That would be cool. Learn and drink cerveza...LOL

Ricardo

By dee

Date 02-09-2004 03:38

Chevy:
Gauges dedicated to pure gas often have a scale calibrated to Boyle's [I think] law's formula for that gas- it is possible to approximate the volume of a known gas at a given pressure and temperature. Mixing gasses tends to change the properties of the gas and a scale for O2 wouldn't be accurate for He, CO2, Ar, or anything else.
Also, there's a critical pressure (different for each gas) at which the gas becomes liquified and occupies much less volume than it would in its gaseous state.
O2 and Acetylene regulators frequently have a scale calibrated in cubic foot- this has to do with the fill, not the rate of flow.

I agree with ssbn that you probably have no way of knowing the actual amount of cubic feet of shield inside your gas bottle, but it is possible if you have, for example, CO2 marked on the gauge and are using the same gas (CO2) inside the bottle. If you see the scale is marked "cu ft." it's cubic feet, and reasonably accurate if THE GAS IN THE TANK MATCHES THE LABEL ON THE SCALE of the gauge. Just understand that that gauge is actually reading pressure and a mathmatical formula is run on it before the numbers are printed on the face; the variables will be different for each gas and the printing will reflect the difference.

It is possible, therefore, to have a practical idea (at least) of how many cubic feet of gas actually do occupy your tank at any given pressure if you know the properties of the gas in question.

Ever forward
d

BTW Weldreality.com may have some info in the gas section as to pressure/cu ft relationships

By MBSims

Date 02-10-2004 01:27

I think the law you are refering to is:

P1 x V1 = P2 x V2

For a 150 cu. ft. bottle at 1000 psig:

P1 = 1000 psi
V1 = 150 cu. ft.

If all the gas were let out of the bottle to the atmosphere, where P2 = 14.7 psi (at sea level), then the volume would be equal to V2. Just rearrange the equation and solve for V2:

V2 = (P1 x V1) / P2
V2 = (1000 x 150) / 14.7 = 10,204 cu. ft. of gas

If your flowmeter is set at 15 cfh, then the example cylinder should last about:

10,204 cu. ft. / 15 cfh = 680 hrs.

At least that's what I remember about Boyle's Law. That would assume that if your using a single-stage regulator that the flow is continually adjusted to maintain 15 cfh as the pressure inside the bottle drops.

By billvanderhoof (****)

Date 02-10-2004 06:45

Correct analysis except I believe the 150 cu ft refers to volume at atmospheric pressure. So a 150 cu ft cylinder should last about 10 hours (using 15 cu ft / hr). In round numbers the remaining content of the cylinder is about proportional to the pressure so that if you start at around 2000 psi (usually somewhat more) then when you get down to 1000 psi your 150 cu ft cylinder should have about 75 cu ft left. At 200 psi in theory 1 hr remains but the regulator probably will not deliver a constant flow rate as the pressure drops.

This is Boyle's law correctly stated as either P*V=K or P1*V1=P2*V2. This assumes an ideal gas. I have ignored the fact that real gasses are not exactly ideal. Monoatomic gasses like argon are really close though.

Above assumes contents of the cylinder are above the critical temperature (That temperature above which compression alone will not cause liquid to form). True at room temperature for most welding gasses and mixtures of them. Not true for pure CO2, propane, MAPP. Acetylene is disolved in acetone (like the CO2 is in water in soda pop) I'm not sure if pressure has a linear relationship to remaining volume or not in this case.

Bill

By dee

Date 02-10-2004 16:04

Chevy,
A final comment to remember, but also has nothing to do with the original question in case the Greek being thrown around here gets confusing... liquids cannot be compressed- this poses a safety hazard when tanks contain too great a volume of liquified gas and they begin to warm- the pressure increases within the tank often beyond the point of a safety venting or explosion...
...the important term is critical pressure- it's defined to account for the critical temperature as well as the critical volume simultaneously, so don't get confused... the best way to fill, charge and buy the gasses like Bill mentioned, such as propane, butane, and automotive refrigerants where overfilling can easily be a hazard is by weight... the proper term would be by mass, which is equal to weight on Earth.

Very little hazard exists if your tanks are filled and inspected by professionals.

Regards
d

By bzzzzzzzzzz (**)

Date 02-10-2004 21:24

half as much pressure, half as much gas. In the same size tank. Unless you get to unbelievable tempertures, this will hold fairly true for most welding gases.

By Slagindaboot (*)

Date 02-11-2004 00:51

Ok Chevy,

Now that you have been given enough information to pass your physics exam lets answer the question at hand:

"Is there sufficient gas in the cylinder to ensure adequate protection of the weld?"

As mentioned earlier the flowmeter is your primary indicator. For practical purposes, if you can adjust the flowmeter to the proper flow then you have a sufficient gas supply. If you notice the flow rate starting to fall resist the temptation to adjust the flowmeter, it's time for a fresh cylinder. You don't need to pay too much attention to the pressure gauge. Often the needle will be pegged out at zero yet there will still be sufficient gas for several hours or even several days of welding. More on the subject follows.

First make sure that you have the right equipment, a regulator/flowmeter combination for the gas that you are using and that it's properly installed. It should look like this: http://www.thermadyne.com/vec/literature/pdfs/020_65-2007.pdf

If it looks like this: http://www.thermadyne.com/vec/literature/pdfs/006_65-2007.pdf STOP! You are using the wrong equipment.

Next make sure that all connections are tight and that the flowmeter valve is closed. Now gently open the cylinder valve until pressure on the gauge starts to rise. Stop! Wait until the needle stops moving, then fully open the cylinder valve.

If your machine doesn't have a purge switch, release the wire feeder drive so that wire doesn't feed while you're adjusting the flowmeter. You can now turn your machine on and pull the trigger or use the purge switch, open the flowmeter valve and adjust to the desired flow.

By dee

Date 02-11-2004 08:28

Slag,
I believe you mean well, but you're probably about to send Chevy out to buy something he really does not need. Aside from the expense it can be embarassing.

There are regulators which may be in use for control of welding gas. These are indeed like the style you indicated were wrong. There are a great many flow meters, particularly those which come stock in single phase GMAW welder kits, which look the same as a regulator from the outside. They are not the same inside. The flow gauge is calibrated in CFH and the scale is held to accuracy by a calibrated orfice. It is this internal orfice which makes the device unique and qualifies it as a flowmeter. This style flowmeter typically provides superior durability and resistance to abuse, better economy, and satisfactory practical accuracy for welding purposes.

By CHEVYTOWN13 (**)

Date 02-14-2004 02:12

Sup fellas.

It's cool Dee. I didn't jump the gun but appreciate Slag's response. I have the 2nd version that was posted.

And folks, looks like I messed up with cubic feet compared to psi...

gotsta lay off the herb I guess...hehehe

Thanks again hometowns...

CT