strange question

[MikeR]

I'm wondering if any of you know how to figure how much arc time you have left on a bottle of gas. Example: I have 400psi of argon left in my bottle, and am running 15cfh. Reason for the question is I need to finish a job I'm doing in order to have enough money to refill my bottles. If I'm going to come up short, I guess I'll .......well, I guess I'm up **** creek.

[Sberry]

Big part of the equasion is missing here,,, What size is the bottle? There may be some guesses but so many variables too.

[MikeR]

oops........sorry, I'm using 154cf bottles. They start out at around 2,000psi and I have 400psi now. I'm just wondering if I'm screwed. I'll need atleast an hour and a half of flow out of it.

[dseman]

Well let's see P1V1=P2V2 (boyles law).
If we assume your flowmeter is based on a 35psi regulator, then:
400psi * 154cf = 35ps * V2
Solving for V2 we get 1760cf. So if you are using a flow rate of 15cfh, then the bottle will roughly last you 117 hours. Actually it will be somewhat less than that since the flowmeter needs a minimum psi to operate.

If 35ps is not what your flowmeter uses, then you can plug in some other value.

This topic was discussed before so you can do a search for 'boyles law'.

In either case, even with a 70psi flowmeter, you will have your 90min. of arc time.

-dseman

[Sberry]

doesnt startting and stopping use more?

[DaveH]

Preflow/postflow

[dseman]

yes. what i mean is he has PLENTY more than is necessary for 90min of welding time. Roughly 110 hours vs. the 1.25 hours that he needs (if his flowmeter operates at 35psi and his flow-rate is 15cfh).

-dseman

[dseman]

last post should be 1.5hours not 1.25hours.

[BillC]

Dseman,

I respectfully disagree with your above analysis for several reasons. First, this is only a 154 cu ft bottle, and I thought we agreed previously that meant 154 cu ft at atmospheric pressure. It is compressed to a smaller volume in the bottle, which is a little over 1 cu ft in actual volume. Second, flowmeters are not rated at 35 psi or 50 psi, they are all advertised as being rated in SCFH Standard cubic feet per hour. SCFH is measured at atmospheric pressure by definition.

So, I would say that the original full 154 cu ft tank was good for 154 cu ft/15 cfh = 10 hrs of welding. With only 400 psi left, which is 20% of the original pressure, you get 20% of the 10 hrs. So I calculate you only have 2 hrs of weld time left.

This is the same place we got stalemated last time we discussed this. Based on dseman's analysis, the 154 cu ft tank should have a total weld time of 585 hours. MikeR, have you already welded 468 hours with that cylinder?

Again, this intended as a respectful discussion. If you professionals out there get years out of your tanks between refills then there must be something I am missing. I just don't know what that could be... How about it those of you with shops? How long do your regularly used cylinders last between refills?

Regards,

[MikeR]

Easy guys!! I was hoping someone would say,"you have plenty.", and you did. That's good enough. I appreciate it!

[dseman]

Bill,

I agree with you on the fact that the bottle has approx. 1cuft of volume (V1). That was a mistake on my part and I thank you for pointing it out. But I do disagree with you on flowmeters being rated soley on SCFH. They are rated in both the preset delivery pressure and SCFH. Smith has 30, 50, and 80 psi preset delivery pressures. Eventually at the nozzle, this becomes atmospheric pressure though. So my original thinking was that (P2) in the P1V1=P2V2 expression needed to be the preset delivery pressure of the flowmeter. But this was flawed thinking on my part since I believe the flowmeter is calibrated for the volumetric rate eventually seen at the nozzle or atmospheric pressure and not the flowrate at the preset delivery pressure.

So going back to P1V1=P2V2 we should have 400psi*1cf = 15psi*30cf. The 1cf for volume is probably a bit of an approximation as well as 15psi instead of 14.7psi for the actual atmospheric pressure.

I think the only time the preset delivery pressure comes into play is if you are trying to mix gases. The flowmeters should have the same preset delivery pressures--otherwise your mix will really be off.

So yes, you are correct about the 30cf and I am wrong. I will now go sit in the corner with my cone hat and await someone else's turn to replace me.

-dseman

[BillC]

Originally posted by MikeR
Easy guys!! I was hoping someone would say,"you have plenty.", and you did. That's good enough. I appreciate it!

Mike,

We've helped you, now help us... If you were to think back, how many hours of welding have you used your cylinder for so far?

Thanks!

[BillC]

dseman,

No cone hats in this group. I'm just still trying to figure this one out, and I appreciate the help.

Whenever a chart or table specifies a volumetric flow rate for welding, all they specify is flow rate, not flow rate at a particular pressure. That leads me to believe that when the ball is floating at 20 in the tube, that is 20 scfh, assuming the flowmeter matches the regulator.

The output pressure of the regulator must be greater than atmospheric, or there would be no flow of gas. If I understand you correctly, you are saying that if the regulator output pressure is 30 psi, then I need to match it with a flowmeter that is calibrated for 30psi input pressure. A flowmeter that was meant for 50 or 80 psi would not read accurately on a 30 psi regulator.

Am I getting closer?

Thanks again!

[dseman]

Hmmmm,
My thinking? is that the low pressure side of the regulator is set for the 30,50 or 80 that Smith advertises. The flowmeter is then calibrated for that input pressure. Smith advertises stand-alone flow meters, and the 30psi model is different than the 80psi model (they don't sell a 50psi model). That's why I don't think that any old flowmeter tube will work on any old regulator--they are a matched set. Now I further think that what is calibrated on the flow tube is the volumetric rate at 1 atm. That would make sense with boyles law, but I'm not absolutely certain of this. Maybe a call to Smith or someone manufacturing these is in order. I do agree that from the flowmeter to the gas solenoid in the machine, there is gas at a pressure above atmospheric. Whether the pressure on the output of the flowmeter is the same as the input side I do not know. Again, a call to Smith would set us straight. Once you open the valve, the gas spews forth and eventually is reduced to 14.7psi atmospheric at the nozzle. I guess what I'm saying is that at the nozzle, after the gas has expanded and is at 1atm---that volume now matches the flowtube reading. But the question becomes: what is the output pressure of the flowmeter? Are they all regulated down to a specific pressure regardless of what the input pressure is at the flowmeter? I don't know, but I'm sure Smith or someone else here can answer that for us. Otherwise, if one flowmeter was 30psi at say 15cf, then another one at 80psi at 15cf would have quite a different effect at the weld pool, right? Because the pool is so close to the end of the nozzle where the pressure is reducing down to 1 atm.


-dseman

[MikeR]

Bill, I'm really sorry but I have no idea how much time I get out of my bottles. I'm usually refiling every month or so, but other than that, it's so crazy in my one man, full time shop that I just simply can't keep track. ****, I usually don't know what day it is!