Purging Question

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Is there a ROT for the time needed to purge a 1/4" argon line at a certain pressure based on length?  I bought a 30' Ar line so I can keep my 330cf bottle stationary while I use my new inverter.  I fashioned a ball valve into the line end to shut the Ar flow just before the machine fitting.  If my math is correct, the hose holds 18 cubic inches of gas or 1/8th CF.  So I calculate 30 seconds at 15 cfh.Last edited by Drf255; 11-09-2011 at 06:04 AM.TA Arcmaster 300CM3XMT 304S22P12 suitcase feederX-Treme 12VSOptima pulserTA161SMaxstar 150STLHypertherm PM45OP setupStihl 020AVP, 039, 066 Magnum
Reply:I get ~18 cubic inch also.A cubic foot is 12*12*12 inches = 1728 cubic inches.  18/1728 is approximately 0.0104 cubic feet.You have 15 cfh, and 0.0104 cf so that is 0.0104/15 = 0.00069 hours.  An hour is 3600 seconds, so if we say 3600*0.00069 = 2.5 seconds.There is rounding error in those calculations, so here it is wrapped up into one, heavily parenthesized algebraic expression.((pi*((1/8)^2)*(30*12))/1728)/15*3600=2.45 secondsIf we proceed with the incorrect number of one cf being 12*12=144 and 18/144=cf .125/15 = 0.008, and 3600*.008 = 30 (seconds).I agree with your computation except for not multiplying by the third 12 to get cubic feet rather than square feet.Thanks for the fun arithmetic problem An interesting note, the difference was a factor of 12.If we multiply the (rounded) correct answer of 2.45 by 12 we get 29.4.The 12 was essentially in the "denominator" of the problem (its portion of the problem), so it was more like a factor of 1/12 rather than 12, so if we say 30/12 we get 2.5Ah, math.  It's so logical and consistent.  Awesome stuff.Last edited by zapster; 11-10-2011 at 03:55 PM."Great spirits have always encountered violent oppostion from mediocre minds."  --  Albert Einstein
Reply:You are right.  Brain fart.  I used SF not CF.  So 1/12th the time at 15 cfh or 2.5 seconds.  Thats alot faster than I imagined.  So that means I set up all my valves for nothing.At this calc, a 6 second purge will go through 60+ feet of line.  So my 17ft of torch line, my 30' supply line, and the 1-2' in the machine will all be purged by one tap on the pedal and the 6 second postflow at 15CFM?TA Arcmaster 300CM3XMT 304S22P12 suitcase feederX-Treme 12VSOptima pulserTA161SMaxstar 150STLHypertherm PM45OP setupStihl 020AVP, 039, 066 Magnum
Reply:Yea, I think that sounds right.One thing with that long of a supply hose, is that it will get pressurized up to 50 psi or whatever the regulator before the flowmeter is set to.  Then when you tap the pedal, you will get quite a cloud of argon out the torch as the hose depressurizes.  You will be able to see the flowmeter ball stay up for some time after the machine shuts off the gas while the regulator repressurizes the hose at 15 cfh.  It won't be a huge deal, but it will be a source of losses.My machine came with 8' of 3/8 hose.  I computed the money worth of argon in it and what it would add up to over the tank, and it was only a few cents."Great spirits have always encountered violent oppostion from mediocre minds."  --  Albert Einstein