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Are low pressure tanks easier on compressors? I assumed that it would be less work and stress on the compressor's final stage to pressurize tanks of equal cf capacity to ~2475 or less rather than ~3300.
Should shops realistically give a small reduction in per tank air fill price for low pressure tank, to the tune of -$0.50 to -$1.00 for low pressure tank users?
Yes, lower pressure generally equates to less wear and tear.
Some shops will charge paint ballers getting 4500 psi fills extra even though very little gas is involved, just due to the pressure involved.
But the reverse is not true in most cases as many shops are banking air in the 3500-4000 psi range anyway, so it does not make much difference to them whether you show up with a 2640 psi LP 95 or a 3442 psi 100.
In any event, it sounds like you are talking about shops that charge for fills on a per tank basis. I prefer shops that sell me gas by the cubic foot rather than using a standard charge per tank, regardless of how many cubic feet the fill involves.
Shops that charge by the cubic foot would charge you for ~52 cf in that example assuming an end pressure of 3000 psi. It's simply based on start pressure, end pressure, and tank size. The speed of the fill is not really part of the equation. The shops that offer this option usually have a higher than normal percentage of tech divers as customers. Tech divers typically end dives with lots of reserve gas in their tanks and also breath some rather expensive gas mixes.
Of course, but that said, is the question "Is it more wear on a compressor to pump higher pressure gas, CF per CF"? LP85, Al80, HP80; end of the day, does it matter to the compressor? (For the sake of keeping it clear, filter and electricity use aside, and really, does 5cu ft really matter in the big picture?)
I have my own opinions (and compressor, LP, AL, and HP cylinders), but I am keenly interested in other's thoughts more so.
More wear and expense? Yes. Much? Eh...
Shops charge what they charge in order to be competitive yet not loose money based on what normally walks in the door. Only one shop I know of sells gas a tick above cost just to get guys in the door. It works for them, but their volume is enormous. I spread my business around (as long as they are O2 clean) when away from my compressor to keep the good guys in business. There are no fills through the internet!!!
The shop I go to charges $4 per tank for an air fill, $2 for a top off. It's just getting kinda annoying from my point of view that I've running 2xLP72s, going in with 800-1100 psi remaining to pay $8 for a warm fill that will usually settle out to about 2100 psi. I would pay $4 there to fill an get 77.4 cf @ 3100 psi in an AL80. But I'm really paying $8 to get a +60 cf boost from 1000 psi to about 2100 psi.
For most shops across the country, air fill rates are not based on anything that makes sense. In 1981 we did a cost breakdown for a tank of air and with the wear and tear, maintenance costs, electricity, space rent, and employee time, we came up with close to $3.00. We charged $5 and the guys across town only charged 99 cents.
In the late 80s, we had a Dacor rep come in with one of his old Skin Diver magazines from the 70s with an article on our area that showed the air rates ranging from 99 cents to five dollars. The air fills at that time for the same area ranged from 99 cents to five dollars. We immediately raised our rate to $6 and got a LOT more air business.
Now, fast forward 25 years and the air rates have drastically changed for the same area. Employee costs have doubled ... electricity has gone up 190% ... compressor parts and maintenance supplies have tripled.
I have never encountered any shop that charges a per ft rate. I've seen HP tanks cost a little bit more (and therefore the 3K tanks cost less).
I've also run into many shops that will top off a tank for the regular rate of an MT tank and some that will top it off for free to encourage you to come back. I'm not sure many shops make much off their compressors. The gear is expensive and they use a lot of juice and maintenance. I think fills are something they have to do to conduct their business.
That's the argument for shopping your LDS. Keep him in business so he'll be there for you.
Dive shops make money on gear service and tank inspections. I don't know what they make on retail sales. Gear service requires special tools and some expertise. Tank inspections, while important, are also kind of superficial. Unless they are doing eddy current analysis for you, they will spend just a few minutes with your tank (like 3).
I don't hold tank fill costs against a dive shop. A top notch fill station is in the vicinity of $20,000 and it runs on at least one 220v electric motor. What I do hold against them, and one reason I will dump an LDS forever, is when they refuse to accept another shop's sticker (whether VIP or O2, either one).
But, as to the particular question, sure there's less wear and tear on the compressor to make a lower pressure. But they are banking 4500-5000psi (probably), so you might reduce the compressor run time a bit but overall it's a small amount.
So, if the guy is filling your tank by the book and it costs a couple bucks extra to get it done right without any hassle, I say pay him, and then tell me where he is!
Last edited by gnominic; June 14th, 2012 at 02:03 AM.
Reason: forgot OP question
Theoretically there is more wear and tear on a compressor pumping to higher pressures. However, after over 10 years with ours and 920+ hours use, I do not think the cost is high. We pump all tanks to 250 bar on the gauge (3500 psi) which cools to about 230 bar which is the fill pressure for steel tanks here. I even pump some higher after cooling for deep dives. Again, never had a problem.
The cost of running my compressor (Junior II) is currently AUS$2.22 a fill (US $ is same). This is made up of $1.76 capital cost and $0.46 recurrent. Of course, we are not paying for labour (sorry, labor for you Yanks). The cost of running a larger compressor should be even lower I would think.