New 119 on the way!!!

[mike_s]

Well,
Unless the shop is a really small shop I don't buy into the "loss leader" on air fills.
OUr club probably does 800-1000 fills a month. We have no sales, no rentals, we are a non-profit org. with about 150-200 members on the books at any one time. Our air system has paid for itself and it's maintanance, and membership is $10 a month, with unlimited 24/7 access to filling station.

Actually in many shops it is a loss item. especially if it's a new compressor and not a surplus piece of junk. One thing to realize that in many markets that the diving season is somewhat limted to 3 months. The rest of the year the compressor system is hardly used. so it's a capital expense that is not providing a profit many months.

In your clubs case, you've got some members that prob dive 10 days a month and if they dive 2 tanks per day, then they essentially get 50 cent air fills. You've prob also got some members who hardly ever dive but still pay the $10/month. IF they don't dive for several months they could essentially be paying $20/fill or more.

Just curious, since you give 7/24 access, I guess this means you let members do their own fills?

Do you require people to maintain a yearly membership (at per month fee) or can they join/stop any time they want? ( I guess in Hawaii though you really don't have a season where you can't dive though right)

How do you keep members from filling their buddies non-member tanks there?



However, back to the LDS loss on air fill sales. One thing they do get even if air fills are a loss is the customer in their shop. They then have the opportunity to sell the customer all kinds of other trinkets, new regulator, those HP 119 tanks, etc. Just like mechanic shops offer $15 oil changes, to get your car in their shop. They make it up in the long run.

BTW... for comparison, there is a LDS on the Gulf Coast I think that will sell you an annual fill rate on your tank of $60 per year per tank. Consider most folks do 2 tanks of diving (per day), that's $120 per year. Pretty good deal if you dive a lot. If you make 24 dives a year then it's paid for (at average rate of $5/fill normally)

 

[mrjimboalaska]

Members are usually asked to pay quarterly. Everyone gets Hazmat training(dealing with compressed air) and trained on the fill stations so we do our own fills.( and getting ready to blend some nitrox, but this may be harder liability wise).

 

[DIR-Atlanta]

Actually in many shops it is a loss item.

I'd guess that it is a loss item in almost all shops. I would be surprised if $5 for an air fill covers the cost of providing the labor to hook the tank up and do the fill. Never mind the additional expenses of operation, maintenance, floor space, etc.

 

[snepdiver]

Well,
Unless the shop is a really small shop I don't buy into the "loss leader" on air fills.
OUr club probably does 800-1000 fills a month. We have no sales, no rentals, we are a non-profit org. with about 150-200 members on the books at any one time. Our air system has paid for itself and it's maintanance, and membership is $10 a month, with unlimited 24/7 access to filling station.

I was curious, Where do you have this set-up? What knind of liability insurance does your club have? Does this even require liability insurance? This is a very interesting concept. I know there are enough divers in my local area that would probably be willing to do something like this. At least you could get a fill when lds is closed. Holidays and Sundays.

 

[paulwall]

I'd guess that it is a loss item in almost all shops. I would be surprised if $5 for an air fill covers the cost of providing the labor to hook the tank up and do the fill. Never mind the additional expenses of operation, maintenance, floor space, etc.

Considering that most of the fills this shop does are for its' own tanks (for training and rentals), I would say that is likely true. So, I dropped the tank off, told them to fill it to 3000 psi with EAN32. I'll get it topped off with air somewhere else. It's mainly a "deep" tank (Oriskany and the upcoming Spearfishing/Lobster dive). If it's not topped off, I'll still have ~106 ft³ of EAN to poke around with.

So, to explore this further, The shop has 3 5000psi bank tanks (unknown capacity), plus a compressor to pump them up to 5000. With a slow fill, how much of that 5000psi bank is used to fill a 119 from 3000 to 3442? I can't imagine it would drain 450ft³ of 5000 psi that much... The draw from 0 to 3000 would be the bigger drawdown, and that would be true for a 119, or for 2 80's. In fact the owner compelled me to upgrade my LP95's burst disks so that he could put 3000psi in them (he routinely overshoots that on a fast fill, I've walked out with 3800psi on an LP95).

The funniest? thing is that they didn't say boo about filling my 2 PST HP80's. I think Mike is more on-track than you're giving him credit for. The fact is, I won't buy much of his equipment (I have too much as it is), and I can't expect FillExpress prices in my area. But to arbitrarily tell me it will cost 2x as much to fill my HP tank an extra 442 psi doesn't sit square with me. They don't offer fill cards, they don't offer per ft³ fills (I pay just as much to fill a tank that has 1000psi as one that has 100 psi), and I have never complained about their fill prices. If there were another Nitrox Stick in the area, I'd try them out. In fact, I'm told that DUDS in Gulf Shores uses a Stick. So, I'll likely get Nitrox fills there the next time I go. I'll see what they charge for a full fill.

Anyway, I'm ok with 3000psi EAN32 on this first fill. They're happy, and so am I (until I get the $12 bill for it).

 

[DIR-Atlanta]

I dropped the tank off, told them to fill it to 3000 psi with EAN32. I'll get it topped off with air somewhere else.

Personally, I don't recommend this practice, unless you are up front with the second shop about what's in the tank. And wouldn't it have been cheaper just to have the first shop do a full 30% fill, rather than pay for a Nitrox fill and then an air fill on top of that?

So, to explore this further, The shop has 3 5000psi bank tanks (unknown capacity), plus a compressor to pump them up to 5000. With a slow fill, how much of that 5000psi bank is used to fill a 119 from 3000 to 3442?

If they are 5000 PSI storage bottles, then they are probably 480 cf capacity. If so, then the amount of pressure loss is pretty easy to calculate. You will add about 15 cf to the 119 to bring it from 3000 PSI to 3442 PSI. Based on straight PV calculations, that translates to about 150 PSI drop in one cascade bottle.

Unfortunately, things are not as simple as that, for a couple of reasons.

First we have the issue of non-linearities in the gas compressibility curve. When you start getting up past 3500 PSI or so, then van der Waals effects start to come into play, and you don't get as much gas for the same increase in pressure. What that means is adding 500 PSI into a tank at 4500 PSI will result in LESS gas being added, as compared to adding 500 PSI into a tank at 3000 PSI. The reverse is also true, so your 15 cf of gas will actually create a larger pressure drop than what is predicted by the Ideal Gas Law (which was about 150 PSI, as calculated above). This has the effect of killing pressure in your cascade system faster, until you get it down to 3500 PSI or so.

The other thing to consider is how a cascade system is actually used. I don't know how much experience you have filling your own tanks, but it is very rare for all the storage bottles to be completely full. Bottles are typically used in a sequence, starting with the bottle that has the lowest pressure (or more accurately, the lowest pressure that is higher than what's currently in the tank being filled). You open and then close each cascade bottle in turn, which increases the pressure in the fill bottle by just a little bit, until you are able to get it up to rated pressure with the last bottle. After some number of fills, you can no longer do that, and then you run the compressor to fill everything back up.

The key to making the cascade system work is keeping the pressure in the last bottle as high as possible. Once that drops below 3000 PSI, it doesn't matter how much pressure you have in the remaining bottles - if you can't get the desired pressure out of the last bottle, then you have to run the compressor. So if you have people bringing in high pressure, high volume tanks to be filled, then that effectively kills the pressure in that last bottle, which forces you to run the compressor more (which kind of defeats the purpose of having a cascade system to begin with).

The whole point of using a cascade system is to avoid having to run the compressor so often. I'd say that a 3-bottle cascade is kind of small for a shop, and I can sort of understand their reluctance to do higher pressure, high volume fills. As you stated, the owner mainly has the fill station to support his own training activities (which is true of most shops), and I'm guessing he has sized his compressor and storage banks according to his needs for his classes. Asking for fills that are somewhat in line with what the system was designed for probably doesn't cause any problems. Higher capacity fills have the effect of skewing the economics, which is why he is charging more for them.

In fact the owner compelled me to upgrade my LP95's burst disks so that he could put 3000psi in them (he routinely overshoots that on a fast fill, I've walked out with 3800psi on an LP95).

Did you mean 2800 PSI? 3800 PSI is not exactly what I would call an "overshoot".

The funniest? thing is that they didn't say boo about filling my 2 PST HP80's. I think Mike is more on-track than you're giving him credit for.

I don't have all the details on the situation, and I'm not really trying to defend or support the owner's price structure - only he can do that. What I am trying to do is explain some of the issues so that you will perhaps have a better understanding of the economics of the situation. In most cases, a low volume, high pressure fill (or a high volume, low pressure fill) is not as big a concern - it's the high pressure high volume fills that cause the problems. That is why shop owners tend to charge more for those.

But to arbitrarily tell me it will cost 2x as much to fill my HP tank an extra 442 psi doesn't sit square with me.

If you feel the price is too high, then you can always invest in your own compressor. That way you can fill as much or as often as you like.

 

[mike_s]

Paul points out something that a lot shops do with their compressor that they don't take into account on what it costs to run the compressor. the fact that many shops tank fills are their own tanks, either through rental tanks or tanks for open water classes.

since each air fill is not at "bar coded item" it's not really tracked in inventory or cost of operations on store owned tanks. (Yes there is prob some shop that does 'bar code' things and track this, but most don't).

Since our local quarry closed to general admission, local fills just aren't needed that much anymore. People who drive to the beach to rent a tank typically rent it from a beach shop so they don't have to carry 2 or 4 tanks down in their car (if they don't own tanks).


Of course this all changes in a vacation destination where people are getting air fills for just everday diving.

 

[paulwall]

Personally, I don't recommend this practice, unless you are up front with the second shop about what's in the tank. And wouldn't it have been cheaper just to have the first shop do a full 30% fill, rather than pay for a Nitrox fill and then an air fill on top of that?

If the tank is stickered at 32% and I ask for a top-off with Air, I don't understand why they should care. they could analyze and verify before the fill, or refuse to top it off.

No, it wouldn't be cheaper to to the 30% fill ($24) than the 3000psi fill ($12 + $5 top-off) if I decide to go that way. I'm pretty content with the volume at 3000psi.


Thanks for the explanation on the cascade bottles/fill sequence. IIRC when watching others fill a series of tanks with different starting pressures, it seems they opened all of the scuba tanks first, to get them all at the same starting pressure, then opened the lines to the banked tanks to bring them all up simultaneously. It makes sense that each of the banked bottles have various pressures as they are used for fills.

As I've said, I use this shop because he has a Nitrox stick. So, for me, they are the only game in town. This situation is pretty low on my irritation factor post-Storm, so if my air fills can help keep him open, I'm all for it. I mainly get air fills, and generally dive shallow.

 

[SuPrBuGmAn]

Down Under uses a Nitrox stick, Underwater Works does partial pressure blending, I'm not sure what method Gary uses.

Underwater works has a deal for Unlimited Fills for the year, $50 per tank, course, this is air. Both my tanks have the stickers, plus 2 of the other 4 tanks I keep at my house(friends tanks) have the unlimited stickers too They come in real handy when you're doing 120 dives a year

 

[algulfdiver]

I will have a Stick soon, at least by Spring if not sooner.