Topping off Tri-Mix with air

[HowardE]

Greetings - I teach at a dive shop in San Diego. We only do air fills (no Nitrox or Tri-mix). Recently a couple of Tec Divers came into the shop with these massive doubles filled with Tri-mix, and wanted us to top them off with air.

OK. The resulting gas would be another mixture which would ultimately need to be analyzed; but it's no problem at all.

The question came up on our side about liability, whether it's safe, etc.

Sure it's safe. Why wouldn't it be? They just need to analyze the resulting gas.

What problems can you run into with an air fill? Filling a tank with pure Oxygen - that could have issues, but air?

I'm sure you could have them sign some kind of waiver, but I'm curious what other's thoughts might be on this in terms of risks or liabilities? Do other shops do this?

None that I get fills at...


Topping with air is doing an air fill. I am not getting a trimix fill, nor am I getting a nitrox fill... I come in to the shop asking for an air fill, and that's what I want. I can analyze the gas myself, and calculate my subsequent dive based on that mix.

 

[Gombessa]

I don't charge anybody for air fills, it comes with the trip, but if I did, yeah, I'd charge them double to fill doubles. Do you get your doubles filled for a single tank price?

Some shops charge by the cuft. Even for shops that don't, if my doubles have >1000psi, yes I get charged a single tank price. If I happen to have ~2000psi, my shop usually tops up on the house (and this is for 32%, not air); while I don't expect that, the key is to be reasonable. A different shop I walked into charged me two full-price fills for a 40cuft top-off for my doubles. I just don't go there anymore.

Remember, we're talking about a top-off here, it's already filled with some percentage of mix, he's not walking in with empty tanks.

While we're on the subject, double-72s hold about 144cuft. Should someone who brings these in really get charged double what someone with an HP130 is charged?

 

[Rainer]

Do you get your doubles filled for a single tank price?

Depends on the shop. Some charge the same, some double, while most charge a bit more than for a single cylinder. It comes down to how the shop sees their fill costs (labor vs. gas pumping). Doubles are one fill (they don't take up two whips, it's only one analysis if needed, etc). In some cases, doubles are hardly more gas than singles (e.g. my double LP72s are basically the same size as my single HP130s) and since they aren't any more work to fill, why charge 2x the cost? Another issue is of course how much gas they have in them already. If I bring them in half full, I'm not looking to pay for two fills, LOL.

 

[Blackwood]

It comes down to how the shop sees their fill costs (labor vs. gas pumping).

As well as compressor/filter costs.

 

[Rainer]

Those would be included in my "gas pumping" costs. Clearly more gas costs more to pump (for many reasons). That said, a fill cost is not entirely dependent on cf of gas pumped. Most shops charge the same for a single AL63 as they do for a single HP130. Why now charge double for a set of double LP72s that hold basically the same amount of gas and took just as much work to pump as the single HP130?

As well as compressor/filter costs.

 

[Gombessa]

As well as compressor/filter costs.

Doesn't that fall under the "gas pumping" category?

Edit: Rainer is faster with a one-paragraph response than I am with one sentence.

 

[Rainer]

Lol...

doesn't that fall under the "gas pumping" category?

 

[Wookie]

Aaargh! I didn't mean to open a debate on fill station costs. Sorry all. I only charge for helium, and by the cu ft used. Air, nitrox, and Oxygen are free at this fill station. It also happens to be the only fill station within 100 miles, usually. No, we stopped charging for fills a long long time ago, and, no, I don't think that a pair of doubled 72's should cost more to top than an empty 130.

Forgive me forgive me.

Frank

 

[Blackwood]

Yah, I skimmed his reply and misread it (thinking it was labor/gas pumping, i.e. a net cost, not two categories).

But yah. When I'm just getting dirty air at the nearby shop, I'll often fill my argon bottle too, and 8 times out of 10 they don't charge me for it.

 

[Rainer]

It's all good, Frank.

Sounds like you run a nice operation. I'll have to check you out.

Aaargh! I didn't mean to open a debate on fill station costs. Sorry all. I only charge for helium, and by the cu ft used. Air, nitrox, and Oxygen are free at this fill station. It also happens to be the only fill station within 100 miles, usually. No, we stopped charging for fills a long long time ago, and, no, I don't think that a pair of doubled 72's should cost more to top than an empty 130.

Forgive me forgive me.

Frank