Partial pressure nitrox fills at home?

[TrimixToo]

Hmmm...can't seem to delete any more.

 

[TrimixToo]

Reading between the lines of previous posts, you will see that a lot depends upon your total situation.I have done this countless times, but I have a booster and a unique situation where it makes total sense.

Not only does a local shop let me do it, that is, in fact, how that local shop gets their own nitrox--from me. In our area there is very little call for nitrox, so they only use it for their instructors doing multiple dives per day. I show up at the shop with an oxygen supply bottle in my van, and they bring me their empty nitrox tanks (so I don't have to bother using the booster for that.) They then top off their tanks. I don't charge them--it only costs me a few dollars per O2 fill to make 36%.

In return for me giving them the O2 for their nitrox, they give me my fills and top offs for free. I put as much O2 and helium into my tanks as I need, and then I get them filled at the shop. (I actually do the filling myself.) If you are talking trimix, a booster is a huge factor--I can pump my tanks up as far as they need to be without having to worry about the pressure differential.

Before a trip to our deep water site (420 miles), I also get some tanks filled with just air. At the site, I use the booster (electric) to mix up the needed helium, oxygen, and air for all our trimix and nitrox fills. We do this in the motel parking lot, with me plugging the booster into our motel electricity. I have only blown a breaker a couple times.

It is good to know that the air you are getting in topping off meets O2 standards, and the air pumped at my shop does.

EDIT: BTW, a nearby shop does offer nitrox fills to the public, and they charge $25 for a cylinder. That makes doing it myself a good idea.

I've seen that van. Great setup for your situation!

 

[_Ralph]

It depends ...... (classic answer)

It depends on if the store will allow you to bring in pure O2 tanks for them to top off with air/mix

It depends on if the store will allow any employee to do such act ("hey new guy, top off all the tanks")

It depends on if the store knows how much pressure you want in the tanks to get the desired mix... are they responsible for getting the mix correct, or you? And what happens if it isn't the correct mix (ie, too lean?)



As for how to do it?

There are several ways. Quite a number of my dive buddies have agreements with their shops, and lease a T of "diving" Oxygen from the gas supplier (~300cuft @ ~2400psi). They transfill to the desired pressure of O2 for their desired mix, tank is labeled as per shops direction, and they fill it up.

Rough estimate, is 450psi of O2 to make 32% (dependent on final tank pressure)... So you would be returning roughly 60cuft of O2 every time you 'finish' a T with this method.

The cost, very dependent on geographical area... but $100-200 on a transfil whip, $50-100 for yearly tank rental, $50-100 for the fill of O2, plus what the shop will charge for their service to make your nitrox.


A better way in the big picture is if your buddy with the dive compressor builds a nitrox stick to continuously blend O2 into the compressor to get <40% Nitrox (dependent on the compressor, manufacturer recommendations and maintenance to run elevated O2 levels through the compressor). The cost on that is highly unknown, as they might need a different compressor altogether to achieve a nitrox mix. But this results in nearly all the O2 being used out of the tank regardless of final pressure.

Another way is to use a booster to use more of the O2, as you can bring the pressure higher into the recipient tank than the donor tank. You would need a way to drive the booster (large shop compressor generally), low pressure and high pressure plumbing, filtering for the shop compressor..... You can do it inexpensively for a few thousand to tens of thousands....

A booster just to make 32% doesn't make sense. Trimix, then returning ~100cuft of helium every time you need to make a few sets of light trimix (transfil only), it starts to make sense. Same with topping rebreather cylinders. Shops will generally charge way too much to bring them up to pressure (ie, it isn't worth their time and effort to run the booster for a few cuft of gas).



_R

 

[rx7diver]

... Could I go get a tank of O2 and fill my tank to the required Partial pressure, then bring it to my LDS and have them top the tank off?

In the post it was never completely clear if you could partial fill a tank with O2 and bring it to the LDS and have them fill it the rest of the way. ...

You'll need 3 or 4 Oxygen tanks ...

First of all, it would be unethical and dangerous if you don't inform the LDS that this (using his/her oxygen-compatible scuba air to top off your oxygen-service cylinder that is already partially filled with pure oxygen) is what you are doing, I think.

Second, yes, an home oxygen cascade system could be set up and used easily enough. I think such a system would be especially useful if you needed to fill, for example, an oxygen deco cylinder and/or an EAN50 deco cylinder, and EAN25 cylinders for a technical nitrox dive to 150 fsw. I wonder, though, about the implications on your homeowner's or renter's insurance if you have an oxygen cascade system in your home or garage.

rx7diver

 

[JFalzetti]

It depends ...... (classic answer)

It depends on if the store will allow you to bring in pure O2 tanks for them to top off with air/mix

It depends on if the store will allow any employee to do such act ("hey new guy, top off all the tanks")

It depends on if the store knows how much pressure you want in the tanks to get the desired mix... are they responsible for getting the mix correct, or you? And what happens if it isn't the correct mix (ie, too lean?)



As for how to do it?

There are several ways. Quite a number of my dive buddies have agreements with their shops, and lease a T of "diving" Oxygen from the gas supplier (~300cuft @ ~2400psi). They transfill to the desired pressure of O2 for their desired mix, tank is labeled as per shops direction, and they fill it up.

Rough estimate, is 450psi of O2 to make 32% (dependent on final tank pressure)... So you would be returning roughly 60cuft of O2 every time you 'finish' a T with this method.

The cost, very dependent on geographical area... but $100-200 on a transfil whip, $50-100 for yearly tank rental, $50-100 for the fill of O2, plus what the shop will charge for their service to make your nitrox.


A better way in the big picture is if your buddy with the dive compressor builds a nitrox stick to continuously blend O2 into the compressor to get <40% Nitrox (dependent on the compressor, manufacturer recommendations and maintenance to run elevated O2 levels through the compressor). The cost on that is highly unknown, as they might need a different compressor altogether to achieve a nitrox mix. But this results in nearly all the O2 being used out of the tank regardless of final pressure.

Another way is to use a booster to use more of the O2, as you can bring the pressure higher into the recipient tank than the donor tank. You would need a way to drive the booster (large shop compressor generally), low pressure and high pressure plumbing, filtering for the shop compressor..... You can do it inexpensively for a few thousand to tens of thousands....

A booster just to make 32% doesn't make sense. Trimix, then returning ~100cuft of helium every time you need to make a few sets of light trimix (transfil only), it starts to make sense. Same with topping rebreather cylinders. Shops will generally charge way too much to bring them up to pressure (ie, it isn't worth their time and effort to run the booster for a few cuft of gas).



_R

So I’ve looked into the nitrox stick a lot more. My buddy works at the dive shop and that’s how I get free fills. I most likely wouldn’t be able to just say hey let’s put this DIY stick on the intake for just a hour to fill our nitrox then take it off so no one would notice… so I’ve been looking into simply purchasing my own compressor to do this, since I’d like to be in control of my own nitrox (get it whenever I want and however much I want), have an issue With paying a shop $17 for nitrox, and they said they’d need to keep my tank over a day to do it, also where the shop is located it’s a lot of gas money… so realistically the nitrox fill is $25+ for a tank. For the sake of numbers we can believe more let’s say it’s only $20 a fill for the math I’m about to do.

I’ve found a couple compressors I might like to get. I’ve heard the alkin w31 is good, it’s $3500.

I’ve also come across the coltri msh6 which is $2100, and I’ve heard people mention them too.

Both of these compressors will take about 25 minutes to fill a tank, so a couple hours of filling throughout the week for the weekend.

Compressors that can handle multiple tanks and fill faster are $5500 I believe, so all in what your cost vs outcome is worth to you.

I hope I can find a used compressor, if anyone knows or has one for sale for cheap please let me know.

So let’s go with the $2100 compressor because I like lower numbers.

$2100 (compressor) + $100 (nitrox stick) + $100 hoses + $100 (whatever else) + $500? (cost of gas with rental?) + $300 for maintenance = $3200

$3200 / $20 (per nitrox fill) = 160 fills to break even.

160 fills / 52 weeks = 3 dives a week

So let’s say it takes two years to have it make sense since I don’t think I will be diving every single week.

Now I’ve been looking into trimix mixing using this same method just adding an additional feed for the helium. It seems like it’s a lot more of an artwork than anything, making sure that the levels are correct throughout the whole fill.

Now if I was able to fill trimix with this method it would obviously make a lot more sense.

 

[johndiver999]

You don’t return the oxygen tanks with 450 psi. You add less to several scuba tanks and the use the other bottle of oxygen or get another full one.

 

[JFalzetti]

First of all, it would be unethical and dangerous if you don't inform the LDS that this (using his/her oxygen-compatible scuba air to top off your oxygen-service cylinder that is already partially filled with pure oxygen) is what you are doing, I think.

Second, yes, an home oxygen cascade system could be set up and used easily enough. I think such a system would be especially useful if you needed to fill, for example, an oxygen deco cylinder and/or an EAN50 deco cylinder, and EAN25 cylinders for a technical nitrox dive to 150 fsw. I wonder, though, about the implications on your homeowner's or renter's insurance if you have an oxygen cascade system in your home or garage.

rx7diver

No worries they would be informed, thanks for looking out for everyone though. I know there’s some risks involved, so everyone should know what’s happening

 

[_Ralph]

You don’t return the oxygen tanks with 450 psi. You add less to several scuba tanks and the use the other bottle of oxygen or get another full one.

True... but assuming "worst" case of 1 T rental, and only 1 scuba cylinder ... so you return with say 400psi because you don't have another cylinder to fill any more O2 into... and need to return the cylinder to swap to a fresh one... or dive weaker nitrox.

Multiple dive cylinders, and multiple T cylinders can mitigate that by cascade filing and the like, but now is that extra $$ worth it? Does one have that much space? It all depends... And with having supply cylinders that low, you're draining your diving cylinders every time for a fill (to use up the last 20% of O2)... is that worth the extra T cylinder rental (or purchase) for returning ~$10 of unused O2?

Also, does that affect your home/renters insurance and/or leases? Does one have a vehicle capable of transporting T cylinders safely? Have a spot to store properly?

As the saying goes, there's many ways to skin a cat... but will it fight back?


_R

 

[JFalzetti]

First of all, it would be unethical and dangerous if you don't inform the LDS that this (using his/her oxygen-compatible scuba air to top off your oxygen-service cylinder that is already partially filled with pure oxygen) is what you are doing, I think.

Second, yes, an home oxygen cascade system could be set up and used easily enough. I think such a system would be especially useful if you needed to fill, for example, an oxygen deco cylinder and/or an EAN50 deco cylinder, and EAN25 cylinders for a technical nitrox dive to 150 fsw. I wonder, though, about the implications on your homeowner's or renter's insurance if you have an oxygen cascade system in your home or garage.

rx7diver

That is a good point regarding the home insurance. I was worrying about the oxygen in my garage last night. One hiccup and the whole house can go down. Do you know if it’s easy to use a fire extinguisher to put out an oxygen fire?

 

[boulderjohn]

That is a good point regarding the home insurance. I was worrying about the oxygen in my garage last night. One hiccup and the whole house can go down. Do you know if it’s easy to use a fire extinguisher to put out an oxygen fire?

Oxygen itself does not burn; it facilitates the burning of whatever fuel is present. The bigger danger is a tank explosion. In all the oxygen incidents I know of, the damage came from an explosion.