Auto-drain for tanks?

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ChrisMBC

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Location
Victoria, BC
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500 - 999
I have a nitrox card with my local shop, and they exclusively partial-pressure blend up from an empty tank. To try and be nice to the people at the shop, I try and bring my tanks in with about 150 PSI left in them, so they don't have to monitor running them down in the shop.

So, what I want to do is build myself some kind of auto-drain system that will stop draining when there's a set amount of pressure remaining in the tank. Is there any off-the-shelf solution, or one I could build with a spare first stage, to achieve this?
 
I suppose you could make a stack of adapters that goes from the HP port of a first stage (or directly from the din valve) to a blowoff valve like this McMaster-Carr. Those are nice because you can pull the ring to vent the remaining internal pressure once you're done and the tank valve is closed so you don't even need a separate pressure relief or second stage attached.

Although I'm not sure what happens if you hit that with a sustained 3000psi. It might ice up.
 
I suppose you could make a stack of adapters that goes from the HP port of a first stage (or directly from the din valve) to a blowoff valve like this McMaster-Carr

Although I'm not sure what happens if you hit that with a sustained 3000psi. It might ice up.
Interesting. What if I hit that from an LP port, perhaps? I could tune the IP to be well above the pressure relief valve...
 
You could, but honestly when I drain tanks I put them outside and turn them on and some back in half an hour. Personally I take no notice of if a tank is full or basically empty. It goes outside until its empty regardless. I basically only drain tanks for VIS/Hydro. I would just give them to them as is and save yourself the hassle.

Seems weird they drain all tanks but I dunno what their fill station setup is. If you're constantly getting Nitrox in your tanks they are kind of wasting O2? Generally I have have just taken an 02 reading when the customer drops off their tanks, used an app to calculate, then filled what is required for the requested mix.

Out of curiosity, do they charge you a flat rate or by the cubic ft?
 
Would a OPV in LP port on 1st stage with higher LP pressure than the OPV is set to work?
 
You could, but honestly when I drain tanks I put them outside and turn them on and some back in half an hour. Personally I take no notice of if a tank is full or basically empty. It goes outside until its empty regardless. I basically only drain tanks for VIS/Hydro. I would just give them to them as is and save yourself the hassle.

Seems weird they drain all tanks but I dunno what their fill station setup is. If you're constantly getting Nitrox in your tanks they are kind of wasting O2? Generally I have have just taken an 02 reading when the customer drops off their tanks, used an app to calculate, then filled what is required for the requested mix.
They don't blend up as a matter of practice. Yes, they're wasting O2, but hey, it's their shop.

Out of curiosity, do they charge you a flat rate or by the cubic ft?

I have a 'Nitrox card' with them that gets unlimited fills for $650 CAD per year.

Would a OPV in LP port on 1st stage with higher LP pressure than the OPV is set to work?

That's exactly what I'm going to try.
 
https://www.shearwater.com/products/swift/

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