Ad-Hoc Tank Blending Risk

[Johnoly]

When I was filling, I never trusted what was inside a customer's tank (even my regulars). Air or otherwise cause if the % or gas is wrong, the filler has the liability for what's in the tank. If I had too many rush tanks to do, then I'd drain the rush tanks and didn't bother with the pre-check. For anyone who's ever done shop fills, I don't know anyone who wouldn't catch 100% (or high %) O2 in a tank. I also would ban the customer from future fills. I'm not going to hold the liability for an Ox-Tox'd diver. Furthermore, Us tank fillers PRIDE ourselves on hitting the requested % EXACTLY on the number >> So we watch you analyze and see how well we did our mixing. So I'll catch it then. Also if you try to leave without testing, then that's a red flag and I'm going to look closer. Tank fillers are not stupid, but we've seen some crazy stuff (like obvious dremel hydro dates & print your own VIP labels) Take your tank somewhere else, I'm not filling it.

 

[Johnoly]

....O2 is released into a closed fill whip, the temperature in the whip will instantly rise to about 1500 degrees due to pressure change. ...

1500 degrees?

I understand there are all different kinds of setups for a fill station. But we really don't want to dump the gas inside of that 4-6 ft long fill whip> mostly because it takes to long to depressurize and we don't have time with a long line of tanks to fill. I also don't want a whip flying about wildly cause a shutoff handle was left open. I have enough problems with old burst disks letting go. Almost every fill whip has a line shut off handle right next to the DIN/Yoke/Bleed assembly. This allows me to just depressurize about 4 inches of line very quickly so I can remove the assembly from the tank valve and put it on the next tank. If your bandit tank full of O2 was hooked up, it would only go 4 inches and that's not enough gas volume to heat up the whip (and certainly not 1500 degrees). There also are backflow preventers at different points in the fill station. Some by the bank bottles, some by the compressor and some by the whips > just depends how it was designed because there's plus's and minuses to everything.

In the end, bringing in a bandit tank full of O2 for an air fill will not have a good ending. The mix will be off, or your tank will be drained or you'll get banned from future fills.

 

[TravisD]

For anyone who's ever done shop fills, I don't know anyone who wouldn't catch 100% (or high %) O2 in a tank.

Serious question - how? Are you putting an analyzer on every single tank before filling? It's not like there's a smell, or or shows up differently on a pressure gauge...

 

[Johnoly]

Serious question - how? Are you putting an analyzer on every single tank before filling?

From a shop standpoint, O2 is expensive and you don't want to waste O2. Since the tank already has a partial O2 in it when brought in, I can save my shop's O2 by NOT draining the customer's tank and doing the quick calculations that re-use the O2 inside the customer's tank. Also what's written on a piece of tape stuck to a nitrox tank may not be accurate and off by 1-2% for a number of reasons. That throws off my numbers when trying to fill it and hit an exact % number requested. When you are filling nitrox tanks you learn to get into a 'rhythm' and do ~4 steps at once on 4 tanks. Hooking it up to my wall analyzer was one of the steps and I check every tank before putting in either banked EAN or pure O2. Many don't but I also checked my final mix on each tank I filled. In Florida we have many different types of divers. The caver's want 32%, the shore divers want air, the Ocean divers want 36% and the tech divers I let the shop owner fill cause it's some wild mixes.

 

[BRT]

From a shop standpoint, O2 is expensive and you don't want to waste O2. Since the tank already has a partial O2 in it when brought in, I can save my shop's O2 by NOT draining the customer's tank and doing the quick calculations that re-use the O2 inside the customer's tank. Also what's written on a piece of tape stuck to a nitrox tank may not be accurate and off by 1-2% for a number of reasons. That throws off my numbers when trying to fill it and hit an exact % number requested. When you are filling nitrox tanks you learn to get into a 'rhythm' and do ~4 steps at once on 4 tanks. Hooking it up to my wall analyzer was one of the steps and I check every tank before putting in either banked EAN or pure O2. Many don't but I also checked my final mix on each tank I filled. In Florida we have many different types of divers. The caver's want 32%, the shore divers want air, the Ocean divers want 36% and the tech divers I let the shop owner fill cause it's some wild mixes.

And you analyze the tanks before filling when people only want air?

 

[jborg]

This thread was specifically about bringing tanks with O2 "to the local (non-Nitrox) dive shop". They'd have an air compressor, no banked O2, and no reason to analyze neither incoming nor outgoing tanks in my experience.

 

[Johnoly]

And you analyze the tanks before filling when people only want air?

Yep ! I've had tanks marked air and when tested had nitrox in them. Never a % to do much harm and probably because the air banks were low for a top-off so they used the full Nitrox banks instead. But when there's any type of accident the lawyers all look for who made any kind of mistake to lay partial blame on. As said above, Just establish a Rhythm and do all the tanks the same way and you'll be covered doing it correctly.

 

[Johnoly]

..... and no reason to analyze neither incoming nor outgoing tanks in my experience.

And we've all seen people who dive tanks they don't test themselves. If you don't want to analyze your tanks, don't test them.

 

[jborg]

If you don't want to analyze your tanks, don't test them.

I analyze every tank before i dive it. I’ve never seen an air-only dive shop analyse tanks they fill with air, before or after.

 

[sod kamus]

ummm yeah. We had a guy that did that.

We usually didn't follow PSI standards concerning emptying a cylinder before filling it. This guy would bring his cylinders about every week for an air fill. He would fill them about 800 psi from his daddy's medical oxygen cylinders with some kind of (un-02 clean) transfer whip he had rigged up in his garage from spare plumbing parts.

He did this for about a year before we ever realized it.

One day I needed to analyze the gas in a cylinder, I grabbed his tank I had just filled with "air" to calibrate the analyzer. I checked the cylinder of nitrox, it read like 60%... WTH? I calibrated it again using his "air" tank, checked the nitrox again, similar result. I thought, analyzer sensor must've gone bad, right. I grabbed another analyzer....same thing. What the heck? I know his cylinder has 21% air in it because I just filled it!

After several readings I realized what must've happened, and questioned him and he fessed up that he was PP blending his own nitrox....in a non-02 cylinder. The "miracle" part of the story is he was a real MacGyver type of guy with a workshop in his back yard who could fix literally anything and he did his own valve rebuilding...and said he used 5w30 motor oil to lube his valve threads. At least he hadn't blown himself or the FSO up yet.

We notified the other dive shop in town as we banned him so we figured he'd go there and try the same thing.

After that we started draining before filling, even if the customer said it was air or nitrox.