Nitrox PPO2 blending waiting period for homogenisation of mix

[fdog]

Yes, I suppose I'm imposing my own standards here, my appoligies.

Still, most cylinders are returned with 500 - 700 psi, and there's usually at least one O2 cylinder that can push in the extra 300 psi...it's not that hard.

My experience with shops that insist on emptying cylinders, is because they lack a blender program, or, don't care to do the math to top off a nitrox fill - hence the "lazy" comment.

Point taken, though - I've occasionally been down to the bottom of the last O2 supply cylinder, and been forced to drain the tank before blending.


All the best, James

 

[iztok]

They may lack a booster. If they are cascading O2 starting with an empty scuba tank makes some sense.

Dumping leftover nitrox is cheap, dumping left over Trimix is another matter.

Tobin

That is exactly the setup. They have cascading O2 but no booster for it.

 

[cool_hardware52]

Yes, I suppose I'm imposing my own standards here, my appoligies.

Still, most cylinders are returned with 500 - 700 psi, and there's usually at least one O2 cylinder that can push in the extra 300 psi...it's not that hard.

My experience with shops that insist on emptying cylinders, is because they lack a blender program, or, don't care to do the math to top off a nitrox fill - hence the "lazy" comment.

Point taken, though - I've occasionally been down to the bottom of the last O2 supply cylinder, and been forced to drain the tank before blending.


All the best, James

Not looking to beat a dead horse, but....

All PP blending involves learning how to "fudge" the numbers to end up with the mix you really want.

How much "extra" O2 do I need to add to compensate for the heating that takes place when adding the O2?

How much do I overfill with Air to end up with the right mix at the right final pressure?

Both can of course be learned, but the starting pressure of the scuba tank is an important variable.

The fudge factors are different when starting with an empty cylinder vs one that's at 1000 psi.

I can easily see where an LDS makes the decision to start all PP fills at "zero" psi to keep things simple and help reduce need to "correct" missed mixes.

It's pretty easy to "fix" a mix if you have a booster, more of a PITA without.

Tobin

 

[fdog]

<snip>

It's pretty easy to "fix" a mix if you have a booster, more of a PITA without.

Tobin

True that.


All the best, James

 

[Rick Inman]

...and no two gauges are alike, so if someone brings in a tank with O2 (or O2+HE) to top, who knows what the gauge differences will be.

I've done it so many time at the shop I use (they have me top all mine and my buddies tanks) that I do know. But it's taken quite a few fills to learn it.

 

[dumpsterDiver]

I've seen it with my own eyes. The mix comes out wrong, you roll the tank for just a minute or so and it reads fine. There is no way, I would have believed it, if i hadn't seen it. If the false reading was associated with the air trapped in the manifold, then when PP mixing, the mix would always read way too low initialy and then slowly climb.

 

[dumpsterDiver]

Not looking to beat a dead horse, but....

All PP blending involves learning how to "fudge" the numbers to end up with the mix you really want.

How much "extra" O2 do I need to add to compensate for the heating that takes place when adding the O2?

How much do I overfill with Air to end up with the right mix at the right final pressure?

Both can of course be learned, but the starting pressure of the scuba tank is an important variable.

The fudge factors are different when starting with an empty cylinder vs one that's at 1000 psi.

I can easily see where an LDS makes the decision to start all PP fills at "zero" psi to keep things simple and help reduce need to "correct" missed mixes.

It's pretty easy to "fix" a mix if you have a booster, more of a PITA without.

Tobin

Before I got nitrox certified and could buy nitrox fills, I used to add oxygen in my garage and then top off with air at the shop. I would never drain my tanks, and just top them off with the correct amount of oxygen based on a spreadsheet I made. I found that my crappy home guage worked better when reading intermediate pressures that would typically be attained when adding oxygen to left over nitrox.

It can save a lot of oxygen if you start with a 1/3 filled tank, but I would have to do a calculation for each tank and it definitely makes the process more complex and prone to errors.

 

[fppf]

Wow, lots of comments all over the place, I think I will throw my 2 cents in for what its worth.
I do mix, so I have first hand experience. I'm also in aerospace and work with high pressure gas and hydraulic systems everyday.

It DOES take time for the gasses to mix in the tank. I have tried it many many times, normally there is about a 2-3% error right after the fill is finished. Rolling the tank or letting it sit a few hours is enough.

When mixing there are no "fudge factors" these are used because people don't take into account ALL variables or don't use them correctly. I see guys calculate numbers based on the fill pressure of the cylinder only to over fill it by a few hundred PSI. Then they blame the calculations on the mix coming out wrong. Another issue is seeing guys use 5% dial gauges, it does not take much pressure error to equal a % of O2.

Starting at zero will give you a more accurate mix. It will also give you an arguably safer mix. When you start with a cylinder that has some mix in it already you are trusting your O2 sensor, and pressure sensor to start your calculations with. Again, O2 sensors are really only 1% accurate, and how about your pressure sensor. After your done mixing you use the same O2 sensor to check your mix. So your O2 sensor better be working correctly.

By starting at zero your letting your math do the mix then just doing a sanity check with the O2 senor. If they don't match you screwed your math or your O2 sensor. Either way your not depending on 1 item to work.

I normally get my mixes within a few tenths. I use a 0.1% pressure transducer and I measure the temperature when filling all gases. Recording the temperature at each step and adjusting the pressure targets will remove your "fudge factors" with real math. Using the same pressure gauge for all steps will help your outcome as well.

 

[cool_hardware52]

Wow, lots of comments all over the place, I think I will throw my 2 cents in for what its worth.
I do mix, so I have first hand experience. I'm also in aerospace and work with high pressure gas and hydraulic systems everyday.

It DOES take time for the gasses to mix in the tank. I have tried it many many times, normally there is about a 2-3% error right after the fill is finished. Rolling the tank or letting it sit a few hours is enough.

When mixing there are no "fudge factors" these are used because people don't take into account ALL variables or don't use them correctly. I see guys calculate numbers based on the fill pressure of the cylinder only to over fill it by a few hundred PSI. Then they blame the calculations on the mix coming out wrong. Another issue is seeing guys use 5% dial gauges, it does not take much pressure error to equal a % of O2.

Starting at zero will give you a more accurate mix. It will also give you an arguably safer mix. When you start with a cylinder that has some mix in it already you are trusting your O2 sensor, and pressure sensor to start your calculations with. Again, O2 sensors are really only 1% accurate, and how about your pressure sensor. After your done mixing you use the same O2 sensor to check your mix. So your O2 sensor better be working correctly.

By starting at zero your letting your math do the mix then just doing a sanity check with the O2 senor. If they don't match you screwed your math or your O2 sensor. Either way your not depending on 1 item to work.

I normally get my mixes within a few tenths. I use a 0.1% pressure transducer and I measure the temperature when filling all gases. Recording the temperature at each step and adjusting the pressure targets will remove your "fudge factors" with real math. Using the same pressure gauge for all steps will help your outcome as well.

Fudge factors do exist and when you the idiosyncrasies of a given system it's pretty easy to get very close.

I "know" for example that I need to add a bit more O2 than the math calls for because the heat of compression of the O2 in the scuba tank.

If I'm adding O2 to an empty tank, don't need much over the calculated pressure. The gas will be pretty cool. OTOH if I'm boosting O2 from say 500-600 psi to put on top of 1500 psi of left overs I "know" I need quite a bit more than the calculated pressure because the O2 will be hotter leaving the booster.

Same goes for the air top. If I do a faster fill I run it up a bit, as total pressure will drop as the tank cools.

Accurate instruments are nice, but what the point of .1% pressure transducer when you can't really measure the temperature of the gas?

If my mix is +- 1/2% I know I'm well within the resolution of my depth instruments anyway.

Tobin

 

[fppf]

Accurate instruments are nice, but what the point of .1% pressure transducer when you can't really measure the temperature of the gas?

Tobin

Because I had it laying around the house
I also have a 0.5 degree F RTD to measure cylinder temperature. But you need to let the cylinder rest for a minimum of an hour to get a good read. Steel cylinders need to sit a little longer than aluminum. I have done lots of experiments understanding all the variables of the system. Did I mention the aerospace geek part?

Anyway another interesting factor for your "fudge" the gas in the core of the cylinder may not be the same temperature as the cylinder it self, it maybe much hotter. I have done tests where the pressure drop of the cylinder is much greater than the temperature of the cylinder it self predicted.