Why is it hard to blend the exact % of O2 for a EANX fill ?

[BlueTrin]

I don’t plan to learn to blend but I would like to know why it seems so hard to blend the exact % of O2 in a fill (I get often within +/- 1%) ?

A naive person who does not know about blending (me) would think that you just need to fill up to a certain pressure with air and top up the remaining with the missing pressure of O2.

I guess that may be related to another thing I was wondering: why is it so hard to get the exact pressure in a fill ? Here we have tanks rated to 232 bar, I got everything from 245bar to 190bar !

 

[tbone1004]

The short answer for the percentages can be inaccurate pressure gauges, usually small form analog gauges which can give about 1% difference in fills.

The real answer is temperature. Very few people fill factor in temperature differentials in filling because the math is essentially impossible to perform since the variables are far too great. If you fill slow enough, you can get VERY close, but it takes forever, read several hours to fill, and no one wants to do that, so they fill the O2 slow, and then fill the air as normal.

In regards to exact tank pressure, the obvious one is still temperature. If they fill fast, that means hot, and if there is a big change in that temp and water temp, you'll see a pretty stark change. If you have a big differential in ambient temperature and water temp *i.e. cave diving in Florida in the winter*, you can actually see the pressure gauge rise while you're in the water and the tanks are warming up from near freezing to 70F. In the summer, if they've been sitting out in the sun, once you get in the water you can see the pressure drop pretty rapidly. The 245bar is usually the overfilled a bit to compensate for temperature, but the temp differential wasn't as big as they planned due to a slower fill rate so it didn't cool off quite as much.
Another potential reason for the low fills is that the banks were low and they didn't turn the pump on.
And another is someone set the regulator to 210bar for filling aluminum tanks and didn't change it when they filled your tank.

 

[jadairiii]

There is a reason why the term "Dive Shop Monkey" was coined.

It is not hard and good shops do it all the time, many times a day. Then you have the shops that still think rolling the tanks on the ground is the best way to blend gases.

 

[Neilwood]

@tbone1004 has pretty much nailed it. You have two variables (pressure and temperature) and a fixed constant (tank volume).

As the tank is filled and the pressure goes up, so does the temperature. Unless filled in a way when that the heat can dissipate to such an extent that it becomes a non issue (either very slowly or in a cooling system where the heat can be conducted away), the tank will always have to be either over filled (to allow for cooling) or filled and topped up (to allow for pressure reduction after cooling). Trying to hit exact pressures is like trying to hit target that is moving in two dimensions - it is possible but takes a lot of skill.

That is also why blending is not 100% effective - you are effectively trying to hit a moving target. Blending calculators and experience help but it is still a case of near enough.

Membrane systems are way to get more accurate filling but it is more expensive than partial pressure filling.

 

[Stoo]

More to the point, who cares. A percentage point one way or the other isn't going to seriously change your dive. Adjust your 'pooter and carry on.

 

[Graeme Fraser]

As well as filling slowly, dunking the cylinder in a water tank whilst charging will help dissipate some of the temperature generated through compression.

If you take a (planned) chamber ride you can feel how warm the inside of cylinder gets, and that's only to about 5bar!

 

[Johnoly]

...The real answer is temperature. ....

During FLA's 2 day mini-lobster season each year, the coastal shops get swamped with literally hundreds of tanks needing fills in a span of 12 hours. Instead of having short fills from rushing the tank fills to clear the line of tanks, many shops will put their ice maker machines to work. They will make a freezing cold ice slurry water trough and chill down the tanks as they fill. The water gets so warm, they have to keep dumping in more ice to keep the slurry going. It works very well during that one time of the year. P.S. before someone posts - NO water does not enter the tanks because all the tanks contain a few hundred PSI that water can't get past thru the valve.

 

[tursiops]

As well as filling slowly, dunking the cylinder in a water tank whilst charging will help dissipate some of the temperature generated through compression.

PSI-PCI - Filling Cylinders In Water - Time to Review

 

[Chris H]

Other folks have answered your question. If the temperature is giving you trouble, you could fill by weight. Gravimetric if you are in the specialty gas business. You'll need a good scale that accurately measures in 1 gram increments. The disadvantage is that you need to know exactly how much your cylinder will hold.

 

[abnfrog]

its not hard if you use a digital gauge and fill slow . by the time my students leave they are hitting the mark with in 1 %.......lds that don't .....stop excepting the fills they will smarten up pretty fast