heliox

[padiscubapro]

Originally posted by omar
I blend for myself and a couple of diving partners. I don’t pump for money and even if I did, I do not need a blender certification. I can demonstrate that there are errors in the IANTD and TDI manuals on blending and I know who wrote the majority of the IANTD manual. I have not looked at the PADI manual. My undergrad engineering degree included P Chem. which covers in a much more useful and thorough manner the behavior of real gases. So there would be no problem for me to demonstrate that I understand and can blend correctly. In fact, I probably have one of the more comprehensive (as far as the ability to mix AND analyze) blending setups around and I log all of my blends anyway.

The helium leaking from a cylinder is typical dive industry BS. It will not leak preferentially from a mix. If you lost gas it will be in equal proportions to what is in the mix. I have a K cylinder of UHP Helium (which costs a fortune) I use for calibration purposes and as a carrier gas. I leave the regulator on it and it will sit for a couple of months unused in some cases. The use is logged for billing purposes so I would know if it has lost any gas. It has never happened. Heck if you get gas from a supplier you would know that it comes in steel cylinders that are for the most part the equivalent of a scuba tank. You can let these tanks sit for ever and you will not lose gas.

I also store in HP (4500 psig) cylinders 30/30 and 20/40, my 2 most used blends for local diving. I do not see a change in the mix over time and I have never seen any changes in my scuba tanks that have sat for long periods with trimix in them.

You can send an e-mail to Tom Mount at IANTD and ask him what he has found about leaving Helium mixes in scuba tanks for extended periods. It is the same - no loss of helium. Helium will not preferentially escape through a tank.

omar

I have to disagree with you.. It did happen the tank was properly mixed and analyzed (both with He and o2 analyzers and readings were stable...measured about 1 hr after final top off) , and the next day it was different. I have been blending my own tanks for about 7 years now, so I know it was done right. Fact is helium is a smaller molecule. Its the same principal that some seals will hold out water but will pass gas through it..
the differance between real and ideal gas laws really aren't a big factor nitrogen and helium have very similiar compressability factors @3000 psi/70 f 1.098 vs 1.058, at 3500 they are even closer, its the o2 thats more of a concern 0.944 , but the contents of o2 are easily and cheaply verifyable.

I'm not saying that compressability doesn't alter a mix but its negligable

filling a typical 20/35/45 mix (o2/he/n2) (using ideal gas laws) will actually be 19.3/36.1/44.6 and even that would be dictated by the accuracy of the guages used in mixing..

thats only a 3.5 % error (of target not total) on the o2
3.05% error on the he and .09% error on the nitrogen, quite acceptable...
The lower the 02 requirement the closer it will be to the target..

(o2 is more compressable than he or n2)

 

[blacknet]

padiscubapro,

Temp/humidity change perhaps?

Ed

 

[padiscubapro]

temp would just effect final oressure in cylinder..

Humidity doesn't make a difference since the gasses are dried..

 

[omar]

Temperature will affect the readings of both the helium analyzer and a galvanic oxygen sensor. A tank one hour after filling will still be hot from fillling and it is not a good idea to use any gas analysis done on a hot mix.

A higher temperature will change the thermal conductivity of the mix which is what the helium analyzer is actually measuring compared to a reference of air. A hot mix will result in the helium content to come out higher than one at ambient. There is temperature compensation for ambient T on the helium analyzer but not for a hot (or cold) tank. The helium analyzer manual will tell you that the mix should be analyzed at ambient T.

Temperature will also result in differences in an O2 sensor. With increasing temperature the oxygen sensor signal rises. This performance is usually compensated with resistors that have a negative temperature coefficient (NTC). Some analyzers/sensors have this, some don't. Again it is important to have the sensor and gas at the same temperature. This is the recommendation from teledyne for their sensors as well.

If you have a leak from a loose burst disk plug or valve there is no membrane that will act as "filter" to let specific molecules to escape. This preferential loss of helium from a scuba tank is BS. The laws of physics work in the Caymans as well as any where else.

I have watched experienced blenders screw up and on occassion I have also. Your experience is more likely a result of an incomplete gas addition and checking a hot mix.

omar

 

[Bob3]

Don't forget that gas blending using the PP method requires the tank to be... [how to phrase this] "shaken, not stirred" in order to assure a homogenous mix. Might've come across a "lump" of He.
All in all, the temperature differential makes more sense.

 

[padiscubapro]

Omar,
I fill tanks VERY slowly, there is very little heat buildup (on my personal blending station my booster pump has a gas cooler), I don't analyze until ther is NO drop in tank pressure. MOST better o2 sensors are temperature compensating (I use an ALPHA which is) so thats not a factor either. NO pressure drop means no temp change since the volume is fixed.
I didn't believe it as well ( I have had tanks sit over winter without loss), until it happend to me.. I never fill faster than 75psi/min and lower rates with He or o2..
I am not saying I never screwed up on a mix, I have but only when I was rushed, no one is perfect.
I dont think there was any type of gas "block" since tanks were filled from 1 side of a manifold and was then isolated and tested, both readings were the same. After getting a reading other than what I expected I analyzed the other side (tank was still isolated) and they WEREN'T the same. I purposely leave the tanks isolated so I can spot any slow leaks since any pressure difference between the 2 tanks will be noticed upon opening the valve and my PG is only on one post.
I understand the theory behind the He analyzer it can be used for other gasses as well...

 

[omar]

If I understand what you have said………

You filled the tanks, isolated them and analyzed with both sides the same. You checked them the next day and the analysis was different from the previous day. Also you indicated that both sides, still isolated had an identical change from the previous day (evidenced by the same analysis). Correct?

To both be the same and to be a result of a leak means that both sides had to have the same leak, the same rate of loss and the same preferential loss of helium. Not likely.

A fill rate of 75 psi/min will still result in enough heat to make a difference one hour later.

omar

 

[padiscubapro]

Originally posted by omar
If I understand what you have said………

You filled the tanks, isolated them and analyzed with both sides the same. You checked them the next day and the analysis was different from the previous day. Also you indicated that both sides, still isolated had an identical change from the previous day (evidenced by the same analysis). Correct?

To both be the same and to be a result of a leak means that both sides had to have the same leak, the same rate of loss and the same preferential loss of helium. Not likely.

A fill rate of 75 psi/min will still result in enough heat to make a difference one hour later.

omar

Omar I edited the previous post it should have read WEREN't the same.. I should have proof read my post, It was a typo.. Sometimes my brain gets ahead of my fingers. When I opened the isolator to reblend the tanks the was a short equalization(my initial check of the day was done on the post without my spg), but it was longer than expected to account for the analysis. The anaysis (the day before) was done 1 hr after a top off with the last gas added (air), so the amount of gas added was VERY VERY small, I don't remember exactly but at most 50-150 psi, and the top offs are just blead into the tank... another note I didn't forget to purge the gas in the regs before analysis, so I was measuring the tanks not some left over gas.

 

[omar]

uh huh,

what else do you want to change?

omar

 

[blacknet]

Hello,

Oh my that was LOW. Quick someone turn on the AC I felt a draft.

BTW Just picked up my copy of the NOAA diving manual on CD and MAN is it jamed with info on an assortment of things. Has a very nice heliox/trimix/oxygen/nitrox section.

Ed