missing helium?

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lermontov

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ok so ive got a mystery I cant solve maybe others can help work it out

on a recent trip I mixed a 17/38 starting with 1 BAR and 21% (empty cylinder)
the calculator stated :
81 BAR He
Fill to 92.1 BAR 02
Fill to 200 Bar air

but when analysed I get 17/20 -thinking I must either have a contaminated He source or somehow dropped some gas , cross checked the He supply and i have over 99%, cross checked the pressure i have 200Bar

second option was the analyser was wrong so cross checked that with another gas source -air ( ok) helium ( ok ) 02 ( ok) cross checked with another analyser and also read 17/20

so if I do the math
81Bar He /200 bar fill = 40.5%
11.1 02 / 200 = 5.6%
air of 108 Bar =108X.79 n2 = 85.3 Bar of N2 / 200 = 42.6%
and 108 Bar x.21 02 = 22.7 Bar of 02 /200 = 11.3% o2

total Gasses are
40.5% He
42.6% N2
16.9 % o2
totals 100%

but analyser says 17/20

the o2 content is spot on so that implies the N2 is also spot on
so wheres my helium gone?
 
ok so ive got a mystery I cant solve maybe others can help work it out

on a recent trip I mixed a 17/38 starting with 1 BAR and 21% (empty cylinder)
the calculator stated :
81 BAR He
Fill to 92.1 BAR 02
Fill to 200 Bar air

but when analysed I get 17/20 -thinking I must either have a contaminated He source or somehow dropped some gas , cross checked the He supply and i have over 99%, cross checked the pressure i have 200Bar

second option was the analyser was wrong so cross checked that with another gas source -air ( ok) helium ( ok ) 02 ( ok) cross checked with another analyser and also read 17/20

so if I do the math
81Bar He /200 bar fill = 40.5%
11.1 02 / 200 = 5.6%
air of 108 Bar =108X.79 n2 = 85.3 Bar of N2 / 200 = 42.6%
and 108 Bar x.21 02 = 22.7 Bar of 02 /200 = 11.3% o2

total Gasses are
40.5% He
42.6% N2
16.9 % o2
totals 100%

but analyser says 17/20

the o2 content is spot on so that implies the N2 is also spot on
so wheres my helium gone?
Very curious.

I am literally clutching at straws here, but if you're diving twins is there any chance the isolator was off and you only filled one post with the other post being pre-filled air.

Apologies, I know this is highly unlikely, but I can't think of any sensible reason why half of your He would vanish.
 
I checked through the maths on excel, and got to the same answer, so that is out of the window. I would try to check the trimix analyser against a known content source to eliminate the possibility of the analyser malfunctioning.
 
Very curious.

I am literally clutching at straws here, but if you're diving twins is there any chance the isolator was off and you only filled one post with the other post being pre-filled air.

Apologies, I know this is highly unlikely, but I can't think of any sensible reason why half of your He would vanish.
good point but no -one cylinder was a 12 l and the other 3 litre
 
I'd expect both He and O2 to be off. But O2 okay and He 20% off?
What about the chance of temperature differences during filling, e.g. rapid helium transfill?
How long did the tanks cool down in between fills?

If the Helium would have escaped somehow (on a molecular level), the O2 percentage would go up over time. So that seems unlikely as well, especially if you see this with 2 different tanks.
 
Compressibility factors for the gases are different. You have to let the cyl cool to ambient before adding a second and third gas. Fill SLOW. Allow several hours before analysing to allow mixing
 
A friend of mine had a similar situation years ago. Helium analysis came out low, but oxygen content was fine.

Turned out his tanks ended up with argon replacing that missing helium. He almost drowned from the narcosis.
 
A friend of mine had a similar situation years ago. Helium analysis came out low, but oxygen content was fine.

Turned out his tanks ended up with argon replacing that missing helium. He almost drowned from the narcosis.
But where would the argon come from in this situation?
 
But where would the argon come from in this situation?
It was bought and sold by the dive shop as suit gas. The outlet fitting on inert gas tanks (n2, argon, helium) is identical. So the dive shop inadvertently used the wrong supply gas to the booster and put the argon in his breathing tanks
 
https://www.shearwater.com/products/swift/
http://cavediveflorida.com/Rum_House.htm

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