Gas Analysis Technique

[msande21]

I was on a dive boat last week and watched a diver analysing Nitrox mix through the second stage demand valve.

Never seen this before but it looked straight forward on wondered if there was a technical reason why this technique is not more widely used?

[Finless]

In my case, I do it that way because my analyser is home built (by Stuart) and I don't have the widget to fit in the cylinder valve.

Also, it's a real PITA undoing the 1st stage when the kit all together and ready to go.

To answer your question, I suspect it is for convenience.

[Lou]

I would suspect it is because the reading will be, to a varying degree, inaccurate. You will be blending air and nitrox on the fly in the 2nd stage as you cannot get a seal on the gas source.

Depends how "accurate" you want to be though - I have to say I wouldn't do it like that personally.

[Woz]

Originally Posted by Finless

In my case, I do it that way because my analyser is home built (by Stuart) and I don't have the widget to fit in the cylinder valve.

Also, it's a real PITA undoing the 1st stage when the kit all together and ready to go.

To answer your question, I suspect it is for convenience.

Van der Graf sell a doo-hicky that screws onto your sensor. You shove it on your cylinder, whack a load of random gas through then the doohicky keeps a slug of it which you then analyse at your convenience a few seconds later. It's idiot (well, me) proof.

[David]

Originally Posted by Finless

I don't have the widget to fit in the cylinder valve.

The widget is a flow restrictor and it also helps give a more stable reading. So Finless should be right.

The other thing that comes to mind is the diver just plain forgot to do it first and picked the lasy way to check his gas. I mean he already knew what was in the cylinder so anything close would confirm his mix I guess

David

[SoggyFox]

i can remember being shown a way to analyse nitrox using a plastic bag and a second stage. seemed to work ok and showed the same readings as if your were using the valve.

[Gareth J]

The advantage of measuring at the second stage. (I do this by putting the second stage & sensor in a bag with a few holes in it). Is that you are unlikely to raise the gas pressure into the sensor above ambient.

Some of the sensors that screw directly into the cylinder can raise the pressure at the sensor above ambient. This changes the reading.

The vadagraph has such a large diameter that this isn't normally a problem (,unless you put your finger over the hole).
Others attempt to use a flow restrictor to control the flow, but this does not control the pressure, especially if you open th evalve to far!

If you are interested in seeing what happens when the pressure is above ambient put your finger over the hole or restrict the gas leavingthe sensor, the % reading will increase. (BE CAREFUL NOT TO RAISE THE PRESSURE TO FAR YOU MAY DAMAGE THE ANALYSER).

Gareth

[Finless]

Originally Posted by Woz

Van der Graf sell a doo-hicky that screws onto your sensor. You shove it on your cylinder, whack a load of random gas through then the doohicky keeps a slug of it which you then analyse at your convenience a few seconds later. It's idiot (well, me) proof.

It's quite an expensive doohicky though, isn't it?

I wrap a 'zip bag' round the post and inflate it (reg also inside the bag) once the contents are flushed through I then take the reading - seems to work OK. I mean, a plastic bag isn't going to suck air back in again is it?

Anyway, I have the blenders measurement to correlate to.

[Woz]

Ummmmm I think it was about £15. Which seeing as the Oxyguess and cells were FOC was not much to fork out.