Question How to check an analyser O2 cell?

[Agro]

My Divesoft analyser seems to give wrong measurement even after 3 point calibration. But sometimes it seems to be OK again. So I do not trust the O2 sensor anymore although it is only 15 month old.

In the Divesoft Liberty rebreather you can check the sensors by making an overpressure chamber out of the head. In a analyser this does not work. Is there any easy way to put the sensor in an overpressure situation and measure pO2 of 2 or 3 bar?

My idea is the same as Divesoft has for the rebreather: if sensor is working fine at 3 bar it will work fine at 1 bar. If sensor is working fine at 1 bar but not fine anymore at 1.5 bar then I can expect the sensor to fail at 1 bar within a short time.

 

[grantctobin]

Build a cell checker.

One potential set of instructions here: Building The Under $50 O2 Cell Checker - wrecklessdiver.com courtesy of @macado . Otherwise, there's a number of threads on RBW about it.

 

[Searcaigh]

@Agro

Are you referring to the Divesoft He/O2 analyser ?

I usually change the O2 sensor annually, they don't appear to last long in my part of the world, even when sealed in a "Pelican" style box.

 

[broncobowsher]

What are your 3 points on your 3-point calibration? If 2 of the points are really close the little error gets magnified.

I just run a 2-point calibration. Air and 100%. No issues with that. Don't do any hypoxic mixes that would need a calibration point that far under air. Any mix between air and 100% is just a mix that has been measured by the same analyzer and really can't be used as a calibration reference.

 

[macado]

I would just swap out the sensor at this point since it's 15 months old. What are you mV readings in Air and O2? You could test it in chamber but it's probably just easier to buy a new cell.

Pure speculation here but if you're not getting consistently readings the sensor might be non-linear but you can also check the molex connector and make sure it's seated properly and that there is no corrosion on the pins.

I usually just do a 2-point calibration but Divesoft seems to recommend your 3-point calibration be on a mix without any O2 if you're diving hypoxic mixes.

 

[Agro]

3 points are: O2/air/He

Exchanging the sensor is an option. But I'd like to check the sensor, no matter if it's old or brandnew. Even a new sensor may fail, that's what append 2 times on my Liberty rebreather.

I'll have a look at the Building-Cell-Checker link tomorrow, looks good.

 

[rjack321]

3 points are: O2/air/He

Gahhhh this is awful!!

Unbreathable 100% helium in an unmarked scuba cylinder? No no no

This is how people DIE.

 

[macado]

Gahhhh this is awful!!

Unbreathable 100% helium in an unmarked scuba cylinder? No no no

This is how people DIE.

I know you're referring to the video in question given the recent tragedy that occured as a result of breathing pure helium. I just do my analysis straight off a T bottle of helium with an adapter to my analyzer but I'm fairly certain that he just did this for the ease of making the video but your point is taken.

TL;DR - Dont put pure helium in a unmarked / unanalyzed scuba cylinder. I know we're both being pedantic here but the unmarked bottle of helium is the issue.

 

[Agro]

Divesoft.com

eshop.divesoft.com

I already have this cell tester for the Liberty. The Liberty head is turned into a pressure chamber. So my idea is to put my analyser's sensor into this pressure chamber, wiring to a mV reader and that's it. I guess the sensor can be anywhere in the head but I must find a way to bring the cable out of the head. This probably means a cable gland in the steel plate.

What do you think about this idea?

 

[broncobowsher]

For a rebreather, having a cell that reads above one atmosphere partial pressure is a must.
Not sure why you need that overpressure for an analyzer?