Question O2 sensor calibration

[LFMarm]

Hello,

I am having doubts about my procedure for sensor calibration with my ChOptima. I calibrate following the procedure described by DiveRite that consists in having a steady low flow of O2 go through the head and calibrating once the mV stabilize.

When doing O2 flushes at the 6m deco stop, I am never able to get to 1.6. The readings are typically about 5-10% lower: i.e., if at 6m I can only get readings to 1.5 (vs expected 1.6), at 5m to 1.4 (instead of expected 1.5) and so on. The sensors are all linear and rather new both in terms of dived hours and of manufacturing date.

I am not too concerned because with readings 5% lower, I am actually on the safe side of decompression. However I would like to understand what is causing this.

Some potential causes:

• Something wrong with calibration?
• Sensors not reading correctly because of humidity in the loop towards the end of the dive
• Unable to fully flush the diligent out of the loop with O2 (so sensor measuring correctly a loop that is not pure O2)
• Need to decrease the correction factor in the Shearwater settings? Currently using 0.98 while calibrating with pure O2

Many thanks for sharing your thoughts.

 

[mr_v]

It sounds about right. By the time you get to deco stops, you probably have some moisture close to your cells and the loop is probably warm. That may affect your readings.

My Meg shows tighter calibrations and is a lot closer to 1.6 PPO2 at 20' flushes compared to the rEvo with the same cells and the same flush profile (i.e., deco ledges at Ginnie or other caves I frequent). I suspect that could be due to rebreather design, as Meg cell tray is on top of the unit and it does not seem to collect condensation as well as rEvo does. rEvo cell surroundings are always extra moist.

My readings in both rebreathers are > 1.54 with Meg being close to 1.58-1.6. Oh, I always want to check the calibration at the beginning of the dive and forget to do that most of the time. I would not worry about such small differences provided that everything else is working well. Do you check your cells at lower PPO2 levels? How do they perform?

P.S. Not sure if you can do this on Chop, but on a Meg I crush the lungs and get a sense that I typically get better, quicker flushes than I do on a rEvo.

 

[Dsix36]

It is difficult to fully crush the Chopitma lungs during a flush well enough to get to 1.6 without doing more fliushes than anticipated. It can be done, but not worth it for such a small difference.

 

[LFMarm]

Oh, I always want to check the calibration at the beginning of the dive and forget to do that most of the time. I would not worry about such small differences provided that everything else is working well. Do you check your cells at lower PPO2 levels? How do they perform?

It's a good suggestion. I should start doing dil flush checks at the beginning of the dive.

P.S. Not sure if you can do this on Chop, but on a Meg I crush the lungs and get a sense that I typically get better, quicker flushes than I do on a rEvo.

I try to flush from below minimum loop volume to make it quicker. Not sure I am able to get enough diluent out to get to pO2 close to 1.6.

 

[LFMarm]

It is difficult to fully crush the Chopitma lungs during a flush well enough to get to 1.6 without doing more fliushes than anticipated. It can be done, but not worth it for such a small difference.

I agree with you. I would not waste more O2 doing repeated flushes to get the last 10% of diluent out. I just want to make sure I am not doing something wrong with settings or calibration procedure.

 

[inquis]

Yes, it's water vapor. Water vapor saturation pressure at body temperature is 47.1 mmHg = 0.06 bar. A fraction of 6% at 6m/1.6 bar is a partial pressure of 0.1 bar, leaving 1.5 bar for other gases. (One can quibble about the temp, but it'll be pretty close.)

 

[rjack321]

I agree with you. I would not waste more O2 doing repeated flushes to get the last 10% of diluent out. I just want to make sure I am not doing something wrong with settings or calibration procedure.

You're not doing anything wrong. To really get all the way up to 1.6 at 20ft takes a serious effort, lots & lots of O2 (to basically dry out the loop), and some breath holding. It's usually do-able in a cave. Midwater forget it.

 

[inquis]

Best bet is to see if you hit a PO2 of 1.6 with a humid loop at 7m.

 

[bardass]

with my Triton (M3S), I am never able to get to 1.6b at 6m
Only 1.5b.
that suits me.

 

[mr_v]

Poor flushes may be one of the reasons for not getting very close to 1.6 at 20' (6m). To do a proper flush you must turn you head all the way up, as if you were kissing the sky, and hammer O2 for ~5 seconds while flushing the loop. That approach will fill the loop with cold O2 and flush out water vapor. If you're in a position to get more O2 after the dive, try doing that and see if your numbers improve.