Question O2 sensor calibration

[LFMarm]

This document has been debated on this forum before.

However all cells are non-linear at some level ... Take all the cockamaimy schemes discussed on this post and run them on this set of cells and tell me how your stategy goes.

View attachment 907795

source: Understanding Oxygen Cells

I agree with you. For me, they need to be linear in the range I will be using them which is 0.5-1.6. I am happy to dive with sensors that flatten out past 2.0 like sensor 1 in your chart but would replace sensors 2 and 3.

 

[LFMarm]

Mods, please move this to Basic Scuba.


To check cell performance at any time, use your computer to select available gas that will produce PPO2 close to or equal to the target PPO2. Note the expected PPO2 and flush the loop. If you hit the expected target, your cells are linear up to that PPO2.

If cells read lower than expected, select the next leaner mix and try again. Determine the PPO2 that cells can read while staying linear and use that PPO2 to finish the dive on the loop, if you want to. Exercise common sense, if your cells are linear only to 0.5, staying on the loop is probably not the best idea.

If cells read higher than expected, there is something off. Select the best emergency procedure for the conditions and turn the dive.

This is the best procedure to figure out what is wrong once you see something is off (see the anecdote from a few weeks ago in a previous post). However I don’t do this as a routine to preserve dilout gas (a proper full flush at significant depth burns not a huge but a noticeable amount of gas) and because it takes a minute to do (again eating into short TTS in serious dives).

 

[mr_v]

@LFMarm, first of all, I am sorry. Somehow my unedited phrase referring to "basic scuba" got into the final draft of the message. It was uncalled for, even in my sarcastic mood.

Secondly, there is a lot to unpack here based on some of your recent replies.

Whether you check / calibrate daily depends on your personal preference. Many divers calibrate only once per trip. Some manufactures suggest a list of events that should lead to calibration, e.g., sensor replacement, new scrubber pour, change in altitude, change in reference gas. Exercise your best judgement.

Reference gas switches and dill flushes should not take much time or effort. If takes more than 30 seconds to select the best gas to check cell linearity and loop flush, you need to practice more. Your gasses (and your team gasses) should be in your computers. Tanks must have easily visible to you MOD stickers. All mechanical and computer work must be second nature. Gas supplies should always be adequate and on a conservative side, e.g., NSS CDS recommends 1.5x of estimated bailout needs.

If your computers or cells exhibit weird behavior on descend, troubleshoot to your best ability and then turn in the dive. It is unlikely that things will fix themselves or get better at depth. Do not set your controller to 1.5 set point, change cables, etc. There are many other procedures that can get you back safely.

 

[LFMarm]

No hard feelings — sensor calibration is definitely one of the basics for closed circuit.

I do all as you say — all hoses are marked with MOD and doing a flush is probably less then 1 minute. However that may be +10 mins of TTS if done at the bottom. This is why I will start doing the quicker tests by manually adding a little of O2 unless I detect a problem in which case I would definitely test with a dil flush and probably also start ascending.

 

[kensuf]

Gas supplies should always be adequate and on a conservative side, e.g., NSS CDS recommends 1.5x of estimated bailout needs.

Only in CCR only teams. In a mixed team, we recommend 2x.

 

[LFMarm]

Only in CCR only teams. In a mixed team, we recommend 2x.

Depends if you do team bailout or individual bailout. I do individual bailout and plan for a conservative SAC rate (4x at the bottom and 2x during deco to account for a CO2 hit) but only for one person.

 

[kensuf]

Depends if you do team bailout or individual bailout. I do individual bailout and plan for a conservative SAC rate (4x at the bottom and 2x during deco to account for a CO2 hit) but only for one person.

Team bailout for bottom gas in caves is stupid. I can see a case for deco gases because you would leave it unless you need it in the event of a buddy separation.

Team bailout for any gases is dumb in places like SE Florida with ripping currents.

 

[LFMarm]

I never liked the recommended calibration procedure for the ChOptima – it uses a lot of O2 as you have a free flow through the head and it's not precise because you get different calibrations depending on the amount of flow which is hard to get always the same (this caused the difference in calibration in the anecdote above).

I have now started using a different procedure also looking at the calibration procedure for other units:

1. Get the readings of the cells in air (very stable as they typically are in that state since the night before)
2. Calculate the expected readings in pure O2 based on the readings in air
3. Add O2 to the closed loop until I match those readings
4. Calibrate the computers

Advantages:

• Reading are very stable becasue the loop is closed and you do not need to rush to calibrate monitor and controller
• You use much less O2
• You already have the loop close to full to be able to do a positive check right after (in the original procedure the loop is open so you lose all O2 when closing it to perform positive)
• You can consistently reach the same ("right") calibration by checking against reading in air

 

[inquis]

2. Calculate the expected readings in pure O2 based on the readings in air
3. Add O2 to the closed loop until I match those readings

Unless I've misunderstood, this will not calibrate correctly, as it assumes zero deviation. Say there's a true deviation of 5% at a PO2 of 1.0 bar, so your cell reads 45.3V instead of the calculated 47.7V. If you continue to add O2 when calibrating until it reads 47.7V (and it will as the loop pressurizes, assuming it's not completely current limited), then the calibration factor recorded in the handset will be 47.7 V/bar rather then 45.3V/bar. During the dive when the cell is generating 45.3 V (an actual PO2 of 1.0 bar), the computer will think that's a PO2 of 0.95 bar. In other words, you're lacking the error correction for small deviations that calibration provides.

OTOH, you have the luxury of having a pressure pot, so you can measure the voltages for a true PO2 of 1.0 bar with minimal O2 usage. If you then calibrate when the sensors are showing *those* numbers, you'd be good to go and retain the various advantages you mentioned. (I'm assuming your pot either doesn't connect to your Shearwater or doesn't fit all the sensors at once, removing the simple option of calibrating with those outputs directly.)

 

[LFMarm]

Unless I've misunderstood, this will not calibrate correctly, as it assumes zero deviation. Say there's a true deviation of 5% at a PO2 of 1.0 bar, so your cell reads 45.3V instead of the calculated 47.7V. If you continue to add O2 when calibrating until it reads 47.7V (and it will as the loop pressurizes, assuming it's not completely current limited), then the calibration factor recorded in the handset will be 47.7 V/bar rather then 45.3V/bar. During the dive when the cell is generating 45.3 V (an actual PO2 of 1.0 bar), the computer will think that's a PO2 of 0.95 bar. In other words, you're lacking the error correction for small deviations that calibration provides.

I thought about this but concluded that if I look at the 3 sensors together and try to "match" all 3 at the same time, it will cancel out problems with the deviation. Assuming the 3 sensors have a random deviation not correlated with each other, if I aim for the average of the 3 sensors in O2 to match the calculated average based on the 3 sensors in air, it should cancel out. Is this reasoning flawed?

OTOH, you have the luxury of having a pressure pot, so you can determine the actual voltages for a true PO2 of 1.0 bar with minimal O2 usage. If you calibrate when the sensors are showing *those* numbers, you'd be good to go, retaining the various advantages you cited. (I'm assuming your pot either doesn't connect to your Shearwater or doesn't fit all the sensors at once, removing the simple option of calibrating with those outputs directly.)

True – I can calculate precisely the sensor constants based on linear regressions on many readings in the pot for pO2 between 0.5 to 1.6. This however I can only do before a diving trip (I don't bring the pot with me). Do you think that the constants from a few weeks before should still be the reference when doing daily calibrations?