Question O2 sensor calibration

[inquis]

Assuming the 3 sensors have a random deviation not correlated with each other

Not happening. Even if they were zero-mean (which they aren't), you'd be relying on the equivalent of not throwing three heads in a row.

Do you think that the constants from a few weeks before should still be the reference when doing daily calibrations?

If you do daily calibrations as you're describing, you will be storing the same conversion factor every time. That's certainly a waste of O2.

I do a variant of your approach to calibrate. While adding O2, I close the loop (let it fill a bit and mix), then open the loop. Then close, then open. When the loop is closed, lungs fairly collapsed (no inflation of PO2 due to pressure), and the displayed PO2 doesn't change, I stop adding. Usually takes about 4-5 cycles. I then take my time calibrating both computers. I also use an 80 of O2 instead of the onboard O2 source, as there's no sense in draining that little bottle.

 

[davehicks]

Not happening. Even if they were zero-mean (which they aren't), you'd be relying on the equivalent of not throwing three heads in a row.


If you do daily calibrations as you're describing, you will be storing the same conversion factor every time. That's certainly a waste of O2.

I do a variant of your approach to calibrate. While adding O2, I close the loop (let it fill a bit and mix), then open the loop. Then close, then open. When the loop is closed, lungs fairly collapsed (no inflation of PO2 due to pressure), and the displayed PO2 doesn't change, I stop adding. Usually takes about 4-5 cycles. I then take my time calibrating both computers. I also use an 80 of O2 instead of the onboard O2 source, as there's no sense in draining that little bottle.

I don't do daily calibration. I dive two three days a week, one or two dives each time. I generally change the O2 bottle and scrubber after three dives. That is usually when i calibrate, with the remainder of my O2 which is usually about 1/2 to 1/3 left. (1000-500 psi) If its been a few weeks since the last dive, or a big dive is planned I will pull a depleted O2 bottle from my fill-pile to calibrate.

 

[inquis]

Since calibration is just storing the conversion factor between mV and PO2, I don't calibrate unless I have evidence that needs to change. Unlike the typical approach, I flood each cell individually with O2 outside the loop (as opposed to trying to get a pure gas throughout the entire loop) as part of linearity testing before every day of diving. If the displayed PO2 is within a couple hundredths of expected (i.e., ambient), that conversion factor is close enough for me. I'll go months between cals if cells are behaving, and cell replacement has actually been the main calibration trigger for me.

 

[lostsheep]

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 ai

You calibrate with a constant flow of o2?

If so, who taught you that? Did you RTM?

.

 

[LFMarm]

You calibrate with a constant flow of o2?

If so, who taught you that? Did you RTM?



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I was thought to calibrate with a constant flow. Either with constant flow or short bursts, I find it hard to get to stable readings. If you get to ~1 and close the o2, values will start dropping below the actual constant (verified with a pressure pot). I find this procedure not very reliable but maybe it’s just me. Keen to hear how others feel about it.

 

[Snusmumrik]

I was thought to calibrate with a constant flow. ... Keen to hear how others feel about it.

Me calibrate cells on rebreather with constant flow (CMF) output connected to blind plug put on sensor housing entry. Another blind plug with small hole seals sensor housing exit.
CMF is set to metabolic rate of ~1lpm, sensor housing has volume of somewhat near glass cup (300ml), so i calibrate to 1.0 after 2-3 minutes of O2 flush, then shut off O2, remove blind plugs and flush sensor housing with air watching PPO2 drop speed and end value.

 

[makar0n]

I was thought to calibrate with a constant flow. Either with constant flow or short bursts, I find it hard to get to stable readings. If you get to ~1 and close the o2, values will start dropping below the actual constant (verified with a pressure pot). I find this procedure not very reliable but maybe it’s just me. Keen to hear how others feel about it.

On this one, I saw someone taking just the head, attaching O2 as normal, then covering the head with a round tupperware cover, one with a rubber seal. Seemed to work pretty ok.

 

[Dsix36]

I was thought to calibrate with a constant flow. Either with constant flow or short bursts, I find it hard to get to stable readings. If you get to ~1 and close the o2, values will start dropping below the actual constant (verified with a pressure pot). I find this procedure not very reliable but maybe it’s just me. Keen to hear how others feel about it.

Calibrating with any flow at all is asking for the final result to be flawed, since flow equals pressure and thus pressure will increase the reading. I realize this may not be a very significant difference but it also depends on the amount of flow. I have never seen or heard of it being taught this way and personally think it a foolish and dangerous way to calibrate. Calibration should always be done at ambient pressure without any flow. The head plugs for the Choptima make this very easy since the exit plug has a mushroom valve to allow freeflow without any reverse once O2 is added and flow is terminated. You can also do it via a head only method using saran wrap with a rubber band holding it in place too (make sure to use the vented cap and add via the solenoid hose).

 

[lostsheep]

Calibrating with any flow at all is asking for the final result to be flawed, since flow equals pressure and thus pressure will increase the reading. I realize this may not be a very significant difference but it also depends on the amount of flow. I have never seen or heard of it being taught this way and personally think it a foolish and dangerous way to calibrate. Calibration should always be done at ambient pressure without any flow. The head plugs for the Choptima make this very easy since the exit plug has a mushroom valve to allow freeflow without any reverse once O2 is added and flow is terminated. You can also do it via a head only method using saran wrap with a rubber band holding it in place too (make sure to use the vented cap and add via the solenoid hose).

I am in complete agreement with you about calibrating and the usage of caps but your statement that flow equals pressure is literally completely wrong; pressure drops with flow.

See the Bernoulli Principle. Flow rates while calibrating are very low so I would expect the difference To be negligible but Flow is not increasing pressure.

 

[inquis]

your statement that flow equals pressure is literally completely wrong; pressure drops with flow.

Depends on the orientation of the flow, laminar vs turbulent, fill & exit rates, etc. I agree for low enough flow it's negligible, but aren't we talking about manual addition?