Calibration question

[joshk]

I know this will shock some people but cells that have been moistened in the head for a 3-4 hour dive just don't respond as well as cells that are dry.

As a result, I'll calibrate on the first diving day after a break with dry cells because it's what's available. However, I'll also re-calibrate immediately post dive with normal moist cells. On the second day I find that all three cells will read very consistently with each other - far better than on day 1 with the dry calibration - and with much closer alignment with what is actually in the loop under real world conditions.

I've found this approach helpful as well. It gives me validation at depth, correct surface air outputs, surface O2 outputs, etc. Dry stable cells don't calibrate well since we're using them in a humid pressurized environment.

 

[doctormike]

I haven't read the manual in years, plus I'm a guy lol

I don't think I could track all the pressure and mV readings simultaneously while pushing 2 buttons. Plus at least on my Meg handset the mV reading screen will time out before the chamber is flushed. So I need even more fingers to toggle back to that. I just flush a whole ton of chamber volumes one at a time.

You don't really need to track pressure readings while you are doing it, you basically are just pushing the in and out button together so that gas flows at a steady rate through the pot. I guess they are designed to be approximately the same flow rate, so that it raises the pressure to some point over ambient, but it doesn't keep rising.

I do wish that there was some way of "locking" the mV display on the Shearwater controller. It times out too quickly, and that means many button presses for each pressure level when you are testing. I don't see why that would be a safety issue. They did incorporate that 0.19 setpoint that I use to keep the solenoid from firing when I'm uploading log data or doing this sort of test.

 

[doctormike]

I've found this approach helpful as well. It gives me validation at depth, correct surface air outputs, surface O2 outputs, etc. Dry stable cells don't calibrate well since we're using them in a humid pressurized environment.

That's a good point. So why aren't we taught to calibrate post-dive instead of during the pre-dive build?

 

[joshk]

I do wish that there was some way of "locking" the mV display on the Shearwater controller.

Careful what you wish for. NERD 1 is no longer receiving updates and the last update was in 2015. Asking for a “locked” mV display could force us to only use Shearwater Cloud™ for divelog uploading! I speak in jest, but Shearwater makes a really usable dive computer and everything else isn't much to write home about.

That's a good point. So why aren't we taught to calibrate post-dive instead of during the pre-dive build?

Good question. I don't want to speculate on the answer since instruction isn't my wheelhouse. But you might want to try it the next time you're at Dutch and doing two dives. Do your normal calibration sequence pre-dive, and then between dives do another calibration sequence. Check the mV outputs during calibration (surface air, surface O2) as well and write them down. Then compare PPO2s on the second dive, focusing on 20' with an oxygen flush and depth with a diluent flush. I'd expect the numbers to be closer to “correct.”

 

[doctormike]

Do your normal calibration sequence pre-dive, and then between dives do another calibration sequence. Check the mV outputs during calibration (surface air, surface O2) as well and write them down. Then compare PPO2s on the second dive, focusing on 20' with an oxygen flush and depth with a diluent flush. I'd expect the numbers to be closer to “correct.”

You mean compare mV on the second dive? On an eCCR the PPO2s would be basically close to high setpoint no matter what was happening with the output. The difference would be what you had recalibrated the conversion factor to be.

 

[rjack321]

That's a good point. So why aren't we taught to calibrate post-dive instead of during the pre-dive build?

Are you going to hold onto your partially completed checklist for weeks?

 

[doctormike]

Are you going to hold onto your partially completed checklist for weeks?

I’m not going to make this change, but if I did, I would probably just add a post dive calibration and take it out of the build checklist.

Basically if we are saying that wet calibration is more operationally accurate than dry calibration, then that’s when we should do it...

 

[joshk]

You mean compare mV on the second dive? On an eCCR the PPO2s would be basically close to high setpoint no matter what was happening with the output. The difference would be what you had recalibrated the conversion factor to be.

I mean compare PPO2s, mVs doesn't matter underwater. When you do an oxygen flush at 20' my hypothesis is you'll hit 1.6 accurately instead of only reaching 1.5. And when you do a diluent flush at depth, say with air at 3 ATA, you'll get an accurate .63 whereas it might have been lower with a dry calibration.

Set your controller to 0.5 for the dives so it doesn't interfere with this.

Are you going to hold onto your partially completed checklist for weeks?

Who fills out checklists?

 

[doctormike]

I mean compare PPO2s, mVs doesn't matter underwater.

OK, I misunderstood what you were saying. You are saying to compare PPO2 during dil and O2 flushes at known depth.

But at high setpoint, comparing mV would show a difference as well (basically, you would see what your conversion factor was reset too). There should be a difference, because there is a difference in output between wet and dry sensors.

 

[joshk]

But at high setpoint, comparing mV would show a difference as well (basically, you would see what your conversion factor was reset too). There should be a difference, because there is a difference in output between wet and dry sensors.

While you're correct, all you'd be checking is that your calibration is correct and your computer is still multiplying X * Y to give you PPO2. I can't think of a single thing checking mV underwater will tell you that the PPO2 display won't tell you.