Calibration question

[rjack321]

I usually calibrate roughly 2x month. But if I move my cells between units I obviously have to recalibrate. But I am also cognizant of the mVs in air and 100%. At my dive rate I see drift over the 3-5 month timeframe which is way longer than I would go between calibrations, so I don't think the concept of "don't calibrate so you can see drift" idea would work for me. That is certainly NOT how we use every other scientific instrument. We calibrate pH meters, Cl- meters and all that kind of stuff (for work) on a regular schedule based on when it was last calibrated and if any drift was expected. The concept of intentionally having less than the most robust measurement possible because you intentionally avoided calibrating is crazy to me.

I calibrate both primary and secondary displays at the same time using the same known 100% source gas. If I am diving multiple days in a row I don't recalibrate unless I see something wonky. If the unit has sat in the garage for a couple weeks I recalibrate.

 

[JohnnyC]

Tom what Meg electronics are you running? My 2.7 electronics never had a 2-point calibration.

I don't start my dive at .7, I go to .7 to check injection on my SF2 when I build the unit then flush the loop and hold a 1.0 on the surface before descent so I can validate on the way down. I'll shut off the ADV down to 6m and validate that way at the start of the dive so I know that it's not current limited from the start. I'll squirt O2 if loop volume gets too low.

Andy Fritz has some weird operating paradigms, and his recommended use of the Pelagian is to O2 flush the loop and descend on OC on the BOV to 6m to make sure the cells validate at 1.6. I don't agree with starting off on OC, but the concept of validating on descent holds merit. Since it's a leaky valve you don't have to worry too much about loop volume dropping to the point where it would trigger the ADV automagically.

 

[rjack321]

Tom what Meg electronics are you running? My 2.7 electronics never had a 2-point calibration.

Apecs 2.7 has 2 point calibration. If you are at the "do you really want to calibrate screen" and hit yes it does the air point. If the cells are over 13mV at that point it will give you an error too.

 

[JohnnyC]

Apecs 2.7 has 2 point calibration. If you are at the "do you really want to calibrate screen" and hit yes it does the air point. If the cells are over 13mV at that point it will give you an error too.

Huh, that's interesting. Mine only ever let me input 70-100% O2. It never let me do anything with air.

 

[rjack321]

Huh, that's interesting. Mine only ever let me input 70-100% O2. It never let me do anything with air.

The first "are you really sure" button press does double duty as the 21% air point. If you have any elevated O2 in there it balks, don't ask me how I know lol

The second button press is the O2 high point with 70-100% as set in the other menu.

 

[JohnnyC]

Ah, I gotcha. That makes sense then.

I know I calibrated once with the O2 off during my MOD 1. It didn't like that. Took me a second and my instructors s***-eating grin to figure it out.

 

[joshk]

Andy Fritz has some weird operating paradigms, and his recommended use of the Pelagian is to O2 flush the loop and descend on OC on the BOV to 6m to make sure the cells validate at 1.6.

How well does this work with a hypoxic mix?

I don't think the concept of "don't calibrate so you can see drift" idea would work for me.

If you turn the unit on and it's reading the same thing as it was last time (air/O2 surface/O2 at depth before and after dive) what is clicking calibrate on the handset helping? Honest questions here

also remember that your specific barometric pressure is going to change the air calibration which is why I don't worry if my cells are spitting out .19-.23 in air.

+1, I said this earlier but you have to include the pressure into these calculations if you want to have better than ballpark accuracy.

@doctormike Shearwater felt that the pressure sensor in the NERD 2 drifting up to 50 mbar was unacceptable and redesigned the computer to use a new pressure sensor. 50 mbar is a difference of 20 inches of water. If your two computers don't read the same at the same depth I would contact Shearwater.

 

[JohnnyC]

How well does this work with a hypoxic mix?

The Pelagian manual actually quotes, "...descend to 6 m while breathing from your off board bail-out regulator or BOV you will bring that known gas sample to a known depth and the result should be around 1.6 BAR Po2.... This should ensure that each dive is started with a confirmation that the sensors are linear from 0.21 to 1.6 BAR PO2 plus confirm that they are not current limited."

If your BOV is plumbed to hypoxic bottom gas, just use something else that's safe to breathe.

 

[joshk]

If your BOV is plumbed to hypoxic bottom gas, just use something else that's safe to breathe.

I suppose you could alternatively breath off a 100% Oxygen loop on the decent and not add any dil. But what do I know, I'm not writing rebreather manuals

 

[JohnnyC]

I suppose you could alternatively breath off a 100% Oxygen loop on the decent and not add any dil. But what do I know, I'm not writing rebreather manuals

It's what I do, O2 purge the loop. I'm assuming that his idea is that you're guaranteed a fully O2 purged loop since people seem to want to debate how fully one can O2 purge a loop.

He also designed a rebreather that only uses two cells. I get the idea behind it, I just don't agree with it, hence added a third cell to my unit.