Were you taught this math in your entry level CCR class?

Were you taught this math in your entry level CCR class?

  • Yes

    Votes: 25 61.0%
  • No

    Votes: 15 36.6%
  • I don’t know

    Votes: 1 2.4%
  • Yes but I did not understand it

    Votes: 0 0.0%

  • Total voters
    41

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

Capt Jim Wyatt

Hanging at the 10 Foot Stop
Staff member
ScubaBoard Business Sponsor
Scuba Instructor
Messages
4,522
Reaction score
4,181
Location
High Springs - Cave Country
# of dives
5000 - ∞
Answer the poll please.


F6E428E8-D8DB-43CB-8968-629C7D1D6ECB.jpeg
 
That was foundational. I almost voted yes but didn't understand it because I've been realizing it's value more and more.

I know it's not taught in some classes because I ask every rebreather diver I talk about it to gain better understanding and application. Some have no memory of this whatsoever...
 
Answer the poll please.


View attachment 429310
I disagree with the mv range in pure O2.
If the range on 8-13mv is used the range of mv in O2 should be 38 - 62mv (38.02 to 61.88 for you pedantic types)
 
I disagree with the mv range in pure O2.
If the range on 8-13mv is used the range of mv in O2 should be 38 - 62mv (38.02 to 61.88 for you pedantic types)

I'm on prism topaz cells. Even higher, like the mk15 etc.
 
I'm on prism topaz cells. Even higher, like the mk15 etc.
Not the actual voltage just the fact that no matter what the air value is the O2 value will be 4.76 X that value.
8 X 4.76 is 38.02 not 43 as shown on Jim's wetnotes.
 
Not the actual voltage just the fact that no matter what the air value is the O2 value will be 4.76 X that value.
8 X 4.76 is 38.02 not 43 as shown on Jim's wetnotes.
I see what you mean, thanks. Good catch.

I think that may very well highlight why some instructors steer away from MV and checking cells like this and just suggest frequent cell replacements. Too easy to mess up the math with life altering consequences.

Regards,
Cameron
 
I see what you mean, thanks. Good catch.

I think that may very well highlight why some instructors steer away from MV and checking cells like this and just suggest frequent cell replacements. Too easy to mess up the math with life altering consequences.

Regards,
Cameron

Being a math nerd, I had noticed that as well. I just figured I didn't know enough about the subject.

It seems like doing these calculations for you would be a feature someone would add to a CCR dive computer.

I mean, if you calibrate at 0.21 ATA, then check it with 1.0, the computer could have an alternate screen to go to to show you calculated versus metered mV values and also show linear deviation. Ditto for checking at 1.6 ATA.

Or something like that. Bottom line: It's a straightforward process and simple arithmetic (or so it seems), so why not have your computer do it for you?
 
Last edited:
I see what you mean, thanks. Good catch.

I think that may very well highlight why some instructors steer away from MV and checking cells like this and just suggest frequent cell replacements. Too easy to mess up the math with life altering consequences.

Regards,
Cameron
Due to the potential for a diver to fail at getting a good O2 flush before calibration I have my students calculate the expected mv in O2 based on the air mv reading. That way they have a sanity check before pushing the button to commit to the calibration.
 
I was taught this in my basic MOD 1, but Jim's arithmetic error in his post is exactly why I don't teach it for underwater use to a new CCR diver. It's too easy to make a math mistake, and look for some value that ends up being erroneous.

With modern CCR electronics, there is absolutely no reason to do this underwater, as with a few button pushes, you can get your FO2 for your given depth and easily validate your cells off this value and a dil flush.

Dave, to expand upon your methods, and avoid the poor O2 flushes - I teach to do the o2 flush, calibrate, and then either flush with your dil, or expose to ambient air. If the cells don't show .21 afterwards, then you didn't get a good flush.
 
3140182F-0843-435F-B327-A63C0D2BEC4E.png
656CE9DB-BFBD-4209-850E-1A0734FFE0CC.jpeg

The numbers 8-13 / 43-62 are numbers I memorized many years ago, and I do not recall doing the proofs as I should have. Thus the egg.

I now use R-22 cells and the output in air is 7-13.

In any event this is the way to proof and predict cell output mV in cells, but do the math correctly.
 

Back
Top Bottom