MOD calculations

[wb416]

The majority of my diving is in freshwater lakes and quarries. In your opinion, is it acceptable/practical to calculate the MOD's using ffw (34) vs fsw (33)?

I've been arguing the point with myself along some of these lines:
*Why not just use fsw for the conservatism, but this seems to imply that it's okay to dive in saltwater w/o the conservatism

*Just use one set of calcs to keep things simple, because the delta in minutes between %O2 called out by ffw vs fsw is pretty insignificant (for example 125ft).

*I currently carry a spreadsheet with all the MOD's between 21% & 50% and it contains calcs for 1.6, 1.4, & 1.2 in both ffw & fsw, so it's not hard to figure out.

*I'm not 'mix certified (yet), but I've noticed that GUE has a list of standard mixes for various depth ranges... implying that simplification might be the key.

What are caveats that I'm leaving out? Am I getting too hung up in the beauty of the math? Since I dive fresh water (primarily), should I stick with calcs for my environ? Thoughts, opinions, comments?

tx, cwb

 

[MikeFerrara]

The math would be correct if you used FFW instead of FSW but why bother? IMO, it's splitting hairs in this case. Standard gases can simplify the task allowing you to memorize mod and exposure limits but doing the calcs is not a big deal. Sometimes a "standard" doesn't fit the bill. Depending on the profiles and number of dives you may want to drop the f02 to limit exposure. When teaching I am often doing a bunch of dives, sometimes in 40 degre water (or colder) and possible increased exertion. Its hard for someone elses standard gases to meet my needs. We sort of have our own standard gases for the dives We do alot. There are lots of ways to simplify planning. Sometimes I dive EANx but use the air tables. Lets say I ask for 32% but get 30.5%. I could have them fix the mix or I could recalculate. Or... since I know the mod for 32% I can stay avove that, which I was planning on anyway, and since I'm diving air tables I'm still ok there.

 

[Ontario Diver]

What is important is that you remain consistant across all of your devices and calculations. Either keep everything in FSW or everything in FFW.

Since my depth guage and all of my tables are calibrated in FSW (even though I dive in FW), I keep my 'puter and do all of my calcs in FSW to remain constant.

 

[omar]

As you are probably aware if you round a number and than do subsequent calculations with that number you are propagating the error and typically causing the error to increase. I do my calculations all the way in the appropriate units and than round at the end. I prefer to "know" what the answer is. However, all these equations are just an approximation at best of what is going on with the interaction of the body and the depth and gases. Also tables and the series of equations used to generate them are just approximations of the biophysical processes as well.

That being said, I have been cautioned in the past when I first got involved in technical diving that there is such a thing as math narcosis.

omar

 

[Uncle Pug]

Math Narcosis.... now that would be a dark narc for me!

Say... the other day I was trying to do an impersonation of Pete's *vertical position of smugness* and I accidentally let my feet get below the MOD...

Well first my left foot starts doing the funky chicken.... and then my right knee started doing the hokey pokey... and then my butt started....

Anyway I was sure glad the upper half of me was above the MOD or I would have been a goner... now I know why Pete prefers the *vertical position of smugness*....

Seriously (You didn't think the above was serious did you? You did? Oh my! You really are cutting that MOD thing razor thin aren't you?!).... but seriously:

KISS.... and use standard gases.... makes keeping track of things a lot easier....