Quiz - Recreational Dive Planner™ - Pressure Group After Second Dive

[TMHeimer]

I'm only familiar with the imperial RDP. I assume there is a (slight?) difference in how things work with metric (due to rounding, lack of decimal points perhaps?)? If so, could you be slightly under the NDL with one and slightly over with the other?

 

[loosenit2]

The scattering of answers so far on this one is interesting....

pretty good indicator of how many folks do not keep current with tables and thus experience skill atrophy. i'm probably old fashioned but I still do a table calculations after every dive, both to maintain the skill and to have my residual nitrogen time documented in case of an unforeseen incident (i.e. computer break etc), that way I can continue to dive.

 

[Pedro Burrito]

Spoiler: Answer

d. Metric V - Imperial X

 

[Brett Hatch]

The folks who voted "I" scare me. It's clear how they got there -- by not taking into account the second dive at all. Yikes.

 

[jonhall]

The scattering of answers so far on this one is interesting....

Mine was one of the scattered. At 6 this morning, I hurriedly looked at the question (or sleepily) and answered only up to after the safety stop. I wanted to change my answer but couldn't!!!!!!!!

 

[scubadada]

Mine was one of the scattered. At 6 this morning, I hurriedly looked at the question (or sleepily) and answered only up to after the safety stop. I wanted to change my answer but couldn't!!!!!!!!

It was a significant improvement when @Pedro Burrito changed the voting mechanism on April 13 to no changing vote and don't see results prior to voting This made it like a real quiz/test . Commit yourself to an answer and give it. I missed a question several days back because I read it quickly and answered without thinking about it. That's part of the exercise, and teaches a lesson.

 

[TMHeimer]

IF you know tables, how can you get it wrong? Like Mrs. Harmon said in my Grade 7 Science class-- check over your work.

 

[-JD-]

OK, first in with Metric vs Imperial for conservatism ...

Trying not to give any spoilers, but 9-10 minutes (13%ish ) of calculated difference in bottom time on the second dive to get the same PG outcome between metric and imperial is ... remarkable.

First, my 13% difference above was calculated based on the effective TBT (Total Bottom Time) including the RNT (Residual Nitrogen Time).

I'm only familiar with the imperial RDP. I assume there is a (slight?) difference in how things work with metric (due to rounding, lack of decimal points perhaps?)? If so, could you be slightly under the NDL with one and slightly over with the other?

Doing the calculations on the imperial table, RNT is 31 and you end up with a TBT of 76 in the 50' column, r-up to 80 -> group X. 80 is NDL for 50'

With the metric, RNT is 35 and you end up with a TBT of 80 in the 14m column, r-up to 82 - group V.
Group W in the metric 14m column is 87 and group X is 92. 98 is Group Y and NDL.

So if you are running the metric tables, to get to group X you would extend your 45 minute dive by between 8 and 12 minutes.

Bumping up against NDL would happen at +4 minutes if you were working in imperial and +18 minutes in metric.

I'm beginning to think I'm crazy.

The whole "Measure with a micrometer, mark with chalk, and cut with an axe" trope about deco theory really seems grossly understated when we are talking about a 14 minute difference in calculated NDL between metric vs imperial version of tables from the same agency, based on the same model.

 

[loosenit2]

The whole "Measure with a micrometer, mark with chalk, and cut with an axe" trope about deco theory really seems grossly understated when we are talking about a 14 minute difference in calculated NDL between metric vs imperial version of tables from the same agency, based on the same model.

While I don't disagree with the quote you are referencing i'm not sure that is really explanatory in this case.

One of the bad things about the tables is that they pick arbitrary depths and times as cut offs so the table can fit into a nice little 5x8 card. If you were to do the same table on a legal sized sheet of paper you could put a lot more data there. Rather than starting at 35 feet and going in increments of 10 feet you could start at 5 feet and go in increments of 1 foot for example. If you were to do this in both the metric and imperial the differences between them would be much more slight.

 

[jonhall]

IF you know tables, how can you get it wrong? Like Mrs. Harmon said in my Grade 7 Science class-- check over your work.

That's what I told my 7th and 8th graders! In my defense at 6ish this morning, I was lying on the couch, laptop on the coffee table, watching the beginning of Maverick on the western channel, eyes half open.....those are my weak excuses.