RMV math…

[UofMI_Divegeek]

Responding to:
You Americans must be amazing at mental arithmetic compared with us metrical peoples!

As the late Bill Hamilton (noted diving physiologist) used to say

"America is slowly moving towards the metric system ... inch by inch!"

and

American students consistently test well below most "wealthy" nations in science and math literacy.
Image from one of my favorite educators below

"

 

[UofMI_Divegeek]

Below is a slide documenting US student math performance from my 2021 slide set on current trends of concern from my intro chemistry course

http://funchemistry.org/CEMDaddy/Chemistry101/01_Course%20Info/21_MyStandards.pdf

US now consistently places in the 30's to the 50's depending on year and number of participants studied.

One interesting comment on US education came from an Australian in our umich research group at

http://funchemistry.org/CEMDaddy/Chemistry101/01_Course%20Info/22_TallPoppy.pdf

When I was in high school (the dark ages of the '60's (g)) , the US was consistently in the top regions of science and math. Those days are long gone. As early as the 1990's Carl Sagan in his "The Demon Haunted World" commented on the vast amount of America workers who could not do 5th grade arithmetic.

 

[Wibble]

It must be awfully difficult to continue with the British legacy of measurements against the overwhelming benefits of a single scientific system of measurement (Metrology) where the individual measurements are related to each other in a logical manner.

In diving this generally means nice easy arithmetic, e.g.

• Depth: every 10m ~= 1ATA ~= 1Bar
• Gas volume = wet volume x pressure [Bar]
• Gas consumption volume = SAC x depth [ATA]
• How long my gas will last = Gas volume / Gas consumption volume
• Minimum gas pressure = Gas volume needed / cylinder wet volume

Even a child can do this arithmetic.

 

[EFX]

<deleted>

 

[Kendall Raine]

You are focused on planning, but diving is also a consideration.

Very true. Diving is the feedback loop to planning. There is a circularity in the pressure/volume issue (which explains the recent discussion ).

So, in the interest of bringing the "tastes great, less filling" discussion to a happy landing (and moving on to such scintillating hijacks as American's ineptitude at math/science, etc.) SAC is computed using pressure changes at depth and converted to a volume consumption at 1 ata metric for use in planning. As a volume metric it can be used across all tank sizes by all members of the dive team, respectively, and factorized for differing conditions. Without the conversion to volume, however, the pressure consumption metric is limited (for planning purposes) to always using the same tank volume (size)-not very useful. Computing each team members respective rock bottom (planning) then requires conversion back to pressure for dive implementation (turn pressure) using an spg.

That's why volume consumption (not pressure consumption) is the basic ("end point") metric since it allows planning by teams across different scenarios.

 

[tursiops]

Very true. Diving is the feedback loop to planning. There is a circularity in the pressure/volume issue (which explains the recent discussion ).

So, in the interest of bringing the "tastes great, less filling" discussion to a happy landing (and moving on to such scintillating hijacks as American's ineptitude at math/science, etc.) SAC is computed using pressure changes at depth and converted to a volume consumption at 1 ata metric for use in planning. As a volume metric it can be used across all tank sizes by all members of the dive team, respectively, and factorized for differing conditions. Without the conversion to volume, however, the pressure consumption metric is limited (for planning purposes) to always using the same tank volume (size)-not very useful. Computing each team members respective rock bottom (planning) then requires conversion back to pressure for dive implementation (turn pressure) using an spg.

That's why volume consumption (not pressure consumption) is the basic ("end point") metric since it allows planning by teams across different scenarios.

Exactly.
And that is why pressure/time is used without any need for conversions to volume by recreational divers during a dive...it tells you exactly what you need to know, with no additional conversions or math, especially if you have a Gas-Time-Remaining display.

 

[60plus]

Seriously? My SAC/RMV numbers over many dives say otherwise. I posted examples of how they vary based on what I’m doing. Exertion requires more oxygen than a leisurely activity. This is true on land and underwater .

I think I’m now understanding your reluctance to knowing your typical SAC. You have no idea what it is. It is certainly not a diver specific constant that never varies based on activity.

You have without realising it strongly supported what I have been saying all along. This is why I argue that trying to know your SAC exactly is pointless. SAC has to be specific to a diver for them to use it in their calculation.

 

[60plus]

Surely you are aware that one must take into account at least two things on all dives, namely your nitrogen status (via NDL, for example) and gas status (do I have enough to finish this dive?).
Your example is about nitrogen status. Fine, glad you are doing that.
But what about your gas status? Why does that seem to be less important to you than your nitrogen status? After all, DCS is rarely fatal. Running out of gas is almost always fatal.
Perhaps you could present a scenario more to the point, one in which gas usage and sufficiency are important?

I constantly monitor my gas status. I have a good idea what I will use on a dive and can tell as I dive if I am starting to deviate. I do this without knowing my SAC because I know what I am likely to use. As soon as there is an incident the any initial calculation based on SAC is redundant

 

[60plus]

More gibberish. Anyone who thinks SAC is a constant doesn't understand the first thing about it. Where do you get this nonsense?

When you use your SAC to calculate how much gas you expect to use on a dive how do put a fluctuating value into your calculator? if as you assert SAC is not a diver specific constant.

 

[Belzelbub]

You have without realising it strongly supported what I have been saying all along. This is why I argue that trying to know your SAC exactly is pointless. SAC has to be specific to a diver for them to use it in their calculation.

Several pages back, you made a statement that pretty much confirmed you have no idea what SAC even is.

It is not a constant like you stated. It varies based on the activity level of the dive. It can also change based on your health. However, it can still be useful. If I’m hunting, I know it’s going to be higher than a leisurely dive. But if I know what I’m going to be doing on the dive, I can figure out how long my gas should last. That doesn’t mean I ignore monitoring pressure during the dive.

A higher than expected SAC might warrant some investigation if the reason is not clear. I don’t think that anyone other than you claimed that SAC is a constant and never varies.