Metric vs Imperial

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Permalink Reply by kevrumbo on March 3, 2009 at 7:01pm

. . .Because of easier math, and mostly because I techdive overseas with buddies who primarily use the metric system. When I dive back home here in SoCal, I'll switch my depth gauges to imperial units if my dive buddy happens to be an American trained here in the States. I keep my analog SPG however always with Bar units both here and abroad.

For me in Double AL80's (aka Twin 11L), my SCR (Surface Consumption Rate) translated to a Bar SPG turns out to be 1bar/min*ATA. So if I spent an easy finning nominal exertion 10 minutes at a depth of 100'/30m (same as 4 ATA), then I know I've consumed 40bar of gas before even looking at the SPG. Worked example: (1bar/min*ATA)(10min)(4 ATA) = 40bar consumed at 100'/30m.

In tropical waters, my SCR is usually 75% better, so using the above example, I would expect to consume 30bar after 10min at 100'/30m.

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Permalink Reply by Marc Blackwood on March 3, 2009 at 7:02pm

I think it's something that is best discussed with your dive team. If you have one, see if they wish to make the switch. If not, that's tough because if you go out and get BAR SPGs and eventually get into a team of imperial divers, you'll have to switch.

The math isn't hard with tanks rated in PSI. It's simple division and multiplication with enough rounding that it's easily done in the water.

Personally, I wouldn't mind going metric, but I see no compelling reason to change.

edit: I just saw that you're in BC. Isn't metric the norm there?

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Permalink Reply by Maciej Arkuszewski on March 4, 2009 at 7:08am

Hi Mike.

If you truly want to be a global diver, you need to be capable of diving both imperial and metric. I find myself diving frequently with people abroad or visiting and being able to switch between two makes whole experience smoother for both parties. As Mark mentioned, whole team has to be on a same page and it's rare to dive exclusively with same team through dive career.

Delia Milliron wrote an excellent article "Battlefield calculations" a while back. It elaborates how to quickly convert between two. I can't find it on a net anymore though.

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Permalink Reply by Marc Blackwood on March 4, 2009 at 7:17am

I hope I'm not stepping on any toes, but here's the document Maciej mentioned.

Personally, I think the most important thing to understand is how tanks are rated, be they imperial (CF and PSI) or SI (L and BAR). Depth is trivial.

Attachments:

• Battlefield Calculations - Delia Milliron.pdf, 258 KB

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Permalink Reply by Mike Barrass on March 4, 2009 at 8:55am

Thanks All, your answers were helpful and gave me some good stuff to think about.

Marc, I live in a mixed up country, we have both metric and imperial here. Scuba gear is in imperial though I have met divers using metric. For now I will be staying imperial due to my buddies but it will be a talk that we will have to have, which will we use.

The battlefield sheet was the one that got me thinking about this, very helpful sheet.

Thanks

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Permalink Reply by kevrumbo on March 4, 2009 at 4:12pm

Marc Blackwood said:

I hope I'm not stepping on any toes, but here's the document Maciej mentioned.

Personally, I think the most important thing to understand is how tanks are rated, be they imperial (CF and PSI) or SI (L and BAR). Depth is trivial.

No Marc, depth is not trivial when you’re figuring breathing gas consumption on-the-fly. And it’s much easier to do it all in metric than in imperial units . . .for example:

Metric: 21meters is 3.1ATA, which means I nominally consume 31bar in 10min.

Imperial: 70’ is 70(3/10) (1/10) + 1 equals 3.1ATA multiplied by 15psi/min*ATA, then multiplied by 10min yields 465psi consumed. (As you can see, much easier & quicker to do in your head using Metric Units above).

If you stay at 70’/21m, it’s also much easier to work in Metric with 31bar segments delta from your initial pressure of 200bar (full twin 11L tanks), than it is to arithmetically work in Imperial with 465psi segments delta starting from 3000psi (full twin AL80 tanks).

[Notes: based on a SCR of 21 liters/min, roughly same as 0.75 cf/min, translated to SPG 1bar/min*ATA, or same as 14.5psi/min*ATA in imperial. IMHO, it's much easier and better to work with unity integer reference values like "1bar" versus numbers like "14.5psi"]

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Permalink Reply by Marc Blackwood on March 4, 2009 at 4:36pm

What I meant was monitoring depth is trivial.

Point being that yes, gas management is somewhat easier in metric, and it's good to understand both systems.

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Permalink Reply by Khaled Moftah on April 10, 2009 at 10:22pm

Hi Mike,


Go Metric..........pleaaaaaaaaaaaaase :D

Mike Barrass said:

Thanks All, your answers were helpful and gave me some good stuff to think about.

Marc, I live in a mixed up country, we have both metric and imperial here. Scuba gear is in imperial though I have met divers using metric. For now I will be staying imperial due to my buddies but it will be a talk that we will have to have, which will we use.

The battlefield sheet was the one that got me thinking about this, very helpful sheet.

Thanks

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