Metric versus Imperial System for Diving?

[agilis]

I agree with you

I don't see the relevance, unless you do SAC/RMV calculations while actually diving. I think these kinds of things are generally done afterward, which was a central element in all my remarks. As far as jumping through several hoops, converting bar to psi is not especially complicated. The dual reading gauge shown in post 126 does it for you in a rough manner, and there are equivalency tables of great exactitude. In any event, the safest thing while diving is, in my opinion, using a direct mechanical gauge with measuring units that are instantly understood by the user.

 

[Tigerman]

I don't see the relevance, unless you do SAC/RMV calculations while actually diving. I think these kinds of things are generally done afterward, which was a central element in all my remarks. As far as jumping through several hoops, converting bar to psi is not especially complicated. The dual reading gauge shown in post 126 does it for you in a rough manner, and there are equivalency tables of great exactitude. In any event, the safest thing while diving is, in my opinion, using a direct mechanical gauge with measuring units that are instantly understood by the user.

How are you planning your gas consumption for a dive or even how much gas you need for a dive and then make sure youre on schesule with that plan if "calculations are done afterwards"?
And what if, god forbid, you decide to replan the dive on the fly because you find that youd rather spend more time with that frogfish?

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[agilis]

How are you planning your gas consumption for a dive or even how much gas you need for a dive and then make sure youre on schesule with that plan if "calculations are done afterwards"?
And what if, god forbid, you decide to replan the dive on the fly because you find that youd rather spend more time with that frogfish?

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I'm not sure how one goes about planning gas consumption. I plan dives, within the limits of the gas I will have available. Replanning in an unfamiliar term, but just about every one of my dives these days has a high degree of unpredictability. My dives are almost all solo, centered around observing fish and other critters, so it's possible for me to spend most of my time in one spot, or to move from place to place, depending on what I come across. The idea of a schedule fills me with horror.

My dive plans are centered around time, tides, depth, and air consumption, but with continual location modifications; free form diving, I suppose it might be called, and my compass/navigation skills are excellent.

I'm now in my 70s, so I no longer dive any deeper than about 25 meters and avoid very cold water and strong currents. Most of my equipment is fairly vintage stuff, and my computer (Gekko) is more a backup to my watch and depth gauge than the other way round.

I have a very, very low air consumption rate, so even in 15 or 20 msw with an 80cf tank I have to watch my time at depth. I am a solo diver almost always, primarily because most of my old dive buddies are either dead or sedentary, but I've learned to love the absolute freedom that comes with diving alone.

---------- Post added October 11th, 2014 at 06:24 PM ----------

To be quite honest, I find that there is more arrogance displayed right here on Scubaboard than I have seen in my travels in real life, comparatively speaking of course.

Ain't it awful?

 

[Kevrumbo]

I'm not sure how one goes about planning gas consumption. I plan dives, within the limits of the gas I will have available. Replanning in an unfamiliar term, but just about every one of my dives these days has a high degree of unpredictability. My dives are almost all solo, centered around observing fish and other critters, so it's possible for me to spend most of my time in one spot, or to move from place to place, depending on what I come across. The idea of a schedule fills me with horror. .. /

It's easier to work with pressure units for gas planning & management in the Metric System, especially if your pressure Surface Consumption Rate (SCR) turns out to be a convenient integer.

I have a cold water reference Surface Consumption Rate (SCR) of 22 litres/min (roughly 0.75 cuft/min in US Imperial units).

The common AL80 Tank holds 11 litres volume at the surface standard of 1 ATA (or 1.01 bar), for a metric rating of 11 litres/bar.

22 litres/min divided-by 11 litres/bar equals 2 bar/min SCR (Surface Consumption Rate) in pressure units --a more useful quantity to utilize during the dive since your SPG reads in bar pressure units. [And 2 bar/min is much easier & quicker to arithmetically manipulate in your head than 29 psi/min equivalent in US Imperial Units.]

Your depth in meters, which converts easily to ATA (simply divide-by-10 and add 1) becomes your multiplier depth factor for your 2bar/min pressure SCR.

Example: 30 meters depth is 4 ATA (divide 30 by 10 and add 1 equals 4); your 2bar/min SCR at depth -or Depth Consumption Rate (DCR)- now becomes 8bar/min. [4 times 2bar/min equals 8bar/min]. So 10 minutes at depth 30m on an AL80 (11L/bar) tank in nominal conditions, you would expect to consume 80bar of gas and your SPG reading to be down or show a delta of 80bar. . .

What is so "horrible" about figuring out a "schedule" like that above? (Are you Jersey People confident at doing anything?)

 

[agilis]

It's easier to work with pressure units for gas planning & management in the Metric System, especially if your pressure Surface Consumption Rate (SCR) turns out to be a convenient integer.

I have a cold water reference Surface Consumption Rate (SCR) of 22 litres/min (roughly 0.75 cuft/min in US Imperial units).

The common AL80 Tank holds 11 litres volume at the surface standard of 1 ATA (or 1.01 bar), for a metric rating of 11 litres/bar.

22 litres/min divided-by 11 litres/bar equals 2 bar/min SCR (Surface Consumption Rate) in pressure units --a more useful quantity to utilize during the dive since your SPG reads in bar pressure units. [And 2 bar/min is much easier & quicker to arithmetically manipulate in your head than 29 psi/min equivalent in US Imperial Units.]

Your depth in meters, which converts easily to ATA (simply divide-by-10 and add 1) becomes your multiplier depth factor for your 2bar/min pressure SCR.

Example: 30 meters depth is 4 ATA (divide 30 by 10 and add 1 equals 4); your 2bar/min SCR at depth -or Depth Consumption Rate (DCR)- now becomes 8bar/min. [4 times 2bar/min equals 8bar/min]. So 10 minutes at depth 30m on an AL80 (11L/bar) tank in nominal conditions, you would expect to consume 80bar of gas and your SPG reading to be down or show a delta of 80bar. . .

What is so "horrible" about figuring out a "schedule" like that above? (Are you Jersey People confident at doing anything?)

You are not describing a schedule, as I understand the word. I used the word to describe where I will be located at any specific time during a dive. Diving solo, I have the option of remaining withing a few meters of my entry point, extending the area I had initially planned, or anything in between, always watching my compass and my gas consumption-- actual gas consumption, not theoretical-- very closely.

You are describing gas consumption in a linear manner which does not include variations in depth, activity level, temperature, or other variables. The number of psi or bar units used per minute will reflect those variables, rendering 'gas management' something best done during actual utilization without meaningless formulas.

I'm not sure why you insist on making sophomoric regional references, like "Jersey People". It may be that you believe they are amusing or clever.

There is a tendency among some divers to infuse scuba diving with technical jargon, giving the impression that it is more complex than it actually is. I'm perfectly capable of "figuring out" the kind of estimated gas consumption analysis you provided. In terms of actual recreational diving it's totally unnecessary. Sounds impressive, though.

 

[dfx]

Here's another, completely different scenario that I stumbled upon a while back.

Let's say you have an object weighing 100 pounds at 60 feet depth. You want to raise it to the surface with a lift bag. How much air from your tank do you need for the lift bag and how much change in pressure does that translate to?

While this is very easy and straight forward to work out in metric units (you can do it in your head), in imperial units the answer comes back as "go !@#$% yourself."

 

[agilis]

Here's another, completely different scenario that I stumbled upon a while back.

Let's say you have an object weighing 100 pounds at 60 feet depth. You want to raise it to the surface with a lift bag. How much air from your tank do you need for the lift bag and how much change in pressure does that translate to?

While this is very easy and straight forward to work out in metric units (you can do it in your head), in imperial units the answer comes back as "go !@#$% yourself."

This is not the sort of thing I would classify as ordinary recreational diving, the kind of diving I made clear was my reference point throughout this discussion. It is also the kind of thing I did frequently back when we used to do a lot of porthole and other artifact lifting from the cold waters of the Graveyard of the Atlantic. I also helped raise a couple of cannon in Jamaica back in the 70s.

The rough calculations can be done without reference to the metric system, but it is much easier to use metric measurement, especially if you know in advance exactly what you plan on doing. For example, it would be helpful to know the weight (in air) of the object to be lifted, something more easily done when lifting a known object like a dropped outboard motor than when lifting a found artifact. In Jamaica we used 72cf tanks brought down for the purpose of inflating multiple lift bags, and watched carefully as lift overcame the degree to which the cannon were embedded, then quickly dumped the excess gas when the artifact broke free, or added lift when a bag failed.

You also did not mention the volume of the scuba tank to which your Bar SPG is attached, though I'm sure that is only an oversight.

Were I to make the calculations you describe, there is no question I would use metric units. The liklihood of being faced with this problem unexpectedly while submerged on a dive is vanishingly remote.

"All theory is grey, but Oh! my friend, evergreen grows the golden tree of life"

(Goethe)

 

[dfx]

This is not the sort of thing I would classify as ordinary recreational diving, the kind of diving I made clear was my reference point throughout this discussion. It is also the kind of thing I did frequently back when we used to do a lot of porthole and other artifact lifting from the cold waters of the Graveyard of the Atlantic. I also helped raise a couple of cannon in Jamaica back in the 70s.

The rough calculations can be done without reference to the metric system, but it is much easier to use metric measurement, especially if you know in advance exactly what you plan on doing. For example, it would be helpful to know the weight (in air) of the object to be lifted, something more easily done when lifting a known object like a dropped outboard motor than when lifting a found artifact. In Jamaica we used 72cf tanks brought down for the purpose of inflating multiple lift bags, and watched carefully as lift overcame the degree to which the cannon were embedded, then quickly dumped the excess gas when the artifact broke free, or added lift when a bag failed.

You also did not mention the volume of the scuba tank to which your Bar SPG is attached, though I'm sure that is only an oversight.

Were I to make the calculations you describe, there is no question I would use metric units. The liklihood of being faced with this problem unexpectedly while submerged on a dive is vanishingly remote.

"All theory is grey, but Oh! my friend, evergreen grows the golden tree of life"

(Goethe)

I fully understand the point you're trying to make. For a normal, recreational dive, it doesn't matter if you measure the depth in Smoots, temperature in degrees Wedgwood, tank volume in Hubble-barns and pressure in Ale Firkins per Outhouse. The question becomes "why would you?" if at the same time you admit that when it comes to doing anything more serious with the measured values, you'd use different units. (Rhetorical question btw...)

 

[KenGordon]

I do the bar/minute calcs in my head on dives. For example on a reef in Cozumel in a group of four or six with a guide imposed max of 55 minutes after 20 I was figuring 'I might not have enough for 55 at this rate, better keep a bit shallower and argue with the current less'.

Of course I have an AI computer so get a view of my gas use to an extra half a decimal place or so.

 

[agilis]

I do the bar/minute calcs in my head on dives. For example on a reef in Cozumel in a group of four or six with a guide imposed max of 55 minutes after 20 I was figuring 'I might not have enough for 55 at this rate, better keep a bit shallower and argue with the current less'.

Of course I have an AI computer so get a view of my gas use to an extra half a decimal place or so.

And following the rule of thirds should make those precise calculations strictly academic. Encountering stronger currents or going deeper than originally anticipated modifies my dive plan instantly, with an intuitive accuracy, no exact calculations needed.

I've been scuba diving for more than 50 years, and am rather good at it by most measures, but I still follow those very conservative safety guidelines, perhaps more strictly now than when I was a young man. I do dive solo most of the time, but I think, all things considered, it's the safer option for me.

I never push the envelope. Maybe that's part of the reason why I've never had a serious incident over the course of several thousand dives. Something's going to get me, sooner or later. It might as well happen while I'm having fun.