Metric or Imperial?

[Neilwood]

Simple answer - You need to use the units that your dive buddies will be using. So long as you are in America, Myanmar (Burma) or Liberia, you will find imperial units in use

But, if you can get them to convert to calculating in metric, things become so much easier (and the rest of the world will agree with you).

 

[beester]

Metric for me... I calculate as well while diving. Mainly during dives to monitor expected gas consumption (in your head) against real consumption (checking the manometer) every 5 to 10 minutes (depending on depth). After a while it becomes natural, doesn't need much brain bandwith to do... so yes also during rec dives. If real consumption is really off from my expected consumption it's a warning sign (equipment? stress? current?). In caves indeed to recalculate thirds.

Tech diving I never calculate backgas as a lost deco gas contingency... unlike with air, with He it's just not practical using backgas (trimix) as a real backup plan. It results for me in a runtime too long to consider.

 

[agilis]

Metric for me... I calculate as well while diving. Mainly during dives to monitor expected gas consumption (in your head) against real consumption (checking the manometer) every 5 to 10 minutes (depending on depth). After a while it becomes natural, doesn't need much brain bandwith to do... so yes also during rec dives. If real consumption is really off from my expected consumption it's a warning sign (equipment? stress? current?). In caves indeed to recalculate thirds.

Tech diving I never calculate backgas as a lost deco gas contingency... unlike with air, with He it's just not practical using backgas (trimix) as a real backup plan. It results for me in a runtime too long to consider.

Checking an SPG to monitor expected gas consumption hardly rises to the level of a math calculation. It's more on the level of a visual observation.

I've always regarded the printing on an SPG as largely irrelevant, since it's really the movement of the indicator needle that's significant. That's why there are 'dual gauges', which are nothing more than the same old gauge with two lines of print. No matter how you slice the loaf it's pressure that's being measured. The fact that you can paste on a different set of numbers and convert an imperial gauge to a metric and vice versa illustrates how fatuous this issue really is.

I've advocated the substitution of smile/frown faces for metric or imperial numbers. At the highest pressure levels there is a bright smiley face (HP tanks will cause the gauge to absolutely beam when full), which gradually changes to a neutral expression and then to a frown of increasing severity as pressures drop.

This will work with dual tanks, small tanks, and large tanks perfectly well. The planning implications of the volume of gas you start with is your responsibility, since it's a fixed and known quantity.

Seriously, scuba related physics are really quite straightforward and readily understandable. For reasons I can only guess at some people like to complicate things to the level of a GRE math section problem. Perhaps it's another case of technical jargon masquerading as complex content.

 

[DevonDiver]

Seriously, scuba related physics are really quite straightforward and readily understandable. For reasons I can only guess at some people like to complicate things to the level of a GRE math section problem. Perhaps it's another case of technical jargon masquerading as complex content.

Again, a caveat - this is a technical diving issue, in the advanced diving forum.

An example: Your deco reg malfunctions and free-flows (unfixable). You have been separated from your team. You elect to feather-breath the deco gas. The process is wasteful of gas, but allows you to access what you have. You have 30 minutes of deco to complete. As you feather breath those 30 minutes, you track your SPG; gas consumption versus time. It becomes important to know whether the consumption rate via feather breathing is inside, or outside, of your planned 1/3rd reserve of deco gas. The only other option is to transfer a regulator - a complex option, best avoided if you have sufficient gas by feather breathing the deco as it is.

This isn't implausible. Deco regulators do fail/free-flow. Divers do get separated from teams (the point of tech self-sufficiency is to mitigate that).

Maths... in-water.... gives you the information you need for the decision making process.

 

[Bennno]

An example: Your deco reg malfunctions and free-flows (unfixable). You have been separated from your team. You elect to feather-breath the deco gas. The process is wasteful of gas, but allows you to access what you have. You have 30 minutes of deco to complete. As you feather breath those 30 minutes, you track your SPG; gas consumption versus time. It becomes important to know whether the consumption rate via feather breathing is inside, or outside, of your planned 1/3rd reserve of deco gas. The only other option is to transfer a regulator - a complex option, best avoided if you have sufficient gas by feather breathing the deco as it is.

This isn't implausible. Deco regulators do fail/free-flow. Divers do get separated from teams (the point of tech self-sufficiency is to mitigate that).

Maths... in-water.... gives you the information you need for the decision making process.

If you have sigificant deco time left, you might as well change the deco reg... it's not a big deal in SM. But granted that is an example. But what about NDL diving?
There is no reason I can think of that would require you to calculate anything on a rec dive. Did anyone have an example in the AI thread?

 

[DevonDiver]

If you have sigificant deco time left, you might as well change the deco reg... it's not a big deal in SM. But granted that is an example. But what about NDL diving?
There is no reason I can think of that would require you to calculate anything on a rec dive. Did anyone have an example in the AI thread?

If recreational diving is defined as "always having immediate access to the surface", then there's no need for maths. The time taken to undertake the maths is probably going to be nearly as long as it'd take to surface.

The only examples I can think of are entrapment/entanglement issues, unplanned/emergency deco and other rare, unforeseen scenarios that limit immediate access to the surface.

The only other scenario is if the diver wants to return to their starting point underwater. The emergency ascent remains available to them, but they may want to use some maths on-the-fly to determine if they can get back to their start point or how far they might return before an ascent is necessary (i.e. until they reach rock-bottom gas etc). Situations that returning to the shot line might be prudent could include boat traffic/jet ski activity, or to ascent the shot/anchor from a wreck if a current was running.

 

[beester]

Checking an SPG to monitor expected gas consumption hardly rises to the level of a math calculation. It's more on the level of a visual observation.

What I mean is I calculate without looking at the manometer. Let's say I'm at 10 minutes with an average depth of 40m...My SAC for a double 12L set is between 3,5 and 4 bar per 5 minute segment. So I'll calculate 4bar*5 (ATA)*2 (2 5 min segments)= 40 bars used. I'll then check this expected consumption rate against the manometer and if it's way off I'll start checking for issues. If I just check the manometer I'm not making the consious effort to evaluate expected against real useage.

Because I dive the same sets a lot (or double 12L or double 8,5L and bottom stages always 80cuft), I tend to know my SAC in bar per 5 minute segment, and calculation is very easy. Doesn't even take 10 seconds, so while I'm putting my light in my right hand to reach for my manometer I'm already doing this little children math in my head. By the time I can read my manometer I know what to expect.

But I digress... you are right, this is a technical diving habit, and not really usefullin recreational NDL diving except keeping the habit up

 

[JohnN]

Up to some 230-250 bar (3350-3500 psi), the nonideal behavior is small enough to be insignificant. It's only in the range from 250 to 300 bar (3500-4350 psi) that it becomes an issue. That's the reason that while a 15L 200 bar tank holds about 3000 surface liters, a 10L 300 bar tank holds some 10% less than that, even if the two tanks have the same nominal capacity.

So up to your normal fill pressures, compressed air does behave like an ideal gas. At least within the limits of measurement and significance. When I use my 300 bar tanks, I take the nonideality into account, but I don't if I'm using a 200 bar or 232 bar tank.

When I look at the data, I get about 8% @ 3000 PSI and about 10% at 3500 PSI. I dive HP120's and 12 cuft is quite a bit of gas

Compressibility factor - Wikipedia, the free encyclopedia

 

[Kevrumbo]

Checking an SPG to monitor expected gas consumption hardly rises to the level of a math calculation. It's more on the level of a visual observation.

I've always regarded the printing on an SPG as largely irrelevant, since it's really the movement of the indicator needle that's significant. That's why there are 'dual gauges', which are nothing more than the same old gauge with two lines of print. No matter how you slice the loaf it's pressure that's beintg measured. The fact that you can paste on a different set of numbers and convert an imperial gauge to a metric and vice versa illustrates how fatuous this issue really is.

I've advocated the substitution of smile/frown faces for metric or imperial numbers. At the highest pressure levels there is a bright smiley face (HP tanks will cause the gauge to absolutely beam when full), which gradually changes to a neutral expression and then to a frown of increasing severity as pressures drop.

This will work with dual tanks, small tanks, and large tanks perfectly well. The planning implications of the volume of gas you start with is your responsibility, since it's a fixed and known quantity.

Seriously, scuba related physics are really quite straightforward and readily understandable. For reasons I can only guess at some people like to complicate things to the level of a GRE math section problem. Perhaps it's another case of technical jargon masquerading as complex content.

So by analogy, this is like qualitatively watching the "gas gauge" on a car: Whenever the needle gets to "E", you just conveniently find a gas station to fill-up.

Breathing Gas Consumption on Scuba at depth is a lot more vital quantitatively than your rhetorical trivialization above. . .

Read and Learn.

 

[Kevrumbo]

When I look at the data, I get about 8% @ 3000 PSI and about 10% at 3500 PSI. I dive HP120's and 12 cuft is quite a bit of gas

Compressibility factor - Wikipedia, the free encyclopedia

BAUE Z Factor Table