SAC vs RMV, revisited

[tursiops]

Yes it does, and you can set a default if you use one tank more than others.

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Nice RMV, by the way!

 

[EFX]

And when do you relate to psi numbers with more than two significant digits? Heck, even in bars I rarely relate to pressure numbers with more than two significant digits. And if you need more resolution, you always have the digits to the right of the decimal point.

Practically, why would anyone use decimals. It's never done while diving using imperial units unless you're a fanatic. Temperature doesn't matter as the change in degC is only 2 degF. Depth, I suppose, could be an issue when precise measurements need to be made: 1 m = 3+ ft. Shearwater had the courtesy of displaying depth to the nearest 10th of a meter on their dive computers. Doesn't matter in my diving.

 

[Manatee Diver]

Nice RMV, by the way!

Thanks, I've been lucky to have pretty decent air consumption from the start, and it has only gotten better.

 

[Storker]

Depth, I suppose, could be an issue when precise measurements need to be made: 1 m = 3+ ft.

When I'm trying to hold a stop in free water without having shot a sausage, I pay attention to the numbers to the right of the decimal point (among other things). Otherwise, I don't care. It just isn't significant.

 

[atdotde]

RMV is more awkward, because it is always "volume" but it may not be "at the surface!" so you need to say the equivalent of "RMV at depth" or "RMV at the surface." Anything else is inviting confusion.

Subsurface lists the actual volume you breath (not multiplied by the current ambient pressure to obtain the amount of gas used per minute) as that number is supposed to stay more or less constant during dives (i.e. independent of depth) so it makes sense to compare it between dives.

To determine how long the gas in your cylinder lasts you will have to multiply this number by the ambient pressure (and of course Subsurface does that when figuring out things like gas remaining, minimum gas etc).

 

[tbone1004]

It has nothing to do with imperial vs metric, but the fact due to compressability that the only accurate way to really measure a gas is by weight, everything else is a calculated estimate. But that really isn't workable in the real world, with the different working pressures and container sizes the standard cubic foot or liter of air is the workaround.

it is more indirect vs. direct measurement of a tank, but comes back to the metric system because we "think" in cubic feet which is not a small enough unit to measure tank capacity. It also comes back to trying to make something sound bigger/better than it is, which then leads to all sorts of confusion on why a HP100 is a quite small tank, but a LP100 is quite large.

 

[Manatee Diver]

it is more indirect vs. direct measurement of a tank, but comes back to the metric system because we "think" in cubic feet which is not a small enough unit to measure tank capacity. It also comes back to trying to make something sound bigger/better than it is, which then leads to all sorts of confusion on why a HP100 is a quite small tank, but a LP100 is quite large.

If we went exclusively by water volume, how do you compare HP steel vs LP steel? You have to do like Europeans, and multiply by the pressure to get the true capacity. The North American way of labeling the tanks by a standard cubic foot simply omits that step. So a HP100 and a LP100 have the same amount of gas in them if filled to their respective working pressures. Where under the European labeling they would have to do the math to find that they have the same capacity even though one is a 12L and the other is 15L.

 

[Bob DBF]

So a HP100 and a LP100 have the same amount of gas in them if filled to their respective working pressures.

Not exactly.

Correct on the HP 100, on the other hand a LP 100 has its stated volume at its service pressure plus 10%. So it would be 90 cuft at service pressure of 2400#. All figures being plus or minus, sometimes the named tank volume is a rounded figure from the actual volume.

There is more than one reason to go to metric. Although I probably won't, old dogs...



Bob

 

[Manatee Diver]

Not exactly.

Correct on the HP 100, on the other hand a LP 100 has its stated volume at its service pressure plus 10%. So it would be 90 cuft at service pressure of 2400#. All figures being plus or minus, sometimes the named tank volume is a rounded figure from the actual volume.

There is more than one reason to go to metric. Although I probably won't, old dogs...

You are right I was just estimating. I spend a lot of time in areas where working pressure is just a ummm... guideline.

Personally I have no issues with metric, I just have an issue with people that say it will cure all our ills. Bad math is bad math regardless or which system it was done under.

 

[Bob DBF]

You are right I was just estimating. I spend a lot of time in areas where working pressure is just a ummm... guideline.

When I moved the new LDS is a bit more literal when dealing with tank pressure, perhaps it will change after they get to know me.

Personally I have no issues with metric, I just have an issue with people that say it will cure all our ills. Bad math is bad math regardless or which system it was done under.

I've been working with imperial and metric since '65, and other than wishing someone would make up their mind, it's a bigger in-connivence than picking a side. A conversion won't cure all our ills, and having two standards to work with has not helped either.

And yes, the math challenged won't be helped regardless.



Bob