SAC differences between AI computer and Manual Calculations

[L13]

Atmospheres is not a metric unit. if you are actually on the metric system, which is "superior in every way," then you don't get to use non-metric units. However, I'll bet you buy litre bottles of wine, no? Litre is not a metric unit. do you worry about eating too many calories? Calorie is not a metric unit. Was your temperature outside today 32 deg Celsius? Celsius is not a metric unit. Etc.

Metric is not one unit system, it is a class of unit systems where effort has been made to keep all relationships between units powers of 10. The SI system is not "The Metric System" it is just one of several metric systems (most of which are easily converted to each other by powers of 10). the CGS system is another example of a metric system. The Calorie fits fairly nicely into the CGS system for some classes of problem. the Joule fits nicely in the SI system. They are both metric units for energy, and are one of the exceptions to the ideal that metric units convert "nicely" to each other. Units for Time are another universal exception. There are other exceptions, but less than the free for all of the imperial system.

 

[tursiops]

Metric is not one unit system, it is a class of unit systems where effort has been mad to keep all relationships between units powers of 10. The SI system is not "The Metric System" it is just one of several metric systems (most of which are easily converted to each other by powers of 10). the CGS system is another example of a metric system. The Calorie fits fairly nicely into the CGS system for some classes of problem. the Joule fits nicely in the SI system. They are both metric units for energy, and are one of the exceptions to the ideal that metric units convert nicely to each other.

I'm not going to get into an arcane debate with you or Boltsnap about the metric system. There are many outmoded metric systems....but only the 7 base SI units and a group of defining constants are internationally accepted as fundamental. There are many derivatives of the 7 base SI units that define things like Celsius, and a list of non-SI units that are accepted for use with SI, such as minute, hour, day, litre, and decibel. Atmosphere is NOT accepted as a pressure unit. My point is that the purest of the pure - the SI metric system -- still uses other units because that is what people use. When Boltsnap says his RMV is (say) 16 l/min and his tank is 11.1 liters, he is using non-SI units...not the currently accepted pure metric system.

And I'm OK with that. My objection is painting metric as superior to all else, when in fact it is barely relevant to much of our diving.....based on atmospheres, liters, minutes and hours. The SI terms would be Pascals, cubic metres, and seconds.

 

[MarkA]

Nope. The OPs main error was forgetting to adjust for surface-equivalent depth..

Just reread OP’s post #6 where the OP said an error was made using 80cuft instead of 61cuft.

Just reread your post #7 and agree with the maths except 1 + 10/33 = 1.303 (typo?, makes no difference to the result)

Its unfortunate that not using the surface equivalent depth and possible volume error produce similar results.

 

[tursiops]

Just reread OP’s post #6 where the OP said an error was made using 80cuft instead of 61cuft.

Just reread your post #7 and agree with the maths except 1 + 10/33 = 1.303 (typo?, makes no difference to the result)

Its unfortunate that not using the surface equivalent depth and possible volume error produce similar results.

Yeah, I had a couple of typos in that post. The point was he missed the 30% correction due to depth. His attempt to get the right answer by using 61 cuft was just flailing about for some other numbers to use.

 

[L13]

I'm not going to get into an arcane debate with you or Boltsnap about the metric system. There are many outmoded metric systems....but only the 7 base SI units and a group of defining constants are internationally accepted as fundamental.
...
The SI terms would be Pascals, cubic metres, and seconds.

You are being inconsistent here, the Pascal is NOT one of the 7 base SI units. It is one of many derived units in SI. Do you think we should be using depth in inches? Or pressure in pound/square foot? All unit systems have derived units.

The Bar is another unit derived from the Pascal. The same principle applies to liter. Wikipedia (not necessarily authoritative but pretty useful for this kind of thing) says:

"The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea level (approximately 1.013 bar)."

It is that last part that is probably the reason it is used for metric diving. Follow the link in the quote above for a definition of "Metric units."

An interesting side note: 4 of the 7 base SI units were redefined in 2019, so anything SI published before then is technically a different unit than SI now (though the changes made no practical difference).

 

[tursiops]

You appear to be using the wrong numbers, AND the wrong calculation.

First, your 80 cuft tank does not hold 80 cuft, but only 77.4, when filled to 3000 psi.
When filled to 2308 psi, it contains 2308/3000 * 77.4 = 59.5 cuft.
At 878 psi, it contains 878/3000 * 77.4 = 22.7 cuft.

So over the course of your 53 minutes dive, you used 59.5-22.7 = 36.8 cuft.
That give 36.8/53 = 0.69 cuft/minute, at an average depth of 10 ft.

But you want a surface-equivalent value....and 10 ft is not at the surface.
At ten ft in salt water, your ambient pressure is 1 + 10/33 = 1.33 ATA.
In fresh water, your ambient pressure is 1 + 10/34 = 1.34 ATA.

Assume salt water: your consumption rate at 10 ft is 0.69 cuft/min, but at the surface that would be only 0.69/1.33 = 0.52 cuft/min. THAT is your RMV.

EDIT: I hit post without refreshing but @tursiops' math is correct and explained better than me.

In the above scenario you used 36.89cuft of gas. AL80 (77.4cuft)
2308/3000 x 77.4 = 59.54cuft available gas
878/3000 x 77.4 = 22.65cuft gas left in cylinder
59.54cuft - 22.65cuft = 36.89cuft gas used
36.89cuft / 53 minutes = .696cuft/minute
10ft / 33 + 1 = 1.303 ata
.696cuft / 1.303 = .53cuft/minute

Just reread OP’s post #6 where the OP said an error was made using 80cuft instead of 61cuft.

Just reread your post #7 and agree with the maths except 1 + 10/33 = 1.303 (typo?, makes no difference to the result)

Its unfortunate that not using the surface equivalent depth and possible volume error produce similar results.

Yeah, I had a couple of typos in that post. The point was he missed the 30% correction due to depth. His attempt to get the right answer by using 61 cuft was just flailing about for some other numbers to use.

My original post had typos. Here are the correct versions (in red) of my equations with a consequently corrected (in red) final result.
----------------------------------------------
First, your 80 cuft tank does not hold 80 cuft, but only 77.4, when filled to 3000 psi.
When filled to 2308 psi, it contains 2308/3000 * 77.4 = 59.5 cuft.
At 878 psi, it contains 878/3000 * 77.4 = 22.7 cuft.

So over the course of your 53 minutes dive, you used 59.5-22.7 = 36.8 cuft.
That give 36.8/53 = 0.69 cuft/minute, at an average depth of 10 ft.

But you want a surface-equivalent value....and 10 ft is not at the surface.
At ten ft in salt water, your ambient pressure is 1 + 10/33 = 1.303 ATA.
In fresh water, your ambient pressure is 1 + 10/34 = 1.294 ATA.

Assume salt water: your consumption rate at 10 ft is 0.69 cuft/min, but at the surface that would be only 0.69/1.303 = 0.53 cuft/min. THAT is your RMV.

 

[saxman242]

There are two kinds of countries in the world... those that use metric and those that have put a man on the moon.


No one mention that NASA did their work in metric and then converted to imperial for the astronauts.

 

[EFX]

 

[Belzelbub]

There are two kinds of countries in the world... those that use metric and those that have put a man on the moon.

I wasn’t aware that Liberia and Myanmar put men on the moon.

 

[MarkA]

I wasn’t aware that Liberia and Myanmar put men on the moon.

No drift here. From “SAC“ to “Myanmar put men on the moon“ in 39 posts.