How to convert tanks sizes from imperial to metric

[Akimbo]

And +1 to @Bob DBF: in the end, it’s all an approximation

This is case of measuring with a micrometer, marking with chalk, and cutting with an explosive shape charge. The purpose of knowing gas capacity is to "estimate" the rate it will be consumed during a dive.

It is pretty pointless to calculate cylinder volume with great precision until you know your RMV and depth, second to second, over your entire meandering dive profile plus how much gas is used for by your BC, drysuit, test breathing, gas analysis, and surface swimming if you don't use a snorkel.

It is possible but you would have to be so heavily instrumented in a hyperbaric lab you would no longer be a diving, you would be a navy diver guanine pig.

 

[acreichman]

What you were describing is the definition, the very definition, of “measure with a micrometer, mark with chalk, cut with an ax“.

This is case of measuring with a micrometer, marking with chalk, and cutting with an explosive shape charge. The purpose of knowing gas capacity is to "estimate" the rate it will be consumed during a dive.

I don't appreciate you trying to inject some reality into the fantasy I'm clearly enjoying.

You're both right, of course, both about the diving aspects and the impromptu personality analysis. The appeal of specific, accurate measurement and comparison was too appealing and I lost the forest for the tree. Thank you for your help and I hope you get exceptionally good diving conditions in the new year.

 

[Wibble]

What's "REE"?

 

[Bob DBF]

It is pretty pointless to calculate cylinder volume with great precision until you know your RMV and depth, second to second, over your entire meandering dive profile plus how much gas is used for by your BC, drysuit, test breathing, gas analysis, and surface swimming if you don't use a snorkel.

And if one calculates rmv like I do, it covers all that. I take the pressure on the boat, then the pressure when I surface. I have a crappy rmv but it's good for planning, unless I'm filling lift bags. Using special conditions, like a 1 minute swim at depth will give an unrealistic rmv, one reason divers double their sac rate when planning for an emergency.

 

[Bob DBF]

What's "REE"?

Another discussion:
How is the REE number on a tank used

 

[Angelo Farina]

This was a very helpful read, thank you.

@tbone1004 @tmassey both of your posts were very helpful, even if I didn't get the simple answer I was hoping for. At this point, I think it makes sense to clarify why I want this information and maybe that will help me figure out what to do. I have a transmitter on my tank and I download my dives, including gas usage, into MacDive, which then tells me my gas consumption rate. Gas consumption rate depends on volume consumed, which is tank dependent since transmitters can only report pressure changes. To make this work, I have to tell MacDive what size tank I'm using, and when I have it set to metric, it looks for a tank capacity in liters.

Here's the way I'm currently thinking about it, please correct me if I'm wrong. Compressibility factors exist because pressure vs. volume is not a linear relationship. MacDive, to the best of my knowledge, assumes volume and pressure have a linear relationship (I'm verifying this in the MacDive forums). More often than not, I'll start dives with full tanks. Given those 3 pieces of information, I should be able to figure out how much actual gas my tanks will have at full pressure (using the compressibility factor) and then work backwards from there to give MacDive a tank size that will be accurate at full pressure (but not at other pressure). If I do that, I'll get the most accurate information for the greatest number of dives. Does that sound correct?

Then the only accurate answer come from MEASUREMENTS, not from calculations.
If you read my previous posts, I explained you how to make vert accurate measurements using a digital scale, and either filling your tank with water or air.
In half an hour you can get the answer.
No calculation method can give you the same accuracy, because manufacturing has some inherent uncertainity. And if in your country the tank is not engraved with the exact values of weight and capacity, there is no other option than measuring these quantities yourself. As said, it takes just half an hour...