A reader who is himself a diver and M.D. contacted me with a question about my articles on gas planning and RMV calculation in Alert Diver. I think his question is very interesting and addresses an aspect of the subject that I didn’t cover in the articles themselves. Here is our exchange.

Q:
Dear Tim
Thank you for your article ‘Let’s talk about gas planning’ in [Alert Diver]; very thought provoking in terms of safety
I am an anaesthetist and Intensive Care Doctor and also enjoy recreational diving
I wanted to ask about the RMV and the value of 15 litres. The average tidal volume (volume of each breath) ia about 70 mls/kg and the breathing rate is about 12-15 breaths. am 70kgs so my RMV would be 500mls per breath x12-15= 6-7.5 litres. For a 90kg diver this would be 630×12-15=7.5-9.5 litres. I do of course appreciate the additional factor of stress that you mention and I am purely referring to RMV
I stand to be corrected of course  
I look forward to reading the other articles on your website
Thank you
Kind regards,
Michael

A:
Hi Michael,
thank you for your kind email, it’s always nice to get feedback on my dive-related ramblings.
As to your question, is the example calculation you provided based on RMV at rest, or even under narcosis? I’m asking because 7.5 l/min would be very low even for a small diver, and I don’t think I’ve seen anyone who breathes a 6, except some kids maybe. (When breathing air, our lungs take up about 5% of the inhaled volume in oxygen. With an RMV of 6 l/min, that would be 0.3 l/min of oxygen available to cover metabolic needs. IIRC that’s too little for most people to live on, at least while diving.) 
The typical range for somewhat experienced, adult divers (I’d guess it covers 90% of the population) is between 10 and 18 l/min. Large men sometimes use more (north of 20). Then again, I personally know a large middle-aged guy who breathes less than 10. Long story short, in practice you’ll find a lot of variance around any value derived from estimates based on physical characteristics. I suppose this is due to there being a large number of extraneous factors in diving compared with the controlled environment of a surgical theater, and all of these factors work to increase RMV compared with a resting state.
So if you want to know your RMV, there’s no way around tracking your gas use and doing the math post-dive. A how-to is linked in the Alert Diver article. On the plus side, there is surprisingly little variance for a given individual between dives – maybe a liter in either direction on a particularly good or bad day, holding everything else constant (gear configuration, workload, environment). So if you make it a habit to calculate your RMV after every dive, you’ll have a pretty dependable value after only a few iterations. 
Hope this helps, thanks again for the question, and please feel free to follow up if there’s anything else I can do for you.
All the best and safe diving, Tim