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On 11/24/95 a post was made which said: >Jason: > >While I don't really care to enter this debate at the present time, I >would like to correct you on a point, for the benefit of others that >might be reading. > >A diver's RMV will not increase at all, due to any stress level >change. > >By definition RMV stands for "Resting Minute Ventilation" and it means just that: resting. I believe you may be confused with "Minute Ventilation" ..... Its a risky enterprise to use the wording "by definition" because you become obligated to reference the source of the definition. The definition of RMV (Respiratory Minute Volume) according to the US Navy Diving Manual, Rev.1 dated 1 June 1985 states "Respiratory Minute Volume (RMV) is the total amount of air moved in and out of the lungs in a minute. Multiplying the tidal volume times the rate gives the respiratory minute volume". I too was under the impression that RMV references the surface pressure ( I think it still is); but someone may have since redefined RMV. Anyhow here is a "definition" with a source reference. No flame here just a clarification, confusing at that. Now to the purpose of this post. Since I have defined RMV in accordance with some recognized referent, I would like to forgo an argument and say that I don't use RMV in my gas consumption estimates. I use something more along the lines of resting minute volume, but I won't call it RMV. I will call it the"X Factor." I'm allowed to do so; it's my post! It would be expected the X Factor will increase with stress, and at different times in a dive the X Factor will vary. One way to incorporate the changes in the X Factor during a dive is to make a careful log of the time, depth, and tank pressure during a dive (water temperature if your more anal retentive) and post-process the information. Rather than approximating the X Factor while at rest on the surface or from a controlled surface swim as some may suggest, I submit you can incrementally account for X Factor in phases of an actual dive to obtain an average X Factor for the type of dive you are doing. The X Factor, depth, and time are the three factors which account for the use of gas during the dive. You can monitor this by the tank pressure; the rate of gas consumed is related to the rate of change of tank pressure (Boyles law). The total amount of gas consumed during the dive is a summation of the incremental gas consumption during distinct phases of the dive. These phases are defined by how often you log the depth, time, and pressure parameters. For example during a one minute period on the bottom you will consume a certain average volume of gas due to the depth, X Factor, and the one minute duration ((ambient pressure/1 atm) x X Factor x 1 minute). The total gas consumed during the dive will be equal to the total (summation) of all the individual phase consumptions. If you do the math you will see the X Factor can drop out of the formula for each individual phase (its assumed an average constant) and the summation now becomes the total of ((ambient pressure/1 atm) x 1 minute)for each phase. The time period can be what ever you make it but shorter is better (how often you log). You now calculate using Boyle's law the total gas consumed during the dive from the beginning and ending pressure in the tank. If you divide the total gas volume consumed by the summation described above, you will get an average X Factor for the type of dive you just did. Remember the X Factor is an average constant that describes your consumption at rest. Why so much trouble? It more closely represents reality in a resting consumption coefficient (X Factor) since it empirically accounts for the whole dive (various levels of stress). How close is it? Really close, as long as you do this periodically to check how your X Factor may be changing. Mine is higher now than before because I've missed a lot of diving due to surgery. Does this mean you can now plan your gas requirements down to the wire? You can but you'll probably croak doing so. Its nothing more than a tool to assist you in proper gas management. You still have to use your brain and call upon prior experience. I typically plan the gas requirements, add the necessary safety margin for me and my partner, and then monitor consumption rate during the dive. If I see gas consumption deviating significantly during the dive, I know something is off and respond accordingly. Doug Chapman
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