RMV/gas consumption.

[InTheDrink]

Maybe boring topic in wrong forum but what the hell...

I've noticed for a few years that my gas consumption at the start of a dive is around three times the rest of the dive.

I blow thru ~50bar in first 10 mins or so on a single 12L. Yet I generally manage between 60 - 80 min dive. Figures above aren't hard and fast but the general trend is.

Anyone else noticed this?

 

[waterpirate]

Anticipation for the dive ahead, and the unknown can elevate sac at the start. After the initial adreneline wears down so goes the sac.
Eric

 

[g1138]

Sometimes I too get excited when that first trickle of water enters my ears. Breathing goes to crap, I piss myself silly, do my little underwater happy dance. My team usually has to remind me to calm myself because I'm embarrassing them, and then after that all is well in low SAC town.

 

[Mr Carcharodon]

Possibly the gas in your tank cooling down and lowering the pressure that way.

 

[DevonDiver]

I've noticed for a few years that my gas consumption at the start of a dive is around three times the rest of the dive.

I blow thru ~50bar in first 10 mins or so on a single 12L. Yet I generally manage between 60 - 80 min dive.

I think many people notice this 'phenomenon'. Time to go back to your gas laws....


Note the last, important, part of the Boyle's law equation.... temperature must be constant.

Going from (warmer) air into (colder) water... and that constant ceases. The gas in your cylinder invariably decreases in temperature. This is especially noticeable in tropical locations... leave the cylinder sitting in the sun on a dive boat deck, then jump in.... even in cold-water diving, there's typically a temperature decrease on initial water entry.

Thus.... Charles' Law.... T/V=K (temperature/volume = constant)



Reduction in temperature decreases the 'brownian motion' of molecules, proportionally reducing the force they exert on surrounding objects. This is noticed as a reduction in pressure.

More information: The Physics of Diving - Scuba Gas Laws

 

[ianr33]

Sometimes my SAC is negative for the first few minutes of a dive.

 

[flyandive]

For what it's worth a lot (but not all) computers factor temperature into the equation. Look through the manual, if it does then its probably some sort of apprehension. If it doesn't mention some sort of temperature compensation then it probably doesn't factor that in.

 

[DevonDiver]

For what it's worth a lot (but not all) computers factor temperature into the equation. Look through the manual, if it does then its probably some sort of apprehension. If it doesn't mention some sort of temperature compensation then it probably doesn't factor that in.

If a computer were to record the SAC (OP doesn't mention how he achieved his SAC figures), then any temperature compensation would be based on ambient environmental temperature. It wouldn't record the temp of the gas itself (I think??). That wouldn't account for the gas/cylinder temp heating due to direct sunlight exposure etc... where the tank temp could become substantially higher than the ambient air temp. Less chance generally of a dive computer being left out in the sunlight, than cylinders..

 

[-hh]

Possibly the gas in your tank cooling down and lowering the pressure that way.

Absolutely.

For the typical temperatures we can expect to dive in, a tank that's been warmed in the sun can easily be 30F (15C) above the water temperature that it will eventually be immersed in (110F in the 80F warmwater tropics, or 25C in 10C waters)...

...and when we remember to use the Kelvin (or Rankine) scales in the formulas...

results in a 5% change: roughly -150psi on a 3000psi AL80, which is the same as -5 bar in a 200 bar tank.



-hh

 

[DivemasterDennis]

The "shrinkage factor" on things that are immersed in cool or cold water is a fact that applies to gas in a scuba cylinder as well as other things. No doubt the more rapid decrease in pressure is attributable in large part to that fact. When we place a 3000 psi filled 80AL tank that has been in 80 degree air into 65 degree water , there is reduction in pressure of at least a couple of hundred psi, giving the appearance of rapid use. As the op is from the UK, it is a safe bet he is diving in rather cool or downright cold water in local diving, where the shrinkage factor will be more pronounced that in warmer water. It is also likely that respiration rate is higher early in the dive due to the excitement of beginning the activity, adjusting to the cooler environment, and moving to the site objective in some cases. So to answer the op's question, yes, Debbie and I notice the same thing, for the stated reasons.
DivemasterDennis