SAC rate & RMV

Hi everybody

I've just been calculating my SAC & RMV and it appears that there is a wide variation. I have just 51 dives and am looking towards the futurehopefully wreck & deep diving certs. At this point how do I apply this info when it seems fairly inconsistent-- or do I just need more time to develop my skills? any input is greatly appreciated.

mjacobydc:

Hi everybody

I've just been calculating my SAC & RMV and it appears that there is a wide variation. I have just 51 dives and am looking towards the futurehopefully wreck & deep diving certs. At this point how do I apply this info when it seems fairly inconsistent-- or do I just need more time to develop my skills? any input is greatly appreciated.

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Rates can vary greatly based on diving conditions, comfort level, temperature, etc. Tracking the info and noticing trends across different conditions will be useful in helping you on future dives.

Keep diving, get comfortable and your consumption will likely drop.

I agree with Cave above on his advise....do the dive time and things will fall into a more predictable pattern.

Keep tracking it as the others have said. Over time you'll see it start to even out. But when you change gear or environment don't be surprised to see it start to fluctuate again. I'm at the point now (almost 300 dives) that I know what those fluctuations are going to be and can adjust for it in my planning. Most of the time I use an average. Swimming hard, easy, resting, dry vs wet, etc. But that took alot of time and attention to the details. Thank goodness I was taught to keep a fairly detailed log.

mjacobydc:

Hi everybody

I've just been calculating my SAC & RMV and it appears that there is a wide variation.

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Your breathing rate is likely to be the primary reason for variation, no surprise. I found that counting breaths for a minute now and then throughout my dive helped me stabilize the rate. Another way is to concentrate on the mechanics of each breath. There are a lot of posts on how this is done. I find that occasional counting also helps in reverse, you develop a keener sense of time passage.

The gas you lose to buoyancy will also have some effect.

Keep a good log and include some indication of activity level. The data you collect now will become more interesting as you improve.

At 50 dives, I would stay away from "breath control teqniques" and just dive. As your comfort and relaxation in the water increases your consumption will drop and eventually stabalize...

As has already been said, your consumtion will also vary depending on if you are in a thin wetsuit in sunny Coz or in a DS in the chilly Atlantic (and a host of other personal and enviromental factors).

As others have said, track your SAC rate over time and you'll see patterns emerge. You'll probably see a cluster of similar rates at the lower end of the range, with other rates above that from time to time.

That cluster represents your baseline SAC, which is your air consumption under best conditions, with the higher rates occurring because of added effort from things like stress, current, cold or other factors that raised your breating rate. If you logged conditions along with your SAC it might give you some insight, but higher breathing rates can also be caused by things you might not have logged such as the 3 cups of coffee you had that morning.

When planning a dive try to use the SAC rate appropiate to the expected conditions, remembering to leave room for error. For example don't use your warm water drift dive low SAC to plan a cold water, low viz wreck dive.

mjacobydc:

Hi everybody

I've just been calculating my SAC & RMV and it appears that there is a wide variation. I have just 51 dives and am looking towards the futurehopefully wreck & deep diving certs. At this point how do I apply this info when it seems fairly inconsistent-- or do I just need more time to develop my skills? any input is greatly appreciated.

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Exertion is normally the main difference.

You must first learn to dive without exertion. This means relaxing, rarely kicking, never fighting your trim, just a gentle floating along with the ever so slight current if any.

And before your dive, you should relax at the surface, breathing with your face in the water (a snorkel helps with this), to trigger the mammalian reflex. Plus you want to clear out as much CO2 from your blood before you begin your descent. Getting your gear on and getting into the water usually builds up a fair amount of CO2. Gravity sucks.

For optimal RMV, get yourself a DPV.

For the ignorant among us: what is RMV?

Rhone Man:

For the ignorant among us: what is RMV?

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Respiratory Minute Volume - the volume of air a diver consumes per minute (while diving)

It's measured by taking SPG readings over time, at constant depths, under different conditions (rest, high load etc)

Then (depth in metres / 10) +1 X RMV = gas usage/requirement

SAC (Surface Air Consumption) is just a reference point - you will breath differently under water. DO NOT use you SAC to calculate gas requirements for diving, despite what seems to have been implied in ealier replies

To make a safe dive plan you should calculate based on your maximum RMV, not your 'without exertion' RMV - everyone knows you can do better, but when it becomes critical will be when you're stressed

nereas:

And before your dive, you should relax at the surface, breathing with your face in the water (a snorkel helps with this), to trigger the mammalian reflex

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From what I've read, mammalian reflex only comes into effect if the water temp is below 21C/70F. And it gets weaker with age. I don't think it is related to breathing either, other than it's easier to get your face in the water by being able to breath.

Regardless, the primary effect (lowered heart rate) would still be triggered when you went below the surface - I don't believe there's any advantage to surface submersion of the face