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residual volume

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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Baron of Breathold
Oct 17, 2001
Sunday I tried to measure residual volume. On a dive to 10 meters, I exhaled completely for over 10 seconds, emptied my mouth and pinched my nose to close off the expanding mask. When I got to the surface, I measured the air that I could exhale in 10 seconds and it was 1.1 liters. Given the method used, it should be read 1.0-1.3, I think. Head down or head up didn't seem to make any difference. Does that sound like a measurement of residual?
Since my vital capacity measures 5.2-5.8 (the higher number is immediately after stretching), I can leave the surface with a total air volume of 6.4 to almost 8 and should reach residual at 43 to 56 meters with the Beuchat mask or get a complete mouthful of air at 36 to 46 meters. When I tried, the 35 meters (no packing) worked OK, but at 45, I couldn't fill my mouth. Carlos, who patiently supervised all this, commented that I lost some air on the deeper dive.
Since a full exhale plus mouth full dive to 10 meters is possible, the lungs must be able to squeeze down to about half a litre. Just thought that I would share and find out if my logic will pass review.
At 10m, once you exhale, the air which volume which remains is RV @ 2atm, so you have a total of 2RV at 1atm.

When you get to the surface, you have 2RV total in your lungs. When you exhale to the max, you exhale down to 1RV, so the amount exhale would be 1RV, so yes, it seems your method should work. Quite ingenious!

It doesn't take into account the volume of your sinuses though. If you closed your epiglottis on the descent and allowed air from the mask to fill the sinuses, it would work better. Also, it doesn't take into account blood shift, which should be quite weak for a 10m dive.

I have an additional idea.

Try this:
Do the test at 5m:
@ 5m, you exhale to RV @ 1.5atm, then upon ascending you have 1.5RV. Once you exhale down to RV, you must have exhaled 0.5RV. So double the amount you exhaled.
Do the test at 10m.
Do the test at 15m: RV @ 2.5atm, then at the surface you have 2.5RV, exhaled to 1RV, meaning you exhaled 1.5RV, so divide the amount you exhaled by 1.5.
Do the test at 20m, dividing the result by 2.
Interestingly, we see a trend:
For depth X, divide the exhaled air by X, then multiply by 10:

@ 5m: RV = ( Exhaled volume / 5 ) * 10
@ 10m: RV = ( Exhaled volume / 10 ) * 10
@ 15m: RV = ( Exhaled volume / 15 ) * 10

Then, you can graph the results, and any errors should show up in the graph.

Eric Fattah
BC, Canada
What a cunning trick - the best measure of residual volume I've heard of so far! I would be willing to bet +/-10% accuracy, which is all you really need.
There is another method too, but more complicated.

This method can measure both total lung capacity (TLC) and residual volume (RV), with no equipment needed!

The method is as follows:

- Get in the water with no wetsuit, and fully wet your hair
- Inhale to the max (and/or pack)
- See how much weight you need to become exactly neutrally buoyant

- Next, at the surface, inhale to the max (and/or pack)
- Go to 10m
- See how much weight you need (or how much buoyancy) you need to become totally neutral

- For exhales, same thing, exhale at the surface, see how much buoyancy you need to become neutral
- Do the same at 5m or 10m

To get an accuracy of 0.1L, you would need 0.1kg weights, and pellets with 0.1kg of buoyancy, all of which would need to be figured out before hand. However, it would be quite accurate, the only inaccuracy is the definition of 'neutral buoyancy.' The surface part should actually be done at 5m, to eliminate parts of your body breaking the surface.

You could make it even more accurate by not using neutral buoyancy, but by weighing yourself with a scale, via a cable to the surface.


- Inhale to the max, wearing a 15lb belt
- Go to 5m, have the surface operator 'weigh' you via a cable to the surface
- Go to 10m, weigh yourself again
- Go to 15m, weigh yourself again

Same for exhales.

This would be EXTREMELY accurate if the scale on the surface is accurate. You could figure out the total volume of compressible airspace in your body to within grams.

Eric Fattah
BC, Canada
Thanks Eric, for the analysis and new ideas. Forgot about the sinuses. I thought of something similar to your ideas and even got one to work in the pool, sort of. No question that a scale with a stable platform could achieve far greater accuracy but in the ocean, even in calm Kona, I couldn't get repeatable results in less than 10 meters with a version of your variable ballast. I was wearing a 3 mm suit though.
P.S. Thanks for the compliment Will.
Bill, that is a really great idea.
And probably the best DIY way.

Have you tried a real spirometer? - not a hand held one, but one that you sit in an air-tight cell connected to a machine that's connected to a computer?
I don't know how accurate they make them though.
They did claim to measure my RV. Was about 2lt. I hope I improved since.

Regarding accuracies, I think it's impossible in none of the ways mentioned above to get it accurate.
Pressure is not exactly 1atm at 10m but 9.8m or so, and it also depends on salinity. Diving in the Red-sea and the Med my bouyancy diffrenciate a bit. Not to mention if I'll ever want to dive in the dead-sea, I would probably still stay on the surface with empty lungs and no suit.:duh

Weighing somebody from a boat would be hard to make accurate. Try weighing yourself on the scales, then put the scales on your bed and weigh yourself. You don't weight the same. The boat's weight/bouyancy changes with your weight. The bigger the boat the less it'll matter though. Not to mention cable stretch for different lengths.
Swell and underwater currants will spoil it easly.
From the surface at a pool, you have to figure out pressure for freshwater with chlorine so you'de know by how much to multiply it. A deep pool would help (pools here are usally under 3m depth).

To sum it, I think that your first method is defenitly the best.
Or just going to the doctor and see what his machines say.
Thanks for the great idea! I gonna seek someone with a spirometer and try it out in sweet waters, without a mask and noseclip. This way I only lose one sinus and ear volume of air. This would be pretty small compared to the total lung volume.

Maybe there is a way to messure those sinusses an ear cavities seperately, collecting the air flowing out of my nose on the way up?

Anyway still a great idea!, thanks!
Thanks again for the info. It took me a long time to get answers on my own. The figures from the doctors' offices were confusing until I found an explaination. One doc used the old formula of Vc*.24=Vr(1.3L) and another used a formula with four variables that gave me a Vr of 2.2L. The third doc said that he could measure it with gas analysis for a few hundred dollars. A weight loss clinic asked for volunteers to sit in a pod but they came up with wierd numbers and told me that I was fat, no news here. Still trying to find an estimation of sinus and dead space volumes.
I discovered many things;
pool water doesn't have a 1.0 density
ocean density depends more on temperature than salt.
lead only weighs 88% in the water.
body density doesn't appear to vary with temp.
if you use an hygrometer to measure density, there is a 3% difference between adjusting the temp. to standard and applying the temp. correction.

The most important lesson, I think, is to spend the time in the water trying to perfect the dive and not playing around.
Last edited:
Another way

I've got another way. Here's what you'll need ....

- 50 feet of Cat5e cable
- 2 Midgets
- A medium giraffe
- 1 semi-competent juggler

Oh wait - wrong thread. ;)
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What is the need for a depth correction depending on salinity, water temperature etc. considering the following:

You probably measure all the deapths with the same deapth gauge/ computer. These machines normally use a fixed water density to calculate deapth (in the manual it mostly says that they are calibrated for salt or seawater use). Normally deapth calculation is not temperature corrected (no corrections are made in water density/ displayed deapth based on different temperatures). If you know what water density the deapth gauge/ computer uses for its calculations there is no need to do corrections for this RV calculation. For the calculation you want to know what water pressure you are at and you do not need to know the exact corrected deapth so you can skip a lot and calculate the water pressure directly. Displayed deapth * water density used by the computer model = water pressure you are at.

If you use lead to do the measurements:
I think that the lead if measured in sweet or saltwater makes no huge difference in the end result of the whole calculation due to small differences and other inaccuracies in other measurements. Just weigh what the lead that you use weighs in the water, which is less than on land, and also depends on things like if it is coated or not.

Great ideas to measure RV. Would be wonderfull to see some more experiences with the various measurements and methods.

Another idea: does the small error that the volume of the sinusses or mask make matter if you equalize them anyway? This could make these methods even more valuable than the (expensive) medical tests.
Bill, if you could tell me how that gas analysis was supposed to be done, I might be able to tell if that's what I have done.
It was a part of my spirometer test in a lab. It was mostly covered by my medical insurance (I think healthcare works better/cheaper here than in the US), I had to cover about 10$ of the cost if I remember correct.
Since I'm a (mostly ex-) asthmatinc, I can ask my doctor to direct me to this test anytime I want.
Come to think of it, I think I can ask her to direct me to most tests that don't cost a fortune. Have any recommendations? ;)

Thanks for an elegant idea. I am going to try the same thing in a pool of known depth and adjust the calculation accordingly. Probably less accurate than using a 1 atm depth, but should be close enough to satisfy my curiosity.

It is my impression that your rv is lower than average. Have you done anything, other than lots of deep diving, to reduce your rv?



That's fantastic (very clever old guy indeed) and you can count me in for a trial of it in a few weeks time. I'm counting the sleeps now until Kona.

Good thinking. No arguement with any of it. The variable density statement concerning water, fresh water, lead and even air explained why my observations didn't match calculations on buoyancy.
I read a statement once that said something about injecting a known quantity of inert gas during inhale and doing a test to find out the percentage of the gas in the exhale. When I asked someone at the hospital doing lung tests about it, he said something like "expensive and not covered".
>"satisfy my curiosity"
Well put. That's what a part of life is about.
I did 4-5 hours of aerobic work, two or three lung stretches and one or two squeezes a week for over a year and increased my vital capacity about 10%. I can now pack over a liter without any damage and it looks like my Vr is about 50% of average. Who says "you can't teach an old dog new tricks"?
One more moon. c u then.
To Kars and all others who would like to measure the volume of the ears, sinuses or even the mask that needs to be equalized.

How about the following method:
Use the method of full exhale on different depths and measure the volume you can exhale again at the surface. Calculate the residual volume in the following cases:

1 when ascending close off the lungs with your epiglottis and let the air in the sinuses, mask etc escape into the water.
2 when ascending suck back into the lungs the expanding air from sinuses, mask etc. so it does not escape and can be measured.
3 dive with a noseclip or close the nose and suck the expanding air from the sinuses etc. back into the lungs so it does not escape and can be measured.

1= residual volume of the lungs
2-1= volume of the equalized sinuses+ ears+ mask
3-1= volume of the equalized sinuses+ ears
2-3= volume of the equalized mask space

I have no spirometer and therefore cannot do the measurements but am very curious what numbers anyone gets and if the volumes differ enough to be able to do the calculations.
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Clever thinking Roland! Maybe we could try on one of our next dives?


love and peace,
This even works in the pool. I calculated 1.7 L for all air spaces, including mask. Repeatability is not very precise, so overall accuracy probably suffers, but it makes a useful baseline for future comparison. Thanks Bill.

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