Air consumption?

[mts0628]

mts0628,

With all due respect, when I had my class "A" flight physical, was put on the treadmill and had a hose attached to my nose and mouth so that my air consumption could be measured, the unit of measure used by the doctors to describe the volume was in cubic feet of air per minute.

If you're breathing is measured in psi, how many psi of gas do you remove from the atmosphere every minute?

The psi method of determining gas consumption is based upon the size tank one is diving and varies with the size/working pressure of the tank.

The cubic foot method of determining gas consumption is based upon the amount of gas aspirated by the person. This method remains constant regardless of what size tank, or its working pressure is used.

the K

Hello Kraken:

My post was not to ruffle feathers, er scales, but to just correct the nomenclature and units of measurement.

So during your Class A physical, I no doubt believe that they reported your consumption of air in ft³/min. What they were reporting was the RMV. Where do you think those hoses were going to? As scientists, precision and accuracy would be paramount so having a measurable gas source attached to the hose would ensure that a proper SAC could be computed and then computing the cylinder constant, an RMV could be reported. This is the most obvious to me but there is probably another way.

Now I know little about aircraft life support, but I would imagine that the RMV would be handy to know if you were flying in a pressurized aircraft and you were required to be breathing off a tank of gas, probably oxygen. Now if you were a fat body or just out of shape, then they would not want to put you in an aircraft that you could suck a standard tank in to a vacuum because you were a gas hog, and thereby jeopardize the mission. I would also venture to say that the tanks on aircraft would be standard, with the exception of mission dependency (and not tailored to each pilot). Again, I know little about aircraft life support.

So, after one of my many Halo/Scuba physicals (where they don't bother with the treadmill and RMV- which I find kinda' odd, but I could never get a Doc to defer the rectal- which I found, "uncomfortable" to say the least), my first hardcore introduction to gas formulas and equations was at the Trumbo Point Naval Annex in Key West, were we had, I say diving aficionados to say the least, as instructors. Nomenclature was drilled into our heads as was a lot of other seemingly now useful information.

Years later, I had the opportunity to participate in scientific diver training- where there is no certification (for the watchful eye of one of my friends ) I was reintroduced to the concepts. Our bible was the NOAA Diving Manual, 4th ed., and Air Consumption Rates started on page 8-11. On page 8-14, there is a simple equation for converting the SAC to RMV. But the moral is, SAC is measured in psi/minute, whereas the RMV is measured and reported in ft³/minute. Evaluating the equation:

RMV=SAC*k, where k is reported as ft³/psi.​

Now let's look at how the measurements march out:

psi/minute*ft³/psi, the psi's in the equation reduce out, leaving ft³/minute- our RMV.​

So, SAC is independent of cylinder constants. It doesn't matter what size of cylinder you're breathing from, you can still compute your SAC [difference in psi/((depth in feet+33)/33). But, in order to compute your RMV, we need to know the cylinder constant, which can be whipped up in short order [cylinder volume/working pressure]. Then we report, "my RMV is 0.6ft³/minute", and mine really is, but I know I can do better.

And finally, NOAA actually has a neat .pdf that has most of the formulas that we could use. This is a bit more organized compared to what I have written, as I do not know if MS Equation Editor is compatible with posting. But I will say, just be thankful you don't have to read my handwriting!

With kind regards,
Thomas

 

[frankmzd]

Where do you think those hoses were going to? As scientists, precision and accuracy would be paramount so having a measurable gas source attached to the hose would ensure that a proper SAC could be computed.

I don't think there is any need to connect the hose to a tank as the gas source, if the gas is air.

Our bible was the NOAA Diving Manual, 4th ed., and Air Consumption Rates started on page 8-11. On page 8-14, there is a simple equation for converting the SAC to RMV. But the moral is, SAC is measured in psi/minute, whereas the RMV is measured and reported in ft³/minute. Evaluating the equation:

RMV=SAC*k, where k is reported as ft³/psi.​

Now let's look at how the measurements march out:

psi/minute*ft³/psi, the psi's in the equation reduce out, leaving ft³/minute- our RMV.​

It is indeed curious that NOAA mention SAC in psi/min (an "alternative" approach to rmv, as they say), but maybe that is simply a description of what people do, not so much a validation or endorsement of this being a useful way of measuring air consumption.
The NOAA manual section you refered to never explicitly states what cylinder size the calculations are based on, but I think the procedure in 8.5.1, especially step 4, assumes that the same cylinder size is used for all 4 steps (meaning, don't measure the SAC on a double 100 and then use a single 62 for dive planning etc.). Also they mention a "standard 80 cubic foot" cylinder in that section, so maybe that is what they have in mind.




So, SAC is independent of cylinder constants.

It certainly is not. As K also suggested, imagine calculating the SAC on a 30 cuft pony bottle , and then on a much much bigger air source (the outside of your house for example - try breathing through the keyhole ).
I think you would agree that the psi of the atmosphere will not change if one person breathes, but that does not mean that your RMV is 0 (if calculated from the SAC rate with the NOAA formula it would be, as the SAC rate in this example is 0psi).
On the other hand, the RMV in cuft /min will be non-zero no matter if you breathe from a 6cuft spare air or from the "outside of your house".

 

[muddiver]

I think you guys missed the point.

 

[scjoe]

Air consumption is measured in different ways for different purposes. If the flight test was determining aerobic capacity, it would have been measuring VO2MAX which is I believe is millilters per kilogram of body weight per minute. In that scenario, higher is better.

 

[alchemist117]

Depending on current, water temp and work load, between 50 and 80 bar.

 

[mts0628]

I don't think there is any need to connect the hose to a tank as the gas source, if the gas is air.

Maybe, maybe not. Reconsidering, there could be a flow meter attached to the hose monitoring the volume of gas passing the detector, but I would think that using a known tank with standard (humidity, temperature, and pressure) gas and topped with an accurate gauge would be more true and not as prone to variables as having free air that would be subject to humidity, temperature, and ambient pressure.

It is indeed curious that NOAA mention SAC in psi/min (an "alternative" approach to rmv, as they say), but maybe that is simply a description of what people do, not so much a validation or endorsement of this being a useful way of measuring air consumption.
The NOAA manual section you refered to never explicitly states what cylinder size the calculations are based on, but I think the procedure in 8.5.1, especially step 4, assumes that the same cylinder size is used for all 4 steps (meaning, don't measure the SAC on a double 100 and then use a single 62 for dive planning etc.). Also they mention a "standard 80 cubic foot" cylinder in that section, so maybe that is what they have in mind.

I don't know where to start. Are you suggesting that a technical textbook used by a scientific agency is just substituting one name for another because of accepted commonality? If so, then you obviously have no background in science. I do not say this with disrespect no matter how it may be read, but in science, or in good science there are no shortcuts and there is standardization (at least).

As far as your example in 8.5.1, especially step 4, that has nothing to do with the question by the original poster. The example is, for those who do no have the NOAA Diving Manual handy:

"4. To estimate how many minutes a cylinder of air will last at depth, divide the number of usable psi in the cylinder (as shown on the [SPG] minus the reserve amount) by the psi per minute used at that depth."

Furthermore, if the cylinder constant is provided, what does it matter what size cylinder is being used? They do mention the "standard 80 cubic foot" aluminum cylinder as you pointed out, but they also compare it to a 71.2 cubic foot steel cylinder pressurized to 2400 psi in Figure 8.3. Both are mentioned in Table 8.8. Further reading reveals that the problem in 8.5.2 provides a set of twin 80 ft³ aluminum cylinders charged to 2400 psig. So there are ample examples of calculating the cylinder constant.

And then, 8.5.3.- thank you for making me read. The first problem: Estimate the total air requirements for a 30-minute dive to 60 feet for a diver with a RMV of 0.92 acfm. So they calculated the RMV in [a]CFM. The solution is reported in scf (82.88 scf for those who want to work it out and check your answers). This was my only correction to the Kraken's post: SAC is reported in force/time while RMV is reported in volume/time (I kept this generic for the metric peeps). I did not sharp shoot his calculations; I just wanted to square away the units of measurement in a respectable fashion.

As more reinforcement for the measurement of SAC and RMV, in The Physiology of Medicine and Diving, 4th ed., on page 44 RMV is reported in a volume per minute (15-75 litres BTPS/min). Another very good book and one of my favorites, is Captain Michael Ange's Diver Down. Captain Ange has very good explanations and definitions in his section on Dive planning, starting on page 82.

I said, "So, SAC is independent of cylinder constants."

You replied, It certainly is not. As K also suggested, imagine calculating the SAC on a 30 cuft pony bottle , and then on a much much bigger air source (the outside of your house for example - try breathing through the keyhole ).
I think you would agree that the psi of the atmosphere will not change if one person breathes, but that does not mean that your RMV is 0 (if calculated from the SAC rate with the NOAA formula it would be, as the SAC rate in this example is 0psi).
On the other hand, the RMV in cuft /min will be non-zero no matter if you breathe from a 6cuft spare air or from the "outside of your house".

I will just reaffirm my comment that SAC is independent of cylinder constants, simply by definition. We can calculate our SAC with any cylinder on our back as long as we have consumed some amount of measurable gas and at a given depth (keeping in mind temperature, physical fitness, task loading, and exertion). Armed with this calculation and then computing a cylinder constant (or if it is provided), an RMV can be calculated.

And finally, I am not going to try and trivialize the understanding of SAC and RMV by using examples that are not real world. I will try to understand your examples so that I may gain an understanding of your way of thinking, but that will be it.

In closing, SAC is measured in force/time (psi/minute) and RMV is measured in volume/time (ft³/minute). The understanding for these two units at least come from my [improperly] cited references, faculty at my local university who taught my scientific diving course, the hardened men of C Company 2nd Battalion, 1st Special Warfare Training Group; and last but certainly not least, Mr. Dick Rutkowsi (of Hyperbarics International and a retired NOAA employee). Surely all of these people haven't been using poor terminology all along.

Sincerely,
Thomas

 

[Doc Harry]

With all due respect to the Kraken, Surface Air Consumption rate is reported in pounds per square inch per minute, while Respiratory Minute Volume is reported in cubic feet per minute. Substitute RMV for SAC in the above, and all is good. And as a good starting point, an average RMV is about 0.75 Ft³/minute.

The units of Surface Air Consumption are CUBIC FEET PER MINUTE, corrected to surface volume.

It doesn't make any sense using units of PSI/min for SAC because the PSI varies depending on the tank volume and rated pressure that was used to measure the SAC.

SAC that is reported in PSI/min is an entirely useless number that has no meaning outside of its context.

On the other hand, a SAC that is reported in cubic feet per minute is a wholly portable number that has its context incorporated into its units.

 

[SparticleBrane]

The units of Surface Air Consumption are CUBIC FEET PER MINUTE, corrected to surface volume.

It doesn't make any sense using units of PSI/min for SAC because the PSI varies depending on the tank volume and rated pressure that was used to measure the SAC.

SAC that is reported in PSI/min is an entirely useless number that has no meaning outside of its context

On the other hand, a SAC that is reported in cubic feet per minute is a wholly portable number that has its context incorporated into its units.

I believe Thomas' argument will make more sense if you read further than the one post you responded to.

 

[muddiver]

Thomas' sgrument (or point) is reinforced by another SB Guru NWGreatfulDiver

NWGratefulDiver.com

See the third paragraph in the third section on the page. This is my "engineering" understanding of the definitions of SAC and RVM.

I do not agree with Doc Harry and Kraken.

 

[The Kraken]

OK, one last question.

You're sitting in your chair at your computer. How many psi are you breathing?

the K