Tank calculations

[V&G]

Some usefull info about tank calculations found in wiki...

1 bar = 14.5038 psi
From here:
Working 200 bar = 2900 psi
Maximum 300 bar = 4351 psi


1 liter = 0.03666 cu ft
Volume of gas at atmospheric pressure = (cylinder volume) x (cylinder pressure) / (atmospheric pressure)
Air pressure is 14.7 psi
From here
the common aluminum cylinder with an internal capacity of 11.1 liters filled at 2900 psi (200 bar) will contain
11.1 * 0.03666 * 3000 / 14.7 = 80.15 cu ft of air - the common "aluminum-80"


air amount in the cylinder = cylinder valume * cylinder pressure
From here
10 liter cylinder filled at 200 bar will contain 2000 liters of air at atmospheric pressure.


The following formula is far from to be precise but for sure might let some estimation...
gas consumed = breathing rate x time x ambient pressure
->
time = gas consumed / breathing rate / abmient pressure
Breathing rate or respiratory minute volume (RMV), in litres per minute (lpm) of the diver in normal conditions will be between 10 and 25 lpm.
(At times of high work rate or panic, breathing rates can rise to 100 lpm)
The ambient pressure at the surface is 1 bar, at 10m is 2 bar, at 20m is 3 bar, at 30m is 4 bar etc.
From here a diver with a breathing rate of 20 lpm will consume at 30 meters (4 bar) the equivalent of 20 * 4 = 80 lpm at the surface (1 bar).
If this diver had a 10 litre 200 bar cylinder (10 * 200 = 2000 liter of air) to breathe from,
the gas in the cylinder would be exhausted after a little over 2000/80/1 = about 25 minutes.

 

[scubadiver888]

Huh? I'm not sure if what you are saying is correct but it sure looks more complicated that it needs to be.

If you are using the Imperial system you get the Pounds / Square Inch or PSI of a cylinder. If you are dealing with the Metric system you get the BAR of a cylinder. Both these are called the Service Pressure of the cylinder.

The amount of air in a cylinder is called the nominal capacity. The nominal capacity is the amount of air in the cylinder if the cylinder is filled to the Service Pressure. So if I have a cylinder with a nominal capacity of 10.1 litres and a service pressure of 230 bar then the cylinder holds 10.1 litres of air when the pressure (at room temperature) is 230 bar.

If we look at the same information in Imperial measurements, If I have a cylinder with a nominal capacity of 99.5 cubic feet of air with a service pressure of 3442 PSI then it holds 99.5 cubic feet of air when the pressure is at 3442 PSI.

When calculating your Surface Air Consumption (SAC) rate, you look at how many cubic feet (Imperial) or litres (Metric) of air you breath per minute.

If I breath 0.7 cubic feet / minute at the surface then a cylinder with 99.5 cubic feet would last me 142 minutes on the surface. At 33 feet or 2 ATA it would last me 71 minutes (twice the ambient pressure means half the time). At 66 feet or 3 ATA it would last me 47 minutes (three times the ambient pressure means one third the time).

If we are working in metric:

If I breath 0.071 litres / minute at the surface than a cylinder with 10.1 litres would last me 142 minutes on the surface. At 10 metres or 2 ATA if would last me 71 minutes (twice the ambient pressure means half the time). At 20 metres or 3 ATA it would last me 47 minutes (three times the ambient pressure means one third the time).

Not too sure why you want to mix metric and imperial. I tend to find it easier to work in one system of measurement.

Regardless, do a search (on this site or with Google) for Rock Bottom Gas Management and you will find some good articles about all this.

 

[jsf35]

In North America the capacity of the tanks in cubic feet i.e. "80" or "95" is calculated using the rated service pressure of the tank (including the "+" symbol on steel tanks)

The metric capacity in litres is based on the amount of liquid that tank would hold. i.e. a standard aluminum 80 tank would hold approx. 11 litres of liquid. (no pressure involved)

If you have a RMV of 0.7 cubic feet per minute, in metric would be approx 15 litres per minute.

 

[knowone]

I grab a tank and get in and go down there and look at that and then look some more and touch things and then go somewhere else and maybe go down some more or stay there or come up some so I can stay longer and go sideways with some more touching and looking and then go up some more and hang around a bit and the after some more I get out again.

With some bars and some psis left having looked at my spg a few times in between.

 

[V&G]

If you are using the Imperial system you get the Pounds / Square Inch or PSI of a cylinder. If you are dealing with the Metric system you get the BAR of a cylinder.
...........
When calculating your Surface Air Consumption (SAC) rate, you look at how many cubic feet (Imperial) or litres (Metric) of air you breath per minute.

That's exactly the reason I started to look at this kind of information. This forum talking in PSI and cubic feet. "My" surround understands BAR and liters. I needed some conversions to build the paralels and to understand what are you talking about here, what I can get on the local market.

That's all. Other info is just some kind of addition, made me to think about these aspects so I shared them with you. It may be useful for someone and completely useless for all others.

I grab a tank and get in and go down there and look at that and then look some more and touch things and then go somewhere else and maybe go down some more....

Yeah, FUN-&-SAFE diving to ALL!

 

[scubadiver888]

In North America the capacity of the tanks in cubic feet i.e. "80" or "95" is calculated using the rated service pressure of the tank (including the "+" symbol on steel tanks)

The metric capacity in litres is based on the amount of liquid that tank would hold. i.e. a standard aluminum 80 tank would hold approx. 11 litres of liquid. (no pressure involved)

If you have a RMV of 0.7 cubic feet per minute, in metric would be approx 15 litres per minute.

Okay, this is making a little more sense to me or at least I understand what I'm missing. When a cylinder's specs say 10.1 litres capacity they are talking about how much water it can hold. I've been, incorrectly, assuming 99.5 cubic feet of air is 10.1 litres of air.

So if 1 cubic foot of air is 28.32 litres of air (did a quick Google), then an RMV of 0.7 cu.ft./min. would be 28.32 * 0.7 or 19.824 litres/minute?

So is it correct that a 10.1 litres tank at 230 bar would be 10.1 * 230 or 2323 litres of air? Something seems a little off for me. If I KNOW I get 142 minutes on this tank and my SAC rate is 0.7 cubic feet/minute then my RMV would be 2323 litres/142 minutes = 16.4 litres/minute? This does not match my conversion above (19.824).

Hmm, I think I'm going to have to pull my books out and re-do my knowledge reviews in metric. It is so confusing growing up imperial, living in a metric country which still uses imperial for scuba.

 

[scubadiver888]

That's exactly the reason I started to look at this kind of information. This forum talking in PSI and cubic feet. "My" surround understands BAR and liters. I needed some conversions to build the paralels and to understand what are you talking about here, what I can get on the local market.

That's all. Other info is just some kind of addition, made me to think about these aspects so I shared them with you. It may be useful for someone and completely useless for all others.


Yeah, FUN-&-SAFE diving to ALL!

I'm going to think about this over the holidays. Maybe the people in Quebec use the litres and bar thing. Even though my country is metric, we still get a lot of stuff from the USA and therefore do scuba in Imperial measurements.

 

[Kevrumbo]

Discovered an interesting one-to-one correspondence between depth and my physiological gas consumption rate in bar/min, while diving with double AL80's (and why I now prefer using the Metric System even in the US). An example:

18m depth, same as 2.8 ATA -->translates directly to 2.8 bar/min
21m depth, same as 3.1 ATA -->translates directly to 3.1 bar/min
24m depth, same as 3.4 ATA -->translates directly to 3.4 bar/min
27m depth, same as 3.7 ATA -->translates directly to 3.7 bar/min
30m depth, same as 4.0 ATA -->translates directly to 4.0 bar/min

So for an arbitrary & convenient time period of say, for example 10 minutes, at a particular depth above, I would expect to consume:

2.8 bar/min(10min) = 28 bar
3.1 bar/min(10min) = 31 bar
3.4 bar/min(10min) = 34 bar
3.7 bar/min(10min) = 37 bar
4.0 bar/min(10min) = 40 bar

Essentially then, all I need to know is depth in ATA and time at that depth, and I'll know instantly how much gas I've consumed --even before looking at the SPG!

Parameters:
Surface Consumption Rate (SCR): 22 litres/min*ATA [My physiological gas consumption rate]
Twin 11 litre tanks (double AL80's): 22 litres/bar total tank rating.

Now dividing SCR by total tank rating (22 divided by 22) yields: 1 bar/min*ATA (and that's the secret easy math trick discovered & applied above --the unity conversion ratio of 1 bar/min*ATA which lets me transform directly from depth to a delta bar per minute rate, and the resulting SPG bar consumed).

For a Single 11 litre tank:
Surface Consumption Rate (SCR): 22 litres/min*ATA
11 litre tank (single AL80): 11 litres/bar total tank rating.
Dividing SCR by total tank rating (22 divided by 11) and I get 2bar/min*ATA rate for single tank diving.

The point of all the above is, I'd rather work with easier unit chunks such as 1bar/min*ATA in metric, rather than 14.5psi/min*ATA in imperial units --especially doing on-the-fly, during the dive gas calculations & remaining gas estimations.

 

[spoolin01]

I grab a tank and get in and go down there and look at that and then look some more and touch things and then go somewhere else and maybe go down some more or stay there or come up some so I can stay longer and go sideways with some more touching and looking and then go up some more and hang around a bit and the after some more I get out again.

With some bars and some psis left having looked at my spg a few times in between.

At last!! THIS, I can remember....

 

[scubadiver888]

Did a little reading and this is all pretty straight forward for me now.

With imperial measurements you have the total volume at the service pressure. With the metric measurements you have the volume of water and this happens to be the volume of air at 1 bar.

So when I try to calculate my SAC rate with imperial measurements I do the following:

- Go to 33 feet
- Note my PSI
- Dive, swim, etc. while staying at 33 feet for 10 minutes
- Note my PSI
- Take the difference in PSI and divide by 10 to get my PSI/minute
- Convert to 1 ATA (the dive was at 2 ATA) by dividing by 2.
- Convert PSI to cubic feet

The last step requires me to know the cubic feet at service pressure plus the service pressure (e.g. HP100 is 99.5 cubic feet at 3442 PSI). This is where the real math comes in. For example, if I calculated that I use 25 PSI/minute then 99.5 cubic feet / 3442 PSI = 0.03 cubic feet/PSI then 25 PSI = 0.75 cubic feet / minute.

Now if I did the same thing in metric it would be:

- Go to 10 metres
- Note my bar
- Dive, swim, etc. while staying at 10 metres for 10 minutes
- Note my bar
- Take the difference in Bar and divide by 10 to get my bar/minute
- Convert to 1 ATA (the dive was at 2 ATA) by dividing by 2.
- Convert bar to litres

The last step requires me to know the size of the cylinder. For example, my HP100 is 12.2 litres. So if I used 1.7 bar I would take the bar used times the size of the cylinder or 1.7 * 12.2 or 20.74 litres/minute. The math seems to be a little easier because there is not converting PSI to cubic feet.

Hey, I think I got it. If I have 20.74 litres / minute and 1 litre = 0.03666 cubic feet then I have 0.76 cubic feet / minute. With the occasional rounding error, I think that is pretty close.

To take it a little further...

On my HP100 at 99 feet I would breath 0.7 * 4 cubic feet / minute or 2.8 cubic feet / minute. If the cylinder is 99.5 cubic feet it would last approximately 35 minutes.

Using metric, my HP100 at 30 metres I would breath 20.74 * 4 litres / minute or 83 litres / minute. If the cylinder is 12.2 litres and I started with 230 bar I would have 2806 litres. At 83 litres / minute it would last ~34 minutes.

I now see what V&G was doing. I just remember my RMV at the surface and the volume of air in the cylinder. I then calculate the usage at depth by multiplying by the atmospheres at that depth.

It is interesting to see when dealing with litres and bar the numbers are so small that rounding errors can built significantly. With an analogue gauge it would be really hard to figure out what your SAC rate was in 10 minutes.