Why Surface with 500 PSI?

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johndiver999

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Looks like you are putting that resource to good use. You seem to be making assumptions which are quite conservative, which I find zero fault with. However, one of the more important input parameters is the stressed SAC rate or multiplier you select.

Since you asked about the RMV multiplier, I encourage you to experiment in a controlled situation. Perhaps in a safe shallow spot, swim like hell for a minute or two and really try to record your air use and determine for yourself what an RMV of around 2 cf/minute feels like or represents. Depending on your size and aerobic capacity that (RMV of around 2) may work out to be overly conservative or perhaps not at all. It is really something that is personal.
 

The Cosmicist

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I was trying to be very conservative, and you are right to guess that measuring my stressed RMV (or trying to get to 2cu ft/min) would be helpful for me. That's a great idea, thanks! I was wondering about that... The RMV multiplier is rather obscure unless you know what range you are working within in real life. For some, a 2 cu ft/min RMV is unrealistically high. For others, it's right after they see a shark haha.

Looks like the homework assignments for my next dives will be measuring accurate calm and stressed RMVs. Just to make sure I understand how to do this well, let me pose a scenario and have you (or someone else) check it.

According to the spreadsheet, the tank factor for AL80 is 38.76 psi/cu ft. If I'm trying to get to 2.1cu ft/min RMV, I need to see 77.52 psi/min disappear. Round that to 80 psi/min since I don't have an AI computer. I'll try that for 2 minutes to see if the needle passes from one hash mark to the next (about 2.6cu ft/min and 200psi), blows right by it (3 cu ft/min or more), or stays within the hash marks (close to 2.1 cu ft/min and 160 psi).

How does that sound? Doing it for longer (or with AI/computer console) would be more accurate, but that should give me an idea.
 

rsingler

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Keep in mind that this is surface consumption. At 33 feet, an RMV of 2 cuft/min is actually 4 cuft/min. It's tough to generate that much effort right at the surface, so I'd pick an easy depth to compute with and measure there.

Then again, that's why the gas disappears so quickly. Get in a high workload jam at 80 feet, and the psi of 2.1CFM x 3.4atm goes fast! Even if you settle down and ascend immediately, it's impossible to get back what you lost in those first minutes at depth.
 

scubadada

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It gave me some perspective on gas consumption when I understood the ANSTI simulator testing of regulators. They use 4 standard conditions for testing. BPM=breaths per minute at 2.5 liters per breath

1)
15 BPM, RMV 37.5 L/min or 1.32 cu ft/min, 132 fsw/5 atm. This is said to be an aggressive breathing rate at the recreational limit.

2)
30 BPM, RMV 75 L/min or 2.65 cu ft/min, 132 fsw/5 atm. This is said to be the rate for a single diver at an extremely heavy work load and to simulate 2 divers breathing at an aggressive rate.

3) 25 BPM, RMV 62.5 L/min or 2.21 cu ft/min, 165 fsw/6 atm. This is the European conformance standard EN250

4) 25 BPM, RMV 62.5 L/min or 2.21 cu ft/min, 198 fsw/7 atm. This is the US Navy Class A test.

It is said that the volume of a normal breath (tidal volume) is about 500 mL, 0.5 L. The normal respiratory rate is about 12 breaths/min. So, increase the volume of a breath 5 times and increase the rate 25-150%, that's a lot of breathing.

Condition 1 would empty an AL80 from full to 200 psi in a little under 11 minutes, condition 2 in half that time! Keep in mind that the density of gas will have also increased 5X, to 6.0 g/L
 

The Cosmicist

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Keep in mind that this is surface consumption. At 33 feet, an RMV of 2 cuft/min is actually 4 cuft/min. It's tough to generate that much effort right at the surface, so I'd pick an easy depth to compute with and measure there.

Then again, that's why the gas disappears so quickly. Get in a high workload jam at 80 feet, and the psi of 2.1CFM x 3.4atm goes fast! Even if you settle down and ascend immediately, it's impossible to get back what you lost in those first minutes at depth.

Very good points. All the more reason to stay calm, keep your skills sharp, and plan well. I'll probably hover around 33 feet and try since that's an easy multiple.

It gave me some perspective on gas consumption when I understood the ANSTI simulator testing of regulators. They use 4 standard conditions for testing. BPM=breaths per minute at 2.5 liters per breath

1)
15 BPM, RMV 37.5 L/min or 1.32 cu ft/min, 132 fsw/5 atm. This is said to be an aggressive breathing rate at the recreational limit.

2)
30 BPM, RMV 75 L/min or 2.65 cu ft/min, 132 fsw/5 atm. This is said to be the rate for a single diver at an extremely heavy work load and to simulate 2 divers breathing at an aggressive rate.

3) 25 BPM, RMV 62.5 L/min or 2.21 cu ft/min, 165 fsw/6 atm. This is the European conformance standard EN250

4) 25 BPM, RMV 62.5 L/min or 2.21 cu ft/min, 198 fsw/7 atm. This is the US Navy Class A test.

It is said that the volume of a normal breath (tidal volume) is about 500 mL, 0.5 L. The normal respiratory rate is about 12 breaths/min. So, increase the volume of a breath 5 times and increase the rate 25-150%, that's a lot of breathing.

Condition 1 would empty an AL80 from full to 200 psi in a little under 11 minutes, condition 2 in half that time! Keep in mind that the density of gas will have also increased 5X, to 6.0 g/L

That's eye-opening. That really makes you think about how easy it would be to run out of air in a panic situation beyond recreational depth. Makes me respect all those guys like John Chatterton and Richie Kohler, diving difficult wrecks at +200' on normal air before Trimix came along! Again, makes me think of the importance of redundancy and planning.
 

formernuke

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My thoughts on gas calculations.

Charter captain and myself say plan 500 back boat. So I plan to be back on the surface with inflated BC at 500. 3000 psi starting.

2500 psi of total diving pressure. Depending on depth I then plan my ascent and turnaround pressure so that I meet that requirement.

For example I was doing a 95 foot dive off cape ann, top of the reef if memory serves was 50 feet. All of us in steel 100 tanks. 3500 pound starting, turn around from 95 feet 2000 pounds beginning ascent from 50 at 1200 leaving a extra safety margin.

As for the spegial Grove well we don't like each other.
 

formernuke

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If a DM / guide tells me I have to get on the boat with a minimum volume, I really don’t pay attention.


Careful I know at least one op that has the mate glance at your gauge as you come back on
 

scubadada

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2100 dives, nobody has ever checked my surfacing pressure.

If the standard is 500 psi in an AL80, that is 12.9 cf of gas. For a HP 100 that would be 444 psi, for a HP 120 that would be 370 psi...
 

formernuke

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2100 dives, nobody has ever checked my surfacing pressure.

If the standard is 500 psi in an AL80, that is 12.9 cf of gas. For a HP 100 that would be 444 psi, for a HP 120 that would be 370 psi...


I wasn't going to get into why the same pressure for different size tanks as the math is already getting hard for some it seems.
 

BLACKCRUSADER

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Careful I know at least one op that has the mate glance at your gauge as you come back on

Would not bother me. I went on a dive lead by an instructor I had done recreational dives with before she did her DM or Instructors certs.
She told all the divers she wanted to see the half tank sign when divers got to 120 bar. There were a few vacation divers who haven't dived for a year so the max depth was 20m. All the other divers gave her the half tank signal during the dive. Normally for most guides its 100 bar to signal half tank.

After the dive ends and we are back on the boat the instructor made a point in front of all the other divers to tell me off for not following her instructions. I showed her my gauge with more than 120 bar and asked if I could get back in the water.

I then asked her if she did know how much air I had left she could have signaled me and asked. Her reply was that as she knows I am an experienced diver she didn't check. I then told the other divers I always follow the guides instructions and we all had a good laugh.
 
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