Cert dives what a PITA

[Roko]

Scubatoys made a neat video regarding curring a high pressure hose vs a low pressure hose, and the rate of loss...

YouTube - Cutting a Scuba Divers Regulator Hoses

 

[jviehe]

Perhaps it was the neck oring that blew? Good job as a student though. You reacted calmy and correctly. Be sure to chew out the rental person for giving you poorly maintained gear.

 

[Crowley]

Isn't 50 bar approximately 735 psi? (50 x 14.7)

er yes - thank you for pointing that out - I had intended to post that 50 bar is approximately 750 psi ( x 15 being the very rough, but servicable, approximation)

I got distracted!

Toodles,

C.

 

[tyesai]

Perhaps it was the neck oring that blew? Good job as a student though. You reacted calmy and correctly. Be sure to chew out the rental person for giving you poorly maintained gear.

I'm pretty sure our instructor has stakes in the rental company which is why he didn't seem to care. My Turkish is limited to reading menu's, asking how much, what, what, what, thats fine, some numbers, and where is the bathroom. LOL

 

[Diver0001]

About a third of the way into it, and he kept his own reg until he ran out, it didn't take to long, it sure as hell wasn't minutes, I'm new to this sport but not new to pneumatics and a tank that small free flowing a couple thousand psi out the back isn't going to take several minutes to be empty.
.

It will depend on the type of problem, how full the tank is and, of course, the depth.

Below is a post made by Curt Bowen on another board. He did an empirical test of various scenarios.

Here is a little test I did at Eagles Nest cave on Feb 12th for a small article to be placed in issue 17 of Advanced Diver Magazine.

The question I wanted to answer was:

How much time would it take to empty a full aluminum 80 cuft cylinder from 3000 psi to 0 for the following situations and would depth affect the times.

1. Failed Burst Disk
2. Failed HP Hose
3. Failed LP Hose
4. Free Flow high performance regulator 2nd stage.

I completed the test on the surface (0 feet) with 4 aluminum 80’s and a Zeagle D50 regulator.

Here are the results

1. Failed Burst Disk = 72 seconds
2. Failed HP Hose = 22 minutes
3. Failed LP Hose = 83 seconds
4. Free Flow High Performance Regulator = 255 seconds


I then repeated the test at 4 atm’s / 99ft

1. Failed Burst Disk = Same within a second or 2
2. Failed HP Hose = Same within a minute
3. Failed LP Hose = Same within a second or 2
4. Free Flow High Performance Regulator = 155 seconds

I then repeated the test at 8 atm’s / 231ft

1. Failed Burst Disk = Same within a second or 2
2. Failed HP Hose = did not complete due to time restraints but would assume it would be with a minute difference
3. Failed LP Hose = Same within a second or 2
4. Free Flow High Performance Regulator = 91 seconds

The results show the small amounts of time a diver would have to solve a major equipment failure while at deeper depths.

 

[iztok]

So how would blown o-ring fit into this scenario above? Comparable to HP hose?

 

[Diver0001]

No, it would be comparable to a burst disk.

Also, this isn't the whole story. Not all K valves are made the same. some of them by chance or by design limit the flow more than others.

I would say the valve on the tanks Curt was using are probably new. IIRC older ones used to limit the flow to about 1000 litres per min, which would mean that if you used an older K valve a full AL80 with no regulator on it would empty in about 2 minutes if you opened up the valve all the way.

Evidently, the newer ones will allow the tank to empty in about a minute.

R..

 

[Crowley]

are the times for the failed HP / LP hoses the wrong way around? seems to me a burst HP hose should empty a lot more quickly than a burst LP hose?

I've never made a test but I reckon a blown o-ring would have a similar sort of timescale to a burst HP hose - possible slightly quicker, depending on the size of the hole created by the failed o-ring.

Cheers,

C.

 

[vladimir]

Isn't 50 bar approximately 735 psi? (50 x 14.7)

So?

So it would be good if everybody knew how to convert from bar to psi. I was sure Crowley knew, but in the New Divers forum, others might not.

 

[Diver0001]

are the times for the failed HP / LP hoses the wrong way around? seems to me a burst HP hose should empty a lot more quickly than a burst LP hose?

C.

No. The aperture (the hole) the air has to pass through is very small on a HP hose like the width of a needle. Take your HP hose off and take a look. On a LP hose it's nearly the diameter of a pencil.

So yes, the *pressure* on the HP is much higher but the restriction is so small that it actually empties slower because the maximum amount of air that will fit through that hole at a given pressure is lower than it is for a LP hose.

There are ..... (get ready to duck) "equations" for working out the net flow rate. I don't know them but if you're interested I can ask a buddy of mine who does.

Conclusions. The numbers are right.

I've never made a test but I reckon a blown o-ring would have a similar sort of timescale to a burst HP hose - possible slightly quicker, depending on the size of the hole created by the failed o-ring.

You can easily test this your self and you'll discover that the flow rate is very high. put a regulator on a tank just loosly enough so the o-ring escapes. I see this a lot because I actually *let* students make this mistake so I can correct it. (better that they see it from me than to see it for the first time under water...). In the time it takes me to turn off the tank (10-15 seconds) they generally lose 30-40 bar. That's a flow rate actually fairly similar to a blown burst disk or even not having a regulator on the tank at all...

R..

edit: You can find anything on google. I don't know much about this so if I'm wrong I'm sorry but it looks like there are more than one "equation" for mapping the effect I was mentioning above. (see attached equation). This is what the variables mean.

t= any time after leak flow starts, in seconds
F= fraction of initial gas weight remaining in source vessel at time t
V= volume of the source vessel, in ft3
C= coefficient of discharge
A= area of the source leak, in ft2
k= cp / cv
gc= gravitational conversion factor of 32.17 ft / s2
P0=initial gas pressure in the source vessel, in lbs / ft2 absolute
d0= initial gas density in the source vessel, in lbs / ft3 absolute

The graph is what you get if you map this equation over the changes in pressure and time.

R..