An experiment - main spring failure

[AquaNSun]

Out of curiosity, I did a small experiment to see what happened if the mainspring in my Scubapro Mk2 failed. Here are the IP readings I got. This was not done in a pool, just dry land tests.

Tank, IP
2500psi, 25.0psi
2000psi, 20.0psi
1500psi, 17.5psi
1250psi, 15.0psi
1000psi, 11.0psi
750 psi, 8.0psi
500 psi, 7.5psi
300 psi, 5.0psi

Since there was no spring in my mk2, it only needed 25 psi to close the valve which was the force acting on the HP seat. This number translated into an orifice area of about 0.008 square inch which was about right. My IP reading was 137.5 psi, with spring at 2500 psi, so I could estimate the spring force to be around 112 psi. I then checked my IP reading at 500 psi, with spring, which read 120 psi. That was consistent as 112 + 7.5 = 120 psi !!!

At 25 psi it was hard to breath on my unbalanced second. At 8.0 psi it became ‘very hard’ to breath. At 5.0 psi, the air was flowing sooo slow but still breathable. Soon I learned that purging my reg was so much easier and my inhalation effort went down to zero instantly – woohoo! If I kept purging my reg, it was actually very nice, a constant, smooth flowing air at 25 psi and down to whispering air at 5.0 psi.

It was great that mk2 was still breathable without a spring. It failed without free flowing and it failed closed but functioning. I don’t know if the same would work underwater? My guess is it might still work. The IP readings should remain the same underwater and as long as you have IP in the 1st stage, you should be able to breath. What do you guys think?

May be the Mk2 is truly indestructible!

 

[herman]

This would hold true for any flow by piston reg, doing a quick what if in my head, a diaphram reg would not be as forgiving. The upsteam seat would be held shut because of tank pressure.....a plus for piston regs I had never considered. On many of the USD regs I have worked with, it was possible to turn the IP down to zero so a spring break would do the same thing. It would be interesting to do the same with a flow thru piston. Without the tank pressure pressing on the piston seat I would think it would slam shut and stay that way, another total OOG. It would work the same underwater, the depth compensation features of the reg would still be working. The IP psia reading would be greater depending on depth- psig would remain the same- although there would be some gas you could not get out at depth as opposed to at the surface but when you are that close to empty you have bigger issues.

I always laugh when I read the "accident" reports of running out of air when the reg suddenly stops working due to an empty tank....they don't, it's a fairly long drawn out process if you are paying the slightest bit of attention and it feels pretty much exactly like you descibe. If you have never breathed a tank to totally empty, you should do so some time, it's an eye opener. In a very safe conditions obviously, I do it at a shore dive 10 ft from shore and in water I can stand up in, usually with tanks that are due a VIP anyway...saves the tech from having to drain the tank and I get a longer dive.

 

[halocline]

I think you could create a bit of a vacuum in the IP chamber of a BP reg by really pulling on the 2nd stage, that might open it and allow the air to flow. That's assuming you wouldn't just be sucking water in from a different 2nd stage, i.e. through a seat saver.

There was a scenario in some early MK15s (I think) where the seat material was faulty and the piston occasionally got stuck in the seat. That would suck!

 

[herman]

You have a point there. This would be easier on a nonbalanced flow thru, at least it has a little piston surface area for the HP to press on. I guess the good thing is, spring failure must be very rare, never heard of it happening to a reg. The other problem would be the exhaust valves on the second stage, I have folded them in playing around to see how much vaccum I could pull on a reg.

Maybe something we should try in the kiddie pool. I love when the name calling starts, it's a sure sign you won.

 

[GrumpyOldGuy]

I have seen many spring fail in non-scuba applications. The mode is a break in the wire coil which effectively makes 2 shorter, weaker springs in series. This would reduce, but not eliminate the spring force. So the results might be even less severe than you experienced. Food for thought. Nice experiment.

 

[captain]

Cut a spring in half and put the two pieces back in and rerun the test.

 

[redacted]

I suspect Herman is right on the balanced piston. I tried to lower the IP in a Mk5 to drive some air tools by shimming under the HP seat. Somewhere around an IP of about 90 psi, it locked up and I could not get to piston to unseat by draining off the IP. Took me a while to get the reg off the tank.

 

[redacted]

Cut a spring in half and put the two pieces back in and rerun the test.

Clearly going to depend on how the two pieces align. The "no spring" condition establishes a limit. Even if the two pieces of spring provide a higher IP initially, I suspect the pieces will shift as they cycle and approach the "no spring" condition.

 

[halocline]

I suspect Herman is right on the balanced piston. I tried to lower the IP in a Mk5 to drive some air tools by shimming under the HP seat. Somewhere around an IP of about 90 psi, it locked up and I could not get to piston to unseat by draining off the IP. Took me a while to get the reg off the tank.

Using an air tool you weren't able to pull any sort of vacuum on the IP chamber, were you? I don;t know if that would do it, maybe you could take one of your many MK5s, take the spring out, put a second stage on it and see how it does.

 

[redacted]

Using an air tool you weren't able to pull any sort of vacuum on the IP chamber, were you? I don;t know if that would do it, maybe you could take one of your many MK5s, take the spring out, put a second stage on it and see how it does.

No airtool involved at that point. I still had a 2nd stage on it and a sucker to operate it.