Scubapro G250: strange free flow, take 2

[Zung]

Just come across another case that started kinda like the one I reported here.

I was checking my #2 son's MK20/G250C, and here's what it does:

1. Air on, all is quiet, IP: 131, slow creep to 133
2. Purge
3. The G250 starts hissing
4. Tighten the adjustment knob half a turn, still hissing
5. Air off, repeat from #1

Expecting some major surgery like the previous case, I play with the purge cover for a while, then I start thinking there may be some kind of connection between the purge and the free flow.

Open the cover: nothing unusual inside, the diaphragm seems to be OK, if slightly dirty. Turn on the air, no hiss. I rub the diaphragm inside and outside with my finger, put it back... AND THE HISSING IS GONE! The reg nows cracks at 18mm/.7in. of water.

So: when you test your second stage dry, the diaphragm may get stuck a little and causes a slight freeflow. A little cleaning is all it takes to fix that. And I'm convinced the problem will go away in the water.

Btw, here's a simplified version of brother couv's commandments that I use:

1. Air on
2. Read SPG
3. Read IP
4. Read cracking pressure
5. Leave it alone for 5 minutes
6. Read IP again for creep
7. Air off
8. Leave it alone for 5 minutes
9. Read IP and SPG again for leak
10. Done

 

[DA Aquamaster]

One of the things I have noted with regs using a balance chamber and flow through poppet is that if the poppet is not sealing perfectly in the balance chamber you'll get symptoms similar to what you report - a good initial seal, then a slight leak on subsequent breaths.

You can check to ensure the poppet is properly sealing in the balance chamber by placing the balance chamber on the poppet without the spring and with the hole in the seat plugged with the pad of your finger (you'll have to push pretty hard on the seat to get a good seal). When you push the balance chamber down it should "bounce" back up and have a springy action to it. If it does not spring, the o-rings or inside of the balance chamber is worn.

 

[halocline]

I'm not sure setting a G250 at .7" is going to work out too well anyway; the case fault geometry is probably at least that much, meaning you could end up with a free flow in some positions.

I've often wondered about those cheap plastic balance chambers; I know there's two o-rings sealing but it still seems like they can't last that long. They must cost all of a nickel to produce, and SP stopped putting them in the rebuild kits in their infinite wisdom.

 

[Couv]

Usually when I consider an intermittent problem like the one you've described, the reg just needs a bit of exercise or a good cleaning. Sometimes the metal lever will make tiny ridges in the diaphragm button that need to be buffed out or rotated perpendicular to the direction of the lever arms. However, if memory serves me correctly, Brother Zung installed a few 2.5mm x 1.0mm poppet o-rings in place of the normal 2.2mm seals. I'm curious to know if this reg is outfitted with the 2.5mm seals causing a bit of drag. Additionally, one of the purposes of having two seals in that balance chamber is to provide a lubricant reservoir.

If the balance chamber were worn, pea sized bubbles would be coming from the mouth piece. However, there could be something sticky in the chamber that needs to be cleaned out.

Persevere mon frère. It's too cold to go diving.

 

[Zung]

... You can check to ensure the poppet is properly sealing in the balance chamber by placing the balance chamber on the poppet without the spring and with the hole in the seat plugged with the pad of your finger (you'll have to push pretty hard on the seat to get a good seal). When you push the balance chamber down it should "bounce" back up and have a springy action to it. If it does not spring, the o-rings or inside of the balance chamber is worn.

This is a GREAT technique! I'll figure a way to pin it somewhere so we don't forget. Thanks DA.

I'm not sure setting a G250 at .7" is going to work out too well anyway; the case fault geometry is probably at least that much, meaning you could end up with a free flow in some positions...

My #2 son always looks up, so CGF is less of a problem, but I just realize I never brief them about the adjustment knob, so now is a good time.

... Brother Zung installed a few 2.5mm x 1.0mm poppet o-rings in place of the normal 2.2mm seals. I'm curious to know if this reg is outfitted with the 2.5mm seals causing a bit of drag... Persevere mon frère. It's too cold to go diving.

Good memory, Bro, but this one's got the current 2 o-rings s-wing poppet with a genuine SP kit, so they're 2.2mm. In this particular case, I'm pretty sure it's friction either in the diaphragm/purge button interface, or the purge button itself: after I remove the cover and the diaphragm, the problem immediately disappears. Actually, I'm starting to prep all the gears for the next trip. I do hope the water will be at a manageable 85°F instead of a chilly 80°F.

 

[knowone]

Yeah, had the same thing going with the metal things going in
the balance chamber withe Apeks and I think Beauchat seconds
about ten years ago with not really old or used much seconds