MK25 IP increase as tank pressure drop

[eelnoraa]

I am not talking about significant change in IP here, but does Mk25 (or any balance piton 1st stage) IP increase as tank pressure drop in general?

I asked because I had a very interesting encounter last weekend. Mk25 was used wth HOG 2nd stage on right post. On previous dives, I haven't drained tank below about 1000~1200psi. Everything operated fine. Breathing of these combo is as good as it gets

Fastward tho last weekend, after a dive, I purposely breathed the tank down to 500psi in shallow water in attempt to do a weight check. I notice this Mk25/HOG combo started to free flow, not in any dramatic fashion, but a constant slow flow of air. So I aborted the weight check. Upon furst examination on land, I found adjusting venturi switch didn't help. I have to close down the breathing effort switch all the way down to stop flow. I then adjusted the allen key screw inside the HOG 2nd, the free flow seemed to be fixed (at least on land).

So what could that be? I first guess was because the HOG was adjusted too agreesively, when IP increase, it starts to freeflow. And I suspend as tank pressure drop, IP increase a bit in MK25.

 

[elan]

Did you actually measure the IP?

Slight increase in IP should not cause a balanced second flowing.

I have never seen the IP on my 25s increasing with the drop of the tank pressure

 

[Zung]

Me neither; the IP drops a bit at low tank pressure, see quote below. Like elan said, it would be interesting to actually measure your IP.

...
Regarding herman's question, here's what I have at low air situation: tank pressure vs IP (bar/PSI)

150/2175: 9.4/136
125/1813: 9.4/136
100/1450: 9.4/136
85/1233: 9.4/136
70/1015: 9.4/136
60/870: 9.4/136
50/725; 9.3/135
40/580: 9.2/133
25/363: 9.1/131
15/218: 6.4/93

At pressure below 60 bar / 870 PSI, when the IP starts to drop a little, the lockup is less snappy and takes maybe a couple of seconds.

...

 

[eelnoraa]

very interesting number. I don't have such accurate gauge to measure IP down to single digital, but I will try to borrow one.

I also agree with elan, HOG 2nd is balance, so unless IP change a lot, it should free flow. So what could be the reason? Can it simply be that the 2nd is tuned too agressively?

 

[redacted]

Sounds like the HOG was just tuned a bit aggressively. A good example of why the user adjustment knob can be very useful. BTW, the venturi adjustment should only effect the severity of a freeflow, and not the initiation; so its performance was as expected.

Edit: But I am not sure why it would take more than a fraction of a turn to stop a slight leak. You may want to take a look and make sure the lever is not too high.

 

[Zung]

very interesting number. I don't have such accurate gauge to measure IP down to single digital, but I will try to borrow one.

I also agree with elan, HOG 2nd is balance, so unless IP change a lot, it should free flow. So what could be the reason? Can it simply be that the 2nd is tuned too agressively?

The numbers are not that accurate, and they don't really need to be. I just read them off in bar, then used the (accurate) calculator to convert them into PSI.


And yes, I believe you just need to detune your second by a bit. To be on the safe side, you may want to juryrig a manometer with a ruler and a piece of piping, and measure the cracking pressure. Anything between 1-1.2 inch of water is good; below means possible freeflow, above means harder breathing.

Worst case: the orifice and/or the seat is shot.

 

[herman]

Come on piston guys, this one is easy'
The answer is no, it does not increase. ALL piston regs tend to decrease in IP as tank pressure decreases. Granted balanced ones are not susposed to change but they do slightly, unbalanced ones more so. It's us unbalanced diaphgram guys that have to deal with increasing IP as tank pressure decreases. You have a second stage issue, slightly mistuned sounds like the most likely answer.

 

[Zung]

The diaphragm guy is right!

 

[DA Aquamaster]

Come on piston guys, this one is easy'
The answer is no, it does not increase. ALL piston regs tend to decrease in IP as tank pressure decreases. Granted balanced ones are not susposed to change but they do slightly, unbalanced ones more so. It's us unbalanced diaphgram guys that have to deal with increasing IP as tank pressure decreases. You have a second stage issue, slightly mistuned sounds like the most likely answer.

Thanks, I was bringing to compose my response to an off track conclusion in this thread until I got to your response.

The major difference between the Mk 20/25 piston and earlier pistons is that the stem is slightly bell shaped with the seating end of the piston stem being slightly larger than the middle of the stem where it passes through the high pressure piston stem o-ring. In earlier pistons (Mk 5, Mk 10, Mk 15, etc) the stem was just a straight tube, so the actual area of the seating edge itself was not balanced creating a very slight imbalance. On the average Mk 10 this only equated to about 4-6 psi of pressure as the supply pressure dropped from 3000 psi to 300 psi.

It was not enough to make any noticeable difference - but as Herman notes, that difference would be a drop in IP, and that drop in IP would reduce the force acting to open the valve and that would have to be made up with greater force on the lever to lift the seat off the orifice to initiate gas flow. Which is to say a drop in IP equals increased inhalation effort.

With the Mk 25 that balancing error is reduced to essentially nothing as the area inside the seating surface equals the area of the stem passing through the o-ring. It's possible a piston stem could be too belled and slightly over balanced, but the 1-2 psi increase in IP should not have any noticeable effect on a properly adjusted second stage.

 

[James R]

Actually, on the MK25 the IP will increase with decreasing tank pressure.

"Intermediate Pressure Limits: IP limits must be between 130-142 PSIG at both 500and 3000 PSI supply. Verify and expect an IP inversion/increase of approximately 5 PSI from 3000 PSI to 500 PSI supply. IP inversion: As tank supply decrease, IP will increase. For best results, set IP @ 142 PSI, with a supply pressure of 500."