IP drop in Piston 1st stages vs Diaphragm

[jwd]

Hi all,

Was doing some servicing on a few regs last night in preparation for a trip.

I've set the IP of all regs to about 150PSI but when testing the dynamic IP (using a gauge on the LP hose) I noticed a big change in the pressure drop when purging the regs.

My Diaphram first stages (Poseidon and Apeks) had very little drop (maybe 10-20 PSI)
My Piston first stages (scubapro Mk10) had a much larger drop (maybe 30PSI).

I've not noticed this before. Is this inherent in the piston design or are my Scubapros overly sluggish for some reason?

Both Mk10s have long hoses on them for sidemount diving so wondering if this might create some type of lag effect due to the amount of gas stored in them vs short hoses?

 

[shurite7]

The drop with the Mk10 is fine. Scubapro's range, though, is 125-145, not 150. I've found 135 to work quite well for Mk10s. Be sure you have good lock up.

 

[rhwestfall]

it very well could be the lube you used.....

As a side note - I don't know why you are at 150 for IP......

 

[axxel57]

30psi dynamic IP drop seems pretty high (I see usually between 15-25psi at 10 SCFM) for a MK10, I would consider changing the main spring.

Balanced piston 1sts seem to have a slightly higher IP drop than diaphragm 1sts, I guess that's also due to the higher air flow they can do.

Diaphragm 1sts often use in one or more LP ports the Venturi assist to 'boost' the air flow and reduce so the dynamic IP drop.

Try on your MK10 the port on top of the turret for your 2nd, you should see a lower IP drop when pushing the purge button.

I wouldn't know why the lube could play a role in the dynamic IP drop.

For sure it makes a difference to use 'normal' Silicone instead of Christo Lube for static IP, but not for the dynamic.....

150psi static IP seems a bit high for the mentioned 1sts, especially the Poseidon, and I just hope you are going to dive in warm waters.

The length of the hose doesn't (at least not the ones used by divers) play a role in the IP drop I think.

In this case I guess that the spring is pretty old and used, causing that above normal dynamic IP drop.

 

[rhwestfall]

I wouldn't know why the lube could play a role in the dynamic IP drop.

If the piston is slow to move (sticky), then wouldn't IP drop quicker?

 

[axxel57]

I don't think so.
While checking the dynamic IP, the valve is all the time open.
The static IP is measured when the valve has closed, so a sticky piston stem or o-ring makes the valve close a bit later, increasing the IP.
When the IP is measured under flow, the valve stays open, so I wouldn't know how lube could affect the dynamic IP, even when it would a little bit slow down the movement.....

 

[АлександрД]

check tank valve. It should be fully opened.
check filter - it could affect on IP drop.

 

[halocline]

I've set the IP of all regs to about 150PSI but when testing the dynamic IP (using a gauge on the LP hose) I noticed a big change in the pressure drop when purging the regs.


Both Mk10s have long hoses on them for sidemount diving so wondering if this might create some type of lag effect due to the amount of gas stored in them vs short hoses?

First, the easy part, the long hose would have no effect, or if anything, a positive effect on IP drop. The hose essentially becomes part of the IP chamber, increasing the volume of IP air that you are drawing on when you take a breath or purge.

Second, the measurement you're experiencing is almost certainly due to the way you're measuring IP drop. The gauge at the end of the inflator hose is subject to venturi effect in the turret; since air is moving rapidly into a 2nd stage hose, past the port where the LP inflator hose is, that movement causes a pressure drop in the LP hose.

The MK10 and your diaphragm reg differ in geometry, so the venturi effect differs.

The only way to get an accurate reading of IP drop during air movement would be to measure it with a flow meter, something that's designed to measure the pressure of moving gas. A regular IP gauge won't work. Even if you have one inline, like on some of the inline adjusters, they still tee off the line and as such are subject to the venturi effect. You would have to use something that the gas flows through.

I found this out years ago when comparing the IP drop under purge on a MK15 vs a MK2. The MK15 has much higher flow than the MK 2, yet the IP drop was quite a bit more pronounced during air movement. It's the only explanation that makes sense.

I wouldn't worry about it in the least. Quick return and solid IP lock up are what matters.

 

[Akimbo]

In general, piston regulators are best when very-high flow rates are required. When operating perfectly and with very precision and fast responding instruments, piston regulators will have the least variation across their operating flow range. The downside is pistons have relatively large surface areas that their dynamic seals act on, which makes them more sensitive to increased friction due to corrosion and loss of lubrication.

Moving parts on diaphragm first stages generally have more isolation from water, smaller dynamic seal surface areas, and lower maximum flow rates. "Lower flow rates" is not the same as inadequate flow rates. Regulators are all about balancing forces across their operating range and physical constraints.

 

[jwd]

Thanks for all the replies guys.

Just got back from a week of diving and have not solved the issue.

The regs reliably provide air but I would describe them as "sluggish" in response at the first stage end.

I tried a couple of excessively heavy drags on them at 30m and it felt a bit like the first stage was playing catch up with my demand (a bit like you experience when the tank valve is not fully open - it was).

There is a mild click when pressurising the reg so I wonder if this is possibly a friction related issue with the piston orings, lubrications, and the worn walls of the piston (they are old Mk10s). Not an immediate problem buy something I would like to solve as in the past I have really liked Mk10s and can't see any real difference in later scubapro piston models (Mk20, etc) beyond cosmetic changes and some minor manufacturing simplifications.

I have a super shiny almost new Mk10 at home that I am going to compare as a control reg to see if it has the same issue.