Question How much of an IP drop is acceptable?

[Jens Bölte]

Hello

I have a MK21 which, when set to an IP of 9.5 bar (137 PSI), shows quite a large drop in IP on inhalation. The IP drops by about 2.5 bar (36 PSI) which I think is quite a lot. I replaced the o-rings on the piston as it was also showing IP creep from 9.5 bar to 8.5 bar (123 PSI) when pressurised over a period of time. The IP remained constant after the o-rings were changed. The drop in IP was slightly less than before, 2 bar (29 PSI) instead of 2.5 bar (36 PSI). Does anyone have any suggestions on what to do about this rather, IMHO, large drop in IP when inhaling? What is an acceptable drop in IP during inhalation?

Thanks a lot Jens

 

[Nemrod]

Is the valve fully on?

The sintered filter is clean?

Are you taking IP at the regulator end?

Where are you measuring IP and IP drop?

The Mark 21 has two so called high flow ports. I doubt it makes a difference but take the IP on a short LP hose connected to the opposing HF port. Or inline on the regulator hose (HF) at the first stage. Depending on a few things, inline at the second stage will sho a larger drop. Inline at the stage may also show a large drop due to Venturi effect. The opposing ports may be more accurate using a short LP hose for the gauge.

 

[Jens Bölte]

Is the valve fully on?

The sintered filter is clean?

Are you taking IP at the regulator end?

Where are you measuring IP and IP drop?

The Mark 21 has two so called high flow ports. I doubt it makes a difference but take the IP on a short LP hose connected to the opposing HF port. Or inline on the regulator hose (HF) at the first stage. Depending on a few things, inline at the second stage will sho a larger drop. Inline at the stage may also show a large drop due to Venturi effect. The opposing ports may be more accurate using a short LP hose for the gauge.

Thank you for your suggestions.

Yes, the tank valve was fully open.

I measured the MK 21 before (IP drop ~ 2 bar) and after the inspection (IP drop ~ 2 - 2.5 bar). The sinter filter was new.

I measured the IP through the inflator hose. We use an electronic pressure bank that does the breathing work via the second stage. It measures the force required to inhale and exhale and measures the IP during a breath cycle. The IP is displayed in addition to the inhalation and exhalation work.

I'll try a different port, but I doubt it will make much difference. I usually use the ports the customer uses for 2nd stage and inflator.

Best wishes Jens

 

[lexvil]

I usually just accept what ever the regulator gives me, basically I don’t think it matters but curious about inlet (tank) pressure, how does it behave with a full tank vs a nearly empty one?

 

[Jens Bölte]

I usually just accept what ever the regulator gives me, basically I don’t think it matters but curious about inlet (tank) pressure, how does it behave with a full tank vs a nearly empty one?

I can give it a try. Usually this results in larger IP drops.

Best wishes Jens

 

[texasdivemaster2012]

Are you using the same tank for all this? If the tank valve has an issue it can act like a partially closed valve with large IP changes.

 

[tursiops]

Hello

I have a MK21 which, when set to an IP of 9.5 bar (137 PSI), shows quite a large drop in IP on inhalation. The IP drops by about 2.5 bar (36 PSI) which I think is quite a lot. I replaced the o-rings on the piston as it was also showing IP creep from 9.5 bar to 8.5 bar (123 PSI) when pressurised over a period of time. The IP remained constant after the o-rings were changed. The drop in IP was slightly less than before, 2 bar (29 PSI) instead of 2.5 bar (36 PSI). Does anyone have any suggestions on what to do about this rather, IMHO, large drop in IP when inhaling? What is an acceptable drop in IP during inhalation?

Thanks a lot Jens

From the Deep6 Signature Service Manual:

13. Purge the second stage and watch the IP drop during purge. Intermediate pressure should drop less than 20 psi during a brisk purge with a full (3000 psi) tank. Excessive dynamic IP drop is caused by a) a tank valve not fully open, b) a partially occluded metal intake filter in the first stage due to accumulated debris or corrosion, c) excess friction among the internal first stage components, likely caused by internal corrosion, or d) low tank supply pressure.​

@rsingler

 

[tursiops]

From the Deep6 Signature Service Manual:

13. Purge the second stage and watch the IP drop during purge. Intermediate pressure should drop less than 20 psi during a brisk purge with a full (3000 psi) tank. Excessive dynamic IP drop is caused by a) a tank valve not fully open, b) a partially occluded metal intake filter in the first stage due to accumulated debris or corrosion, c) excess friction among the internal first stage components, likely caused by internal corrosion, or d) low tank supply pressure.​

@rsingler

From Vance Harlow's "Scuba Regulator Maintenance and Repair":

Once the IP is adjusted, and while the gauge is still attached, try breathing the regulator, and watch how it affects the IP. It will always drop somewhat, but how far it drops, and how quickly it recovers, can give a good indication of the internal health and performance capabilities of the regulator.​

​

Then repeat the test using the purge button, noting how great the pressure drop. Regulators will vary quite a bit in this respect, and until one has tested enough to get a feeling for how they perform it is a mistake to put too much significance on minor variations.​

To give a few numbers, a Scubapro Mk10 and an Apex - two top performing 1sts that just happened to be on hand - drop about 5 psi (33 kpa) when I take a breath, and about 9 psi (60 kpa) when I hit the purge. Recovery is almost instantaneous. An older Scubapro MK2 - a basic medium performance NBP 1st - drops 9 breath/11 purge (60/73). But a Spiro NBP that appeared to be a virtual clone of the Mk2 shows drops of 30,/30 (2/2 bar).​

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And then an old Dacor Dart - a twenty year old regulator that had a reputation as a hard breather even when new - drops a full 20 psi breath and 40 purge (1.3/2.6 bar).​

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Of the two figures - the drop for taking a breath and the drop for purging - the first is by far the more meaningful, because the effectiveness of the purge button - that is to say, how wide it opens the demand valve - varies so much from one 2nd to another. The purge on the Odins, for example, produce prodigious flow that’s substantially greater than most other regulators, and not surprisingly, it shows a greater IP drop when purged.​

​

A reasonable rule of thumb, then, might be that if a regulator’s IP drops more than 15 psi (1 bar) on taking a breath, or shows any hesitation before recovering, something is probably amiss inside - or it may just not be a very good regulator. High performance regulators should be held to a higher standard - say 10 psi (6.6 kpa)​

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Some manufacturers give specs for drop and recovery in the service literature, but most don’t.​

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If the regulator’s IP drops precipitously, or takes a long time to recover, that’s a good clue that something’s wrong inside. It may be dirt or corrosion, or the O-rings may be kinked, worn, damaged, improperly lubricated, or maybe even the wrong size.​

​

Also check that the 2nd is attached to the right port - some regulators, newer ones especially, have a dedicated regulator port for the primary 2nd stage, and hanging it on the wrong port might just cause a drop in performance.​

 

[Nemrod]

At this point I am thinking, knowing we are looking at a balanced flow through piston design of a Mark 21, that there is a lube problem or O-ring stiction on the piston shaft. And yes, that IP drop would seem excessive to me for a flow through piston regulator.

I do want to correct my earlier post, I was confused in the head, happens sometimes as we get older . The Mark 21 does not have dedicated HF ports. I had just gotten through buttoning up a Mark 11 which does so had my brain cells crossed.

Another suggestion, remove the regulator from the machine and use the purge and see if the drop is still excessive. Just a curiosity on my part. What I am wanting you to do specifically is depress the purge and hold it down for a few seconds (or more), not just a tap. Some regulators will show a transitory or momentary higher drop and then in an instant recover while still under demand. That too is usually the result of stiction in the system.

 

[rsingler]

The Mk21 was designed by Scubapro as their "value" Mk25.

Sacrificing the turret for manufacturing simplicity and lower cost, it was designed in part by a former motorcycle engine designer; hence the odd (abortive) external design that never really took off.
But in almost every internal part, it's a Mk25. The seat, springs and piston shaft seal are all Mk25. The piston shaft ID is identical, though it's a shorter part.

The posters above have hit all the key points. The IP drop MUST be related to one of four things: 1) knife edge/seat flow restriction, 2) friction, 3) input pressure/flow restriction or 4) measurement error.

I couldn't quite discern from the description of the test setup, but tapping the IP from a bcd hose is reasonable. The first thing I'd do is remove the (new) sintered filter and throw the reg set on a tank with the largest visible gas outlet in the valve, making sure it's full and wide open. That will eliminate 3). Eyeballing an analog IP gauge on a bcd hose is good enough for 4).
The only potential weakness in the Mk21 is its shorter piston. Much like the transition from the Mk5 to the Mk10, ensuring proper piston alignment was part of improving flow. The Mk21, like the Mk10 & 25, has its piston head land inside the reg body (instead of the screw-on turret). But it is so much shorter that (theoretically), there could be slightly poorer axial tracking, and internal friction could play a role.

If the knife edge is perfect and the seat is new, then gas flow should be nominal at the interface if IP is at specification  unless for some reason your spring constant is much higher, and valve opening is restricted at a given IP. So to eliminate 1), I'd replace the mainspring.

Finally, internal friction should be revealed by using a cheap USB microscope to inspect the piston shaft, the OD of the piston head and the piston head land in the body. Any scuffing would implicate 2).

Unless this is a new phenomenon in a reg that has shown better performance before, I'd pick the the spring.

If it truly is a new change and there's no scuffing inside, I'd pick the tank valve flow or the sintered filter. It's just logical that if the IP flow can't keep up with the wide open second stage valve, there must be an obstruction. A clogged or poorly manufactured replaceable filter would seem to be the statistically likely culprit.

A fascinating case!