cressi IP adjustment problem

[pointy]

I have a new Cressi T10 chromo first stage that was giving the shop tech a problem in setting the IP. He removed the retaining ring over the sealing disk to lower the IP, but as he tightened the retainer, the IP increased by about 20 psi. He was able to get the IP where it should be by setting it lower to compensate for the increase that happens when the retainer is tightened, but he wasn't happy about having to do it that way. It does seem a bit dodgy and he wants to discuss the issue with Cressi.

The sealing disk is compressing the part that transmits ambient pressure to the guts of the first stage. Does anyone know if this could present a problem for the diver?

 

[Tanks A Lot]

I have never had a T10 in my hands, but a fair amount of other Cressi regulators.

I assume that we are talking about the retainer (HZ800079) pushing down on the diaphragm (HZ800011), which in turn pushes down on the transmitter (HZ800010).
On an MC9, which looks rather similar on the schematics with regards to the environmental seal, Cressi has very specific instructions on how to assemble the retainer and diaphragm mentioned above.

After inserting the silicone seal inside the S.C. cap, connect the first stage with a pressurized tank, open the valve and start turning the disk with a compass spanner, as shown in the picture, while pressing the seal and discharging the connected second stage with every small turn of the closing disk. This will avoid the formation of any air-pocket in the first stage seal chamber, that would cause the seal to bend and the regulator to work badly.

Most manufacturers don't mention much about installing environmental seals, besides that the first stage is pressurized when installing it. Apeks goes as far as saying to "depress the diaphragm lightly with your thumb when tightening the retainer".

I always wondered what was meant by "seal to bend and the regulator to work badly". I assumed this was the usual end-goal of preventing the diaphragm from flexing outwards and thereby protecting it a little better and allowing it to transmit pressure changes right away without an air pocket needing to be compressed beforehand.
I unfortunately do not have a T10 service manual on hand, but I assume it will say much the same as for the MC9.

I can't see how this would alleviate the issue you are describing if the diaphragm physically pushes down on the transmitter, but it may be worth a shot double checking if this would make any difference in case the technician has assembled it differently.

Another possibility would be poor assembly of the parts that sit between the diaphragm and the environmental seal. If the spring for example does not sit flush on the little washer that protects the diaphragm, the whole mechanism gets pushed further towards the environmental seal.
This could result in less space for the transmitter, hence the environmental seal pushing down on the transmitter when the retainer is tightened.

 

[Tanks A Lot]

I've taken a quick glance at the cutaway of the T10 and something seems off with what you describe.

As with any other drysealed diaphragm first stage, the environmental seal should not be pressing on the transmitter without outside force, e.g. pressure.

I can't be sure without having that particular regulator in my hand and I never had it in my hand as mentioned above, but I suspect there is something assembled wrong.
My best bet is the spring not sitting flush on the washer that protects the main diaphragm. For whatever reason your transmitter is getting pushed too far to the environmental seal end.

 

[pointy]

Tanks A Lot, for all that useful information. You understood the issue better than I did. Now I'm equipped to fix the thing myself rather than driving 2 hours to the shop and back.

I can see that the Cressi logo on the transmitter is not perfectly centred under the seal, and is likely tilted in such a way that its edge is being pushed down, (thereby increasing IP) when tightening the retaining ring. Depressing the seal while tightening the ring would keep the transmitter shaft centred so the seal won't be bearing down on the transmitter. You noted that a dry seal transmitter should not be making firm contact with the transmitter unless ambient pressure is increased.

I'll forward your response to the shop tech. He'll be grateful too.

 

[Tanks A Lot]

You noted that a dry seal transmitter should not be making firm contact with the transmitter unless ambient pressure is increased.

I worded this poorly. They will be in contact at all times, but there really shouldn't be any pushing towards the main diaphragm going on, except in changes of ambient pressure.

Think about the dry seal diaphragm as just an "extension" of the main diaphragm. As the main diaphragm can't sense changes in pressure by being cut off from the water, the transmitter/transducer is there to well, transmit, these pressure changes. All three, main diaphragm, transducer, and dry seal diaphragm should be in constant contact with each other.
But any excess force on either one of the three will be transmitted to the others. They move as a unison in operation.

The cutaway of the T10 doesn't do it justice, it is a poorly made cutaway with the dry sealed diaphragm not being in touch with the transducer.

Maybe this cutaway of a Aqualung Legend can show better what my poor wording is trying to convey.

 

[pointy]

Your wording was fine, and I understood your message that the sealing mechanism should not interfere with the accurate transmission of ambient pressure.

 

[Tanks A Lot]

Do keep us posted on how this goes, I'm very much interested in the end-result.

 

[axxel57]

I found it's quite common that there is a small difference in IP when tightening the retainer, but it normally doesn't exceed 2-3psi.
20psi is way too much.......

 

[rsingler]

It's very common for the IP to rise as the shop tech screws on the environmental seal. Assuming the installation was done with the reg pressurized, which is the way it should be, it's almost impossible to not put pressure on the transmitter as you hold the seal against the transmitter while screwing down the outer ring.
As you do that, you simulate increased ambient pressure, the valve opens, and IP goes up. Once the seal is in place, one simply depressurizes the set. Upon repressurization, the reg should show its normal IP.
In a couple of brands, the tolerances in height between the seal, the transmitter and the pressure plate below the spring are so tight that screwing in the seal causes final IP to be a few psi higher. For those brands, one simply starts with an IP a few psi lower than desired, knowing that when the env seal is applied, you'll end up where you want to be.

I think this is maybe the tech not fully understanding the mechanics of the transmitter and why IP goes up but does not come back down until you cycle the reg. Unless there's a piece of junk between the transmitter stem and the pressure plate, there is NO reason for the IP to be 20 psi different after the seal is applied, and the reg is fully depressurized and cycled.

Ah, lol! As usual, @axxel57 beat me to it.

 

[pointy]

I was able to reassemble the dry seal mechanism the way Cressi describes, by pressing on the diaphragm while tightening the ring. It seems to be working fine. I'll take it on a dive Monday.

The Cressi logo still looks slightly off centre, but no part of the transmitter is restricted in its movement as far as I can tell. I'm not sure now that was ever a problem.