Question Advantages of a high or low IP on 1st stage regulators?

[Nemrod]

I wonder when all of that became en vogue, since my crazier ice diving days, began in the early 1980s, and no mention of lowering an IP for cold water was ever offered by anyone.

In fact, most of us routinely used regulators with IPs which ranged from 10 -12.5 bar, without any icing issues . . .

The less the delta between ambient and Interstage pressure the less the adibatic cooling. It is just physics. And yes the delta is greatest at the surface where icing is often encountered.

Here is from the Conshelf manual, the Supreme (sealed and intended for cooler water diving) has a lesser IP as an example:



I do not know how universal the admonition is from other manufactuers to lower the IP for cold water usage but there it is from AL.

 

[Nemrod]

So, the predominant ideas are that it might effect free flows and icing.

Here are my ideas, based on theory not experience so take that into account:

1. IP higher than 2nd tuning IP --> increased chance a free flow
2. IP lower than 2nd tuning IP --> increased WOB
3. IP lower --> less mechanical stress on 2nd stage structures
4. IP higher --> possible lower WOB at high gas densities at depth
5. Average tank pressure is ~100 bar, typical IP is ~10 bar, so typical IP splits the pressure drop to a factor of ~10 per stage.

This is true for simple downstream regulators without pneumatic balancing. Pneumatic balanced regulators would not benefit as much (I did not say at all).

AL SB 1985 #5 basically inferred using the wide range of IP available for the Conshelf regulator first stage to tune cracking force, well, you have to read between the lines . They state the manual recommendation of 140+/- psi but then state they acceptable range is 130 to 160 despite that being in conflict with the Supreme recommendation of 125+/- 5. Clear as mud but I have used IP to fine tune the 1085 second stage. So, the good old Conshelf first can range from 120 to 160 psi. They also infer to use care below 100 feet where sufficient flow could be compromised (between the lines by low IP ranges).

 

[L13]

The less the delta between ambient and Interstage pressure the less the adiabatic cooling. It is just physics. And yes the delta is greatest at the surface where icing is often encountered.

The same physics applies to the adiabatic cooling in the 1st stage, suggesting that a higher IP would reduce icing in the first stage at the same time that it would increase icing in the 2nd stage and vice versa.

I have been the buddy for 3 occurrences of free flowing regulators due to cold at depth. All three occured due to the 1st stage (both 2nds were affected).

 

[L13]

IP settings seem to be more critical with unbalanced 2nd stages especially going deep where a diver might want to overcome ambient pressure changes and air delivery. They might want to ride on the higher side.
With pneumatically balanced second stages it makes virtually no difference since they have a very light spring and rely on air pressure to press the poppet shut.
They will breathe great all the way down to 10 psi, (kinda scary actually!)
The IP could be 100 or 150 and a balanced second stage could care less.

It does seem like the pneumatically balanced 2nd should theoretically be fairly impervious to IP variations. However, the must be some reason for picking one value over another.

 

[Nemrod]

The same physics applies to the adiabatic cooling in the 1st stage, suggesting that a higher IP would reduce icing in the first stage at the same time that it would increase icing in the 2nd stage and vice versa.

I have been the buddy for 3 occurrences of free flowing regulators due to cold at depth. All three occured due to the 1st stage (both 2nds were affected).

The pressure drop in the first stage from tank pressure to IP is much greater by tens of times than the 10 to 20 psi we are talking about for the second stage. It would not be significant as an ice prevention scheme for a first stage, thus they employ other measures for first stages to reduce the risk of icing, some work better than others.

 

[Bigbella]

The less the delta between ambient and Interstage pressure the less the adibatic cooling. It is just physics. And yes the delta is greatest at the surface where icing is often encountered . . .

I fully understand the physical rationale behind the idea; but the old Poseidon Cyklon 300s were set at an IP of 12.5 bar -- and that was what we exclusively used under the ice, without any problems whatsoever. We often used antifreeze first stage caps filled either a glycerin concoction or with vodka; but were also told that the most important aspect of ice diving was the care of gear, before it ever hit the water.

Ironically, it had been the 300s that Cousteau took to Antarctica, along with Unisuits -- in lieu of anything from the Aqua-Lung / US Divers catalogue.

None of our instructors, most of whom spent years ice-diving in the Northeast, had ever suggested lowering the IP-- nor did any of the Poseidon service manuals that I had ever used . . .

 

[Beau Holden]

In fact, most of us routinely used regulators with IPs which ranged from 10 -12.5 bar, without any icing issues . . .

And I'll bet no one was over breathing their regulator.

Take a tank and open the valve. Crack it and see how much ice (frost) forms. Open a little more and check. Open full and cool the beer down. It's about restricted high flow and at a big pressure differential that causes the frost to form

All I know is that in all the ice, deep Midwest quarries and great lakes diving I have done. I have never had a 1st stage ice up. I dived NE Indiana, Michigan and Ohio for 10 years and all 12 months of the year. From October to May was some of our best inland diving with water temps going as low as 34 degrees.

 

[halocline]

Doesn't that just shift the icing problem from the 2nd to the first? Though I guess as a percentage it is smaller on the 1st than the second.

I think that's true, but I believe that the primary reason for the old 'drop the IP in cold water' idea is to slow down the flow a bit, which I guess is a result of less pressure drop in the 2nd stage, or maybe it's just that lower pressure in the hose means less flow.

Anyhow, I think that lowering IP for cold water use is not as widely done these days as it used to be. I'm kind of guessing here because I am a warm water diver only.

Getting back to the OP's question about IP, personally mine tend to stay on the lower end, usually between 125 and 130 PSI. That's on MK5s and 10s with the lowest seat (+) and on MK15s which only have one seat. I'm honestly not sure why they tend to stay lower, but I don't mind at all. It is theoretically less wear and tear on the seat, because remember that IP is the force that closes the 1st stage valve by pushing the piston into the seat, so lower IP means less force on the piston. How much that matters in the real world in terms of seat longevity I don't know, but I can say with confidence that my 1st stages go years-multiple- without creeping or exhibiting any problems.

There is an argument that higher IP means higher flow rate in the hose and potentially quicker recovery, so better performance, and that might actually have some validity, but in my case I am very happy with the performance of my 2nd stages and so I don't bother with adding more shims to raise IP.

 

[Beau Holden]

We often used antifreeze first stage caps filled either a glycerin concoction or with vodka; but were also told that the most important aspect of ice diving was the care of gear, before it ever hit the water

When I first started diving is was with the Mares V-16 Proton if I remember correctly, the cap was filled with vegetable oil. Still got that reg some where deep down in the old gear sack. Then went to AL Titan. The cap was not filled with anything but air. Then went with DR RG1208s and never looked back or forward. Gear care, don't over breath the reg and warm water. Now warm water is relative.....20 degree air temp and 34 degree water is warm water, lol. Just being an arse.

There is an argument that higher IP means higher flow rate in the hose and potentially quicker recovery, so better performance, and that might actually have some validity, but in my case I am very happy with the performance of my 2nd stages and so I don't bother with adding more shims to raise IP.

I set IP at spec then tune the second at the cracking effort I want. I can set that from free flow to sucking a milkshake through a straw, defining performance. like you, no need to change IP.

 

[Eric Sedletzky]

It does seem like the pneumatically balanced 2nd should theoretically be fairly impervious to IP variations. However, the must be some reason for picking one value over another.

Time tested manufacturer protocols I suppose?

Low IP means the unbalanced second stage spring can be set softer because it doesn’t have to hold back as much pressure, so reg will breathe easier longer down to lower tank pressure, but where this falls apart is when you’re deep.
High IP with unbalanced second stage means the second stage spring needs to be tighter to hold back the pressure and this is great with a full tank when you’re deep. But then when the tank begins to get low and IP drops the second stage will begin to draw hard earlier because of the tight spring. So they decided somewhere in the middle is perfect, like 130-140.
I’m sure the manufacturers tested the crap out of their regs to come up with their numbers.
Poseidon Cyklon had an IP of over 160