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

[L13]

The service manuals for my SP regulators specify an IP of 9-10 bar (130-145 psi). Are there any advantages/disadvantages to having an IP on the high or low end of this range (assuming the 2nd stage is tuned using the actual IP)?

Are there any reasons you might set up a 1st stage with an IP outside this range? Again, assuming the 2nd is tuned to match.

 

[Nemrod]

Regulators used in cold water often specify a lower IP for the first stage, less drop from IP to ambient in the second stage reduces the chance of icing.

SP and others generally call out a range because it is not always needed to be exact, especially with air balanced second stages. And during break in of a new seat the IP might change so the tech will set the initial IP with that process in mind, maybe. Some SP regulators (and others) use shims under the spring or different seat heights (thickness) to adjust IP so being exact is not always easily possible and it does not need to be.

I favor the higher end but if upon assembly and break in the first stage IP sits to the lower end of the range, but still within, it is what it is. Not going to tear down a regulator just rebuilt and within manual specifications to fuss over a few psi this way or that.

 

[Beau Holden]

I cannot comment on SP, as a dive Dive Rite regs. My spec says 140. After rebuild I have had a few PSI one way or the other and set it to 140. Then I will cycle the regs a few times and then leave them pressured up an hour or so to check for creep. I might have to dial up or back couple of PSI.

 

[Eric Sedletzky]

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.

 

[L13]

Regulators used in cold water often specify a lower IP for the first stage, less drop from IP to ambient in the second stage reduces the chance of icing.

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.

 

[d^2b]

As far as I understand, higher IP increases the danger of free-flow near the surface, but perhaps that can be compensated by appropriately (de-)tuning the second stage.

 

[SlugLife]

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.

Perhaps a little, but I'm not familiar with 2nd stages icing up. You also have your warm-breath passing through the 2nd stage.

 

[Bigbella]

Regulators used in cold water often specify a lower IP for the first stage, less drop from IP to ambient in the second stage reduces the chance of icing.

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 . . .

 

[Beau Holden]

Doesn't that just shift the icing problem from the 2nd to the first

The only time I seen a 2nd stage freeze was on surface in 20 degree air temps. Regs was breathed on the surface, guy went to get into the water and it free flowed when he inhaled. He immersed the reg in 36 degree water and it thawed out & stopped. Worked fine the entire dive.

 

[L13]

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.

Conclusions:

1. Tune 1st and 2nd together
2. Unless WOB at depth is and issue, lower is possibly a little better if not overdone
3. I think I will probably shoot for 9.0-9.3 bar/130-135 psi, at the lower end but within spec.