Vintage Double Hose

Lets say you set you set your IP on an unbalanced 1st stage with NO IP gauge using only a cylinder with 200-500 psi, adjusting the spring tension till you have a slight free flow and backing it off 1/8 of a turn and then call it good…..What I’m interested in is…..Using an IP gauge and a full cylinder what pressure does the IP end up being when you are done using this method?

Wow! That would be hard to answer with any precision.

There are at least three major variables (from any one regulator to the next) involved in this scenario that is almost impossible to accurately estimate.

1st) The actual IP at which any regulator will start free flowing can vary substantially from one second stage to the next. This affected by the spring and to a lesser degree by the rubber seat hardness (also by the volcano orifice size, but that is fairly constant).

I have seen second stages leaking as low as 140 psi and as high as close to 200 psi (in Rob’s Sportsways / New England Divers down stream DH regulator).

2nd) Turning the IP adjustment screw 1/8 of a turn doesn’t change the IP a constant amount on any two regulators. Actually, if you notice the IP doesn’t change linearly (and/ or consistently) with any fixed amount of rotation. This is mostly caused by the course threads, the diaphragm compression, the uneven spring compression, etc. When I adjust the IP I often go past the desired amount and then check it as I back it a bit. I will repeat it from both directions (going up and down). This phenomenon is referred to as hysteresis and it applies to many adjustable mechanisms (depending on how precise they are).


3rd) The change with IP as a function of tank pressure varies with different regulators. In an unbalanced regulator it is mostly affected by the size of the first stage volcano orifice (and to a lesser extent the size of the IP diaphragm or piston head). The larger the volcano orifice the higher the unbalance and the higher the pressure swing.

I have measured several DA Aqua Master first stage volcano orifices and have found that US Divers changed the size several times during the years of fabrication. Some of the nozzles I measured were the ones you send me during the early stages of the Phoenix project; therefore I have no idea at the years different orifice sizes were used.

My experience (if I recall correctly) with several DA is the IP change with tank pressure change can vary from around 25 psi to as high as close to 40 psi pressure swing. This would be interesting data to collect from all the DA owners (IP at 500 psi and IP at 2500 psi).


I am sorry if this doesn’t help you much, but that is what I have observed.
Your results will vary.

If this is a competition I am going to guess 120 psi (+ or - 20psi).

I agree with the variables you are describing 100% Luis....I just finished doing 3 DA's over the last week and all of them came out to be 119-121 psi when I went back and put an IP gauge on them before they went back to their owners....I don't remember ever paying much attention in the past to the differences in them.....It is odd that USD says 110 psi for the IP pressure....I'm guessing that was a failsafe number for them???

Bryan wrote:I agree with the variables you are describing 100% Luis....I just finished doing 3 DA's over the last week and all of them came out to be 119-121 psi when I went back and put an IP gauge on them before they went back to their owners....I don't remember ever paying much attention in the past to the differences in them.....It is odd that USD says 110 psi for the IP pressure....I'm guessing that was a failsafe number for them???

If I recall correctly, in the old US Divers service manual, the IP recommendation of 110 psi does not include the tank pressure to be used.

To me an IP of 110 psi (for a DA) would be with a full tank at 2250 to 2500 psi. That would translate to an IP of around 130 to 145 with a tank close to empty or around 300 to 500 psi.

Since there were no regulator work benches with adjustable air source in the old days, I am assuming that US Divers was just playing it safe and specifying a low IP in case the tech used a full tank to make the adjustment. If the tech used anything but a full tank the IP would just be lower when the tank was full. It is just my guess, that they were playing it safe and making it easier on the techs.

IMHO, it is terrible that they did not include, in the repair manual, the required tank pressure for the corresponding IP (for an unbalanced regulator), but in the early days, they were just learning.




You said the 3 DA’s turn out to have an IP of 119 to 121 psi, at what tank pressure?
So, my 120 psi guess was not too bad.

That reminds me of the “theory of cancelling errors”. That is when you have errors in two or more variables and they end up cancelling each other, so the end results works out to be correct by pure luck. You may ask if I have personal experience to such an occurrence. Well, the standard practice is that if you find that situation, you correct the errors, but since the final answer doesn’t actually change, no one really needs to know.

luis wrote:
You said the 3 DA’s turn out to have an IP of 119 to 121 psi, at what tank pressure?
So, my 120 psi guess was not too bad.

Cylinder pressure 300-350 psi......I'd say you were right on

I remember doing a test years ago to see the IP change between 400-500 psi and a 2500 psi supply pressuere. If I recall correctly the IP was about 130 psi at 400-500 psi and 110 psi at 2500psi.