First stage

Question
QUESTION: Hi,I would like to ask ;
The intermediate pressure of the first stage is 140 psi (average) till the second stage. Is this pressure vary depending on the water pressure. 140 psi at the surface and 195 psi at the 100 feet or is it constant for all dept?
Thanks & regards
Hakan

ANSWER: Hi Hakan

That's a good question and I can tell that you've already given it some thought.

You are correct in that the intermediate pressure is around 140 psi and can vary somewhat depending on the manufacturers specifications.  Since this pressure is controlled only by spring compression, it is independent of the effects of water pressure.  This means that the intermediate pressure will remain unchanged regardless of the depth.

That is not the case with the second stage.  Second stages, as you may already know, are designed to deliver air at "ambient pressure".  This means that they will compensate for the change in depth (water pressure) so as to deliver air to the diver without the need for increased breathing effort as the depth increases.  This is accomplished by the diaphragm found in the second stage.  On some second stages, there may be an additional adjustment which can "fine tune" this compensation to further reduce the effort of breathing at depth.

Hakan, I hope this provides the information you were looking for.

Thanks for the question!!

Mike Giles
Mike's Dive Center
mikescuba.com

---------- FOLLOW-UP ----------

QUESTION: Dear Mike, thank you for the information,
I'm a little bit confused about the intermediate pressure of first stage. As I've seen the diagrams at the net (e.g. http://diver.net/capture/musac.cs.man.ac.uk/regs/first.htm) it shows that the intermediate pressure of the first stage is affected by the ambient pressure. It says "it is 140 psi (average)over the ambient pressure"
Is there different types of first stages as open to water pressure or close to water pressure?
Your answer is highly appreciated.
Kindest regards
Hakan

Answer
Hi Hakan

Your confusion is well-placed!!  Your follow-up question caused me to do some research of my own and I must revise my previous answer.  You are correct in that ambient pressure does figure into the delivery pressure of the first stage.

 There are two basic designs for first stage regulators...piston and diaphragm.  The piston type has a spring chamber which is exposed to the surrounding water pressure.  In the early days before environmental kits were available for these first stages, the spring chambers (ambient pressure chamber) were frequently packed with silicone grease to keep water out of them, thus preventing ice from forming in extremely cold water.  This silicone packing didn't seem to have any effect on the functioning of the regulator (it still worked just fine) but it must have had some effect on the pressure distribution inside the chamber.  Since the regulator worked and the manufacturer even recommended the silicone packing procedure, we assumed that the ambient pressure didn't play any significant role in the functioning of the first stage.  Your question and subsequent research causes me to rethink that assumption.

Before I continue, let's briefly discuss the other major type of first stage regulator, the diaphragm type.  This type eliminates the ambient pressure chamber altogether and replaces it with a diaphragm which is directly exposed to the ambient water pressure.  It is the regulator of choice when the system is to be used in very cold water or where heavy silting is likely to be encountered (which could plug the ambient pressure chamber).  In the case of the diaphragm first stage, the ambient water pressure still plays a role but the water does not enter the regulator mechanism.

So, Hakan, let me restate my original answer.  The ambient pressure does have an effect on the operation of the first stage.  It will assist the spring pressure to try to maintain the intermediate pressure regardless of the depth and supply pressure.  The intermediate pressure to the second stage will always be around 140 psi, however, the ambient pressure will affect the valve mechanism in the first stage to make the delivery of this pressure easier for the valve mechanism.  This is assuming that the tank supply pressure is in excess of 140 psi.  Such would not be the case if the tank were almost empty.

The ambient pressure assist we've discussed is something that regulator manufacturers have added to make the regulator perform more uniformly at varying depths.  The original double-hose regulators that I learned to dive with almost 50 years ago didn't have this system and were significantly harder to breathe from as the water got deeper.  So, while I think the regulator would still function if the ambient pressure were not assisting the spring pressure, it would not breathe as easily.

Hakan, thank you very much for making me dust off the old manuals and do a little research of my own!!

Please write back if this answer has evoked further questions or if you're not sure of my explanation.

Sincerely,
Mike Giles
Mike's Dive Center
mikescuba.com