regulator airflow capabilities primer

[Phil]

Just to put this all in a little more phyical context, in 1972, we (Oceaneering) used a venturi-modified Scubapro Mk2 fed by a redesigned Scubapro 1st stage - diver doing hard hard work - at 970 feet and got good ventilation. Three years later, we used the same set-up at the U of Pennsylvania at depths approaching 2000 feet with similar results.
The reg was used in our standard 'RatHat' -with mouthpiece inside the helmet (and, most importantly, the 2nd stage diaphragm inside the helmet!) We used these rigs between 1966 and about 1986 and logged many tens of thousands of gas dives deeper than 100 meters. At the high point of that period, we operated in 25 countries and employed more than two thousand commercial divers!
It was a helluva run, but by 1980 we could already hear the whirring of the ROV propellors!
Phil

[Greg Barlow]

Just to put this all in a little more phyical context, in 1972, we (Oceaneering) used a venturi-modified Scubapro Mk2 fed by a redesigned Scubapro 1st stage - diver doing hard hard work - at 970 feet and got good ventilation. Three years later, we used the same set-up at the U of Pennsylvania at depths approaching 2000 feet with similar results.
The reg was used in our standard 'RatHat' -with mouthpiece inside the helmet (and, most importantly, the 2nd stage diaphragm inside the helmet!) We used these rigs between 1966 and about 1986 and logged many tens of thousands of gas dives deeper than 100 meters. At the high point of that period, we operated in 25 countries and employed more than two thousand commercial divers!
It was a helluva run, but by 1980 we could already hear the whirring of the ROV propellors!
Phil

Just out of curiosity, I am assuming that you were using heliox or trimix blends that yielded a partial pressure of oxygen of 1.4AtA or less. My guess is trimix blends due to HPNS. With such mixes, did your firm ever calculate at what equivalent air depth would the breathing gas density be the same? For example, at a depth of 970', you might use a mix of 4/96 heliox or a trimix blend yielding an equivalent narcotic depth of 130' or so. Due to the decreased gas density would it be the same as if you were breathing air at 150' or shallower? I've always wondered if there is a commercial diving software program that can give a gas density/depth equivalent. One ballpark figure that I have heard thrown about is that a reg delivering air at 200' breathes about the same as delivering mix at 600'.

I really enjoy reading your posts, and your articles in HDS. They are always a great source of knowledge.

Greg Barlow

[Nemrod]

Just to interject, not a disagreement with anything here said. The interstage pressure (intermediate pressure) does affect the flow volume of a second stage, for any given displacement of the second stage seat from the volcano orifice, the volume flow potential will be greater with a higher interstage pressure. Since the typical first and second stage regulators are capable of flowing significantly more volume than a diver would normally demand or even at maximum demand require. The second stage demand lever is not an On and Off switch but instead more like a throttle, the more it off seats from the volcano orifice, the greater the volume that can flow for a given interstage pressure.

How does this concern us, when even an energetic diver with a properly adjusted and functioning regulator would at sport diving depths, not have a demand that would exceed the delivery capability of any modern SCUBA regulator? Well, let just say, maybe, you have a regulator which is NOT properly adjusted and is not functioning correctly and let us just say the culprit is a low lever height. The low lever does not travel through an arc sufficient to fully open or off seat the second stage seat from the volcano orifice. The diver notices the reduced flow, say at 60 feet, and increases the IP and on a subsequent dive the regulator appears to be fine and his demand is fully satisfied. However, the next dive is to 110 plus feet and working into a current the diver becomes aware that there is inadequate delivery.

The moral of this story is that if a diver notices that his regulator is not supplying enough volume even at shallower depths, please check the IP, of course. However, if the IP is anywhere in the range of 110 or above then jacking the IP up will only mask the actual condition. The condition of fault is not low IP but something else and it could be a low lever height. Increasing the IP only masks the condition and could indeed result in a dangerous situation where the diver cannot satisfy his required demand for air at depth.

James