Impact of the first stage on breathing performances

[Falco]

I am in the process of buying my first set of regs, so I have done a fair bit of reading on different models and their characteristics. All this said and done, I have a few questions that are still floating in my head.

The breathing performances af a given regulator is mostly dictated by the first or second stage.

What would be the consequence of coupling a second stage from a easy breathing regulator (a Mares, Apex, Atomic for exmple) to a first stage of not so well breathing redulator (say a Sherwood Maximmus first stage).

Thanks in advance

Jeremie Laplante

 

[Greg Barlow]

I am in the process of buying my first set of regs, so I have done a fair bit of reading on different models and their characteristics. All this said and done, I have a few questions that are still floating in my head.

The breathing performances af a given regulator is mostly dictated by the first or second stage.

What would be the consequence of coupling a second stage from a easy breathing regulator (a Mares, Apex, Atomic for exmple) to a first stage of not so well breathing redulator (say a Sherwood Maximmus first stage).

Thanks in advance

Jeremie Laplante

You would have a regulator that is average in performance. The first stage flow rate is a critical factor in a high performance regulator.

Greg

 

[scubatoys]

It's really going to depend on what kind of diving you are doing. Basically, the second stage determins how easy a reg is to breath. And all the first stage does is supply about 145 psi of pressure up the hose to the second stage. That second stage has no clue if that air is coming from an atomic, or sherwood, or mares 1st stage. So as long as the 1st can keep up with the air demand - it is not too critical to the ease of breathing of the second.

Now that being said, if you have a wimpy non balanced, low output 1st stage, and you are at 200 feet with low tank pressure, 2 guys trying to breath off the same first stage while swimming into a current being chased by a shark.... we'll then the first stage may not be able to push enough air to fill the demand of the second.

Make sense??

 

[DA Aquamaster]

Second stage flow rates vary from about 30 cfm to about 70 cfm. So a high peformance second stage will have the potential to put more demand on the first stage particularly, as Larry pointed out, at depth, under high workload and/or stress and when paired with an in use octo.

What happens then is that the intermediate pressure drops from a normal 120 to 145 psi to a much lower figure. This lower IP then has an adverse effect on the volume of air flowing through the second stage as its flow rate is more or less proportional to the IP. If this volume falls below that required for the diver(s) to immediately get what they need, the first stage is being over breathed and the diver will feel he is not getting sufficient air.

A high perfromance second stage is not always the same as an "easy breathing" second stage as some low performance second stages breathe very well. But in sistuations where the IP drops excessively, a high performance second stage has more margin to spare as if a second stage with a 60 cfm flow rate is only delivering half of that potential under reduced IP conditions, it is still going to be asymptomatic compared to a low performance second stage that delivers the same 30 cfm air flow under ideal conditions but is only delivering 15 cfm under the same reduced IP conditions.

So a high performance second stage on a low performance first stage is a mixed blessing. With a single diver using the regulator, it will ensure better performance as it raises the bar on second stage performance which is normally the limiting factor. For the most part the second stage's greater flow rate will compensate for any IP drop that may occur in high demand situations.

With a diver and an OOA buddy however it creates more potential to outbreathe the regulator and a diver is well advised to try to breathe out of phase with the other OOA diver so you are not both inhaling at the same time.

With an unbalanced piston regulator (which tend to have low flow rates) or with a diaphragm regulator (which tends to have slightly slower repsonse times and greater drops in IP than balanced piston regulators) it is a good idea to set the first stage up to operate at it's maximum allowable IP (normally 145 psi for most brands). This will allow the first stage to deliver maximum flow rate and will provide a lot more cushion in terms of IP for the second stage compared to the same first stage operating at 120 psi.

 

[halocline]

Aren't there other factors in determining first stage performance besides max flow rate? Is there anything like a "response time" or something like that which is dependent on the quickness with which a 1st stage can "recover" to supplying standard IP after a breath is initiated?

I'm asking because my 1st instructor, who was a big SP fan, said that piston 1st stages react more quickly to the initial demand of the 2nd stage, which in turn results in a more natural feeling of inhalation. (all other things being equal)

It seems that if there were not subtle differences in performance beyond the max flow rate, virtually all 1st stages would be indistinguishable from each other, as they all have flow rates exceeding that of the 2nd stage under all but the most extreme conditions.

edit-never mind; this is addressed in DA's previous post.

 

[bottomrunner]

I am in the process of buying my first set of regs, so I have done a fair bit of reading on different models and their characteristics. All this said and done, I have a few questions that are still floating in my head.

The breathing performances af a given regulator is mostly dictated by the first or second stage.

What would be the consequence of coupling a second stage from a easy breathing regulator (a Mares, Apex, Atomic for exmple) to a first stage of not so well breathing redulator (say a Sherwood Maximmus first stage).

Thanks in advance

Jeremie Laplante

Hi Jeremie
all the info you got in this thread is good
the only thing i would add is what does your local dive shop carry
you will have to service them some where
and get parts for them. also all regs can be tuned or detuned to suit your type of diving.
see what all your friends are using and ask them to try thier regs
Most regs on the market will work for sport diving
It all depends what you want to do in the furure.
hopes this helps

 

[Falco]

Thanks for the input

My LDS can get Mares and Sherwood regulators. I have experience diving with the Sherwood Maximus (various years from the big black things to the sleeker 2004 model), the Blizzard and the Mares Abyss. I like the reliability and simplicity and of the Sherwood first stage (no winter kit, little maintenance) but they aren't quite as good breathers like the Abyss. What I want te find out is if I attach an Abyss/Proton Ice to a Sherwood first stage will they keep their "easyness" of breathing.

I am presently doing some rec diving in cold to very cold conditions (-2 to 5 Celcius in the water and it can get well below the freezing point on the surface) and I value the safety that Maximus gives me (neither I or my instructor has seen one freeze) and like it's compact size (the newer ones) but it has a less enviable performance record.

Jérémie

 

[spectrum]

Second stage flow rates vary from about 30 cfm to about 70 cfm.

No wonder I drew the tank down when I tried breathing off a freeflow holding by the purge!

 

[uberspeed]

DA, or anyone else with the information that is willing to help, where can I find the cfm flow rates on first and second stages? Is it something that I can calculate with a formula and/or equation given other data? Are there "benchmark" ratings that I should be aware of? I have looked on manufacturer's sites but have come up empty.

For example: a good, balanced diaphragm first stage will flow xxx cfm and a good balanced second stage will flow xxx cfm, the system as a whole will have IP variances of xxx psi and a WOB of less than 1.4 joules - or something along those lines.

Thanks in advance!