what is the flow rate of tank valve

[shogan]

Ok,

I keep trying to analyse 1st stages. Thought I had the sherwood picked out as having solid proven balanced piston design, enviornmentally sealed, and cheap to maintain (28 each service including parts). But I keep hearing some say it's not as technically great as current models.

It has 2 HP ports plenty of lp ports and is envioronmentally sealed. So the only thing I can deduce is when I hear that it won't perform as well as some more current regs I can only assume folks are referring to the air flow as the reg has 1/3 to 1/4 of the air flow capability of current 1st stages. But then I read about tank valves not being able to give enough flow rate to meet the regs.

So what is the flow rate of my typical AL80 valve. Just trying to figure out how much more air the other regs are providing over the sherwood.

 

[herman]

You are way over thinking this. Pretty much any modern...and I consider the older Sherwoods to be modern...first stages and valves can flow way more gas than any 2 divers can use. What you are really hearing is sales hype and uneducated chatter from those who believe the sales hype. The old Sherwoods firsts are solid regs and will hold their own with any reg on the market today in a relevent test.....a max flow rate test is irrelevent. Will other flow more gas? Sure, but it's like compairing 2 cars, one will go 130 mph and the other will go 150 mph but both have rev limiters to prevent them from going over 100 mph so their actual top end is irrelevent. Same goes for first stages, as long as it can supply gas at a sufficient volume to supply 2 divers, any extra is of no use.
Another thing you have to realize is, as long as a first stage can deliver enough gas at close to the second stages design IP without dropping in IP pressure it has almost no effect on the performance of the second stage. The second stage is where you need to concentrate if you want the best performance. Reguardless of the hype, as long as a second stage is getting plenty of flow and an IP at anywhere close to it's design pressure it will preform well, reguardless of what it's hooked to, be it a top of the line Brand X first stage or a bench test hose. Here is a thread you might find interesting. http://www.scubaboard.com/forums/re...difference-between-2-first-stages-help-2.html

A slight correction, the Sherwoods are not technically balanced pistons. They are flow by pistons with a floating HP seat that keeps IP constant as tank pressure varies. The floating HP seat is what "balances" the reg. Bottom line is they are balanced, just do it differently. Connect a Sherwood first to any of a number of quality second stages and it will serve you well.

 

[rstofer]

Assume a non-panicked SAC rate of 1 SCFM. OK, take it to 100' or 4 SCFM then double it for 2 divers to 8 SCFM and then double it again for the heck of it to 16 SCFM. Regs that advertise capabilities over 100 SCFM are just doing marketing.

Now, take your full Al 80 and open the valve all the way. See how long it takes to drain the tank. If it takes more than 5 minutes, your tank can't deliver 16 SCFM. And we're giving the valve every opportunity because it is basically venting to 1 ATM. The flow rate will be less when it is delivering air at 4 ATM.

In any event, the tank will not drain in 48 seconds so there is no possibility it will keep up with a 100 SCFM regulator. And I'm not convinved you can shove 100 SCFM through the hose in any case!

Richard

 

[herman]

Assume a non-panicked SAC rate of 1 SCFM. OK, take it to 100' or 4 SCFM then double it for 2 divers to 8 SCFM and then double it again for the heck of it to 16 SCFM. Regs that advertise capabilities over 100 SCFM are just doing marketing.

Now, take your full Al 80 and open the valve all the way. See how long it takes to drain the tank. If it takes more than 5 minutes, your tank can't deliver 16 SCFM. And we're giving the valve every opportunity because it is basically venting to 1 ATM. The flow rate will be less when it is delivering air at 4 ATM.

In any event, the tank will not drain in 48 seconds so there is no possibility it will keep up with a 100 SCFM regulator. And I'm not convinved you can shove 100 SCFM through the hose in any case!

Richard

Nor can you shove it through a second stage orifice which is the most restrictive point in the assembly after the HP orifice.

Something to consider here. While using SCFM to estimate the time a tank will last at a given depth works well, it has no bearing on flow rates at depth. The actual flow rate in cfm (or L/min) a diver inhales at 100 ft is exactly the same as it is at the surface. If a diver breathes in 1L of gas at the surface in a normal breath he will also breathe in 1L at any depth. (these numbers are random values chosen for easy math-they are not intended to be accurate volumes/rates) Assuming a breathing rate of 20 breaths per minute and a 1L/breath volume at the surface that gives you a 20L/minute flow rate, that same diver will still breathe in 1L per breath at depth and assuming he is still breathing at 20 breaths per minute, the flow rate is still 20L/minute. The total number of molecules of gas removed per breath from the tank is 4 times greater at 100 ft which is why the tank empties faster but because the gas is being breathed at a higher ambient pressure, the volume per breath is the same regardless of depth even though the total molecules of gas removed from the tank has increased by 4x (at 100 ft). The wide open flow rate of the regulator is also going to be basically the same regardless of depth. While ambient pressure has risen, the PSIG of the IP has remained constant (PSIA has risen to compensate for the greater ambient pressure) so flow should be the same...granted the increased air density will slow the flow some but not enough to be detected by a diver.

 

[Texfrazer]

So what is the flow rate of my typical AL80 valve. Just trying to figure out how much more air the other regs are providing over the sherwood.

I knew I'd find it around here somewhere. Check out this post.

Looks like 28-32 cfm for a yoke (though that sounds a hair low to me).

 

[shogan]

I knew I'd find it around here somewhere. Check out this post.

Looks like 28-32 cfm for a yoke (though that sounds a hair low to me).

Which is exactly where the sherwood is if I understand the sherwood information correctly.

So this being said if I'm understanding everyone correctly as long as everything is tuned/functioning properly my sherwood blizzard 1st stage with my fav 2nd stage should function similar as my mk 16 1st, or any other 1st that I might pair up with it.

 

[Bubbletrubble]

So this being said if I'm understanding everyone correctly as long as everything is tuned/functioning properly my sherwood blizzard 1st stage with my fav 2nd stage should function similar as my mk 16 1st, or any other 1st that I might pair up with it.

I think a better way to state things is that the tank valve will most likely be the limiting factor in determining practical, "real world" max flow rates for a given first stage.

It would be silly to base your reg purchase on "max flow rate" though, since pretty much any modern regulator on the market will provide enough gas for two "stressed" divers sharing gas at depth. Better to look at other features: subjective breathability, inhalation adjustment knob (cracking pressure control on second stage), reliability, price, weight, adjustability, warranty policy, number/location of ports, ease of maintenance, ease of tuning, ease of repair, ability to service locally, parts availability, ability to DIY service, performance in cold water, environmentally-sealed first stage, etc.

Good luck...

 

[halocline]

I think the way 1st stages are tested for flow rates involves taking all the port plugs out and hooking the reg up to a constant 3000PSI source with very high flow. So, for example, the SP MK25 claims to have a 300CFM flow rate, that's actually 60CFM out each of the five ports. (assuming each port flows the same, I know they don't but that's a different discussion)

I seriously doubt there are many second stages that flow 60CFM, maybe some with very aggressive purges, who knows. It doesn't matter.

The tank valve is almost certainly the bottleneck in flow rates, especially as the tank loses pressure.

I have experienced first stages directly affecting reg performance; when I converted my DA aquamaster to a RAM, the only difference is in the first stage nozzle, and there was a VERY noticeable difference in performance while diving. This was due to the much smaller drop in IP under inhalation and quick recovery. The second stage with that reg is an unbalanced diaphragm, and relies on a lot of venturi effect to smooth out the delivery down the big ambient pressure hose to the mouthpiece.

 

[LeadTurn_SD]

This is an interesting discussion (and there have been some other very good threads recently).

My question is sort of an open one:

Qualitative versus Quantitative performance: How closely are they related?

What I mean is, do subtle improvements in 1st stage design actually lead to improved breathing performance as perceived by the diver? Things like a designated/dedicated primary LP port, more direct airflow path, greater overall 1st stage airflow potential, etc.

In theory, if the first stage can provide a stable IP, it shouldn't matter whether it is a $100 brand "X" or a $600 brand "Z" first stage.

But, are there large differences between current low-end, mid-range, and high-end first stages when it comes how much the IP drops during inhalation, and how quickly it rebounds for example?

I honestly don't know... I currently own 2 brands and 4 models of regulators, and 1st stage performance as far as I can measure it with an IP gauge and by the "feel" of breathing is identical (but all are mid-to-high-end 1st stages).

So I'm curious....

Best wishes.

EDIT: I was posting at the same time as mattboy, and his post above partially answers what I'm asking.

 

[shogan]

Ok so outside of physical attributes of port placement, special features, cost and ease of repair etc, it sounds like the biggest factor of performance is not air flow per say but more if the ability to maintain constant IP under demand

Is there anyway to quantatively check this. The reality of actually checking this via LDS is not very realistic. 1st a pool is nothing like being 40'-60' down. 2nd I've found the LDS tends to be biased towards their highest end line.