is there a 'downside' to boosting the IP to increase 2nd stage performance ?
- Thread starter scubafanatic
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
It's common to reduce the IP in a cold water regulator as it reduces the flow rate which in turn reduces the amount of adibatic cooling which is one of the major causes of first stage freeze ups in the first place.
Car guys are pretty familiar with engines and horse power so I'll try this example. To get more power from an engine, you enlarge the bore and/or lenghten stroke and/or smooth the flow passages and/or you increase the RPM and/or you add a supercharger or turbo charger to compress the air going through the engine and increase the manifold pressure. With any of these approaches you improve the mass flow of the enigne and power output is directly proportional to the mass flow (not exactly, but more or less).
Regulators work the same way. The higher the pressure of the intermediate pressure air going through the first stage passages, the higher the flow rate of the regulator since the higher pressure of the air allows more mass to flow through the regulator's passages in a given amount of time.
On the other hand, there is only so much you can do with a small engine or a "small" first stage. You can increase the intermediate pressure but you have to keep in mind that the pressure of the air inside the regulator is also increased by ambient pressure by about .5 psi for each foot of water depth, so eventually you go beyond the point of diminishing returns as the increased viscosity of the gas under increased pressure at depth begins to reduce the flow of gas through the passages in the first stage more than the increased pressure increases it. With an IP of 145 psi, at 150 ft, you will have an actual pressure inside the first stage of 225 psi. If you bumped it up to 170 at the surface, it would be 245 psi, at 190 psi it would be 265 psi, etc. That air is a lot thicker than the air you are going to put through even the most highly turbo/supercharged car engine - and that much thicker air starts moving a lot slower.
As a teen who liked to go fast, there was only so much I could do with the very limited displacement of the 4 cylinder in my Chevy Vega, so it got thrown out and was replaced with a 350 small block. It was a much bigger engine with much more performance (and growth potential) and was a far more efficient solution than trying to get seriously fun performance out of a production 4 cylinder motor.
In the same manner, if you have to boost the IP above the normal maximum IP range of about 140-145 psi in a first stage to get adequate performance, you really need a small block...I mean a bigger first stage.
Also, to be able to benefit from the additional horse power in a Vega, it needed a new transmission (Turbo 350), much larger tires and ladder bars as the drive train became the limiting factor when the motor was replace.
Similarly, in a second stage the limiting factor in all cases is second stage performance. Even the highest performing second stage will not flow more than about 70 to 75 SCFM, so if you have a first stage that will flow 175 SCFM at an IP of 145 psi or alternatively will flow 145 SCFM at a lower IP of 120 psi, who cares? At the reduced flow rate of 145 SCFM, you still have 70 to 75 SCFM more flow rate than you can use.
Car guys are pretty familiar with engines and horse power so I'll try this example. To get more power from an engine, you enlarge the bore and/or lenghten stroke and/or smooth the flow passages and/or you increase the RPM and/or you add a supercharger or turbo charger to compress the air going through the engine and increase the manifold pressure. With any of these approaches you improve the mass flow of the enigne and power output is directly proportional to the mass flow (not exactly, but more or less).
Regulators work the same way. The higher the pressure of the intermediate pressure air going through the first stage passages, the higher the flow rate of the regulator since the higher pressure of the air allows more mass to flow through the regulator's passages in a given amount of time.
On the other hand, there is only so much you can do with a small engine or a "small" first stage. You can increase the intermediate pressure but you have to keep in mind that the pressure of the air inside the regulator is also increased by ambient pressure by about .5 psi for each foot of water depth, so eventually you go beyond the point of diminishing returns as the increased viscosity of the gas under increased pressure at depth begins to reduce the flow of gas through the passages in the first stage more than the increased pressure increases it. With an IP of 145 psi, at 150 ft, you will have an actual pressure inside the first stage of 225 psi. If you bumped it up to 170 at the surface, it would be 245 psi, at 190 psi it would be 265 psi, etc. That air is a lot thicker than the air you are going to put through even the most highly turbo/supercharged car engine - and that much thicker air starts moving a lot slower.
As a teen who liked to go fast, there was only so much I could do with the very limited displacement of the 4 cylinder in my Chevy Vega, so it got thrown out and was replaced with a 350 small block. It was a much bigger engine with much more performance (and growth potential) and was a far more efficient solution than trying to get seriously fun performance out of a production 4 cylinder motor.
In the same manner, if you have to boost the IP above the normal maximum IP range of about 140-145 psi in a first stage to get adequate performance, you really need a small block...I mean a bigger first stage.
Also, to be able to benefit from the additional horse power in a Vega, it needed a new transmission (Turbo 350), much larger tires and ladder bars as the drive train became the limiting factor when the motor was replace.
Similarly, in a second stage the limiting factor in all cases is second stage performance. Even the highest performing second stage will not flow more than about 70 to 75 SCFM, so if you have a first stage that will flow 175 SCFM at an IP of 145 psi or alternatively will flow 145 SCFM at a lower IP of 120 psi, who cares? At the reduced flow rate of 145 SCFM, you still have 70 to 75 SCFM more flow rate than you can use.
OP
scubafanatic
Contributor
- Messages
- 5,090
- Reaction score
- 914
oxyhacker:
....well, this somewhat confirms what the dive shop told me...but I'm still confused, this statement doesn't specify if lowering the IP from 145 psi to 130 psi is MANDATORY or not......I think Mares is specifying this be done under the assumption anyone using the CWD kit is diving 'cold' water, they are not making an allowance for someone who's not necessarily interested in the cold water advantages for the kit, but is really looking only at the 'environmental sealing' feature to protect the 1st stage from salt/particulates.........so I'm assuming there's no reason I can't use the CWD kit and still set the IP @ 145 psi, is there ?
Karl
A couple of points here. First, the upper range of the IP is determined by the spring tension on the LP seat for a downstream second stage regulator; if the regulator is set above that, the second stage leaks. So we're talking about maybe 10 to 15 psi difference, not in the 195 psi range mentioned above. Even at that, there may be some leaking due to the difference between the exhaust valve and the diaphragm, which I mentioned above. So it's not a great amount of psi that they would be increasing in the intermediate pressure.DA Aquamaster:
Second, wouldn't simply putting a long hose on the regulator be the same as that "small block" analogy you were talking about, with the hose being the reservoir? But again, this is probably academic as the amount of psi adjustment is rather limited.
SeaRat
Unless you are going to dive it cold... don't bother with the kit. A diaphragm reg keeps all the water on the outside anyway... Just a diaphragm and spring ever feel the water - and as previously said, the amount of possible salt build up would be so very insignificant that you do not need to worry at all. I've carried an abyss all over the world for about 3 years I was using one... and never bothered to seal it... or rinse it... or rebuild it... 
It is probably one of the most indestructible work horse regs, you can run into.
And as far as this whole salt build up thing... The very small cavity at the top of the reg can hold maybe a few spoons of water... not that much salt there, and the next time you jump in... if there is any - it dissolves. If water didn't dissolve the salt, you would see huge mounds of salt on the beach... salt crusted on palm trees... never seen it.
Just use your reg, don't worry about it, and rinse it good at the end of the trip. If you do that... you are fine.
It is probably one of the most indestructible work horse regs, you can run into.And as far as this whole salt build up thing... The very small cavity at the top of the reg can hold maybe a few spoons of water... not that much salt there, and the next time you jump in... if there is any - it dissolves. If water didn't dissolve the salt, you would see huge mounds of salt on the beach... salt crusted on palm trees... never seen it.
Just use your reg, don't worry about it, and rinse it good at the end of the trip. If you do that... you are fine.
oxyhacker
Guest
- Messages
- 1,314
- Reaction score
- 36
I don't see any reason why increasing the IP should be mandatory - I think it is just one other thing you can do at the same time as installing the kit, to improve cold water performance.
BTW, Mares are funny, because they are mostly, even the top of the line ones, single adjustment 2nd stages - you can adjust the lever height but not the cracking pressure. So Mares service literature suggests using the IP to fine tune the cracking pressure. But if you install the cold water kit and set the IP as recommended, you cannot do this. So performance may take a small hit, if you follow the Mares instructions to the letter. This is also a good reason NOT to try and increase the performance on a Mares by cranking up the IP.
BTW, Mares are funny, because they are mostly, even the top of the line ones, single adjustment 2nd stages - you can adjust the lever height but not the cracking pressure. So Mares service literature suggests using the IP to fine tune the cracking pressure. But if you install the cold water kit and set the IP as recommended, you cannot do this. So performance may take a small hit, if you follow the Mares instructions to the letter. This is also a good reason NOT to try and increase the performance on a Mares by cranking up the IP.
scubafanatic:
Not it's not a great amount of increase or decrease in IP if you stay in the recommendend IP range for most brands (120-145 psi). You'd want a REALLY good reason to go beyond that though.John C. Ratliff:
Any second stage adjustment needs to be done after the first stage IP is set as it does effect the spring pressure needed in the second stage - even with a balanced second stage as they are all biased to the downstream side to allow them to vent excess pressure (from a leaky HP seat) by freeflowing.
A larger internal reservoir of air will help supply more air immediately if there is a brief lag while the poppet opens before air begins to flow but it won't do anything to improve the maximum flow rate once the system is running wide open. So with the car analogy, a larger internal reservoir of IP air would help compensate for slow throttle response but would not increase maximum horsepower.John C. Ratliff:
Similar threads
