Intermdiate Pressure

[goldenwingk]

im fairly new to diving and i dont understand the Intermdiate pressure (ip).
what does this mean and how do u adjust it to certain diving environments.
for example i dive in cold ice water and also dive in deep water.
how do i adjust my "ip" to this type of diving and for diving in warm carribean waters?????
i'm diving with a MK17

 

[Isaac-1]

IP is the pressure level in the hose between the 1st stage and 2nd stage regulators, in general it should be somewhere between 125-150 psi. In general it should not be considered "user adjustable" and should only be adjusted by your service tech or those that know what they are doing. There are those that adjust it when diving in cold water to lower the chance of icing induced freeflow by lowering the pressure change they also lower the chilling effect of a gas coming out from under pressure.

 

[spectrum]

IP is the intermediate pressure and sometimes referred to as low pressure.

You have High pressure = tank contents
IP = set to 125 -140 over ambient range, give or take with brand. This feeds your second stages, BC &, drysuit if you have one.
Ambient pressure= your second stage output equal to ambient.

Only in the most specialized cases would anyone bother to finesse the IP . It will be set by the servicing technician and the second stages will be adjusted around it.

For a deep dive the IP of a depth compensating first stage (most decent ones) will automatically raise the IP to keep that same setting above ambient pressure for pretty consistent second stage performance.

For cold water, like under ice I have heard of intentionally dropping the IP to the bottom of the range to limit the adiabatic cooling.

Pete

 

[halocline]

Actually, lowering the IP increases the difference between HP air and IP air, thus at least theoretically increasing the adiabatic cooling in the 1st stage. I suppose it does decrease the difference between IP and ambient, but this is a much smaller pressure difference already. I believe the reason ice divers have been known to lower the IP is to slow the flow to the 2nd stage and (theoretically) lower the chance of freezing at the 2nd stage.

With a MK17, you should have no worries about freezing. I work on my own regs, and set the IP at around 125 (SP balanced pistons) which is the low end of the scale. IP will rarely decrease in use; almost always as the seat wears the opposite occurs. There's still plenty of flow at 125psi.

 

[Xaryo]

For cold water, like under ice I have heard of intentionally dropping the IP to the bottom of the range to limit the adiabatic cooling.

Pete

In fact I think it would be the opposite. The more the pressure drop, the more the cooling caused by the adiabatic expension. So if the IP is at the lowest, say 125 PSI, the delta P will be greater than an IP on 150.

Ptank=3000
IP=125
delta=2875

Ptank=3000
IP=150
delta P=2850

************
One of my friend doing ice diving says that he don't fill his tank to the service pressure to avoid that too great delta P between the tank pressure and the IP, exactly to avoid a frozen 1st stage and a free flow.

it is all basic thermodynamics.

 

[spectrum]

As Mattboy clarified my statement was with reference to easing the drop in the second stage. In my ice diving to date the second stages seemed to be the trouble spot for some divers. The second stage is where some environmental moisture is apt to be.

You both do make a point that it cuts both ways. Percentage wise an adjustment to the low end of the normal range is a much higher % change for the second stage.

Pete

 

[Isaac-1]

.... IP will rarely decrease in use; almost always as the seat wears the opposite occurs. There's still plenty of flow at 125psi.

With balanced piston regulators I have found this to only be partly true, as the lubrication fails/gums up the set point for the IP will usualy drop, but if there is a problem with the HP seat the pressure will then creep up until it either seals or blows off at the second stage. This is why if you take a first stage that has went a few years without servicing, but seems to be working ok, check the IP and find it to be low (say 120 psi with no creep), simply disassemble, clean, lube, and reassemble, and you will likely find the IP up in the 135-140 psi range without any part changes, or even touching the HP seat.

Ike

 

[widget]

Its very easy to check your IP yourself.

If you feel the need to check it yourself, buy a small IP gauge that plugs into your BC low pressure hose with the same male as your BC, (you could even make one yourself if you wanted to) attach your Reg to a tank and turn on the air - the gauge will read your IP pressure.

Obviously adjusting it is something you need to be a bit more carefull about or IF YOU ARE NOT CONFIDENT LEAVE WELL ALONE, but its just basic engineering, some regs have internal adjustments with shims which means you need to strip it down, but others are quite easy with external screws, plugs or nuts which one turns in or out to increase or decrease the IP, no disassembly is needed on these regs.