Free flow at depth

[Diver003]

This is how I understand IP or Intermediate Pressure. The IP (Intermediate Pressure) is the pressure level that the 1st stage is "regulating" down to between 9-10 bar (around 132psi to 147psi) from the pressure in your Cylinder (Tank). Remember your cylinder pressure can be 3000psi for an AL80 for example or 3442 for a High Pressure 130 as another example. Imagine trying to take a breath from an air stream pumping out at you with 3000psi Now we can go into great detail about balanced and non balanced first stages (which I am not experienced enough to lead), however the key takeaway for me is that a properly balanced 1st stage will be able to maintain this IP of say 140psi from the beginning of your dive with your tank at 3000psi all the way down to 500psi.

Here is a link to the Wikipedia explanation:
Diving regulator - Wikipedia, the free encyclopedia

HTH,
Mike

Ok, this part makes sense to me But when applied to this scenario, why is checking IP so important? It seems if the regulator was properly serviced, the IP would be correctly adjusted - unless temperature, moisture, etc. affects it? This is where I get confused...

 

[lowviz]

Ok, this part makes sense to me But when applied to this scenario, why is checking IP so important? It seems if the regulator was properly serviced, the IP would be correctly adjusted - unless temperature, moisture, etc. affects it? This is where I get confused...

Your first stage controls (lowers) the changing pressure from your tank to a constant pressure (the intermediate pressue) that allows your secondary reg to function with nearly unbelievable accuracy.

Your IP is constant but it is also a range. On the low side of this range, you work to breathe but less chance of free flow. On the high side, your secondary throws gas at you, but you risk free-flow.

I used to have my tech set mine high during the summer and low during the winter. Now just set them mid-range all year.

Reg techs tend to set them high for vacation divers.

-Just one of the many reasons why some encourage you to work on your own regs...

 

[String]

Ok, this part makes sense to me But when applied to this scenario, why is checking IP so important? It seems if the regulator was properly serviced, the IP would be correctly adjusted - unless temperature, moisture, etc. affects it? This is where I get confused...

Just because something is set right at a point in time doesn't mean it stays right. Like anything with moving parts a regulator first stage has some movement and loosening. Springs lose their tension slightly, seats get compressed or moved. Tiny adjustment threads move slightly. All of this through the thousands of repeat cycles in normal use.

From my experience usually IP drifts up as opposed to down and the result is a constantly flowing second stage after a few seconds of the IP creeping upwards.

 

[Diver003]

lowviz and Sting ... your comments have cleared up a lot for me.

 

[DevonDiver]

Don't breathe your reg on the surface even once as the air has a lot less "thermal mass" than water. Do a controlled entry with your first breath underwater. Remove your secondary and with the mouthpiece pointing down, look for a steady stream of bubbles. If bubbling, it won't get any better. Return and fix what will become a free-flow. If no bubbles, ok to go. Do not add air to wing or suit while taking a breath, spread out the load on your primary. Breathe more slowly. First from diaphram then expand chest. Slow down.

+1

Air and water temps don't need to be freezing, for your regulator to ice.

One of the biggest causes of cold-water freeflow is pre-breathing the regulator (dry) on the surface. This can cause a tiny sheen of 'frost' in the regulator. Cold air temperature + chilled air rushing from your tank under pressure = cold enough for icing.

When you subsequently start the dive, this sheen of 'frost' will rapidly form ice once it is contact with water. Over the space of a few minutes it can completely fail your regulator.

This was the cause of my first and only deep, cold water free-flow many years ago.

Solutions to this problem are:

1) As lowviz says - amend your pre-dive safety check so that you are not breathing your regulator 'dry'. Postpone the 'air check' until you are in the water.

2) Ensure that your regulators are stored in a warm location prior to the dive. Don't leave them sitting around in the cold air. I've even gone so far as to get a picnic 'cool bag' and place it over the regulators, along with a chemical handwarmer.

 

[Subcooled]

Well, when the gas pressure drops from 200 bar to 10 bar to 2 bar (multiply by 15 to get psi), then gas is going to cool down. A lot. The air could be substantially below 0c. It could be -10c. Any moisture in the regulators starts to cool down until it finally freezes. Of course, exhaled air is warm and moist, and inhaled air is (hopefully dry and) very cold. These effects may cancel each other, or not.

The deeper you are, the denser the air you breathe through the regulator.

The denser the air, the better it cools down the regulators diaphragm.

Free flow probability increases with depth/pressure and also with time (as humidity from exhaled air builds up).

 

[Subcooled]

And its warmer on the surface, 9 vs 6.

You've found the tipping point

 

[Plotek]

It seems to me that free flow and freezing regulators are something like mystery

Sorry but 9 or 6 degrees of C is too much to have freeflow from frozen 2 stg, otherwise in some countries it should happen all the time, but it doesn't happen. Sorry guys but advice to do first test in the water is ok, but when You have less than -5 C on surface. The +9 C is pretty hot for some divers You should see some movies, for example made by Cousteu, when the divers goes under the ice on Antarctica. They always started breathing before descent.

The first reason why it happens is some water inside the bottle. So I always try to fulfil my bottles in well known dive centres. If I can not, then I am using personal filter just to have dry air. If Your air is not enough dry, then even with best reg's, the free flow can happen.

If You have troubles with IP then You should have constant flow of air, but quite small and all the time even on the surface. It is very far from free flow.

It is true that air temperature is going down when we start to breath, but considerable amount of time is necessary to frozen yours first stg, and it can happen only if there is some water inside.

 

[Subcooled]

The water in the bottle explanation seems most probable.

What regulators did you use?
Did you get your air/bottles from the same place?

 

[Saudi-Diver]

I suspect that the compressor your using is giving wet fills, air in your cylinders should be dry.