Deadly Down Current

[Gombessa]

You said earlier: "The regs they are breathing from will probably start breathing pretty hard by 350psi, and stop completely at 250psi..." Have you ever done that, even as a test - to see if it's true?

I know this is completely off-topic, but I drained my tank to 290psi (surface pressure) last Friday to adjust my buoyancy at 10ft - regs were no harder to breathe than at 3200psi.

 

[Ayisha]

If a reg is balanced, it is not supposed to breathe harder at depth or when nearly empty. It is supposed to require the same effort of breathing throughout.

Some people say that a pressure gauge/computer may not be completely accurate in the last 300 - 500 psi. Apparently some people have said that a gauge may show 200 or 300 psi but they have gotten nothing from it, so to be cautious of the last few hundred psi remaining. That may be what B1gcountry was referring to.

 

[DandyDon]

I know this is completely off-topic, but I drained my tank to 290psi (surface pressure) last Friday to adjust my buoyancy at 10ft - regs were no harder to breathe than at 3200psi.

It's probly a good idea to do that now and then to ensure your reg works as it should on low pressure and your SPG reads correctly. I have not sucked one empty in years; my guage and reg worked well right up to the last as I recall, but that has been a long time.

If a reg is balanced, it is not supposed to breathe harder at depth or when nearly empty. It is supposed to require the same effort of breathing throughout.

Some people say that a pressure gauge/computer may not be completely accurate in the last 300 - 500 psi. Apparently some people have said that a gauge may show 200 or 300 psi but they have gotten nothing from it, so to be cautious of the last few hundred psi remaining. That may be what B1gcountry was referring to.

Perhaps? I can't read minds or I wouldn't be a bachelor - just read what he typed. I agree on managing air well and returning to the boat with a good safety margin unless it is needed unexpectebly. I have a full tank due for a viz; maybe I should test my reg and Spg? It'd be a dry test, so I want it to breath well down to 50#. I guess I could test all my pony regs and Spgs at 100# too?

 

[b1gcountry]

Most regs run an IP of around 150 PSI. You cannot generate this with tank pressures below 150psi. Also, the psi is 150 above ambient, so to generate the proper pressure in the LP hoses, at 15', you need 150 psi, plus 1.5 ata, or about 23psi. Some regs are better than others, but you will definitely notice any reg breathing harder at about 300 psi. The point they stop working will be different for each reg, but it can't be below your IP plus your Atmospheric pressure (173psi in my example.) I said you have a pressure of 250 psi because I was being slightly conservative, but I don't think it was overly so. Air will not rush past the HP orifice as fast if the pressure differential is low between the tank and the LP hose, so even if a first stage is balanced, you will still notice that it takes longer to draw the air into your lungs.

And yes, all SPGs are pretty unreliable below 500psi. Mine has no markings below 350 even. If you are getting this low, then you had better not be relying on the gas to get you to safety.

Tom

 

[dave4868]

Most regs run an IP of around 150 PSI. You cannot generate this with tank pressures below 150psi. Also, the psi is 150 above ambient, so to generate the proper pressure in the LP hoses, at 15', you need 150 psi, plus 1.5 ata, or about 23psi. Some regs are better than others, but you will definitely notice any reg breathing harder at about 300 psi. The point they stop working will be different for each reg, but it can't be below your IP plus your Atmospheric pressure (173psi in my example.) I said you have a pressure of 250 psi because I was being slightly conservative, but I don't think it was overly so. Air will not rush past the HP orifice as fast if the pressure differential is low between the tank and the LP hose, so even if a first stage is balanced, you will still notice that it takes longer to draw the air into your lungs.

Tom, I agree with much of what you're saying, but I'd say it differently to keep it simple for myself.

I find it helpful to remember the 1st stage regulator is a downstream design that allows the pressurized air from the tank to flow through the high pressure chamber to the low pressure chamber until the pressure on the low pressure side overcomes the spring's force and shuts off the flow at the high pressure seat.

If the tank pressure is below the normal IP for the spring, etc, the flow is not shut off at the high pressure seat. That means the high pressure chamber, low pressure chamber and the tank will all have the same pressure.

Therefore, if tank pressure drops below a particular regulator's normal IP, but is still above the ambient pressure at the diver's depth, the first stage will supply gas to the second stage on demand, but at a lower than normal pressure equal to the difference between ambient and tank pressure, as you've pointed out.

I also agree the delivered pressure will be less than normal IP, but "proper pressure" is usually quite a bit higher than we normally need unless breathing heavily.

At shallow depths, like 15', the first stage will supply plenty of air for normal, relaxed breathing practically down to zero psi, but you may be right that it will seem to take longer to fill one's lungs, yet it's hardly noticeable, in my experience. For certain, one would become aware of the diminished performance when overbreathing the regulator with rapid, deep breathing.

My feeling is that the tank pressure doesn't have to be much above the ambient pressure to provide enough gas for normal, relaxed breathing.

And yes, all SPGs are pretty unreliable below 500psi. Mine has no markings below 350 even. If you are getting this low, then you had better not be relying on the gas to get you to safety.

I don't know about other brands, but my Sherwood spg's seem quite accurate and I often breathe my tank down to near zero (in the shallows at the end of a dive). I'll just call it gauge testing....

I know this is completely off-topic, but I drained my tank to 290psi (surface pressure) last Friday to adjust my buoyancy at 10ft - regs were no harder to breathe than at 3200psi.

That's been my experience, too, even when close to zero psi.

Heck, one can even reach a point where the regulator won't provide any gas unless sucked out, hence the expression, "sucked the tank dry".

But none of would ever do that, would we?

End of hijack....

Dave C

 

[b1gcountry]

I find it helpful to remember the 1st stage regulator is a downstream design that allows the pressurized air from the tank to flow through the high pressure chamber to the low pressure chamber until the pressure on the low pressure side overcomes the spring's force and shuts off the flow at the high pressure seat.
Dave C

Most second stages are downstream, normally closed.

First stages can be upstream or downstream, but are normally open.

Tom

 

[dave4868]

Most second stages are downstream. First stages are upstream.

Tom

Guess I was locked into thinking about my Sherwood piston 1st stage, which is a downstream design.

Apparently, after reviewing a reference, I see that most piston 1st stages are downstream, whereas most diaphragm 1st stages are upstream.

Told you I needed it simple! I shouldn't have generalized beyond my own experience.... Sorry about that!

I hate when that happens....

EDIT: Nevertheless, I believe the rest of my observations and conclusions hold true for both designs.

Dave C

 

[japan-diver]

A few things:
A reg will deliver air till the gauge reads zero- diaphragm or piston. Most modern regs will breathe the same at 200-300psi as they do at 3000psi. So whoever taught you how a reg works go get a refund- if you don't believe me take a reg to a shallow site and breath it zero.

On the original post:
We deal with down currents and upcurrents at many of our sites here in Okinawa and especially in Yonaguni.


1st. Not always caused by tidal currents- so just looking at the tide charts isn't enough. Local knowledge a pointed out goes along way.

2nd A tech BC and tons of lift not normally necessary- good planning and skills are.

Your first line of defense is avoid if possible- downcurrents are usually next to or close by walls or pinnacles and by watching particulate and coral on the wall you can normally pick up the current before getting sucked into it.

If you do get caught in one you normally have two routes of escape

1. Grab the wall and climb out of the current- this can use a lot of air and energy depending on the strength of the current. Holding the wall and waiting for it to stop would not be a good idea- most open ocean down currents will (even if tidal) will not stop anytime soon.

2. Swim away from the wall to escape the current. The further you get away from the wall of pinnacle the less the current and the easier to work your way up. The trick is not to panic or work to hard to get up.

When a down current is really going deploying a lift bag is usually very difficult or impossible. Had a case once where a 50lb bag fully inflated actually went down not up.- Luckily the bottom was only 150ft and we could get away from the wall easily and did a nice slow ascent.
Your best reaction is normally a quick a decisive action if you can not immediately grab the wall, swim away from it. Don't worry about lift bags or markers until you are stable.

We also dive the convergence of the East China and the Phillipine Sea off of Yonaguni and this puts us in some mid-water downcurrents sometimes- luckily this tend to not be as strong as the ones near walls and can normally be overcome by inflating the BC. They also tend to be somewhat compact so you can swim out of them easily.
They can be a bit unnerving as suddenly your bubbles are going down not up- its a little freaky but again stay calm - inflate swim forward and it will stop. Also make sure you are ready to dump air as soon as you break free from the current.

I dive a BP/W with a 30lb bladder and most of or clientele in Japan use jacket style BCs with limited lift and we have not had any problem dealing with downcurrents.

 

[dave4868]

A few things:
A reg will deliver air till the gauge reads zero- diaphragm or piston. Most modern regs will breathe the same at 200-300psi as they do at 3000psi. So whoever taught you how a reg works go get a refund- if you don't believe me take a reg to a shallow site and breath it zero.

It's not clear to whom your comments refer, but I'm obviously in complete agreement.

My extensive testing certainly corroborates your statement that modern regulators will have adequate output at near zero psi in shallow water.

I've even done some testing in deeper water, 200 psi at 80', but that was only because a bunch of scallops distracted me....

Breathing was about as easy as with the full pony when I switched over.

Dave C

 

[b1gcountry]

Alright, I'm now ignoring this thread. I had some comments about dive planning, and if you can take anything away from the point I was trying to make, then please do. My intention was never to start a discussion on reg performance, I was just explaining why I added 250psi to my gas planning calculations. Regardless of how the reg breathes, it is critical to plan your dives without relying on the last 250-350psi in your tank. If you consider this conservative, it was meant to be.

Tom