Why upstream?

[James-S]

I am not meaning to cause an argument about this, nor offend any die hard Poseidon fans, but I would like to ask, what is the benefit of having your second stages upstream? They are more complicated, and so more to go wrong, and they also free flow the rest of your gas away when you get to a certain tank pressure. Why do you recommend the Xstream for deep diving, down to 200m? At 200m, the tank pressure only need to be 60 bar before it is all lost. I know that you would leave more of a reserve than that, but in an emergency you would need all you could get.

Thank you in advance for your hopeful response.

 

[Jorgen Nilsson]

Dear Sir.

This is something that has been argued and debated amongst divers for as long as there's been upstream and downstream regulators.
And I wouldn't be surprised if this thread will turn in to a debate again, as it seems to be a topic that stirs up emotions.

To begin with, my personal opinion is that an upstream 2nd stage as the Jetstream or Xstream isn't more complicated than a downstream regulator. It's a bit different, yes, but not more complicated.

Secondly, if an upstream 2nd stage fails, it doesn't automatically means that it fails in such a way that it free flows and empties your tank. It can also fail "closed". It all depends on what causes the failure.

The most commonly argued reason why you should use an upstream regulator like the Jetstream or Xstream, is that they are quite easy to breath from. It requires little inhalation effort to open the 2nd stage valve and you get quite a lot of air.

The Jetstream 2nd stage has now been around for about 25 years and it's still a favorite among many divers who dive really deep on a regular basis. The Xstream 2nd stage is based on the Jetstream design and it seems it too is well liked amongst deep diving divers. But at the end of the day, it all comes down to your own personal preferences. Some like Poseidon regulators some don't and I respect them both.

I do not quite follow your statement about having to have 60 bar pressure in the tank, at 200 meters.
The Jetstream & Xstream 1st stage is set to an intermediate pressure of about 8,5 bar above ambient pressure. So even at 200 meters depth, the intermediate pressure is around 8,5 bar. So theoretically, you need to have more then 8,5 bar in your tank, to maintain the intermediate pressure. But in real life, you should not go below 20 bar tank pressure, as this might cause problems with a Jetstream or Xstream. Usually in the form of a free flow, as the intermediate pressure then don't have enough "umphfff" to seal up the 2nd stage.

I hope that this reply is helpful to you.

If you have further questions about our regulators, please feel free to contact me at any time at jorgen.nilsson@poseidon.com

 

[James-S]

Thanks very much for your reply. It answers most of my questions, but not about the minimum tank pressure required for the second stage to keep working. I read here that the tank pressure needs to be at least twice that of the absolute interstage pressure of the first stage. This means that at a depth of 200m, to which the regulator is certified, if the tank pressure drops below 59 ((8.5+21)*2) bar then the second stage(s) will free-flow the remaining gas away. I know that 59 bar isn't a big reserve for such a deep dive, but surely if you were in a position where you needed extra gas in an emergency, you would want to be able to access it instead of having it thrown away?

 

[scubadiver888]

James-S,

If you are at 200m with only 59 bar of gas in your cylinder, you have a LOT more to worry about. Anyone with the appropriate training to be at that depth would NEVER let their cylinder get that low.

Things to think about (I'll use imperial as I'm a north american diver):

• At 60 feet per minute, it would take you ~11 minutes to get to the surface.
• You would be obligated to do safety stops. So that 11 minutes becomes a LOT long.
• If you have a good SAC rate (0.5 cu.ft./min), at 200m you'd be breathing 10.5 cu.ft./min.
• 59 bar would be 855 PSI.
• The largest tank I use is 130 cu.ft. @ 3442 PSI or ~0.04 cu.ft./PSI
• 855 PSI would be ~32 cu.ft.
• At 200m or 21 ATA, this would be ~3 minutes of gas (32 / 10.5 = ~3 minutes).
• If the pressure outside the cylinder is greater than the pressure in the cylinder, the gas will not come out.

So (a) you are going to have to suck that gas out of the cylinder and (b) ignoring your safety stops, you have 3 minutes of gas and over 11 minute swim to the surface. You are going to run out of air and/or get bent.

Darrell

 

[Jorgen Nilsson]

Hi again.

As the link you provided seems to be "dead", I can't really comment on the content of that document or who wrote it, but I have checked with our engineers here, and they all agree with my initial post, that the intermediate pressure is around 8,5bra over ambient pressure and that 20 bar in your tank should be enough to prevent the 2nd stage from freeflowing.

If you have that "xstream.pdf" document, please send me a copy at jorgen.nilsson@poseidon.com

Thanks very much for your reply. It answers most of my questions, but not about the minimum tank pressure required for the second stage to keep working. I read here that the tank pressure needs to be at least twice that of the absolute interstage pressure of the first stage. This means that at a depth of 200m, to which the regulator is certified, if the tank pressure drops below 59 ((8.5+21)*2) bar then the second stage(s) will free-flow the remaining gas away. I know that 59 bar isn't a big reserve for such a deep dive, but surely if you were in a position where you needed extra gas in an emergency, you would want to be able to access it instead of having it thrown away?

 

[SDAnderson]

James-S,

If you are at 200m with only 59 bar of gas in your cylinder, you have a LOT more to worry about. Anyone with the appropriate training to be at that depth would NEVER let their cylinder get that low.

Things to think about (I'll use imperial as I'm a north american diver):

• At 60 feet per minute, it would take you ~11 minutes to get to the surface.
• You would be obligated to do safety stops. So that 11 minutes becomes a LOT long.
• If you have a good SAC rate (0.5 cu.ft./min), at 200m you'd be breathing 10.5 cu.ft./min.
• 59 bar would be 855 PSI.
• The largest tank I use is 130 cu.ft. @ 3442 PSI or ~0.04 cu.ft./PSI
• 855 PSI would be ~32 cu.ft.
• At 200m or 21 ATA, this would be ~3 minutes of gas (32 / 10.5 = ~3 minutes).
• If the pressure outside the cylinder is greater than the pressure in the cylinder, the gas will not come out.

So (a) you are going to have to suck that gas out of the cylinder and (b) ignoring your safety stops, you have 3 minutes of gas and over 11 minute swim to the surface. You are going to run out of air and/or get bent.

Darrell

A 200 meter (or 100 meter) dive is not a single tank dive. If you're carrying multiple bottles you want to be able to utilize as much gas from each cylinder as you possibly can before you have to switch. The greater the unusable reserve required, the less the efficiency of your plan.

 

[scubadiver888]

A 200 meter (or 100 meter) dive is not a single tank dive. If you're carrying multiple bottles you want to be able to utilize as much gas from each cylinder as you possibly can before you have to switch. The greater the unusable reserve required, the less the efficiency of your plan.

True, I was assuming with "Logged Dives: 25 - 49" that James-S is not a technical diver.