My "new" independent doubles setup

[AfterDark]

I’ve been given more thought to this change that I’m making to my Independent Doubles rig. If the testing goes well this change may alter my entire paradigm on diving independent doubles.

Since I started diving IDs during the 1980’s I’ve done regulator switches to keep the air supply in the tanks balanced or to regulate the air supply as the dive may require. Using 2-second stages it is the only sensible way lest you empty one tank and have a failure with the other setup and be OOA with a full tank on your back. So for the last 30 years that’s how I used my ID setup on hundreds of dives.

Recently I decided to change part of my rig replacing the 2-second stages with one, a Scubapro A.I.R. second stage. The AIR 2nd stage can be connected to 2 separate 1st stages at the same time. When both 1st stages have similar IP the tanks will drain evenly which is the reason for regulator switches, to keep the air supply in the tanks balanced or regulate the air supply as the dive may require.

After posting about this change on the board I received many comments all of which I read and have considered each on its merit and credibility. The one that kept coming back to me was Anglo’s comments and story about Marco and his pilot 2nd stage. At first the implications of this and how it could change my protocols didn’t click because I was stuck in my old paradigm of swapping regulators. It caused me to look at ID air management with the AIR from a different perspective.

It came to me that the inline shut-off’s best use isn’t replacing regulator switches. The best use is to isolate the AIR from the 1st stages in case of a 1st stage failure and a runaway free flow. Shutting off the inline to the failed 1st stage will redirect the air to the OVR on that 1st stage and make the AIR useable with the other tank.

The take away is if the AIR can and does allow the tanks to drain evenly when connected to separate tanks then my new protocol for diving my ID rig with the AIR will be both inlines opened. Only in the event of a failure or a change in the dive plan that requires more gas in one tank towards the end of the dive do I need to close an inline.

This makes diving with my rig even simpler than in its original 2-second stage form! Now I’m actually getting excited about this change!

 

[AfterDark]

So now that this thread has quieted down I would like to thank everyone especially the negative commenters for your input. Even the posts that were rude, and ridiculing have value to me, there's food for thought in each one even if it's only a fleeting thought.

It's been a fun thread and have enjoyed it very much, learned a few things I didn't know and confirmed a few things I wasn't sure if I understood correctly that alone is valuable.

I am going to try to take the AIR for a test spin today but the weather is making it hard to work up the enthusiasm for a dive knowing I'll be a Helen Keller impersonator for an hour.

I posted yesterday in the NE section that I remembered a site that is sheltered from the SSW SW winds we've been getting that destroys the vis around here. So last night the winds changed to NE and are forecasted to change to E winds both which of course blows directly off the water into that same site! So now there isn't anywhere in RI where the ocean isn't a efn mess!

THANKS AGAIN EVERYONE! I'll be updating post testing so there is more to come because as always. I'll be back!

 

[flymolo]

Actually I do dive a bereaver.

Oh god now I will never be able to forget that nickname for it

 

[AfterDark]

Oh god now I will never be able to forget that nickname for it

I didn't catch that! LOL! They have caused a fair share bereavement thru the years.

 

[halocline]

I didn't read this entire entertaining thread because I saw the other, also entertaining one, where I posted my opinion of this set up. OP, I have to wonder, why are you posting this? You obviously have your own opinions about it and when someone criticizes it, you argue. So why bother, what do you care what anyone else thinks? Especially idiots like cave diving instructors. What do they know?

Anyhow, the reason I'm posting here is to address the issue of unequal IPs in the 2 first stages. Forgive me if someone else beat me to it.

Basically, if the inline valves are in the 'on' position, or didn't exist, what you have is one humongous IP chamber that spans from the IP chamber of one 1st stage, through the hoses and 2nd stage (upstream of the valve) and on to the other 1st stage. The pressure in that whole system must equalize, that's just physics. Therefore, the IP of each 1st stage will be identical. You're basically measuring IP at two different points in the same pressure chamber.

In case anyone needs an example, here it is: Assume 1st stage A has an IP of 130, and 1st stage B has IP of 140. (measured separately, of course) IP is, by definition, the pressure required to close the 1st stage valve. Any lower pressure in the chamber, and the valve stays open, allowing more HP air into the chamber until it reaches IP, then shuts the valve. Everyone should understand this, correct? (Sorry if I'm being too basic for the experts among us) But since they are connected together via LP hoses, air will keep flowing into the chamber until both 1st stages are closed, so in this case, that would be 140 PSI. This also means that until the pressure drops below 130PSI in the IP chamber, 1st stage A will not open and all the air will be coming from 1st stage B.

 

[Angelo Farina]

I didn't read this entire entertaining thread because I saw the other, also entertaining one, where I posted my opinion of this set up. OP, I have to wonder, why are you posting this? You obviously have your own opinions about it and when someone criticizes it, you argue. So why bother, what do you care what anyone else thinks? Especially idiots like cave diving instructors. What do they know?

Anyhow, the reason I'm posting here is to address the issue of unequal IPs in the 2 first stages. Forgive me if someone else beat me to it.

Basically, if the inline valves are in the 'on' position, or didn't exist, what you have is one humongous IP chamber that spans from the IP chamber of one 1st stage, through the hoses and 2nd stage (upstream of the valve) and on to the other 1st stage. The pressure in that whole system must equalize, that's just physics. Therefore, the IP of each 1st stage will be identical. You're basically measuring IP at two different points in the same pressure chamber.

In case anyone needs an example, here it is: Assume 1st stage A has an IP of 130, and 1st stage B has IP of 140. (measured separately, of course) IP is, by definition, the pressure required to close the 1st stage valve. Any lower pressure in the chamber, and the valve stays open, allowing more HP air into the chamber until it reaches IP, then shuts the valve. Everyone should understand this, correct? (Sorry if I'm being too basic for the experts among us) But since they are connected together via LP hoses, air will keep flowing into the chamber until both 1st stages are closed, so in this case, that would be 140 PSI. This also means that until the pressure drops below 130PSI in the IP chamber, 1st stage A will not open and all the air will be coming from 1st stage B.

True. That's the reason for using two closely matched, identical 1st stages, for getting a balanced air supply from both tanks.
This will never be PERFECTLY balanced, one tank will always be drained more quickly than the other.
But this could even be a good thing, for example ensuring a bit more air in the tank supplying the BCD.

 

[AfterDark]

I didn't read this entire entertaining thread because I saw the other, also entertaining one, where I posted my opinion of this set up. OP, I have to wonder, why are you posting this? You obviously have your own opinions about it and when someone criticizes it, you argue. So why bother, what do you care what anyone else thinks? Especially idiots like cave diving instructors. What do they know?

Anyhow, the reason I'm posting here is to address the issue of unequal IPs in the 2 first stages. Forgive me if someone else beat me to it.

Basically, if the inline valves are in the 'on' position, or didn't exist, what you have is one humongous IP chamber that spans from the IP chamber of one 1st stage, through the hoses and 2nd stage (upstream of the valve) and on to the other 1st stage. The pressure in that whole system must equalize, that's just physics. Therefore, the IP of each 1st stage will be identical. You're basically measuring IP at two different points in the same pressure chamber.

In case anyone needs an example, here it is: Assume 1st stage A has an IP of 130, and 1st stage B has IP of 140. (measured separately, of course) IP is, by definition, the pressure required to close the 1st stage valve. Any lower pressure in the chamber, and the valve stays open, allowing more HP air into the chamber until it reaches IP, then shuts the valve. Everyone should understand this, correct? (Sorry if I'm being too basic for the experts among us) But since they are connected together via LP hoses, air will keep flowing into the chamber until both 1st stages are closed, so in this case, that would be 140 PSI. This also means that until the pressure drops below 130PSI in the IP chamber, 1st stage A will not open and all the air will be coming from 1st stage B.

Posts like yours is why I post. I don't cave dive. Thank you!

 

[AfterDark]

AD why don't you like SM?

I can't believe non-cave divers dive this way and like it?! We are not all the same. I think I'd rather snorkel.

 

[Kupu]

True. That's the reason for using two closely matched, identical 1st stages, for getting a balanced air supply from both tanks.
This will never be PERFECTLY balanced, one tank will always be drained more quickly than the other.
But this could even be a good thing, for example ensuring a bit more air in the tank supplying the BCD.

If IP on the both MK5s is reasonably matched (say 5 PSI or less difference) with full tanks, it should not take long to reach the point where the MK5s have matching IP? Assuming the inline shutoffs both remain open.

IP drops as tank pressure is reduced on the MK5 with highest starting IP, which would be initially feeding the system. Soon the 1st stages reach a point of matching IP and both begin feeding the 2nd stage equally. Both tanks draw down at an equal rate from that point onwards and both tanks reach empty on the same breath.

Is my thinking correct?

 

[AfterDark]

If IP on the both MK5s is reasonably matched (say 5 PSI or less difference) with full tanks, it should not take long to reach the point where the MK5s have matching IP? Assuming the inline shutoffs both remain open.

IP drops as tank pressure is reduced on the MK5 with highest starting IP, which would be initially feeding the system. Soon the 1st stages reach a point of matching IP and both begin feeding the 2nd stage equally. Both tanks draw down at an equal rate from that point onwards and both tanks reach empty on the same breath.

Is my thinking correct?

The MK 5 is a balanced 1st stage the IP stays the same regardless of tank pressure above the IP.
The IP is changed by adding or subtracting main spring shims not by tank pressure. The IP needs to be similar on both MK5s as it is on mine 135 psi on both MK5. The IP hasn't changed since I last serviced them in 2017. They are over due but that's one of the beauty things about MK5's they are very forgiving of neglect to point.