#1 I have never used a rebreather so I cant really relate to when people say "position of the diver affects DH reg performance". Is it a negative? How significant is it? Again, since I haven't experienced that myself, any info will be very much appreciated
Positioning effects can be a negative, but are generally just different. Bad is if the can (the reg itself) is not positioned properly (as Ghost95 said, between shoulder blades directly against your back). The VDH backplates are great for this, but I also use backplates I cut from a $5 cutting board. The "Different" aspect is that position of mouthpiece relative to the cans has a perceived work of breathing effect. If the mouthpiece is higher than the cans, it will feel like it is trying to feed you air (puff your cheeks out), but the exhale will take more effort. If the mouthpiece is lower than the cans, it will feel more like you have to pull the air in, but the exhale will be effortless. Ghost95's preferred slightly head up trim keeps the mouthpiece level with the cans, minimizing either effect.
#2 Are there any limitations on tank pressure? Meaning, can I take a DH reg, put it on a 200-230bar tank and off i go?
Depends on the DH reg. There are 2 factors to consider.
1) Type of reg; Single stage regs (like an Aqualung Mistral or a Healthways Scuba) are harder to breath the higher the tank pressure, so most wouldn't recommend them for over 200 bar at all, and they won't really come into their own until you breath it down to 100 bar or so.
Unbalanced 2 stage regs (Aqualung DAAM and the like) IP increases as tank pressure drops.... the US Navy used them on 3000 PSI tanks (208 bar) with no issues, but they will breathe better as tank pressure drops (but not to the degree of change a single stage will).
Balanced 2 stage regs (Royal Aquamaster, VDH cyclone, VDH Phoenix, Kraken, Voit Trieste) will maintain IP throughout tank pressure, and as such should be fine in that regard with any thing (within reason).
2) Yoke design; Early scuba gear was designed for 2250 psi tanks, and as such had thinner yokes. Later DH's had thicker yokes and tank pressure is a non issue in that regard with those, but with a skinny yoke it's a judgement call. I'll run skinny yokes up to 3000psi (208 bar), but some won't.
#3 Perhaps a stupid one, but if I am donating air, does it affect the regs performance?
Doesn't affect performance, but your options to donate depend on the reg. Without an octo, buddy breathing is the only option and is a trained skill. Single stage regs have no way to add an octo. I have octo's on almost all my DH regs, and when I dive with others and use one without an octo I bring a slung pony tank.
#4 It looks like there are 2 main sellers:
a)
Home | Vintage Double Hose [currently not working]
b)
Welcome to DiverDown Double Hose Regulators
anyone I missed?
The Scuba Museum (as Rhwestfall pointed out).
To be fair, though, if you pick a relatively common one (such as a DAAM or RAM or Mistral) they aren't rocket surgery and parts are mostly available. And when nobody has a part, some of us will find a way. Look into
Home | Vintage Scuba Diving Community Forum forum see the lengths people will go to sort them out and help each other in keeping these regs in the water.
#5 This question might require a longer answer, so a special thanks to the person who answers it! What would be "perceived" negatives of a DH reg (i.e. little things that are in reality irrelevant) vs "true" negatives (i.e. more significant cons that a future owner should be aware off?)
Some will claim old=inferior... but fundamental reg design hasn't changed in roughly 60 years. The Aqualung RAM first stage is parts interchangeable inside with all the aqualung balanced diaphragm first stages up to the 2008 Titan.
Some will claim the breathe like garbage.... most who say that haven't used one positioned correctly, or just don't appreciate that "different" isn't always "worse" (see above comments on positioning effects).
True negatives; Some dive ops won't let you use them unless they have all the accessories. Short yoke regs won't be able to use a banjo fitting (to hook up a pressure gauge), but I have some tanks with vintage valves that have an HP port just for diving those regs. Very few shops will service them for you, so it's almost necessary to learn to do it yourself (but it isn't hard and doesn't have to involve a lot of special tools). Parts availability in the long term is not guaranteed (but some of us are working on it). They are addictive.... I bought one to try out, and now I have 5 that are dive ready and 7 more in various stages of "I'll get to it"!
#6 Anything else important I should ask?
Can you link up with someone who dives them to show you the ropes? If you're ever down my way (lower Alabama/Florida panhandle) I'd be happy to link up and share what I know. I self taught from reading old manuals and forum posts, and that works, but the learning curve is far easier with help on hand.
How to clear a DH reg (there's no purge button). 2 step process and step one can be done 3 ways.
1) Clear the mouthpiece itself:
a: exhale through it if you have the air to exhale
b: hold mouthpiece above can level with opening down (this will cause a freeflow and having opening down will keep water out of the mouthpiece) then rotate the mouthpiece into your mouth.
c: "pump" your cheeks in and out, each "pull" will pull a little air from the intake side, and each "push" will push a little water out the exhaust side, and quickly clear it that way.
2) Clear the water from the exhaust hose. Roll slowly left while exhaling. This allows the water to follow the low point in the hose around to the actual exhaust valve in the can.
#7 Can the DH reg be installed upside down? So instead of the hoses coming from the top of the second stage, you flip it 180 degrees so they come down from the bottom. Visually, I think it will make for a nicer routing of the double hose as it can run closer to the shoulder (maybe add a piece of bungee on the shoulder bca webbing to keep the double hose from swimming away)
In theory yes, but probably not a great idea from a clutter/freedom of movement stand point, and would make the "induced freeflow" method of clearing the mouthpiece far harder. On the other hand, my 12 y.o. doesn't like how floaty the hoses are so I put some stainless steel rings on them as hose weights to offset it, and now he loves them.
Respectfully,
James
