I'm putting it here mostly because it's technical and rebreathers have double hoses.
The KISS DSV feels a lot larger than it is mostly because of the quick disconnects used between it and the hoses on either side of it. The biggest flaw however is that KISS charges rather obscene amounts for spares. The little plastic clips that retain the QD fittings are $40 each as are the simple mushroom valves used on each end of the DSV. They are also almost impossible to source from someone other than KISS and KISS isn't always the fastest shipping company on the planet, so if you don't have them in your spares kit on your trip, you are not going to get replacement parts on your trip.
With that in mind I thought I'd see how well the new VDH DSV works on my KISS Sidekick.
The KISS hoses fit, requiring just a very slight stretch to go on VDH DSV. It's not much of a stretch and not hard to accomplish, but I'll rely on the material experts here to advise whether that might be an issue with the EPDM hoses used by KISS.
It is slightly lower profile than the KISS DSV:
In use, the drain port to purge the mouthpiece prior to opening the loop works just as well as it does on the KISS, but with a bit less resistance due to the slightly larger hole. The work of breathing with the VDH DSV also doesn't appear to be any greater than the KISS DSV despite the smaller mushroom valves, but that's not a surprise as it's not the limiting factor in the design of the rebreather.
The mushroom valves them selves seal just as well, which is even more important in the rebreather given that the one way flow of the gas is essential to ensure it all goes through the scrubber.
There are however two potential, or at least theoretical concerns:
1. with no quick disconnect between hoses and DSV, disinfecting the hoses in the loop can be easily accomplished but drying the hoses takes longer without the ability to easily separate the hoses from the DSV without using tools; and
2. the KISS DSV uses a lever that takes 7 3/4 pounds of force to move on one of our KISS DSVs and 9 pounds on the other, while in contrast, the VDH DSV will start to move with about 1 pound of force on the mouthpiece with one of ours and 4.5 pounds with the other one.
I'm not real concerned about the quick disconnect issue as it's a mixed bag already with the KISS. A great deal of care has to be taken with the KISS design to ensure you don't pinch one of the o-rings, and the plastic retainer clips can be lost or broken (although I've found a suitable length of weed leader line works fine in a pinch). Thus I don't have a lot of heart burn in forgoing a quick disconnect feature and in practice I'm only using it periodically to inspect the o-rings and do deep maintenance. I do like the idea of the much smaller profile of the overall arrangement with the VDH DSV, particularly, in smaller passage.
The second issue is much more of a concern as a slight rotation of the hose could close the DSV just enough to allow water to start leaking through the drain hole while breathing on the loop[ during the dive. The KISS has the same weakness as if it is in any position other than fully open or fully closed, it will allow a small leak of water into the breathing loop, and over the course of even a 1hour dive that can create enough water in the loop to overwhelm the water trap if the diver does not notice the seepage. The KISS avoids this issue by using a lever, and having a 7 to 9 pound opening and closing force for the DSV, which makes unintentional operation very unlikely. On the other hand, it increases the potential for the diver to unknowingly stop about 1/8" short and cause a slow leak.
I'm not sure how big an issue unintentional closing would be with the VDH DSV and I''ll have to do some more diving to find that out. 99.9% of the time the mouthpiece is in your mouth and the loop is open when under water, so it's mostly an unintentional partial closure of the loop and the resulting least through the drain port that worries me, although an unintentional opening while gearing up in the water would be bad too as it would flood the loop and scrubber and force me to start over.
To increase the force needed to close the loop, I have considered this minor addition, using a small piece of stretched bungee to increase the force needed to close it, and in this case it raises the force to about 7 pounds before the bungee snaps over the top of the screw and allows it to close. With this in place I think it's stiff enough to warrant more OW trials on the rebreather.
A simple loop of cave line around the mouthpiece hooked to the bungee from the other side with the DSV in the closed position would also serve to help lock it closed on the surface to prevent any flooding.