So I went back to square 2 and decided to remake the original brass valve body in lighter ABS plastic using our Makerbot 3D printer.  The beta version looked like this:

Remade underwater solenoid valve with 3D-printed structure.

I re-used the gasket, screws, core, and shaft from the original valve (where the solenoid sits), and replaced the big brass parts with light ABS plastic that is customized to fit onto my hull.  The steel shaft press-fits onto the shaft holder at the top of the photo.  The shaft holder screws into the connector piece, which implements the same idea as the brass valve piece, but instead of the input being a pipe fitting the input is open to the inside of the robot hull.  Note that in the photo I’m holding the valve upside-down, the solenoid shaft will hang down from the top of the robot hull, and the other end of the valve will bolt onto the hull’s top using the existing screw holes, pressing the base of the conical shaft against the hull’s top hole (apparently I’m a terrible photographer too).  When the valve opens, the air inside the hull will flow through the conical shaft out of the hull’s top hole and will be replaced by water flowing in through the bottom hole.

The new valve did in fact work.  The solenoid successfully pulled back the gasket to open the valve when activated, and it almost sealed when the power was disconnected.  The seal wasn’t perfect because it printed over a bump in the 3D printer’s build platform.  The plastic was 277 grams less massive than the original brass, and with a tweak or two the new valve will bolt onto the hull efficiently.

The valve had a few problems.  The steel shaft’s wide screw part was too wide for the hole it was supposed to screw into, so I had to file the hole out until I could press-fit the shaft in.  The support structures on the interface piece are too spindly, as you can see in the photo.  Also the square plate on the interface piece was a little thin, and it sat a little too short.  The corners  of the square hit other structures inside the robot hull, and it needed to be about 1 mm farther away from the nut holders.  In the new version I have tried to rectify these issues:

Updated valve connector structure.

The square plate is now half again as thick, and the nut holders (the bits sticking out near the top of the conical shaft) are 1 mm farther away from the plate.  The supports are a lot thicker, too: they’re 4 mm wide instead of 2 mm, and the six supports that don’t connect to the nut holders flare outward stylishly.

There’s one other issue that I foresee.  The heavy copper solenoid and the steel shaft/magnetic core sit pretty high in the hull, and raise the robot’s centre of mass.  This is a potential problem because if the centre of mass is too high the robot will roll too much in the water.  I can add extra ballast to lower the centre of mass, which may or may not be fine, depending on what kind of buoyancy the rest of the hull has.  I might actually have to do some engineering to figure that out.  Or, I can try it and see what happens.

Underwater solenoid valve from Nuoling Pneumatic

The valves were a lot bigger and heavier than I expected.  You can see it’s basically a big chunk of brass connected to the waterproof electrical components.  Fortunately it’s easy to take apart:

Inner workings of this solenoid valve.

The valve assembly consists of three parts: the brass base; a rubber gasket connected to the spring-mounted iron solenoid core; and a metal sheath that encloses the core, bonded to the upper brass component.  (The solenoid is mounted on the metal sheath.)

Inside the solenoid valve's brass base.

The above photo shows the structure inside the brass base.  Notice the arrow indicating the expected direction of flow through the valve.  The rubber gasket seals the inner aperture, and the pressure of the incoming fluid is dispersed around the perimeter of the large outer chamber so that it doesn’t push the gasket up.  When the solenoid is activated it pulls the iron core up, which pulls the gasket away from the inner aperture.  Fluid flows under the lifted gasket into the aperture until the solenoid is de-energized, at which time the iron core is released and the spring return pushes the gasket against the aperture’s lip to seal it back up.

Solenoid valve specification.

This label shows the valve’s specification, including the desired 12 V power supply.  In fact, I tested with a fully charged 7.2 V Li-ion battery and it still actuated in air.  The operating pressure label is a little disconcerting, as the datasheet the supplier sent me specified a working pressure of up to 500 kPa.  A 0.10 kgf/cm^2 pressure corresponds to less than 10 kPa.

The valve's waterproof solenoid.

The solenoid is made of a copper winding connected to some nice, solid leads.  The winding is coated in an encapsulant, which is a pretty thin layer of some kind of urethane-like material, and wrapped in a fabric mesh to hold everything together tightly.  I haven’t tried it underwater yet, but this part (housed in the black casing in the photo at the top of this post) is exposed to the environment so I assume it’s actually waterproof.

This may turn out to be the most useful part of the valve.  I don’t think I can justify putting all that brass and steel in my underwater robot, it’s just too heavy.  By a happy coincidence, the valve I made previously fits perfectly inside this structure, so hopefully I can just mount this solenoid on my existing valve (or maybe an elongated version of the current valve.  I tried, and my spring return is too strong–the compression spring on the solenoid’s iron core is surprisingly febrile–but this gives me hope that I can cut it down further and reduce the amount of force needed to pull the valve open.

One problem I foresee is that in my configuration, the transvalve pressure will be pushing the valve closed, so the deeper the robot, presumably the more force is needed to open the valve.  It’s not a big deal, it will just limit the robot’s depth, which is better than making the robot unrecoverable.  I’ll be happy if the stupid valve opens at all.