The finished pressure chamber will be 1 1/2” tall. It will be made from the top and the bottom of the paint can.
Remove the paint can lid and mark a line around the side that is exactly 5/8 down from the top. Mark another line around the outside that is exactly 5/8” up from the bottom. Cut carefully along this line. Cut carefully so that you also preserve the center section of the can. You are going to use the center section to cut some additional parts.
I use a sheet metal nibbler to cut my paint cans. I drill a small hole (about 1/4”) to start the cut, insert the nibbler, then work my way around the can.
Use tin snips to cut the center band of the paint can. Make a vertical cut parallel to the weld that is in the side of the can. Measure and cut a strip of metal from the bottom of the can that is exactly 1 3/16” tall. This cut is parallel to the cut made when you removed the bottom of the can. This piece of metal will be used to make the pressure chamber sidewall.
The pressure chamber sidewall will be glued to the inside of the top and bottom can parts. Use the high temp silicone glue for this. Glue all three pieces together at the same time (top, side, and bottom). Start by gluing the sidewall to the inside of the pressure chamber top. (The top is the piece with the lid.) Use enough silicone to get an air tight seal everywhere. I used a small piece of metal (about 1/2” x 1 1/8”) to reinforce the seam in the sidewall. Set this assembly aside until the glue is completely cured.
The displacer is cut from a piece of 3/16” thick foam core poster board. Measure the opening of the paint can. The displacer will have to fit into this opening, so it must be made a little smaller. Use a compass to mark a circle on the poster board that is exactly 1/8” smaller in diameter than the opening of the paint can. This will provide 1/16” clearance between the edge of the displacer and the opening of the can. Cut carefully on the line.
Cut a 3” piece of 1/16” music wire. The easiest way to cut music wire is to score it with a file and then bend it and break it. Good quality music wire is very hard and will break like tool steel when it is scored and bent. Place a 90 degree bend in the music wire 3/4” from one end. This bend should be as close to 90 degrees as you can get it to be.
If you have a drill press you can use it to create a simple jig for gluing the displacer rod to the displacer. Simply drill a 3/32” hole through a flat board. This hole will hold the displacer rod at the correct angle while the glue is curing.
Press the long end of the displacer rod through the center of the displacer and down through the hole in the jig. Apply enough pressure to compress the poster board just a little bit around the center. Fill the depression with epoxy glue and set it aside until the glue has cured. Once the glue cures remove the assembled part from the jig and check the alignment. The rod must be perpendicular to the surface of the displacer and in the exact center of the disk.
Drill a 7/16” hole in the center of the bottom of the film can. This hole is drilled to fit the short pipe nipple that is used to connect the drive cylinder to the pressure chamber lid.
Drill a hole in the center of the lid and carve it out until it is as large possible, until only a ring remains. This ring will be used to seal the diaphragm to the drive cylinder.
Use a compass to find the center of the paint can lid. To do this, set the compass to match the radius of the lid, and then use it to draw a line near the center of the lid. Rotate the lid 90 degrees and repeat the process. Continue to rotate the lid and draw marks with the compass until you have established the center point of the lid.
Drill a 1/4” hole through the center of the lid.
Locate the position of the drive cylinder. The drive cylinder will be positioned as close to the edge of the lid as possible while still being mounted to the flat surface of the lid. Mark the position of the hole using the film can as a guide. Drill the hole with a 7/16” drill bit.
The “gland” allows the displacer rod to pass through the top of the pressure chamber without causing a leak and without creating significant friction. UHMW plastic is a good choice for this purpose. It is naturally slippery.
Cut a piece of UHMW plastic to be 1” x 1” x 3/8”. Locate the center of the square and drill a 1/16” hole using a drill press (if you have one). Test the fit with a piece of 1/16” music wire. If it is a snug fit that does not allow the music wire to slide easily, the hole must be enlarged. Increase the drill size by 1/64” (to 5/64”) and drill it again. Test the fit again. 1/16” music wire is actually slightly oversized and should be a good match with a 5/64” hole.
Use RTV silicone to attach the gland over the center hole of the lid. Keep the silicone away from the center hole in the UHMW as it will cause friction if it gets in the way of the displacer rod.