The cheap solution is to brew in plastic bottles, so you can squeeze the bottle to determine how much pressure has built up. This works well, but purists will turn their noses at the idea. The theory goes, that plastic has two disadvantages:
1) Plastic is not as impermeable to gas exchange as glass.
2) Plastic can easily develop scratches, which can harbor bacteria that affect taste.
For me, the decision is aesthetic: I really like the look of brew fermenting in glass. So I wanted to build a way to monitor pressure in a bail-top bottle without sacrificing looks. I was really happy with the product -- I think it has a nice steampunk look to it. That said, this project was a lot of work for a problem that can be solved with a plastic pop bottle.
The basic outline of steps is as follows (refer to pictures for more explanation):
1. Take the cap from a bail-top bottle, remove the harness, and cut a welding rod to fit through the hole, sticking out on either side. Close up the hole with clay.
2. Create a plaster mold of this cap construction.
3. Make wax positives and clean them up.
4. Invest the waxes and burn them out in a kiln.
5. Cast bronze with a centrifugal jewelry caster.
6. Clean up the bronzes. Drill and tap a hole for the pressure gauge.
7. Bend a piece of welding rod to interface this new cap with part of the old harness.
8. Put on a rubber gasket and brew up some goodness!
My batches with this bottle have been great -- It's so nice not to have to worry about the bottle blowing up! I usually brew to a little over 20 PSI, so the gauge is pretty grossly mis-scaled, but it was the only one I could find (it has a 200 PSI maximum!).
The other reason I wanted to pursue this idea was to investigate the possibility of building a forced carbonation set-up. A set of caps with ports (think bike valve) would be the first step, I think.
Let me know what you think, and be sure to post links to any similar projects. Thanks for reading!