Having seen very tiny pressure gauges on fire extinguishers (diameter less than 25mm / 1in.), I contacted an extinguisher service facility - yes, they had spares for sale at a very attractive price, just a few dollars each!

The only problem: These gauges are designed for a substantially higher pressure than we live steamers use: the scale goes up to 20 bar / 300 psi. Half of that would be closer to what I'd need...

The service engineer knew how these gauges were constructed, and could offer a suggestion: Remove one of the two cup-formed spring washers inside the gauge, and the full scale reading will probably drop...

How right he was! I disassembled the gauge (had to break the pressed-in plastic cover), took out all the parts, discarded one of the cup springs, and - success! Max. scale reading is now around 10 bars (140 psi), eminently suitable for a working pressure of about 6 bars (85 psi).

I made a threaded retaining ring, and corresponding threads in the gauge body to hold everything together - originally, the mechanism was held in place just with a piece of springy steel wire in a groove in the body. This picture shows the parts, a new plastic window still needs to be made, as well as a new bar/psi scale...

Clockwise from top left: Gauge body (with membrane), gearing mechanism, retaining ring, spacing washer, cup spring.

Here's a drawing of how the gauge is constructed. The membrane is soldered to the body (needs to be air/steam tight!), but all the other parts are held in place by the retaining ring (not shown here).

The mechanism has a tiny lever that contacts the membrane through holes in the washers. Having removed one of the spring washers, the gear mechanism came a bit closer to the membrane, which caused the pointer to rise above the zero mark when no pressure was applied. This was easily corrected with cautiously grinding the lever shorter, by as much as the thickness of the washer - about 1/10 mm (that's just a few inch "thous").

NOTE: There is another type of diminutive gauge, also used in extinguishers, that works on the "Bourdon tube" principle, i.e. it contains a flattened, spring wound bronze tube that expands when pressurized - this type is NOT adaptable for lower pressures, at least not as described here...

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