The Air-Operated Double-Diaphragm Pump: It Stalls Instead of Bursting

Print two flanged dishes in PETG, 90 mm across the flange, each shaped as a shallow bowl 10 mm deep. One is the **liquid side**: give it two hose ports near the bottom, one for an inlet check valve and one for an outlet check valve, pointing opposite ways. The other is the **air side**: one port at the centre. Cut a disc of rubber sheet for the diaphragm, or use a ready-made pump diaphragm of that size, and clamp it between the two flanges with eight machine screws tightened evenly in a star pattern. The diaphragm is the only seal between air and liquid: an uneven clamp leaks at the flange, and a crease in the rubber cracks early. Put a light compression spring on the air side, behind the diaphragm, so it returns to the air side when the air is let out. That spring stands in for the second diaphragm and its shaft.

Connect the air side to the compressor through the regulator, set to **0.5 bar**, through a tee with a push-button or a finger over an open bleed hole as the air valve: finger on, air pushes; finger off, air escapes and the spring returns the diaphragm. Put the inlet hose in a bucket of water and lead the outlet into a measuring jug. Pump twenty strokes and divide: that is your stroke volume. Now the test. Tee the pressure gauge into the outlet hose and close the outlet with a ball valve. Press the air button and hold it. The diaphragm moves a little, then stops, and the outlet gauge settles at about the air pressure — 0.5 bar. Raise the regulator to 1 bar and the outlet gauge follows to about 1 bar. The pump is a pressure transformer of ratio one, and it cannot exceed its air supply. Keep the air at 1 bar or below for this printed model, and wear safety glasses.

Hand-operated diaphragm pumps — a rubber sheet moved by a lever, with two flap valves — were long used to drain trenches, bilges and excavations, because grit and mud pass through them. The air-operated double-diaphragm pump put two of them back to back on one shaft and let compressed air do the stroking, with a self-switching air valve. **Jim Wilden is credited with the invention in 1955**, and the company bearing his name still makes these pumps. His original patent number is not asserted here: it could not be read from a primary source, and the mechanism does not depend on it. It filled a gap no motor-driven pump filled: a pump that can be left running into a line that may block, may run dry, may carry grit, and may sit in a place where an electric motor is a fire risk. Paint shops, mines, sewage works and chemical plants took it up for exactly those reasons. **Honest limits.** It is costly to run: the notebook shows that at best a small part of the compressed air's energy reaches the liquid. The flow pulses, twice per cycle. The diaphragms are flexed on every stroke and are replaced as wear parts, and a burst diaphragm sends the liquid out of the air exhaust. And it can ice up: the expanding exhaust air is cold, and moist air can freeze in the air valve.