I've been working a couple weeks now on solving the problem I posted here on how to improve my pump design by alternating the feed of ethanol vapour. I got good feedback but ultimately all the solutions involved mechanisms which were going to be somewhat tricky to build and source, which is against the brief of the project I'm working on; being an open source solar tracker concentrator makeable from scrap. In the end I solved the problem by largely redesigning the whole pump. Since it's driven by boiling ethanol, rather than add an extra mechanism for turning the feed of vapour off and on, I reduced the amount of ethanol being boiled, so that it boils itself out after an appropriate period. The vapour is then able to collapse fully, which sucks in more liquid ethanol and refills the system. 1. At the bottom right is the boiler, which holds about 2-3 ml ethanol. 2. This boils and the vapour enters the 'chamber' (the half blue, half white (liquid and gas)), forcing out the liquid, which pours into the wheel, ending up in the main reservoir. 3. This continues until the eths in the boiler has boiled away to the extent that it can no longer overcome the rate of re-condensation in the chamber, which starts to suck, so to speak. 4. This draws liquid from the reservoir, which passes through the boiler, shutting off the boil, the pressure drops quickly and the chamber and boiler refill with liquid. 5. Two valves (the only moving parts, besides the wheel) keep all this going in the right direction. 6. Repeat. The wheel provides the motion for the solar tracker. It's not in by any means powerful or efficient, but the whole thing can be made from a bit of metal tube, some thin pipe, a glass jar and two valves from bike inner tubes (plus a paint tin, bike wheel bearing and some drinks cans for the wheel). I haven't had a chance to try it in the field yet, but powered by a candle it seems to work fairly well. Blog entry here, will post photos and videos when available.