A DIY Oxalic Acid Evaporator

Thought I'd pass this info along in case it's of interest ...


I've been asked to make a custom OA 12V Evaporator, which can be inserted into a 22mm diameter hole in a beehive - i.e. the same size as a standard wine cork, which are used by many alternative beekeepers.

Firstly, a few useful bits of data:

At standard atmospheric pressure:-
Oxalic Acid melts at: 102-3 C (216 F)
Oxalic Acid sublimes at: 157 C (314 F)

JB Weld:
max continuous working temperature: 260 C (500 F)
max temperature for short periods: 300 C (572 F)

60/40 solder melts at 188 C (370 F)
pure lead melts at 327 C (621 F)


Ok - I've already made several 24V Evaporators, using ceramic soldering iron elements - but right now I'm playing with some 12V Diesel Glow Plugs.

There are two basic principles involved when cobbling together one of these devices - the first being to ensure that the thermal transfer from heater source to the pan holding the OA is as efficient as possible, and secondly - to make the thermal mass of the apparatus as small as possible, thus enabling a fast warm-up and cool-down.

If I owned a TIG set, then I'd work with thin stainless steel as a material of choice - but I don't, so I can't, and am working with copper instead: basically the device consists of a copper tube surrounding the glow-plug, with a shallow copper tray sitting on top.

Now although JB Weld is very useful in many ways, it would be so much better to have some kind of metal-to-metal bonding for maximum thermal transfer, and I've come up with what I believe to be a novel idea for replacing silver solder, which I'd happily use if only it wasn't so bl##dy expensive.

The technique is to wet the copper to be bonded with 60/40 standard solder, which is then wiped off, leaving but a thin layer on the surface of the copper. Repeat for both pieces. Then, after priming the surfaces with acid flux, a thin sliver of pure lead (ex. roofing sheet) is melted between the two copper pieces to be joined. The result is very nearly pure lead and the joint has already been tested to 290 deg C, without any signs of failure.

At the moment, the copper apparatus weighs just 15 grammes, and has an OA melting time of 34 seconds, with sublimation occurring at 1 minute 2 secs.

Attached here are a couple of pics of the experimental lash-up, but as I have some long reach Land Rover glow-plugs arriving any day now, I'm waiting until these arrive before working on the handle, etc.

http://i61.tinypic.com/27xgwet.jpg

http://i59.tinypic.com/300znz6.jpg

More pics as work progresses.

'best
LJ