Well - Johno has pretty-much cracked a DIY version of the Sublimox/ProVap design, and it's very hard to imagine this being further improved upon.
However, I thought there might be some mileage in developing a Non-Inverting Band-Heater Vapouriser which could sit on top of the Crown Board (inner cover) - perhaps even with the operator just pressing a button and walking away.
I can see two possible advantages with such a press-button non-inverting design: firstly, there would no longer be any need to get 'up close and personal' with any Oxalic Acid dust (not that I've personally had any problems in this regard with Johno's design) - for this type of applicator could be operated remotely - from 10, 20 feet away - or even from within the next field if you're really paranoid; and secondly there would be no need to stand there like a lemon holding the applicator while it performs it's magic - indeed, with a timer added it would then be possible to automate the process such that it does indeed simply become a case of 'pressing a button, and walking away'.
There's a third advantage for myself: at 6'4" I invariably need to stoop when using a hand-held inverting band-heater vapouriser - depending on the hive height, of course - and I'd really like to eliminate that.
There is, of course, one major disadvantage of the 'cold > hot > back to cold again' sequence of the Varrox protocol in that it's very much slower than that of the inverting band-heater design. On initial testing I found this to be somewhere around 9 minutes from cold to cold - but as these non-inverting units promise to be insanely cheap to make - it could well become cost-effective for several units to be made and then operated at the same time (perhaps on the Master-Slave principle ?). But - for anyone interested in this approach, please DO be aware from the outset of this limitation.
Johno has discovered that injecting OA dust at or near the top of the hive is preferable to dosing from the bottom - so for now I'm sticking with this principle - and the lower operating temperature of this design means that construction can be far less demanding for the DIY-er. Ok, enough chat ...
Here's a sketch of the basic concept:
The narrow-bore delivery tube has a ID of 4mm and extends upwards inside the can by 25mm (1 inch), and protrudes downwards until flush with the base-plate, which enables the assembly to then be slid into position over the feed-hole in the Crown Board (inner cover).
The prototype 36mm OD brass 'reaction vessel' I'm using is fitted with a 150W Band-Heater - which will be replaced by 28mm copper fittings and a 120W Band-Heater providing this trial shows some promise. A standard (and therefore low cost) demi-john cork will fit perfectly into a 28mm fitting - but as a bung for this 36mm tube would cost as much as a PID controller (crazy, or what ?), I've made a simple weighted gravity closure thusly:
So - onto the prototype build itself ...
Bearing in mind that this is still very much at the experimental stage - here's the woodwork of the first prototype rig:
I've fitted a heatshield to protect the controller, which is probably unnecessary, but rather safe than sorry ...
And a pic of the 'reaction vessel' itself, which I've already posted:
The joints of this vessel have been made with Lead (Melting Point 327°C, 620°F) NOT solder, using a bog-standard propane torch, and it has already been successfully soak-tested at 250°C (480°F). In use, the normal operating temperature of this assembly is not expected to exceed 200°C (390°F).
Of course, brazing or silver-soldering is a far better technique to use should you have the necessary equipment.
So here's a shot of the state of build as at this morning:
The can is held firmly in position by a pair of s/s wires. A toggle switch has been fitted so that the controller will still act as an electronic thermometer whilst the reaction vessel is cooling (with the heater switched off), post vapourisation. The small shield over the connection block is to prevent accidental contact with the 220V present on those terminals.
To the left of the assembly is the gravity closure already mentioned, together with a simple cover made from squashed copper tube which will prevent any dispensed Oxalic Acid from falling down the delivery tube - thusly:
And finally, with the gravity closure in place:
So - all that remains now is to wire the thing up and insert a K-type thermocouple in-between the bottom of the can and the rubber insulating washer below it. I'm sure that'll perform adequately for now, although I'll rig-up something more workman-like for the finished product (assuming that ever happens ... ).