As an addendum to the previous message which I wrote at 3:30 AM:
I should add that I am not particularly impressed with many aspects of the the units we used. They do work and work well, but it seems to me that the ergonomics are poor. These units are are heavy and awkward to maneuver and it is necessary for the operator to lean over repeatedly to load the acid and again to activate the fog.
A lot of work and expense has gone into the electronics of heat control and the fan setup, but the measurement and injection system is primitive.
It seems to me that it would not be that hard to design a dispenser which would push a measured amount of the powder into the heat chamber from a standing position and activate the fog.
The use of a 110 volt generator with a step-down to lower voltage adds bulk and noise. It seems to me that batteries charged from the vehicle alternator would be better in many ways. We used such a system for our hive loaders and it worked. There are some technical details to keep from running batteries too low and blowing alternators by trying to charge too many batteries too fast but these problems can be managed, especially for those who drive diesels and idle them constantly (one of my pet peeves).
The low-pressure blowers IMO could be replaced by compressed air pulses from a central compressor or tank and reduce weight and moment of inertia of the applicator units to allow easier handling and positioning. An entrance blocker sponge could be attached to the nozzle and fit into placeeach time the nozzle is pushed into an entrance.
The ideal air volume desirable for each hive to ensure distribution in large hives is probably about 2 cubic feet, since a triple has an interior volume of only about 4 cubic feet before the frames and bees are considered, so the actual empty airspace could be as little as 1 cubic foot. These units apply far more air than that, resulting in considerable fog escaping the hives. Perhaps if the hives are closed up, there is less air delivered, though.
When air is injected, using this system, as opposed to the verdampfer method, it should be obvious that the extra air has to go somewhere or no new air can be pushed in. Thus the need for either some open entrance or open top holes or a lifted lid when forced air is used.
The Varrox unit does not introduce outside air and relies on the natural convection in the hive, plus bee activity to distribute the fog, and from the literature that seems to be very adequate, at least in small hives.
IMO, where air is used to drive the fog trough the hive, a short strong pulse of limited volume is more likely to be useful than huge volumes of low pressure air which send huge clouds outside the hives indicating waste and presenting an adverse working environment. The waste is not costly but anything that reduces the fog outside the hives cannot be anything but good.
Additionally, if an air delivery system could deliver smoke in advance of the fogging pulse to drive the bees up and loosen the cluster, that could be a good thing, too. A central air source could conceivably send smoke to the applicator units.
All in all, although a number of people have refined and re-designed the system demonstrated, it has not come a long way from the early European designs and the mobile multi-hive unit that Cor Dewitt first built in Canada and presented to our 2002 convention. The construction is more sophisticated, but the basic design is unchanged.
http://www.honeybeeworld.com/diary/articles/cor.htm
http://www.honeybeeworld.com/diary/articles/cor2004.htm
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