Hi Vaquero44
You're looking at the Thomas Creek Toprunner style sluice for putting the perforated over the catch medium, and creation of self classifying feature in the sluice.
Which sets up an alternative for fluid beds possibly with some advantages and some disadvantages depending upon circumstances.
As it happens the TopRunner appears to my eyes to be an innovative improvement upon a previous sluice called the DamnFineSluice for which the website is gone but an archived version remains here :
http://web.archive.org/web/20030803093305fw_/http://home.att.net/~ajsmiles21/
This sluice was supposed to be extremely good at retaining very small gold down to flour size, sub #200 mesh, and I think Andy Scheurer was aiming for similar results with self classification added in his TopRunner II design.
http://thomascreek.net/ (I haven't tried other of these yet myself)
The questions are: how well can a modern fluid bed compare to a TopRunner when the available gold is extremely small? When water conditions are getting a bit low and the gold is small-to-flour size, does the fluid bed's strong points fade slightly compared to eg a TopRunner type? Also: did the TopRunner achieve the performance of the DamnFineSluice for flour gold in the first place?
I plan to make a couple of test sluices to work this out for myself because I don't see all that many beach or sand sluicers talking about their Bazooka, despite what the fluid bed fans say about how good it is. As far as I can see the king of sand (which is the ultimate pre-classified size gangue material) and flour gold still seems to be something between a PopAndSon lengthy expanded metal+carpet sluice, a shaker table, or a Gold Cube.
In the time before I actually do my own comparisons I have at the design stage a preference for wire mesh over finer size perforated steel as the smallest gold appears to be able to go round the neater round holes in some kind of a surface tension avoidance fashion. Also the lack of water down there at the start (top) of the under sluice may not clear black sand as well as required and push the flour downstream some, but wire will get water in better. Thirdly there's an argument for having flour drop a very very short vertical distance to the catching medium because the current is taking it away to a much greater degree than say #100+ mesh gold.
Now all the while there is the possibility that a tuned fluid bed may be catching a high percentage of sub #100 gold, but the presence of flour gold in the cleanup does not mean that a % of flour gold was not lost out the back. So I suspect the beach guys have figured their game out and don't use Bazookas for good cause.
But fluid beds have distinct advantages. I still want to sluice as unclassified a material grade as I can get away with! However there seems to be no reason why a trap for superfines might not be located under a fine classification mesh and within the slick plate's middle area in a bazooka type design. (Look at Andy Scheurer's TopRunner 1 where he had a fine mesh then more slick plate, then another grizzly)
http://thomascreek.net/?page_id=428
A middle fines catchment prior to the main "baxooka" grizzly could be cleaned with the sluice in situ easily if a small plywood board is placed over the slick plate water intake shutting off the flow temporarily, after which the classifier is lifted the moss removed and dunked and subsequently replaced. Remove sluice gate plywood and ready for action again. Meanwhile flour gold losses from the first catchment upon their escape will move down into the fluid bed for second attempt at recovery.
Likewise but differently, a TopRunner style sluice could have a fluid bed located at it's downstream end for entrapment of whatever drifts across an upper trap area due to black sand clogging or unexpected water flow surge causes. You could say I'm interested in hybrids of the main versions because I can make my own and I suspect the more different entrapment media used the more overall strengths the total performance of the sluice will have.
Just some thoughts while I'm at the making up the drawings stage.
