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re: prices, diff pumps
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I don't know how other folks work, but a long time ago I stopped bidding
jobs in the break-down / calculate / add-together mode.... for the average
retail job that comes my way I sort of size it up with a shrug and say "what
is it worth to me to have to do this" as well as "what will the market bear"
and "what type of client is this". As Kenny points out, I don't want to be
cheap, would rather be known for quality...
The part of this "pricing" system I like is that once engaged on a job, I'm
not thinking "cost" any more, just "make it nice". Sure, sometimes I eat it
-- but generally it averages out...
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As for diffusion pumps, I still think the best deal is to get one of the
bottom -dollar Duniway Stockroom used metal-type units ($200-$400) and set
it up with a self-contained water-cooling apparatus. There's not all that
much heat to dissipate -- I rigged a five-gallon plastic bucket as a
"resevoir tank" with a $40 mail-order-surplus water pump doing the
circulation; cooling of heated water is provided by an automotive
heater-core takeout and a small fan. The water never gets more than mildly
warm...
The other part to rig on a unit like this is the connection to your
manifold. Mine has a 4" bore and came with an extra piece of "pipe" on top;
I capped this pipe at about 3" tall and bored a 25mm hole in the side,
attaching a 25mm NW-KF stainless fitting. Had all the tig welding done by a
retired NASA welder for all of $30.
Granted most shops aren't set up with NW-KF type manifolds but it's a simple
step to go from there to glass. Eurocom Imports sells a glass fitting -- in
either soda or borosilicate -- that has a heavy NW-KF casting on one end
that clamps right up with a pertinax clamp (which they also sell). Other end
of the fitting is just an open glass tube, to weld to whatever you like;
varying diameters are available. (These are what I use to connect my
all-metal NW-KF system to a glass "fore" piece, to which tubes for pumping
are attached.)
There's a 4"-diameter flanged seal between the top of the diff pump and the
top cap, held with bolts and sealed with a giant O-ring -- when I couldn't
get my O-ring to fit back in (majorly epanded through time) I just laid a
heavy bead of "torr-seal" in place (high-vac epoxy) and three years later
the torr is still sealed...
The heater coils on this pump are also internal, but they resemble more a
stovetop electric burner element than nicrome wire. I control the heat with
a 10-amp variac; I also have a "overheater preventer" in the way of a
coffeepot-type heat sensor ($3, surplus) that's wired in series with the
heater-coil input; the sensor is tie-wired in place about half way up the
pump and cuts the power in the event of the pump reaching, oh, 220 degrees F
or so... This way if my water cooler fails I don't bake the diff pump.....
Also, on the Welch 1402 pumps: recently mine lost it's
oil-backflow-prevention ability and filled my diff pump with its oil... I
installed a HPS $400 anti-suckback device that, in the event of power
failure, immediately seals off the diff pump (still under vacuum) and vents
the forepump to atmosphere.... no more oil sucking back, works great.
-Ted Pirsig