5.0
OPERATION
5.01
Trial Run
Once you’ve mastered the controls of the 6750 Freezer/Mill, we suggest several
test-grinds with samples typical of those you will be grinding. The general
operating procedure is outlined below.
5.02
Electrical Hookup
Plug the mill into a standard 3-prong grounded electrical outlet; we recommend that it be a fused
20-amp circuit for the 115V/60 HZ version. If you have the 230V/50HZ version of the mill,
make sure the cord and plug conform to local electrical codes; we furnish the 230V/50HZ 6750
Freezer/Mill with a standard European 2-prong plug with cord.
5.03
Adding Liquid Nitrogen Coolant.
Liquid nitrogen is likely to be available to you either in a large tank (e.g. 160 or
240 liters) with a flexible steel hose, or in 20-liter or 30-liter Dewar flasks, or
both. Either will serve. The 6750 Freezer/Mill consumes a moderate volume of
liquid nitrogen: 4 to 5 liters for initial cool-down, with a maintenance rate of 2
to 4 liters per hour, depending on use. We recommend an adequate supply.
Liquid nitrogen is added to the 6750 Freezer/Mill with the lid open. There is a
fill mark about an inch below the lip of the tub. If the liquid nitrogen is much
above this mark when the lid is closed, the nitrogen is likely to splash outside the
tub. Always close the lid slowly, to avoid splashing.
5.04
Filling the Tub with Liquid Nitrogen
Unlock the latch by pushing the thumb tab at its top away from the cabinet, and then release the
lid by pulling the bottom of the latch away from the cabinet. If the bottom of the latch is still
engaged, push the lid down to free it. Lift the lid slowly; a pneumatic cylinder controls its
movement. The coil mechanism (Diagram “C”) is attached to the lid and will move upward with
it. If there is no vial in the coil, the coil gate will probably drop open when the lid is lifted. If
the coil gate remains closed, pull the coil gate handle outward to lever the gate open.
When the Freezer/Mill tub is empty, pour liquid nitrogen into the tub about a third of the way.
Initially the nitrogen will boil off vigorously, but as the tub cools the boiling will subside. Add
more nitrogen gradually, to avoid splashing and boiling over. When the nitrogen is up to the fill
mark and boiling is subdued, push the lid down gently, stopping if splashing becomes excessive.
When the lid is closed and latched, further nitrogen boiling will be visible as a stream of
condensed water vapor (“fog”) rushing from the vent in the rear of the Freezer/Mill; this is
normal. When the vapor stream has subsided, lift the lid again and top off the liquid nitrogen in
the tub, filling it to the mark on the inside wall of the tub.
Page 14 of 28
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