Filter
Top Products
Page 20 of 25 SPEX SamplePrep LLC 9.24.07
6.4 Checking The Liquid Nitrogen Level
Every time you remove or insert a vial, note the level of liquid nitrogen (LN) in the tub. You
should top off the LN when needed as a matter of routine. If your programmed grinding cycle
includes more than twenty or thirty minutes of actual grinding time, you should visually check
the nitrogen level at about that point and add more if needed. We do not recommend cumulative
grinding time of more than thirty minutes per run, as after that much grinding the LN level is
close to the point where the LN sensor will shut down the mill. However, if you must run the
mill for an extended time, and the LN sensor stops the grinding program, the mill can be refilled
with LN and the grinding program resumed where it stopped.
The 6870 Freezer/Mill can be refilled automatically with the 6820 Auto-Fill System, which adds
LN to the tub when the LN level is low. The 6820 Auto-Fill System must be installed and
calibrated at our factory, so if you did not order your 6870 Freezer/Mill with an Auto-Fill System
and want to order one later, the Freezer/Mill must be returned to SPEX SamplePrep LLC for
installation and calibration.
6.5 6820 Auto-Fill System: Description, Installation, Operation
Description: the 6820 Auto-Fill System automatically transfers liquid nitrogen (LN) to the 6870
Freezer/Mill during operation, making hand-filling unnecessary. The Auto-Fill mechanism
includes a cryogenic valve linked to LN sensors in the tub. When the LN level is low, the valve
opens during Pre-Cool or Re-Cool stages to let LN into the tub. The Auto-Fill System as
supplied also includes a safety valve for the LN transfer hose.
LN transfer hoses can vary with the LN supply tank or line, so one is not included with the 6820
Auto-Fill System. Four-foot and six-foot transfer hoses of standard design (part numbers 6906
and 6907) can be purchased from SPEX SamplePrep, or custom transfer hoses can be designed
and supplied by the user. The LN inlet is a male ½ inch JIC fitting placed low on the right side
of the Freezer/Mill, so the outlet of the hose requires a female CGA295 fitting. The 160 liter and
240 liter LN tanks used by many of our customers generally have a male 3/8” (9.5 mm) NPT
outlet, so SPEX transfer hoses come with a matching female 3/8 in. NPT fitting. We do not
recommend excessively long transfer hoses.
LN tanks come in two types, high-pressure and low-pressure. Some LN tanks have valve
systems to switch from low pressure to high pressure. Always use a low-pressure LN supply,
with maximum pressure of about 20 psi.
Installation
: install the safety valve on the outlet of the tank or LN line, with the gooseneck tilted
up and the valve outlet down. Then attach the LN transfer hose to the safety valve, and the other
end to the inlet on the Freezer/Mill. The safety valve should be higher than the LN inlet on the
Freezer/Mill. The purpose of the safety valve is to have an outlet for nitrogen vapor in case the
mill and supply tank valves are both shut, and pressure builds up inside the hose.
When installing the safety valve and transfer hose, use Teflon plumber’s tape on all joints, and
tighten the nuts with a wrench. If the joints leak, tighten them further and/or use more tape.