Leica Biosystems LN22 - Manual (Page 11)

11
• Once every three months check the actual evaporation rate of your
Dewar vessels and compare the measuring results to the evaporation
rates specified by the manufacturer. The evaporation rate of an un-
damaged metal Dewar is far less than 1 liter per day. Defective
Dewars with evaporation rates higher than that are a safety risk and
should be taken out of service or repaired.
• Standard Dewar vessels, such as the metal Dewar of the LN22 sys-
tem, are not pressure vessels, therefore make sure they are closed
only with a special Dewar topper (Fig. 2). These stoppers fit loosely
onto the Dewar neck and leave a sufficiently large gap to allow for
sudden evaporation of GN
2
. Check the stopper frequently to make
sure it will not be stuck to the Dewar neck by ice deposits (Fig. 3).
Never close a Dewar vessel with stoppers that you have crafted
yourself (Fig. 4). Through exposure to shock or vibration (e.g. trans-
port), a gas-tight closed Dewar vessel can release GN
2
in an explo-
sion-like fashion and burst in the process. In that case the entire LN
2
content (35 liters = 35m 3 GN
2
) will evaporate at once (that may e.g.
happen inside a vehicle during transport). When a Dewar moves, thus
causing a sudden evaporation of LN
2
, an appropriately designed stop-
per will rise due to the gas pressure, thus opening a wide enough gap
for the GN
2
to escape (Fig. 5). The small quantities of LN
2
which may
escape as well in such situations do not have to be regarded as a sig-
nificant safety risk.
GN
2
GN
2
Fig. 4 Fig. 5
Tightly fitted stopper
with seal
Fig. 2
Loosely fitted stopper
Fig. 3
Ice deposits
2. Safety