Controlled-Rate Freezer Safety Precautions | 3
Handling Liquid Nitrogen
Handle liquid nitrogen carefully.
The extremely low temperatures can freeze human flesh very
rapidly. When spilled on a surface, the liquid tends to cover it
completely, cooling a large area. The gas issuing from the
liquid is also extremely cold. Delicate tissue, such as that of
the eyes, can be damaged by exposure to cold gas which
would be too brief to affect the skin of the hands or face.
Never allow any unprotected part of your body to touch
objects cooled by liquid nitrogen.
Note: Always wear gloves when handling anything that is, or
may have been, in immediate contact with liquid nitrogen.
Insulated gloves are recommended, but heavy leather gloves
may also be used. The gloves should fit loosely, so that they
can be thrown off quickly if liquid should splash into them.
Such objects may stick rapidly to the skin and tear the flesh
when you attempt to free yourself. Use tongs to withdraw
objects immersed in the liquid, and handle the object
carefully.
Wear protective clothing.
Protect your eyes with a face shield or safety goggles (safety
glasses without side shields do not give adequate protection).
When handling liquid in open containers, it is advisable to
wear high-top shoes. Trousers (which should be cuff less if
possible) should be worn outside the shoes.
Introduction
The safe handling and use of liquid nitrogen in cryogenic
freezer and dewar flasks is largely a matter of knowing the
potential hazards and using common-sense procedures
based on that knowledge. There are two important properties
of liquid nitrogen that present potential hazards:
1. It is extremely cold. At atmospheric pressure, liquid
nitrogen boils at -320°F (-196°C).
2. Very small amounts of liquid vaporize into large amounts
of gas. One liter of liquid nitrogen becomes 24.6 cu. ft.
(0.7 m3) of gas.
The safety precautions in this manual must be followed to
avoid potential injury or damage which could result from these
two characteristics. Do not attempt to handle liquid nitrogen
until you read and fully understand the potential hazards, their
consequences, and the related safety precautions. Keep this
information handy for ready reference and review.
Note: Because argon is an inert gas whose physical
properties are very similar to those of nitrogen, the
precautions and safe practices for handling and use of liquid
argon are the same as those for liquid nitrogen.
Use containers designed for low temperature liquids.
Cryogenic containers are specifically designed and made of
materials that can withstand the rapid changes and extreme
temperature differences encountered in working with liquid
nitrogen. Even these special containers should be filled
SLOWLY to minimize the internal stresses that occur when
any material is cooled. Excessive internal stresses can
damage the container.
Do not cover or plug the entrance opening of any liquid
nitrogen freezer or dewar.
Do not use any stopper or other device that would interfere
with venting of gas. These cryogenic liquid containers are
generally designed to operate with little or no internal
pressure. Inadequate venting can result in excessive gas
pressure which could damage or burst the container. Check
the unit periodically to be sure that venting is not restricted by
accumulated ice or frost.
Use proper transfer equipment.
Use a phase separator or special filling funnel to prevent
splashing and spilling when transferring liquid nitrogen into or
from a dewar. The top of the funnel should be partly covered
to reduce splashing. Use only small, easily-handled dewars
for pouring liquid. For the larger, heavier containers, use a
cryogenic liquid withdrawal device to transfer liquid from one
container to another. Be sure to follow instructions supplied
with the withdrawal device. When liquid cylinders or other
large storage containers are used for filling, follow the
instructions supplied with those units and their accessories.
Do not overfill containers.
Filling above the bottom of the neck-tube (or specified
maximum level) can result in overflow and spillage of liquid
when the neck-tube core or cover is placed in the opening.
Never use hollow rods or tubes as dipsticks.
When a warm tube is inserted into liquid nitrogen, liquid will
spout from the top of the tube due to gasification and rapid
expansion of liquid inside the tube.
As the liquid evaporates, the resulting gas tends to displace
normal air from the area. In closed areas, excessive amounts
of nitrogen gas reduce the concentration of oxygen and can
result in asphyxiation. Because nitrogen gas is colorless,
odorless and tasteless, it cannot be detected by the human
senses and will be breathed as if it were air. Breathing an
atmosphere that contains less than 18% (or) 20% oxygen can
cause dizziness and quickly result in unconsciousness and
WARNING: Contact of liquid nitrogen or cold
gas with the skin or eyes may cause serious
freezing (frostbite) injury.
WARNING: Nitrogen gas can cause suffocation
without warning. Store and use liquid nitrogen
only in a well-ventilated place.
