2.Very small amounts of liquid vaporize into large amounts of gas. One
liter of liquid nitrogen becomes 24.6 ft3/0.7
m3 of gas.
The safety precautions as outlined 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 a print
out of this webpage handy for ready reference and review.
Maintenance:
Keep the unit clean and dry at all times. Do not use strong alkaline or
acid cleaners that could damage the finish and corrode the metal shell.
Note:
Because argon is an inert gas whose physical properties are very similar
to those of nitrogen, the precautions and safe practices for the handling
and use of liquid argon are the same as those for liquid nitrogen.
Handling Liquid Nitrogen
Contact of liquid nitrogen or any very cold gas with the skin or eyes may
cause serious freezing (frostbite) injury.
Protect hands at all times when working with liquid nitrogen with
SPI Cryo Gloves.
Handle liquid nitrogen carefully
The extremely low temperature can freeze human flesh very rapidly. When
spilled on a surface the liquid tends to cover it completely and intimately,
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 an exposure to
the 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.
Such objects may stick fast to the skin and tear the flesh when you attempt
to free yourself.
Use tongs, preferably with insulated handles,
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). Always wear
cryo gloves when handling anything
that is, or may have been, in immediate contact with liquid nitrogen. The
gloves should fit loosely, so that they can be thrown off quickly if liquid
should splash into them. When handling liquid in open containers, it is
advisable to wear high-top shoes. Trousers (which should be cuffless if
possible) should be worn outside the shoes.
Any kind of canvas shoes should be avoided because a liquid nitrogen spill
can be taken up by the canvas resulting in a far more severe burn, in fact
that would occur if the feet were essentially open or bare! Now we don't
advocate going bare foot when using liquid nitrogen, but we also don't think
that the wearing of canvas shoes is a safe practice either.
Use only 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 ever cover or plug the entrance opening of any liquid nitrogen 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. Use only the
loose-fitting necktube core supplied or one of the approved accessories for
closing the necktube. 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 necktube (or specified maximum level) can
result in overflow and spillage of liquid when the necktube 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 bottom of the tube due to gasification and rapid expansion of liquid
inside the tube. Wooden or solid metal dipsticks are recommended; avoid using
plastics that may become very brittle at cryogenic temperatures which then
become prone to shatter like a fragile piece of glass.
Nitrogen gas can cause suffocation without warning. Store and use liquid
nitrogen only in a well ventilated place.
As the liquid evaporates, the resulting gas tends to displace the 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 percent oxygen can cause dizziness and quickly result
in unconsciousness and death.
Note:
The cloudy vapor that appears when liquid nitrogen is exposed to the air
is condensed moisture, not the gas itself. The gas actually causing the
condensation and freezing is completely invisible.
Never dispose of liquid nitrogen in confined areas or places where others
may enter.
Disposal of liquid nitrogen should be done outdoors in a safe place. Pour the
liquid slowly on gravel or bare earth where it can evaporate without causing
damage. Do not pour the liquid on the pavement.
First Aid Notice
If a person seems to become dizzy or loses consciousness while working with
liquid nitrogen, move to a well-ventilated area immediately. If breathing has
stopped, apply artificial respiration. If breathing is difficult, give oxygen.
Call a physician. Keep warm and at rest.
If exposed to liquid or cold gas, restore tissue to normal body temperature
98.6° F (37° C) as rapidly as possible, followed by protection of the
injured tissue from further damage and infection. Remove or loosen clothing
that may constrict blood circulation to the frozen area. Call a physician.
Rapid warming of the affected part is best achieved by using water at
108° F/42° C). Under no circumstances should the water be over 112° F/44° C,
nor should the frozen part be rubbed either before or after rewarming. The
patient should neither smoke, nor drink alcohol.
Most liquid nitrogen burns are really bad cases of frostbite. We don't mean
to belittle the harm that can come from frostbite, but at the same time, we
wanted to keep the dangers associated with liquid nitrogen burns in
perspective. Indeed,
liquid nitrogen burns could be treated as frostbite.
Handling Liquid Nitrogen Dewars
Keep unit upright at all times except when pouring liquid from dewars
specifically designed for that purpose.
Tipping the container or laying it on its side can cause spillage of liquid
nitrogen. It may also damage the container and any materials stored in it. If
tipping is anticipated,
be sure to purchase a dewar that can be outfitted with
a tipping stand.
Rough handling can cause serious damage to dewars and refrigerators.
Dropping the container, allowing it to fall over on its side, or subjecting
it to sharp impact or severe vibration can result in partial or complete loss
of vacuum. To protect the vacuum insulation system, handle containers carefully.
Do not "walk", roll or drag these units across a floor. Use a dolly or handcart
when moving containers, especially the larger portable refrigerators. Large
units are heavy enough to cause personal injury or damage to equipment if
proper lifting and handling techniques are not used.
When transporting a liquid nitrogen dewar, maintain adequate ventilation and
protect the unit from damage.
Do not place these units in closed vehicles where the nitrogen gas that is
continuously vented from unit can accumulate. Prevent spillage of liquids and
damage to unit by securing it in the upright position so that it cannot be tipped
over. Protect the unit from sever jolting and impact that could cause damage,
especially to the vacuum seal.
Keep the unit clean and dry
Do not store it in wet, dirty areas. Moisture, animal waste, chemicals,
strong cleaning agents and other substances which could promote corrosion
should be removed promptly. Use water or mild detergent for cleaning and dry
the surface thoroughly. Do not use strong alkaline or acid cleaners that
could damage the finish and corrode the metal shell.
Protect Dewar Contents
Materials stored in a liquid nitrogen dewar with a wide mouth are protected
by the extremely low temperature of the liquid nitrogen or the gas that
issues from the evaporating liquid nitrogen. When all of the liquid nitrogen
has evaporated, the temperature inside the unit will rise slowly to ambient.
The rate at which the liquid nitrogen will evaporate depends upon the pattern
of container use and the age and condition of the container. Evaporation
increases as insulation efficiency deteriorates with age and rough handling.
Opening and closing to insert and remove materials and moving the unit will
also increase the evaporation rate.
To protect valuable material stored in a liquid nitrogen refrigerator:
Check the liquid level in unit frequently
Great damage could result to laboratory equipment that requires constant
cooling to protect some critical part of the equipment such as the Si (Li)
detector on a modern EDS system. Or important experiments could be delayed,
or critical samples spoiled, if one unexpectedly ran out of liquid nitrogen
and then could not perform their cryo ultramicrotomy. So it is of the
greatest importance to check the liquid nitrogen level constantly in order to
anticipate any such kinds of problems that could arise.
Condensed moisture or frost on the outer shell of a refrigerator and
abnormally rapid evaporation of the liquid nitrogen are indications of
vacuum loss.
If vacuum loss is evident or suspected, start thinking immediately about the
procurement of a replacement dewar. It is just not cost effective to continue
to use a dewar with a bad vacuum and waste valuable liquid nitrogen in the
process. There is also the safety issue of excessive boil-off in an enclosed
area that is not large enough to "absorb" the higher rate of nitrogen boil
off.
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Monday March 22, 2010
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