Danger of poor handling of laboratory Cryogens
Hand damage with liquid Helium
temperature but have very low boiling points. Nitrogen has a boiling point of
-196^0C and helium has a still lower boiling point of -269^0C which is
just 4 degrees the above absolute zero temperature. Such materials at extremely
low temperatures pose unique hazards and care needs to be exercised for mishap-
free handling.
Rapid Vaporization
As the boiling points are well below room temperatures these cryogens may
vaporize and boil violently at room temperatures. This feature can cause
splashing when transferring from one container to another.
The liquids on evaporation produce colourless and odourless vapours but
because of their extreme low temperature they result in condensation of
moisture present in the air to form dense clouds of fog. This fog can be seen
around equipment or containers holding extremely low temperature materials. The
dense fog should not be mistaken for the cryogens but indicates release of
vapour from extremely low temperature sources. Such dense fog is not toxic but
can reduce visibility to almost zero and can result in serious laboratory
accidents.
Due to large expansion ratios from the liquid to gas phase the cold
vapour can lead to condensation and freezing of atmospheric moisture and
blocking of openings of dewars. Due to pressure buildup of trapped gas inside
it can lead to sudden explosions unless pressure relief valves are provided.
Splashing
As cryogens boil vigourously at room temperature splashing can result
when transferring from one container to another. It is advisable to use an
appropriate transfer line or a wide mouth funnel. Rapid transfer can result in
splashing as the receiving container is at a higher temperature. The transfer
rate should be kept low so that the recipient container gets time to cool down.
The lower end of the transfer line should not be high but should remain below
the liquid level while transferring.
Asphyxiation
Cryogenic liquids evolve large volumes of gas on vaporization. Liquid
nitrogen,for example, vaporizes to nearly Awareness of hazards associated with liquid cryogens helps prevent labora700
volumes on expansion to gaseous state. This rapid expansion readily displaces
the oxygen in the air. In case of leakages and spills the affected area should
be evacuated to prevent asphyxiation when oxygen levels fall below 20 percent.
Installation of oxygen level monitors is advised in .areas housing cryogens
Burning
It is true that flames and high temperature surfaces cause burns but you
can also get burn injuries through contact with materials at very low
temperatures. In addition your skin can stick to the cold surface and when you
try to move away the skin will get torn and stick to the surface. It is
therefore strongly advised to avoid contact with such surfaces at extremely low
temperatures
Fire Hazards
Liquid nitrogen and liquid helium do not pose combustion hazards but on
rapid evaporation the extremely cold vaour can condense oxygen in the air and
liquid oxygen is highly reactive and combustible. Materials coming in contact
with liquid oxygen can ignite vigourously and even cause explosions. The space
around cryogens should be kept free of flammable solvents, oils, grease, etc
and any hot surface or heat generating equipments. Such materials can cause
spontaneous fires and explosions in oxygen enriched environments.
Cryogens can be used safely provided stipulated precautions are taken in
their handling and storage. Such precautions will be discussed in a subsequent
article.
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