h a l f b a k e r yI CAN HAZ CROISSANTZ?
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The -80C lab freezer is something of a standard feature of
medical/bioscience research lab. I use them for stable storage of
tissue/protein samples, DNA archives, transgenic bacterial
archives, sensitive chemicals & the good vodka. One problem in
a busy lab is that frequent opening leads to moist
room
temperature air making it's way inside. This immediately forms a
layer of frost and accumulates as inches of snow in available
gaps. It coats almost everything making the labels difficult to
read. You solve this by wiping away the frost on your box of
choice, often partially melting it. Now your frequently used box
will begin to develop a much tougher layer of solid ice. Labeling
is lost behind layers of this ice, label tape falls off everything at
this temperature, nothing writes on a very cold frosty surface.
Existing ink, even specialty stuff is easily rubbed off plastic
surfaces.
Ultimately a poorly maintained freezer will build up enough ice
to compromise the seals and accelerate failure. This can happen
in a few weeks. So how to mitigate this?
The problem is humidity in the air. We can remove that by
moving all freezers to the antarctic, or mountain peaks,
aggressive air conditioning or dehumidificaiton. These aren't
practical for obvious reasons - noise and the coffee supply
situation for example. What we can do is preferentially fill the
freezer with a totally dry gas. Labs often have inbuilt vacuum,
natural gas and crucially a nitrogen supply. A regulator on the
freezer would allow a small constant flow of N2 into the rear of
the freezer. This would ensure net outward movement of gas
through the valve in the door that exists to compensate for
temperature-induced pressure changes. This should mitigate a
most of the humid air influx.
If a nitrogen line isn't available, cylinders can be used. A further
opportunity exists to use liquid nitrogen. Many labs have this
constantly on hand. A port in the freezer would allow a weekly
top up of liquid nitrogen into a vacuum-insulated chamber. The
liquid slowly boils off and supplies the freezer with it's dry gas
feed. Filling liquid nitrogen cell-storage is already a weekly
chore, so this is a modest addition.
[link]
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Why only for lab freezers? Have sympathy for users of domestic installations |
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I like it. It is a step more convenient than filling the entire
lab with nitrogen. |
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//more convenient than filling the entire lab with nitrogen.// |
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That has it's appeal though. There are lots of chemicals that
are oxygen sensitive, a nitrogen atmosphere would be a robust
guard against fire and undergraduates. As a downside, the
bunsen burner wouldn't work, and you can't really do science
without one of those available. |
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I was thinking a glovebox/airlock system to minimise moist
air transfer, but this is much better!
As per [kdf], could you use a convenient 79/21 mix of
nitrogen & oxygen instead, or is that getting too
complicated for the temperature control side of things?
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I'd keep the nitrogen flow relatively modest. It will likely
not bother a standing person anyhow, for an upright
freezer, when you open the door, the cold nitrogen, just
like air, will just sort of slump downward and spread over
the floor. You can feel the effect on bare feet if you open a
freezer. It CAN displace oxygen, but this isn't the same level
as, say spilling liquid nitrogen in an elevator. Most labs have
a howling gale blowing through them anyway, just because
of multiple fume hoods extracting air. Now, if you put your
head RIGHT inside a chest
freezer, it might end badly, but that person has bigger
problems. |
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