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Stopcocks come in different
forms; they all have a formed shell, with a plug which effects a seal
which can be opened to some maximum diameter, which diameter is used
as a reference for size.
Ground glass stopcocks have
a glass plug with a hole or holes through it, allowing closure or
opening with a turn of the plug. Ground glass stopcocks require stopcock
grease to lubricate and seal the plug and shell; this can be a problem
if there is a danger of contamination from the grease. There are a
variety of configurations of arms and bores: straight, T-bore, Y-bore,
oblique bore, double oblique, etc., and different grades for high
vacuum use.
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are stopcocks in the same forms as the ground glass ones, but
which have Teflon plugs. This allows for sealing without grease,
and are probably the most common kind - think separatory funnels.
They are not as effective for vacuum. |
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Threaded
stopcocks have a glass shell with a threaded end which accommodates
a plug which screws into (or over) the threads, sealing the
stopcock into the shell with o-rings, and effecting the seal
at the tip of the plug. Teflon plugs are most common; glass
plugs are available in some forms. Some plugs have extended
conical tips which provide greater control of the aperture for
flow. |
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