Stopcocks
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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There 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 high vacuum, although they work for low vacuum applications, such as vacuum filtration. |
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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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