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Glass Blowing for Vacuum Devices

Pump Unions

Most electronic devices made from glass need to be evacuated or gas filled to operate. This requires some form of union to attach the glass device to the vacuum pump, the pipe work of which is usually stainless steel.

The unions above were made by modifying some standard couplings using a small lathe. There are two different sizes. One is dedicated to glass tube with an 8mm outside diameter. The other is dedicated to 12mm diameter tube.

They are shown assembled above, with the glass tubes extending from the top. The threaded flange at the bottom connects to the vacuum rig.

Above, the unions are shown dismantled. The brass ring above the O-ring was made on the lathe. Its purpose is to compress the O-ring against the end of the union and against the wall of the glass tube.

The inside edge of the union is chamfered so that the O-ring is compressed against the union and the outside wall of the glass tube, making a seal. The internal diameter of the union is just slightly larger than the diameter of the glass tube. There is also a shoulder 10mm down to stop the glass tube dropping right down the pipe.

For room temperature operation, standard Neoprene O-rings can be used. For operation at higher temperatures, Viton O-rings are more suitable. Viton has a vapour pressure of 5x10-8 mbar at 150C. This is useful when the device being evacuated needs to be heated to out-gas it.

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