Frequently Asked Questions
The component units of measurement are:
| Unit | Atmosphere equivalent (approx.) | Abbreviation |
|---|
|
|
1000 |
mbar |
|
|
760 |
torr |
|
|
105 |
Pa |
|
|
7.6 x 105 |
µ |
There are usually two separate vacuum systems on an EB welding machine - one for the electron gun chamber, and another for the welding chamber - each with its own vacuum gauges. The electron gun must operate at 1 x 10-5 mbar or better, but the welding chamber vacuum can be specified as -
|
|
in the 10-4 to 10-5 mbar range |
|
|
in the 10-2 to 10-3 mbar range |
|
|
in the 10 to 10-1 mbar range |
For measurement in the 'rough' vacuum range - down to 10-3 mbar - a Pirani gauge or a thermocouple gauge is used.
Pirani gauge
This consists of a wire supported in a tube exposed to the vacuum. The wire is heated by an electric current and loses heat by thermal conduction to the residual gas in the tube; the more gas there is (higher pressure), the more heat will be lost. Thus, the amount of heat supplied to the wire to maintain it at constant temperature is a measure of the pressure.
The gauge can be calibrated at atmospheric and high vacuum according to the manufacturer's instructions to give accurate reading throughout the measurement range given above. The wire is delicate and can become contaminated with oil; this can be removed with solvents and gentle agitation (see service instructions); it can also be damaged by venting too fast.
Thermocouple gauge
This operates in a similar way to the Pirani gauge except that the filament temperature is monitored by a thermocouple using the Seeback effect to generate a voltage at the junction of two dissimilar metals at elevated temperatures. It is more robust but slower acting than the Pirani gauge and has a smaller operating range of 5 mbar to 10-3 mbar; it can be cleaned using the same techniques as above.
Penning gauge
To measure lower pressures, in the 10
-4 - 10
-6 mbar range, a different principle is required. In this pressure range, the residual gas can be ionised (made to conduct electricity) by applying a high voltage (about 2.5kV) across two electrodes; with sophisticated design to stabilise the discharge, the ionisation current between the electrodes is a measure of the number of residual gas molecules, i.e. the pressure.
The electrodes can become contaminated leading to unstable or inaccurate readings; in this case, the manufacturer's cleaning routines should be followed.
If there is a deterioration in the vacuum level, it may be helpful to look at FAQ: Why won't my electron beam (EB) welding chamber pump down to the usual vacuum?
For further information see Joining Technologies or please contact us.