Effect Of Focal Spot Size. The area, or focal spot size, bombarded by the electrons affect the heat dissipation capabilities of the anode. This limits the tube rating or the milliamperes at which the tube may be safely operated.
- Heat Dissipation. The method of removing heat from the X-ray tube anode affects the tube ratings. An X-ray tube dependent upon convection cooling has a lower limit of operation than the same tube where water or some other coolant is used to conduct heat away from the focal spot.
- Operational Considerations. When a new X-ray tube is put into operation, it requires a warm-up period. A new tube may have been stored for a period of time and a very small amount of gas may have been released into the very high vacuum by the metallic parts within the tube. These gases can be driven back into the metal components by operating the tube at low kilovoltage and slowly heating the anode to high temperatures. Therefore a new X-ray tube should be energized at low kilovoltage, and the kilovoltage slowly increased until maximum rating has been obtained. The same procedure should be used when a unit has not been operated for 30 days or more.
Recalibration. Recalibration should be accomplished when either a new tube or new components have been installed in an X-ray generator. Quite often technique charts need slight changes to compensate for the new conditions. This may be due to large quantities of tungsten that have been deposited on the inside walls of an old tube that increased the inherent radiation filtration. Differences in the filtration of different windows in the X-ray tubes may cause some small variations in radiation output. Due to the special equipment required to re-calibrate, this procedure is not normally performed at field level. Recalibration is desirable to insure that X-ray tube output falls within the tolerances specified in the manufacturers operations and maintenance manual. Failure to calibrate may result in over or under exposure of radiographs when following X-ray exposure techniques provided in the various aircraft/engine NDI inspection technical orders.
Tube Ratings. Several variables affect the maximum rating of an X-ray tube. These should be carefully inspected to assure the X-ray tube rating is not exceeded. Some of the more important variables to be considered are listed below.
- Focal Spot Size. The size of the focal spot usually dictates the milliamperes that can safely be conducted across the X-ray tube.
- Method of Cooling. The method of heat removal from the anode affects the length of time the tube may be operated under a standard operating condition. The operation is extended by the use of external coolant.
- Type Of Circuit. The type of circuit design used in the X-ray generator affects tube rating. Where self-rectified circuitry is used, the inverse voltage applied to the x-ray anode limits the operation of the tube. Usually, the maximum operating conditions are much greater where full wave circuitry is used, in comparison to self-rectified generators.
Source: KARTA TECHNOLOGY, INC
