Good work practices can significantly prolong tube life.
These recommendations should be followed whenever possible:
Minimize filament boost (“prep”) time
Boost time will usually exceed the actual exposure time. High
filament current applied for too long will shorten filament life and
will lead to unstable operation as evaporated tungsten from the
filament is deposited onto the glass envelope. This is especially
the case at high mA stations.
Use lower tube current (mA)
The high filament current required to produce high tube current (mA)
will shorten filament life and will lead to unstable operation as
evaporated tungsten from the filament is deposited on to the glass
envelope. Whenever possible, use a lower mA station and a longer
exposure time to arrive at the desired mAs.
Follow rating charts and anode heating/cooling curves
Operation beyond published ratings will result in premature focal
track wear or damage. Even moderate etching of the focal track will
result in a fall-off in radiation output, because electrons from the
filament which strike in micro-crevices in the target material
produce radiation that is mostly absorbed in the surrounding target
material. More severe etching, or melting, results in the liberation
of gasses from the target material which causes tube instability.
Excessive heat transfer from the target into the rotor body will
cause bearing failure or slow rotation which will result in melts on
the focal track.
Limit operation to 80% of maximum single exposure ratings
Although higher power levels are both possible and permitted, this
reduction will help assure long focal track life. Also, it will
minimize the reduction in radiation output associated with a
roughened focal track.
Do not exceed anode thermal capacity or dissipation rate of the
target
The greatest danger is to heat flow into the bearing structure, as
discussed above. In addition, gasses may be emitted from the various
metals within the tube if the temperature reached during clinical
use is appreciably higher than that used during the “outgassing”
stage of manufacture. If outgassing occurs during clinical use, the
tube will become unstable. (“Aging” the tube may reverse the process
but this is not assured.)
Do not make high mA exposures on a cold target
Uneven expansion caused by thermal stress from a high power exposure
can result in a cracked target. Do not assume that a “thermally
relieved” target design provides absolute protection. Always follow
the recommended warm-up procedure. The procedure may need to be
repeated between patients, if the “idle” time is long enough, in
addition to being performed at the beginning of the work day.
Avoid long intervals between spot-films
Most systems provide for a “holdover” period of up to approximately
25 seconds between spot-films, during which the rotor is kept at
high speed before the rotor brake/reboost cycle is allowed to occur.
In some systems, the filament current remains at the exposure value
during this period, thereby causing evaporation of filament material
and resultant tube instability.
Limit rotor start/stop operations
Rotor start/stop operations especially to/from high speed (150/180
Hz) generate considerable heat in the stator windings, which will
lead to stator damage in extreme cases. Generally there should be a
minimum of 30 to 40 seconds between starts. Tubes equipped with a
heat exchanger will be less sensitive to this potential problem
because oil circulation will help prevent hot spots from occurring
around the stator windings.
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