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Overloading the tube
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The maximum load a tungsten
target will safely take for a period not exceeding 1 second is 200 w per mm2.
Thus a 1 k w tube will have a focal area of 5 mm2. Normally X-ray apparatus is
so designed as to prevent the use of excessive electrical output for a single
exposure but overloading can occur if a series of repeat exposure are made at
high output.
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Such overloading can cause
tube damage in at least two ways. The tungsten may vaporize and be red posited
on the glass wall which will eventually render the tube inoperable, or the
heated target will emit electrons like a filament and cause the high-tension
voltage to flow in the reverse direction on the normally suppressed half-cycle
of the alternating current and wreck the delicate cathode. Therefore the tube
must always be operated within its rating.
Steps to extend tube life
Following precautions help to
extend tube life:
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Anode should be warmed up
before actual exposure is made.
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Do not switch on mA or
kVp settings while rotor is engaged because this causes torque force on the
anode and bearing assembly and increased filament evaporation.
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Use high kVp and low mA
settings to minimize overheating of the anode.
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Consult correct tube
rating chart to avoid overheating of the anode.
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Do not run anode
unnecessarily because this shortens the life of bearings.
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Adequate cooling of the
tube housing must be ensured to avoid excessive heating of oil in the tube
housing.
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Do not allow overheating
of the filament by repeated exposures in a short time.
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