Villa Sistemi Medicali ENDOS AC, User Manual

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USER'S MANUAL
Technical data
ENDOS AC/ACP - CE (Rev. 7) 24
5.3 5.3 5.3 5.3 Correct use of dosimeters to measure exposure Correct use of dosimeters to measure exposure Correct use of dosimeters to measure exposure Correct use of dosimeters to measure exposure
times times times times
The spread of non-invasive equipment to measure the functional
parameters of RX equipment has introduced a series of interpretation
problems when measuring exposure times.
The source of the problem is in the characteristic rise curve of the RX
tube’s anodic current which is represented in the picture:
According to IEC60601-2-7 (1998) regulations, "in equipment where the
filament is switched on and high voltage is applied simultaneously, the
exposure time is calculated as the interval between the instant when the
anodic current exceeds 25% of the nominal value and the instant when
it goes below such value".
This method is defined as invasive because it requires that the anodic
current flowing through a resistance inside the tubehead must be
measured.
Non-invasive methods are definitely easier and faster compared with the
invasive method, but they are prone to errors which can be considerable
when determining exposure time. In fact some of these devices start
counting exposure time as soon as a small quantity of radiation reaches
the measuring chamber with the result that they take longer times than
the ones determined by the invasive method applied by the
manufacturer.
Consequently, calculations obtained by these non-invasive methods can
erroneously lead to the conclusion that the equipment timer is not
accurate. Actually the difference is connected to the method adopted in
measuring the exposure time.
Time
Anodic
current