Siemens MOBILETT Plus - Page 51

17 - 1
Siemens-Elema AB Register 6 SPR8-215.850.02 Page 1 of 2 MOBILETT Plus/ Plus E
Solna, Sweden Rev. 01 01.02 SPS-UD
Tube-loading calculation 17
The block diagrams and the wiring diagram X037E, sheet 140 are the basis for the func-
tional description given below.
The tube-loading calculation is performed by the 80C535 microprocessor on CPU board
D1.
It fulfills two tasks:
1. it prevents the thermal loading of the anode plate from exceeding a maximum of
80 % of the permissible loading (in HU = Heat Units) and
2. it limits the long-term (one hour) mean value of the X-ray tube heat reduction to
73 W.
This makes use of a so-called HU memory, in which the heat units calculated from the kV
and mAs values used in each exposure are summed.
The memory contents are continuously compared with a HU limiting value of about 80%
of the permissible thermal loading in HU.
Fig. 1
Between exposures, the contents of the HU memory are reduced by 13 Joules every 250
ms in order to simulate the cool-down characteristic of the X-ray tube assembly. 13 Joules
represent about 52 W cooling capacity.
"Ready" off
100%
80%
67%
Last
exposure
"Ready" on
Calculated anode plate
heat energy
Calculated cool-down of
anode plate heat energy