Pressure Sensors

Because the Linac is using an open ionisation-chamber, dosimetry has to be compensated for air-pressure changes. A change of 10mBarr in air pressure induces a change of 1% in dosimetry. For safety reasons there are two separate dosimetry systems. Each system has its own pressure sensor. The pressure sensor 1 (Item 558) of DOS channel 1 is located on the DOS1 PCB The Pressure sensor 2 (Item 559) is located on the pressure board (Radiation Head Control Area) To make calibration of the pressure sensors more easy, for each sensor part 10 is printed on the PCB. During calibration part 11 has to be changed until the reading matches the actual pressure.

Usually this calibration is good enough. Sometimes the readings of the pressure sensors differ more than 3 mBarr from each other (extreme pressure changes). This will induce the inhibit: Dos Press Diff.

A quick fix is to alter part 11 of sensor 2

A more accurate pressure calibration can be achieved by recording the air pressure, pressure reading, gain and offset from each channel at a very high and low air pressure. A simple calculation will then give a more accurate gain and offset.