Leaking Dos. Channel

One of the few expensive habits of the 25's are the leaking Dosimetry channels. Especially with the old ``Aluminium'' ionisation chambers the ladies are (were) frequently forced to clear the added dose before they could start a treatment. Sometimes leaks are that high that dosimetry accuracy is influenced. In most cases, these leaks find their cause in the chamber itselve. This can be determined by disconnecting the ionisation chamber. By removing the wedge the ionisation chamber connector can be reached.
(Disconnect connector SK12F1 on the RHCA. Make sure that the cable is ok. )

To get an impression of the leaking rate:

• Start an energy and push the interrupt button (otherwise the readings of the Dosimetry channels are cleared every five minutes).
• Go to the calibration page.
• Enter in Cal-Item1 the desired Dos Channel. For example 44 for Dos Chan 1.
• Enter in Cal Part Select 152.
• Note cal part value.
• Wait five minutes and do another readout.
• Subtract both values.
• Max number of count is 142 (for every monitor unit there are 64 counts).
  (In the new manual ``Supplementary setup and test procedures'' chapter 8, a value of 64 counts is given? I don't like the described procedure as dos. refs are changed and saved.)

When an ionisation chamber starts to leak or gives 300 Volt errors it may help to knock gently with a small hammer on the side of the chamber. This trick can expend the life of the ion chamber from 1 hour to one year. These problems start due to oxidation of the Aluminium spacers used between the foils. (When the chamber is removed and you shake it gently you can hear the Aluminium oxide moving through the chamber).

Leaking can be caused by high humidity. Usually this fault occurs in the morning after a raining weekend. It can be solved by removing the wedge and apply hot air for several minutes. Be careful!!

In older machines: the cable tree from the ionisation chamber is also suspect: disintegrating isolation!

In exceptional cases the DOS PCB are faulty.

For a correct functioning of the 2T, 2R and Hump systems it is important to check if there is a leak (offset) at one of the measuring channels at regular intervals. This can be done at the first seconds after the start button has been depressed, or by opening the fascia doors.