[APD] pH controller is flipping

["pieter cool" <pieter_cool at student.kuleuven.ac.be>]

Hello all,
I am currently using the seramic ph controller (produced by sera).  Recently the ph measurements started swinging from 10 to 4 without any reason.  I tried to reinstall the whole thing, using the test solutions and all.  It still isn't working properly.  I suspect the ph probe to be broken and the test solutions outdated as well.
I'm now trying to fix everything.  The easiest solution would be to go to an LFS and buy a new probe and test solutions but I was wondering if it wouldn't be possible to use another probe (possibly cheaper).  Has anyone any experience with this?
My second question is about the test solution of 7 pH.  Wouldn't demineralised water do the trick.  I thought it has a pH of 7 also.  Am I wright about this?

I have added a link to the probe I am talking about.

http://www.sera.de/English/autoframe.htm?url=http://www.sera.de/english/start-e.htm

Thanks
Pieter Cool
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