bluelab Guardian Monitor Care And Use Manual page 9

Bluelab pH Probe replacement
The Bluelab pH Probe is the only part of the Bluelab Guardian Monitor
that requires replacing.
pH probes do not last forever. They age through normal use and will eventually fail.
To ensure you receive a long life from your pH probe, please read the instructions
provided with it.
When the time comes to replace your Bluelab pH Probe all you have to do is order
a replacement from your supplier!