BLUELAB - IN LINE PH PROBEReplacement Probe for Bluelab Guardian Monitor Continuous pH Meter If you own a Bluelab Guardian Monitor and need a replacement probe for the continuous pH meter, you've come to the right place. pH probes can degrade over time, and with regular use, it's normal for them to need replacement. However, with proper care and maintenance, you can extend the life of your probe. The Bluelab replacement probe is specifically designed for the Guardian Monitor