Bluelab pH ProbeA replacement probe for Bluelab Combo and pH Meters as well as the Bluelab Guardian Monitor. The Bluelab pH Probe is a replacement pH probe (electrode) suitable for use with Bluelab products that test the pH value of a solution. Unfortunately pH Probes do not last forever as they age through normal use and will eventually fail. With good care and use (keeping the probe tip wet and cleaning once a month) you will extend its life. The Bluelab pH Probe