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 has a double junction reference which provides a longer life than many other pH probes and is a gel filled non-refillable probe, so apart from cleaning is maintenance free. A replacement probe for Bluelab� Combo and pH Meters, and Guardian Monitors.