Neptune Systems Lab Grade pH Probe Replace your pH probe or add another for another tank, calcium reactor, etc. Buy this probe if you are: – replacing a worn out pH probe (we recommend you replace your pH probe every 18 months). – adding another tank to your system and purchased a PM1 to do so. – want to have a back-up probe on hand Lab Grade Probes tend to last longer, give more accurate results, stay calibrated longer, and need replaced less frequently