The Neptune Systems ORP / Redox Electrode – Double Junction is a probe with which you can monitor ORP, replace your ORP probe or add one for another tank. The probe measures ORP in your aquarium with an Apex System or a PM1 module. You can replace a worn out ORP probe (its recommended to replace your Lab Grade ORP probe every 18-24 months). This probe can be used when you are adding another tank to your system, want to measure ORP in it, and purchased a PM1 to do so.
Lab Grade Probes tend to last longer, give more accurate results, stay calibrated longer, and need replaced less frequently.