Sludge and soap residue build-up on the sides of RV black and gray water tanks can lead to false readings. The sludge acts like a conductor between the positive content level sensors down the tank to the negative probe that is located at the bottom of the tank. This will trigger the indicator panel to show that there is liquid level in the tank even when there is none. The patented design of Horst Miracle Probes prevents similar false readings in the following ways: Horst Miracle Probes are considerably longer than standard probes and the sensor is surrounded by a Delrin body and teflon tube which prevents any liquid residue or sludge from setting up a conductive path.• Compatibility: Gray Water Tanks• Location: 3/8 Inch Diameter Hole For Existing Probes• With Wiring Harness: No Specially Designed To Prevent False Probe Contact & Related Reading For Use With Gray Water Tanks Easy To Install As They Fit The 3/8 Inch Diameter Holes Of Existing Probes | Valterra Waste Holding Tank Level Sensor in Black