I have found the sensors installed at PI to be very accurate and reliable. In fact, with over 2.5 years of full time use I have never experienced a single failure. I do use 2 cups of Calgon water softener in the black tank and a cup of liquid laundry detergent after every dump. (The GEO Method) If you do that, you will most likely never have an issue with the black tank or the sensor in it.
Bob and Sharon Steele