Yacht Products Intl, Inc.
Offers Two Versions of Tank Level Monitors
Wired and Wireless (nowire™)
Both versions use SensaTank II Mirus detector cells that mount on the outside of polyethylene tanks and Fiberglass tanks up to ½ of an inch thick. They are great for fresh, grey and black water tanks. No holes in the tank. These sensors are time tested and very reliable.
Read more about SensaTank II sensors:
Marine Holding-Tank Sensors: How do you Know When
the Black Water Level is at Red Alert?”, Practical Sailor
Magazine, Volume 34, Number 2, February 2008
Note: nowire™ Tank Monitor Systems also offer an equally reliable ultrasonic sensor option for tanks other than plastic and fluids other than water.