5 eTape Liquid Level Sensor + extras : ID 3827 : $39.95 : Adafruit Industries, Unique & fun DIY electronics and kits
Liquid Level Sensor, AC/DC 0~220V Stainless Steel Liquid Water Level Sensor Liquid Float Switch Tank Vertical Water Level Sensor for Pool Can 100mm: Amazon.com: Industrial & Scientific
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WISY Universal Pneumatic Level Indicator | Buy a Universal WISY Pneumatic Fill Level Indicator - Rainwater Management Solutions
Capacitive Liquid Level Sensor Probes
Amazon.com: BTER Water Tank Level Indicator, 0-15Meter/0-49.2Ft Measurement Liquid Depth Meter Sensor Alarm Function ±0.03m Accuracy Digital with Temperature Display, Timer : Tools & Home Improvement