The Chemical eTape Liquid Level Sensor is a solid-state sensor with a resistive output that varies with the level of the fluid. It does away with clunky mechanical floats, and easily interfaces with electronic control systems. What separates this from adafruits other eTape Liquid Sensor is the Teflon (FEP) jacket that is rated for use in chemical, petroleum, and food safe applications
The eTape sensor's envelope is compressed by the hydrostatic pressure of the fluid in which it is immersed. This results in a change in resistance that corresponds to the distance from the top of the sensor to the surface of the fluid. The sensor's resistive output is inversely proportional to the height of the liquid: the lower the liquid level, the higher the output resistance; the higher the liquid level, the lower the output resistance.
This is a very unique sensor, adafruit haven't seen anything else that is affordable and accurate for measuring liquid level. This particular sensor is the 12" model, adafruit also include a 4-pin connector and 560 ohm resistor. The connector is so you don't have to solder directly to the delicate pins: instead, just solder to the connector and plug it onto the sensor.
Since the sensor is resistive, it is easy to read it using a microcontroller/Arduino ADC pin. Check the tutorials tab for a quick-start pointer.
Connect pin #2 of the sensor to ground, then pin #3 to a 560 ohm resistor. The other side of the 560 ohm resistor to VCC (3.3V or 5V for example) to create a resistor divider. The ADC pin connects to the point between the resistor and sensor.