Best way to determine if a 5 gallon water jug is getting empty

Using a Force Sensor to Measure a Quantity of Water

To satisfy your requirement of having the sensor on the exterior of the water container, I would recommend using a force sensor to weigh the bottle at set intervals. A force sensor, such as the FlexiForce A401, is a type of resistor that changes resistance in response to the amount of force applied. While not as accurate as a scale, it should provide enough accuracy (+-3% according to the datasheet) for the application described.

The basic procedure would be to hook up the force sensor to the Arduino and first measure the resistance with an empty bottle, then measure again with a full bottle. This will give you an idea of the force that your specific platform is actually applying to the sensor, and you can then fine tune the voltage (as per the datasheet) for your installation.

A basic tutorial using a similar force sensor with an Arduino can be found here.

Other Possibilities

• Optical Liquid Level Sensors
• Ultrasonic Level Monitoring
• Thermistors and Heating Coils at various levels to Measure the Thermal Mass (i.e. water) near the Sensor

Another option is to measure the current going into the pump; when it runs dry, the current goes up, which generates the heat that fries the pump.

Or measure the temperature on the pump, but measuring the current will detect the beginning of the pump's meltdown, while the temperature may only detect the middle or end of the meltdown. :-(

Or install a current limiter or breaker inline with the pump, just in case everything else fails. :-)

I think you want a "liquid level switch", possibly also known as a "fluid switch" ... I realize this may not satisfy your requirements, but this is one way to solve the problem for >$10. I would probably hack something together that would put this sensor inline to the bottle. it's probably too hacky, but you could have a disc of plywood the same diameter as the water bottle, with a pipe fitting in the middle where you could mount the switch, so that the water bottle routes water through it so that when it runs low, it would trigger the switch before it runs to the pump.