I am currently working on a project that involves recording the amount of automotive coolant in an overflow reservoir (essentially an open container) to display the level. The reservoir is a cylinder 1 foot tall and 2 inches in diameter, and I would like either continuous measurement or an accuracy or at least 0.5 inches. I'm having a little trouble deciding what type of sensor to use.

I think the easiest sensor to use would be an ultrasonic distance sensor. The problem here is that hot coolant reaches 200 f, so the ambient temperature and therefore the speed of sound in the container may be significantly different at times. So, this solution would need an additional air temperature sensor to compensate.

Other ideas I've come up with include:

• Adafruit's e-tape: temperature max is too low

• An assortment of optical liquid level switches at various heights: gets expensive very quickly

• Radar distance sensor: usually useful for sensing things in the range of 1-20 feet due to the sensor's blind

• Pressure sensor at the bottom of the tank: again is sensitive to temperature, and probably more complicated than ultrasonic

• Continuous float sensor: too large for my reservoir (only about 2 inches in diameter)

Are there any other ideas that I've overlooked? And is an ultrasonic sensor with temperature correction the most reasonable?