Accuracy of liquid level measurement using sonar sensor Is it a good way to measure liquid level in a water tank using a general purpose sonar sensor? How can echoes from the tank walls affect measurement accuracy?

I have a cylinder shaped horizontal 700L water tank, used for emergency water supply. I need to measure the water level with approx. 5% accuracy and display it in my kitchen. 



UPDATE! Today I found this helpful page: Home Heating Hacking Part 1 or How to Measure an Oil Tank.
I had no time to realize my idea, but it's exactly what I wanted to do. So take a look.
I've implemented this on my 2 x 23,000 litre water tanks using 2x hc-sr04 sensors mounted on the tank lids and a nano in the pump house. Yes water is a good hard reflective surface to bounce the ultrasonic sound waves off. My tanks have an overflow outlet but not sure how well the eletronic will last? However they are cheap to replace. It is only early days but the overall the results are looking good.
Couple of points:

1.polling occurs hourly. 

2.I only power the ultrasonic sensor just before taking a reading.

3.Has a sound detection sensor to alert on water pump overrun, ie a leak.

good luck