If you are still interested, let me know and I'll go and experiment with it in the kettle

Think in a kettle you have a rising very high humidity (100%) and rising temperature:

- affects electronics as posed earlier
- which changes the speed of sound through the air (lets do some research)

**Enviromental Effects on the Speed of Sound**
Source - ~~http://www.rane.com/pdf/ranenotes/Enviromental%20Effects%20on%20the%20Speed%20of%20Sound.pdf - http://bit.ly/omGpzb -~~
- http://www.rane.com/pdf/ranenotes/Enviromental_Effects_on_the_Speed_of_Sound.pdf -

[u]Temperature effect[/u]
float sos = 331.45 * sqrt(1 + t/273); // t in Kelvin

This means sos == 331 at 0 Celcius and 385 at 100 Celcius. An increase of 17% !!
(on a hot day 30C the change is allready 5%)

[u]Humidity effect[/u]
Humidity is a tougher job as the effect of humidity on the speed of sound is temperature dependant.
However the effect is substantial smaller than temperature for the range 0..40C, however looking at the numbers in the 100% RH colums I see an exponential pattern.
at 100%RH every 10 C increase, give an ~1.82 times higher number.
furthermore if the temperature is constant the effect is almost linear with the RH

increasePercentage = RH * 0.11 * power(1.82, (t/10)); // t in Celsius; RH in %

For 100C the increase is 45% !! so more than
==>
increaseFactor = (100 + RH * 0.11 * power(1.82, ((t-273)/10))); // t in Kelvin, RH in %

[u]combined[/u]
float sos = 331.45 * sqrt(1 + t/273) * (100 + RH * 0.11 * power(1.82, ((t-273)/10))); // t in Kelvin

This formula is based upon the [u]assumption[/u] that the exponential pattern goes on until 100C

[u]If this assumption is correct[/u] the cummulative effect 100C and 100% RH
==> 1.17 *1.45 = 1.69 so almost 70% increase in speed of sound from 331 @ 0C,0%RH to 560 @100C,100%RH

**conclusion**
Using an ultrasound sensor for measuring the waterlevel is both temperature and humidity dependant.
Depending on the ranges the speed of sound increases with a factor up to 1.7. (under above assumptions)
That means that an uncorrected measurement will show up too short with this same factor.
In practice this could mean that an almost empty tank could appear half full.

*disclaimer: there might be some mistakes in my math as it is getting pretty late here ;*

fun!