Anybody still interested in this topic?
I've started work on it for a different reason, I've now got two natty display units and two dead sensors! The ultrasonic sensor appears to corrode after a year or two and stop working, so after fruitlessly searching for a replacement sensor, I decided to construct my own using a waterproof ultrasonic module (typically used for car reversing sensors), thermistor using a nano and a 433MHz transmitter.
So far, using the data from this thread (thanks Kinotech for the decoding!) I have successfully been able to transmit any temperature I like, but I have not been able to decode the depth sensor data, such that I can have the display show the value I want.
I think the sensor is sending the tank depth in metres * 100, but I have not been able to figure out the 8 nibbles used to send this data.
Here is some data I've extracted from the thread - If i send it exactly as-is the display reports the correct values.
Common data: Tank Depth (TD):1.2m, Airgap:0.51m
The sensor should be sending raw distance (D), so when the tank is full it will be sending 0.51m, when the tank is empty 1.2m The display unit shows water depth (WD), so it will be showing WD=TD-D = 0.69m when full and 0m when empty.
Data from Senor (extracted from previous posts and .wav files, this will need bitswapping etc per earlier posts)
T=21.8 WD=0.69 (D=.51)
010111110001110010001000010000001011000001010000011001000000000000001111
T=21.9 WD=0.69 (D=.51)
010111110001110010001000010000001011000011010000011001000000000010001111
T=22.5 WD=0.69 (D=.51)
010111110001110010001000010000001011000010000000111001000000000001101111
T=23.8 WD=0.46 (D=.74)
010111110001110010001000001000000101000001110000111001000000000000101111
T=23.6 WD=0.0 (D=1.2)
010111110001110010001000000100001000000000110000111001000000000010001111