Hallo Farshid,

currently i am searching for their datasheets.

Have a look at here:

http://www.vishay.com/docs/88301/15ke.pdf

Sometimes this transzorb is written '1.5KE6.8A', with points instead of comma. Maybe you will find it now. Have also a look at www.vishay.com. There are a number of other transzorbs that can be used, for instance BZW04.

...and i think they will limit the maximum voltage and so the seried resistor should be greater than 450 ohms to read the sensor correctly .am i right on this ?

If you keep the voltage drop across tank sensor (which is a rather low resistance, I assume) less than 0.2V or so, no relevant current will flow through the diodes and degrade the reading.

After reading all of the info i feel a bit scary now ! and i will be more carfuel with it.

That's what I wanted to make you...

Good luck,
Kai