Can you be more specific on what type of instrument signal are you dealing with? If you are using analog signal, let's say 4-20mA then you have to use an engineering formula. This is basically a ratio of the signal change and the unit of measurement you want to get, such as volume, height, or whatever else you want. It also depends how you would be interfacing the transmitter.
Hi, I asked this same question somtime ago and was directed to past responses on the same issue. Scroll through the past threads and you will find alot answers to this question. But on the other hand, if your own situation is different from mine and you want specific answer. What is the height of your tank, the specific gravity of the fluid, what type of transmitter( DP or Radar ), if it's a DP transmitter, where is it mounted on the tank?