In addition to the suggestion Wally made, another possibility is that you have a blanket pressure in the tank that is not being controlled properly, which in the application you describe should be a constant pressure. You mentioned that the tanks are insulated, which would suggest that these are closed tanks. When you use a GP transmitter to measure the level in the tank, any fluctuation in the pressure inside a tank would be seen as a change in level inside the tank. We would typically measure a closed tank with a DP transmitter, versus a GP transmitter (as indicated by the G in the model code). Using a DP transmitter allows accurate measurement of the level in the tank regardless of any fluctuations in the blanket pressure.