Thermal transfer measurements are useful for measuring either a mass flow rate or the specific heat of a fluid. The ratio of the passband captured by a target detector to the passband captured by adjacent detectors is determined and used to calculate the amount of light to add back to the amount reported by the target detector. The interface circuit is implemented on the first IC, operatively couples to the reference circuit, receives the reference signal and a data input, and generates the interface signal. The upper-most deck is to supply a third type of fluid and a fourth type of fluid to the fluid-ejection printheads.