Abstract: A system including a first element to provide a first parameter value that varies with a first temperature of fluid adjacent the first element, a second element to provide a second parameter value, the first element connected to the second element at an output such that output signals at the output correspond to differences between the first parameter value and the second parameter value, a heater element situated between the first element and the second element to provide heat to the fluid, and a circuit to evaluate the output signals at the output and a threshold value that corresponds to a fluid flow set point and to determine whether flow of the fluid, in at least one direction, across the first element, the heater element, and the second element exceeds the fluid flow set point.