This design measures the differential temperature between two sensors. Sensor Tl, located before the heater resistor, assumes the fluid`s temperature before it is heated by the resistor. Sensor TZ picks up the temperature rise induced into the fluid by the resistor`s heating. The sensor`s difference signal appears at Al`s output. A2 amplifies this difference with a time constant set by the 10 MO adjustment. Fig. 33-lc shows AZ`s nutput versus flow rate. The function has an inverse relationship. A3 and A41inearize this relationship, while simultaneously providing a frequency output.
A3 functions as an integrator that is biased from the LT1004 and the 338-KO input resistor. Its output is compared to AZ"s output at A4. Large inputs from A2 force the integrator to run for a long time beforeA4 can increase, turning on Ql and resetting A3. For small inputs from A2, A3 does not have to integrate long before resetting action occurs. Thus, the configuration oscillates at a frequency which is inversely proportional to AZ"s output voltage. Since this voltage is inversely related to flow rate, the oscillation frequency linearly corresponds to flow rate.