Low Flow-Rate Thermal Flowmeter

Reprinted with permission from Rudolf Graf.
For this and over 1000 other electronic circuits, review and order Rudolf Graf's Encyclopedia of Electronic Circuits on CD-ROM Vol-1 McGraw-Hill ISBN0078528119

Fc33-1.jpg (49709 bytes)

FC33-1

Description:

This design measures the differential temperature between two sensors. Sensor T1, located before the heater resistor, assumes the fluid's temperature before it is heated by the resistor. Sensor T2 picks up the temperature rise induced into the fluid by the resistor's heating. The sensor's difference signal appears at A1's output. A2 amplifies this difference with a time constant set by the 10 M adjustment. Fig. 33-1c shows A2's output versus flow rate. The function has an inverse relationship. A3 and A4 linearize this relationship, while simultaneously providing a frequency output. A3 functions as an integrator that is biased from the LT1004 and the 338-K input resistor. Its output is compared to A2's output at A4. Large inputs from A2 force the integrator to run for a long time before A4 can increase, turning on Q1 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 A2's output voltage. Since, this voltage is inversely related to flow rate, the oscillation frequency linearly corresponds to flow rate. (Linear Technology Corp., Linear Applications Handbook, 1987, p. AN5-5.) From figure FC33-1 from Encyclopedia of Electronic Circuits Vol. 1.