handbook of modern sensors

366 11 Flow Sensors
(A)
(B)
Fig. 11.6. Bridge circuit for a thermal flowmeter (A); sensor responses for different fluids (B).
whereas the temperature differential (T s − T 0 ) is used as the output signal. In the
second method, the temperature differential is maintained constant by a control circuit
which regulates the heater’s voltage e. In the latter case, e is the output signal.
This method is often preferable for use in the miniature sensors where self-heating
temperature detectors are employed. A self-heating sensor [it can be either a resistive
temperature detector (RTD) or thermistor] operates at high excitation currents. That
current serves two purposes: It measures the resistance of a detector to determine
its temperature and it provides Joule heat. Figure 11.6A shows that both temperature
detectors (heated and reference) can be connected in a bridge circuit. At very low
flow velocities, the bridge is imbalanced and the output signal is high. When the flow
rate increases, the heated detector cools down and its temperature comes closer to
that of a reference detector, lowering the output voltage. Figure 11.6B illustrates that
the sensor’s response is different for various fluids and gases. A sensor manufacturer
usually provides calibration curves for any particular medium; however, whenever
precision measurement is required, on-site calibration is recommended.
For accurate temperature measurements in a flowmeter, any type of temperature
detector can be used: resistive, semiconductor, optical, and so forth (Chapter 17).
Currently, however, the majority of manufacturers use resistive sensors. In industry
and scientific measurements, RTDs are the prime choice, as they assure higher linearity,
predictable response, and long-term stability over broader temperature ranges. In
medicine, thermistors are often preferred because of their higher sensitivity. Whenever
a resistive temperature sensor is employed, especially for a remote sensing, a
four-wire measurement technique should be seriously considered. The technique is a
solution for a problem arising from a finite resistance of connecting wires which may
be a substantial source of error, especially with low-resistance-temperature sensors
like RTDs. See Section 5.8.2 of Chapter 5 for the description of a four-wire method.