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9.2 Thermal Flow Sensors 219

9.2.1 Research Devices

9.2.1.1 Anemometers (Heat Loss)
Anemometers consist generally of a single element, which is heated, and the influ-
ence of the fluid flow on that very element is measured [Figure 9.5(a)]. Hot wire or
hot film anemometers have very fast response times due to their small thermal mass,
but they are not bidirectional. They are operated generally in:

• Constant power mode: In the constant power mode, heat is dissipated from
the resistor element into the fluid flow, and the resulting temperature of the
resistor is a measure for that flow. With increasing fluid flow, the temperature
of the element decreases.
• Constant temperature mode: The temperature of the heater is directly meas-
ured and kept constant above ambient temperature. The electrical power
needed to maintain a constant temperature is a measure of the flow. In this
mode, the flow sensor is very fast, but an additional control system is
necessary.
• Temperature balance mode: (Recently proposed by Lammerink et al. [63].) In
this concept, the temperature difference between two anemometers (up- and
downstream) is kept constant at zero. This is done by a controlled distribution
of a constant total heating power. The ratio between the up- and downstream
heating power is a measure of the fluid flow. The absolute temperature will
not be constant. At constant total power, the average temperate of the up- and
downstream sensors will decrease with increasing flow velocity. However, the
concept allows nonlinear temperature sensor transfer function as long as it is
symmetrical for the two sensors. As it is a balance measurement, the tempera-
ture sensor pair should only indicate if the temperature difference is smaller
than, equal to, or larger than zero. An advantage of this operating principle is
that the system output does not depend on the sensitivity of the sensor. Hence,
highly sensitive metal/semiconductor thermopiles, which are strongly nonlin-
ear but with good symmetry, can be used.

Hot wire anemometers have a limited lower range of measurement due to the
convection caused by the heat out of the wire. They are sensitive to contamination
and therefore need calibration at certain intervals, or they can be damaged by parti-
cles. They are kept very thin to achieve fast response time, but at the same time they
become fragile. It is important to have a temperature reference resistor in order to
make compensation for fluctuations in fluid temperature.
Stemme [55] reported a gas flow sensor where the sensing area was thermally
isolated from the silicon body via a polyimide trench [Figure 9.6(a)]. A different
anemometer setup is used by Wu et al. [44, 45]. The sensor uses a boron-doped
polysilicon thin-film heater that is embedded in the silicon nitride wall of a micro-
channel, which is formed by surface micromachining [Figure 9.6(b)]. Three sensor
designs have been studied to obtain the best sensitivity: (1) the polysilicon heater
–3
18
19
–3
boron doped at a concentration of 2 × 10 cm ; (2) 2 × 10 cm , to increase the
temperature coefficient of resistance; and (3) the channel suspended to improve the
thermal isolation. As a result, the relative sensitivities for (1), (2), and (3) are 8, 40,
and 180 ppm/(nl/min), respectively. This shows that the less doped (higher TCR),