Measuring Air Velocity Basics
Operating Principle
The E+E air velocity sensor is an hot film anemometer. An electrical current is increasing the temperature of a resistor on the substrate. The flowing air causes a reduction of this temperature. The cooling effect is directly proportional to the mass flow and consequently to the air velocity and inversely proportional to the air temperature. At equilibrium, the temperature of the sensor’s surface is the measure for mass flow.
For temperature compensation a second temperature sensor is
placed in the same air flow.
In the new E+E design both sensors are on the same substrate.
The sensitivity of the air velocity sensor is strongly related to the difference between the temperature of the heated element and of the flowing air.
The higher the ∆T, the higher the sensitivity.
On the other hand, the electrical power consumption
should be as low as possible.
The designer’s target is to maximize the ratio temperature difference
∆T to power consumption
.
 is the sensor’s characteristic number
"Thermal Loss Resistance", also called "Natural Thermal Coefficient".
depends on the design of the sensor, as well as on its practical installation.
Sensor Design
The most important negative phenomenon is the thermal loss through the sensor’s terminals.
Appropriate sensor geometry and careful selection of material are reducing this source of errors to
a great extent.
Substrate Material
A substrate material with very low thermal conductivity allows for higher temperature difference ∆T.
Substrate Thickness
The electrical power consumption of the sensor is directly proportional to the thickness of the
substrate.
Electrical Terminals
The electrical power consumption is inversely proportional to the distance between the heating
element
and the terminals
The design of E+E air velocity sensors:
Substrate material : glass
Substrate thickness : 0.15 mm
Stripe form, typical 2 x 10 mm, with a terminal at each end
(see also ‘Air Velocity Sensor Elements’)
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