Thermoelectric anemometer
The thermoelectric anemometer is a meteorological measuring instrument.
Basically, a two-dimensional thermoelectric anemometer determines wind direction and speed in a similar way to our finger, which we hold in the wind. The finger cools down due to the heat dissipation of the air flowing past, whereby the strength of the cooling can give a subjective indication of the wind speed. The finger side, which cools down more, shows us roughly where the wind is coming from. A thermoelectric anemometer makes this objective and much more precise.
The measuring method usually used in a thermoelectric anemometer is also known as hot wire air mass measurement, hot film (air) mass measurement HFM, hot wire anemometry or hot film anemometry and was used, for example, in almost all diesel engines to measure intake air.
convection
The hot film anemometry uses the cooling that the surface of a warm body experiences in a colder flowing medium as a measuring effect.
In a thermoelectric anemometer, this warm body is a tube and the medium flowing past is normal air. The heat transfer that takes place from the warm tube to the air is called forced convection. Convection is always associated with the transport of particles (e.g. air molecules). With forced convection, the particles are transported by wind or other external influences. In contrast to free or natural convection, in which the air warms up on the tube and rises by itself due to the difference in density between warm and cold air. Together with forced convection, there is always a small proportion of free convection, which, however, can be neglected. The amount of heat transported away by forced convection essentially depends on the variable parameters of flow velocity, pressure (density) and the temperature difference between the body (tube) and the medium (air).
Measurand
In a hot-film anemometer or hot-wire anemometer, a current-carrying sensor surface or a wire is used as a probe and heated electrically. The materials used for the probe have a temperature-dependent resistance that increases as the temperature rises ( PTC resistance ). The electrically supplied heat output is partially transported away by a flow as heat dissipation. As the flow velocity increases, so does the heat loss. The electrical power can therefore be used as a measured variable for determining the flow velocity.
Two methods have become established for measuring the heat loss due to flow: the constant current anemometry (CCA - Constant Current Anemometry) and the constant temperature anemometry (CTA - Constant Temperature Anemometry).
Constant Current Anemometry (CCA)
With constant current anemometry, the heating current is kept constant and the voltage drop across the resistor is measured. The measured voltage is proportional to the temperature of the probe. As the flow rate increases, the probe cools down further and the measured voltage decreases. A constant current anemometer has a simple structure and is well suited for measuring small flow velocities; however, the sensitivity decreases sharply with increasing flow velocity.
Constant temperature anemometry (CTA)
The thermoelectric anemometer uses the method of constant temperature anemometry and works with an excess temperature of the sensor surface that is kept constant compared to the ambient temperature. In order to be able to set a constant overtemperature, the ambient temperature must also be measured. If the flow speed increases, the current is increased until the heat loss caused by the flow is compensated and the temperature of the sensor surface is again constantly above the ambient temperature.
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With the measurement of the ambient temperature to control the excess temperature, the temperature dependency of the convection is already compensated and the measured electrical current is a measure of the air mass flow. If the air pressure and thus the air density are also measured, the flow speed can be calculated from the air mass flow. Fig. 1 shows the control circuit of a constant temperature anemometer schematically. The implementation of a constant temperature anemometer is technically more complex, but enables sensitive and precise flow measurement over a large measuring range.
Direction measurement
For simple flow measurements, hot-film anemometers usually work with just one heated resistor and one temperature sensor. A direction measurement is not possible with such a hot film anemometer shown schematically in Fig. 2.
The cooling behavior of a sensor surface or a wire, however, depends on the angle of attack. As shown in Fig. 3, the heat loss is greatest when the flow hits the surface at a right angle .
The thermoelectric anemometer uses this effect. The sensor works with a sensor element that consists of 4 individual sensor surfaces that are evenly arranged on the jacket of a pipe. If this sensor element, shown in top view in Fig. 4, is exposed to a flow, each sensor surface experiences a different heat loss. The flow direction can be determined from the ratio of the energy fed to the respective sensor surfaces. The sum of the tracked energy of all sensor surfaces is the measure of the flow velocity, taking into account the air pressure.
The schematic structure of a 2D hot-film anemometer shown in Fig. 4 corresponds to that of the thermoelectric anemometer, which has an integrated air pressure sensor. This means that the wind measurement of the anemometer is independent of the altitude and the prevailing air pressure.
Summary
With the thermoelectric anemometer, four surface segments of a tube are heated and their cooling behavior is measured in a flow. The total cooling is a measure of the wind speed and the distribution of the temperature on the tube surface is a measure of the wind direction. The influence of the ambient temperature and the air pressure on the measurement of the flow velocity is compensated by integrated sensors, so a universally applicable sensor has been created, which offers a very high measurement accuracy and measurement dynamics at a favorable price-performance ratio.