The main use of anemometers and how to use them
1. Measure the velocity and direction of the mean flow.
2. Measure the pulsation velocity of the incoming flow and its spectrum.
3. Measure the Reynolds stress in the turbulent flow and the velocity correlation and time correlation between the two points.
4. Measurement of wall shear stress (usually carried out using a hot film probe placed flush with the wall, similar to the principle of hot-wire velocimetry).
5. Measurement of fluid temperature (measured beforehand probe resistance with the fluid temperature change curve, and then according to the measured resistance of the probe can determine the temperature.
In addition to this, many other specialised applications have been developed.
1. Observe whether the pointer of the meter is pointing to zero point before use, if there is any shift, you can gently adjust the mechanical adjustment screw of the meter to make the pointer return to zero point;
2. Place the calibration switch in the off position.
3. Plug the measuring rod into the socket, place the rod vertically upwards, press the screw plug tightly to make the probe seal, the "calibration switch" is placed in the fullness position, and slowly adjust the "fullness adjustment" knob, so that the meter pointer points to the fullness position;
4. "Calibration switch" is placed in the "zero position", slowly adjust the "coarse", "fine" two knobs, so that the meter pointer in the zero position. So that the meter pointer in the zero position
5. After the above steps, gently pull the screw plug, so that the rod probe exposed (length can be selected according to need), and so that the red dot on the probe face to face with the wind direction, according to the meter readings, access to the calibration curve, you can find out the measured wind speed;
6. In the determination of a number of points (10min or so), must repeat the above 3, 4 steps once, so that the current in the instrument to be standardised
7. After the measurement, the "calibration switch" should be placed in the off position.
Anemometer will flow velocity signal into an electrical signal of a speed instrument, but also can measure the fluid temperature or density. The principle is to place an electrically heated thin metal wire (called hot wire) in the airflow, the heat dissipation of the hot wire in the airflow is related to the flow rate, and the heat dissipation leads to changes in the temperature of the hot wire and causes changes in the electrical resistance, and the flow rate signal is converted into an electrical signal. It has two working modes: ① constant current type. The current through the hot line remains constant, when the temperature changes, the resistance of the hot line changes, and thus the voltage at both ends changes, thus measuring the flow rate; ② constant temperature type. The temperature of the hot wire is kept constant, for example, 150℃, and the flow rate can be measured according to the current to be applied. Constant temperature type is more widely used than constant current type. Hot wire length is generally in the range of 0.5 ~ 2 mm, diameter in the range of 1 ~ 10 microns, the material is platinum, tungsten or platinum-rhodium alloy. If a very thin (thickness of less than 0.1 micron) metal film instead of wire, that is, the thermal film anemometer, function and hot wire similar, but more for measuring liquid flow rate. Hot wire in addition to the ordinary single-wire type, can also be a combination of two-wire type or three-wire type, to measure the velocity component in all directions. Electrical signals from the hot line output, amplified, compensated and digitised into the computer, can improve the measurement accuracy, automatically complete the data post-processing process, expanding the function of velocity measurement, such as the simultaneous completion of the instantaneous value and the time-averaged value, the combined velocity and the velocity, the turbulence and other turbulence parameters of the measurement. Compared with the Pitot tube, the hot-wire anemometer has the advantages of small probe volume, small interference to the flow field; fast response, can measure non-constant flow velocity; can measure very low speed (e.g., as low as 0.3 m/sec), and so on.
