A few years ago, we explained how to create an anemometer, but there is another solution to measure wind speed: the Pitot tube. This week, we are going to see how to measure wind speed with a Pitot tube for scaled modelling and a Yocto-0-10V-Rx.
There are many many posts which explain in details how a Pitot tube works but, to make it short, a Pitot tube (or probe) is a device made of several tubes which allow you to determine the speed of a fluid by measuring the pressure difference between two parts of the device. However, in the opposite to an anemometer, a Pitot tube allows you to measure wind speed in one direction only. Pitot tubes are often used in aeronautics to determine the relative wind speed.
The Pitot tube and the MPXV7002DP
For some time, Pitot probes for scaled models designed to be used with drone flight controllers are available. We ordered from HobbyKing the HK Pilot Analog Air Speed Sensor model, containing both the Pitot tube and the MPXV7002DP differential pressure sensor enabling you to measure the pressure difference in the Pitot tube.
The Pitot probe available from Hobbyking
The MPXV7002DP pressure sensor has three pins: two for 5V power supply and one output pin corresponding to the pressure difference. The output voltage varies between 0.5V and 4.5V for a pressure difference of -2 to 2 kPa.
Connecting the Pitot tube on the Yocto-0-10V-Rx
You can use only a Yocto-0-10V-Rx to measure the output value of a MPXV7002DP, but you must use some cunning to connect them. You cannot connect the MPXV7002DP to the Yocto-0-10V-Rx terminal, because the later provides only 23V. However, you can "steal" 5V from the USB bus. Simply solder two wires on the 1.27mm port of the Yocto-0-10V-Rx. Take care: the 5V is connected directly on the MPXV7002DP +5V pin, but the 0V must be connected to the Yocto-0-10V-Rx terminal on top of the MPXV7002DP.
Connecting the MPXV7002DP to the Yocto-0-10V-Rx
When everything is connected, you can test how the sensor is working by using the VirtualHub. With a null wind, the input of the Yocto-0-10V-Rx displays a value of about 2.5V and this value increases by blowing in the direction of the Pitot tube. If this value decreases, it simply means that you inverted the two tubes connecting the sensor to the Pitot tube.
Calibrating the Pitot tube
Now that the device is assembled, we must convert the voltage, corresponding to a pressure difference, into speed. Instead of computing the Bernoulli equation, we simply used the calibration functions of the Yocto-0-10V-Rx to estimate speed.
The Pitot tube and the Yocto-0-10V-Rx assembled
To compute the 5 calibration points, we simply fixed the Pitot tube on a car and we took a series of measures at different speeds. Note, to get pertinent measures, the Pitot probe must be far enough away from the turbulence created by the car. And to diminish the influence of the wind on that day, we performed the measures in both road directions.
The Pitot tube and the Yocto-0-10V-Rx mounted at the end of a pole
When the series of measures is performed, you must select the 5 best calibration points and save them in the Yocto-0-10V-Rx flash memory. To select the best 5 points, you can compute the sum of the errors or use our calibration tool. When you have applied and saved the calibration, you can use the Yocto-0-10V-Rx directly as an anemometer in any application working with Yoctopuce sensors.
When calibration is performed, the Yocto-0-10V-Rx publishes the speed directly
For a limited budget, you can measure the wind speed with a Pitot tube. This solution is simpler than creating your own anemometer but measures speed in only one direction.