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Construct and test an anemometer.

Extracts from this document...

Introduction

Physics Coursework – Toby Parnell

Instrumentation task - Anemometer

Planning

I have chosen to construct and test an anemometer. An anemometer is a sensor that is able to measure wind speed. Some anemometers measure wind speed and temperature, which I had initially planned to do. I decided this would be too complicated and not viable with the time and resources available.

Background information:

An anemometer is a type of flow sensor; flow sensors are used to measure the rate of flow for many applications for example in chemical industries to measure the rate of flow of liquid or gas in a pipe. An anemometer is specific to measuring wind speed, or the speed of the movement of air. The type of anemometer that I have constructed does not depend on a certain wind direction so in essence it is just a device for measuring the speed of the wind, and not the velocity. This is because no directional information is gained from the results.

An anemometer works by ‘catching’ the wind in ‘cups’ that are attached to a central axis. The wind causes the cups to move around the axis at varying speeds, which is dependent upon the speed of the wind. Complex sensors could be added to the wind-catching ‘cups’ to sense at what point the cups are accelerating the most, but as the cups are all attached to the same axis, they would all accelerate. Instead, the sensors could be added to cups to detect which cups are catching most wind, enabling wind direction to be calculated. Again this is far too complicated with respect to the available resources. There were several ways that I devised to measure the wind speed and temperature before deciding upon a final solution. These are listed below.image00.jpg

To measure wind speed:

...read more.

Middle

To overcome this I used a stroboscope, a device which creates a constant flashing beam, whose rate can be altered. The device was able to produce 200 flashes per minute, to over 2000 flashes per minute. Necessary for my calibration was around 200 to around 500 flashes per minute. Care had to be taken as a stroboscope can trigger fits of epilepsy, as it produces high frequency flashing lights.

To record the rpm of the cups, I had to use a constant and even wind source; for this I used a fan. I then placed the anemometer with the motor attached to a multimeter into the wind stream. I then adjusted the position of the anemometer in relation to the wind source to achieve a constant potential difference created by the motor. Once a constant voltage was being produced, I could then adjust the stroboscope to produce more or less flashes per minute, until the cups appeared frozen in motion. To make this easier the practical was done in a dark room so the stroboscope was the only source of light. Once the stroboscope was set so the cups appeared frozen, I counted the number of flashes per minute produced by the stroboscope.

To do this, I used a stopwatch and counted 20, or 40 flashes depending on the rate of flashes and measured the amount of time it took for the stroboscope to produce the recorded number of flashes.


During the practical, each cup was sighted in one revolution when the cups appeared frozen. It was possible to show this as one cup had a black mark on it.  This cup appeared every third ‘frozen cup’. To calculate rpm for a given potential difference the following calculation had to be followed:image11.jpg

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Conclusion

For this same reason, the sensor’s resolution was limited. In conjunction with a high level of random variation caused by the multimeter’s insensitivity in measuring such a small amount of potential difference, the smallest degree of potential difference that I could accurately measure was 10 mV; this is ten, one thousandths of one volt. Therefore the resolution of the sensor is around 0.15 ms-1; this is roughly the wind speed calculated from the calibration results for 10mV. This is irrelevant because of the fact that results can be calculated to 2 decimal places, as I can only be sure of results to the nearest 0.15 ms-1 due to the sensor’s relatively large resolution. In comparison, the datum can accurately measure to 2 decimal places, e.g. 2.42 ms-1.

I was able to detect and explain the systematic error due to the fact that my sensor was relatively inaccurate and I had access to a much more accurate sensor designed to measure the same thing. The use of a datum enabled me to effectively analyse my results. Overall to create a more successful sensor, I would need to review the complexity of this sensor and devise a method that reduces the margin for error as the current design encompasses too many opportunities for the results to be affected.

Bibliography

www.uq.edu.au – Picture of Reed switch

Advancing Physics AS – Institute of Physics

Lonsdale Science Revision guide – The essentials of OCR science double award.

64398.doc

Toby Parnell

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