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Instrumentation: Pitot Tube Static Anemometer, Part 2

November 21, 2009, 5:45:10 AM EST

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I have been talking about corrected vs. uncorrected wind gusts over the last couple days. Before I elaborate more on that, I need to finish my discussion on the Pitot Tube Static Anemometer, otherwise some things will not make sense to you. First of all, if you missed part 1 of this discussion or need to review it, go here. Part 1 focused on the instrument itself, part 2 today will focus on the rest of the system and how we use the instrument to get a wind speed.

So running from the instrument into the Weather Room (what we call the room we do our work in) are a set of two copper tubes. One tube is measuring the total pressure, which is the pressure of the wind blowing into the front of the pitot tube. The second tube is measuring the static pressure, which is the pressure of the air around the instrument, measured through the small ports on the side of the pitot tube. Below is a picture of what the network of tubing looks like when it comes in behind one of the walls in the Weather Room we have appropriately dubbed, the Weather Wall.


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Behind this wall, the tubes connect to several devices that can measure the differential pressure created by the pitot tube and through calibrations and calculations we can get a wind speed. The first of these devices that I want to point out is also the simplest. It is the manometer, pictured below.


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I mentioned in part 1 that I tell kids that this system is basically like blowing into a straw in a glass of milk; the manometer is the equivalent of the straw. When the wind speed increases, a column of red water in the tube on the manometer rises up. When the wind lulls, it falls back down. The numbers on the side are how we measure the differential pressure, in inches of displaced water column, or inches of water for short. Here is a closeup of the scale that I am talking about. You can also see the red column of water in this picture, measuring a value in this case of 0.6 inches of water.


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The next device is the one that we use most often for our observations and is also the device you will see reporting our wind speed on our website most often. It is called the Setra pressure transducer, and is pictured below.


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This device measures the differential pressure created by the pitot and turns it into an electrical voltage. The key thing with this device is that this voltage can then be sent to a computer and through a calculation, be turned into a wind speed. This device is the one we use the most often because it is the most precise and accurate way to calculate the wind speed since it is an electronic device that outputs a digital signal and is capable of very high resolution data.The next device is the Hays chart. This is a much older piece of equipment and is strictly analog, meaning there is no digital output from this device to a computer. The Hays has been around the Observatory for a long time, dating back to the late 50s. It graphs wind speed on a circular piece of paper through the course of one day, from midnight to midnight. Here is a picture of the chart in action:


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The red line you see is the graph of wind speed. There is a pen on the left side of the graph that is drawing that line. Basically, when the wind speed is higher, therefore creating a higher differential pressure, the pen is pushed away from the center of the graph and in turn draws the red line further away from the center of the graph. The red line will be thicker if the wind is gustier, since the pen will fluctuate more rapidly toward and away from the center of the graph. Here is a closeup of the chart and the pen that I am talking about:


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The Hays chart reads an inches of water measurement just like the manometer. It is calibrated to graph wind speeds up to 140 mph, or around 9 inches of water. For wind speeds above that, we use the Barton chart. It works exactly the same as the Hays, it is just calibrated differently and is able to measure to 30 inches of water, or about 270 mph. Here is a picture of the Barton chart:


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Keep in mind that our Static Pitot Tube Anemometer does not in itself directly measure the speed of the wind. It simply measures the force that the wind exerts, which is what the inches of water measurement represents. This inches of water measurement can then be turned into a wind speed using certain equations and calculations. This will be important to remember for tomorrow when I talk about how and why we correct big wind gusts, like the one from last Friday, for temperature and pressure.

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