Instrumentation: Pitot Tube Static Anemometer, Part 1

November 21, 2009, 5:33:50 AM EST

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Today kicks off a series of entries that I will do over time (not necessarily on consecutive days) highlighting the instrumentation we use at the Observatory. Some of the instrumentation I will talk about may already be familiar to you, but I guarantee that a lot of it will not.

The instrument I am highlighting today is one I am 100% sure you have never seen. I talk a lot about the winds that we experience up here, and that is probably what Mount Washington is most famous for. So, I figured it would be appropriate to begin this series by talking about the instrument that we use most often to measure winds up here. I am actually going to break up the discussion on this instrument into two parts since it is a complicated system. The first part today will be about the instrument itself. The second part will be about how we actually get a wind speed from the instrument.

This particular instrument is very unique; it was designed specifically for us, by us. It is so unique in fact, that you will not see it used anywhere else but on Mount Washington. It is called the Pitot Tube Static Anemometer, and is pictured below (my apologies our wireless users):


The only piece of off the shelf equipment used in the making of the anemometer that you see pictured is the pitot tube (WIKI) itself. Pitot (pronounced pee-toe) tubes are more commonly used on aircraft to measure their airspeed. In that application the tube is flying through the air. Here on Mount Washington, the air flies through us.

A pitot tube, including the one in use on our anemometer, works by measuring a differential pressure. Basically, the tube faces into the wind, hence the need for the tail you see in the picture, and the wind blows into a hole on the front of the tube. This creates a higher pressure than the air around the instrument, being measured through small holes on the side of the tube. Here are two pictures; the first shows the hole on the front of the tube to measure the pressure of the wind (the dynamic pressure), and the second shows the small holes on the side measuring the pressure of the air around the tube (the static pressure).



Here is how I explain it to kids: it is just like blowing into a straw in a glass of milk. The level of the milk in the glass will rise because of the difference in pressure you are creating by blowing into the straw. It is the same principle that the pitot tube works on.

To wrap part 1 up, I want to highlight the main reasons we have chosen to use such an instrument to measure wind speed:

1. It records high wind speeds accurately. Considering the frequency that we see high winds on Mount Washington, this is a big reason we use this instrument. Remembe, a pitot tube is usually used to measure the speed of an aircraft traveling at speeds of several hundred miles per hour.

2. We can heat it. Just like when it is used on an airplane, we heat the pitot tube, as well as the rest of the body of the anemometer. It is heated to somewhere around 100 degrees to try to keep rime ice from forming on it.

Tomorrow I will talk about the rest of the Pitot Tube Static Anemometer system, and how we get a wind speed from the instrument and the differential pressure it creates.

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