Ultrasonic Anemometer Part 1 – Math Concepts

Index


With the first stage of the weather station project nearing completion I’ve decided to explore building some additional sensors to collect more readings. One of the things that I’ve been very interested in measuring is wind speed and direction. A device that measures wind speed is called an anemometer. Wind direction is usually measured with a wind vane.

There are a few problems with traditional designs of these instruments, especially with anemometers. One of the biggest problems is that they usually have a minimum wind speed threshold. If the wind is below this threshold the device will not be able to measure it. This is usually because the wind is not strong enough to spin a propeller of some kind. To overcome the problem of low speed wind measurements I’ve decided to go with a solid-state (no moving parts) alternative. This alternative involves measuring the difference in <a href="https://en have a peek at this web-site.wikipedia.org/wiki/Time_of_flight” target=”_blank”>time-of-flight (TOF) of a sound wave across a known distance.

The overall approach to measuring the wind speed using this method will be:

  1. Create a pulse of sound at point A
  2. Measure the time it takes to arrive at point B
  3. Create a pulse of sound at point B
  4. Measure the time it takes to arrive at point A
  5. Use the difference in the two times to determine wind speed

This method will determine the wind speed in along one axis. In order to get wind direction you must measure the wind speed along another axis, then use trigonometry to calculate overall speed and direction. To make these calculation easy we will measure the wind speed in the North-South direction and the East-West direction. In doing so the equation for overall speed is

Total Speed = SquareRoot(VNS2 + VEW2

where VNS is the wind speed in the North-South direction and VEW is the speed in the East-West direction. The formula is a direct application of the Pythagorean theorem. The equation for overall direction is

Angle = tan-1(VNS/VEW)

where tan-1 is the inverse tangent function. These equations are relatively simple because of the fact that we intend to make our wind speed measurements exactly 90 degrees apart. It would still be possible to calculate wind speed and angle if we had chosen another angel, but the math would not be as simple as this.

Now that the theory is out of the way, we will begin to actually implement this method in part 2 using some cool electronics and sensors.

OSHPark.com PCB Fabrication

OSHPark PCB
OSHPark Purple PCBs

It wasn’t until the last few years that ordering small-batch printed circuit boards became so affordable for the average low-budget hobbyist. Over the last few years several US and Chinese companies have sprung up that cater to the small quantity of circuit boards that a hobbyist usually needs for their projects.

One of these companies is OSH Park. OSH stands for “Open Source Hardware”. OSH Park is based in Portland, OR where members of the local maker/hobbyist scene founded Dorkbot PDX, which grew into a PCB service, and eventually OSH Park.

The nice things about OSH Park is that they have no setup cost. You pay a flat $5.00/sq in. for your circuit boards. They send you three copies of the finished product with free shipping about two weeks after you submit the order. The result is a nice quality purple PCB, with gold plated pads.

I’ve been using them both at work for cheap prototyping, and at home for my own personal projects. This particular board is part of a weather station kit that I’m working on. More info will follow when it’s closer to being complete.

Calibrating Test Equipment

<img class="wp-image-148" src="http://n1qq.com/wordpress/wp-content/uploads/2015/10/simpson_260-3a_02-300×154.jpg" alt="Simpson 260 Multimeter" width="351" height="180" srcset="http://n1qq.com/wordpress/wp-content/uploads/2015/10/simpson_260-3a_02-300×154.jpg 300w, http://n1qq.com/wordpress/wp-content/uploads/2015/10/simpson_260-3a_02-720×369.jpg 720w, http://n1qq.com/wordpress/wp-content/uploads/2015/10/simpson_260-3a_02-224×115 generic lexapro.jpg 224w, http://n1qq.com/wordpress/wp-content/uploads/2015/10/simpson_260-3a_02.jpg 741w” sizes=”(max-width: 351px) 100vw, 351px” />
Simpson 260 – A classic multimeter

For the average hobbyist equipment calibration is not something that would be of any great importance. However, if you do commercial electronics it is usually a requirement that you have your test equipment professionally calibrated. This usually means your multimeter and other test and measurement equipment needs to be sent to a calibration facility on a regular basis. Usually once a year, give or take.

Different types of projects will demand different levels of calibration. However, this is an interesting topic. In my work experience I have often seen requirements that calibrated equipment be used for testing devices. However, tolerance is not always given in these requirements. A person could use a meter calibrated to within 50% and (I suppose) would meet the requirement, even though 50% is a laughable tolerance.

Costs will vary depending on the equipment being calibrated, level of precision, and turnaround time needed. For a simple multimeter (volt-ohm-amp) expect to pay around $50 for a simple calibration.

My usual experience with having calibration done on equipment involves sending a meter to a calibration lab. They usually send it back with a new sticker on it, but don’t make any adjustments. This is because modern meters are remarkably stable. Almost always the meters are still well within tolerance levels when I send them off for calibration, so they do not require any adjustment.

In the end the average ham radio operator, or amateur electronics hobbyist will not likely have a need to spend the money to have their equipment calibrated. However, it is not prohibitively expensive, and there is certainly value in knowing that your test equipment is giving accurate measurements.

APRS

APRS (automatics packet reporting system) is a digital system designed to pass location and other telemetry and information between radio stations. Tom Hayward shared his experience with this system during the evergreen intertie youth net. Recorded (I believe) sometime in 2008.

Tom is also an avid mountaineer, and participates in search and rescue operations around the pacific northwest.

Moonbounce Communications

MoonBack when I was in high school I used to coordinate a weekly on-the-air meetup of younger ham radio enthusiasts. We called it the Evergreen Intertie Youthnet.

It was a fun group of people, and I remember having some interesting conversations about all kinds of things relating to ham radio. Even though the main focus of the net was to discuss things that young ham radio operators would be interested we would usually have hams of all ages participate.

Every now and then we would have a ‘special feature’ where a guest would share a little bit about a project, or something to do with ham radio. I just happened to record this one of John, W7OE, talking about earth-moon-earth communications. This was recorded some time in 2008 I believe.