We wanted some more “objective” methods to confirm the numbers on our growing collection of anemometers, so we naturally thought about pitot tubes. This is the way that aircraft tell their airspeed. I don’t know how much they get used anymore, but the great virtue of a pitot tube is that it can be entirely mechanical, although that would of course depend on the gauge that is used to translate the pitot tube’s pressure into a number.
The pitot tube actually has two connections. The “high side” connection is exposed to the oncoming air and generates a positive pressure. The “ambient” connection, takes into account any static pressure in the system. The two lines are then read differentially, similar to a differential connections for an op-amp, so that that the output is the high side pressure minus the ambient pressure.
Isn’t this a wonderful looking set of gauges? These are called “Magnehelic” by Dwyer instruments. They are totally analog (although an electronic one is on top) and are hooked up to the pitot tube with small rubber tubes.
Here is some raw data from the Rev P Wind Sensor at 4 different temperature points.
It would be very nice if static pressure was just proportional to wind speed, but few things in life or in physics are so simple. The pressure generated is dependent on the density of the air, which makes sense if you think about it. The density in turn, is dependent on barometric pressure, temperature and humidity.
Here’s a graph that shows the temperature dependency.
So I implemented the math to correct for temperature and barometric pressure. One little hiccup was that “Absolute Temperature” was denominated in the little-used Rankine scale (F + 460). Once I entered the correct values for temperature, some wind speeds that looked very plausible came out the other end of my Excel spreadsheet, whose chart looks like this:
A couple of conclusions that I’m drawing is that at lower air speeds, the ambient correction circuit in the Rev P wind sensor is doing an admirable job, for some reason at higher wind speeds there is still some correlation between output and ambient temperature. It’s curious that the higher temperatures are reporting greater output, because normally one thinks of it taking more energy to get cold air up to temperature.
My current focus is on the ambient temperature correction circuit. The thermistor doing ambient correction is a 10k which is not by my choice. I would have desired a much higher value, but 10K was the highest value available in the thermistor line that I am using. The 10K thermistor dissipates about 3mW which is enough to raise its temperature almost 1 degree K, according to the datasheet. (Datasheet is 4mW/K). This would tend to be velocity sensitive also as at higher wind speeds the self-heating would be swamped by the power of the wind speed to enforce the ambient temperature. It’s just a hypothesis at this point.
Another hypothesis is that the “active” (heated) thermistor and the ambient sensor, just don’t track each other perfectly, leading to some variation over temperature. Indeed I would be shocked if there was no variation over temperature. I can cure the self-heating problem fairly easily, but only a lookup table will compensate for the variation in sensor response over temp, and that is the direction in which we are heading.
The new wind sensors are in the shop here: http://moderndevice.com/product/wind-sensor-rev-p/