Davis Instruments Anemometer

ljbeng · Joined: 10 Feb 2004 Posts: 205

Has anyone used a Davis Instruments Anemometer (not sure part number) with a PIC A/D? The wind speed is simple. The wind direction uses a non-linear pot. (Anyone know why?) I use a 4.7K pullup to +5 so I only get a range of 0-828 A/D reading. The Wind direction pot is ~20K pot. Here are some reading:

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

I do not know the anemometer in question but..

How have you connected it?

Have you connected it as a simple voltage divider with the wiper going to the A/D input, one end of the pot going to ground and the other end of the pot going to the 4.7K resistor?

Why did you use the resistor?

When testing the anemometer did you test for a dead band around 0? If so how wide is the dead band?
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Guest · Posted: Tue May 06, 2008 9:48 am

Hi,

First, some terminology, an 'anemometer' typically refers to an instrument that measures wind velocity, not wind direction.

You are apparently referring to the Davis wind direction sensor which consists of a 360 degree potentiometer connected to a wind vane. The pot is certainly linear, and you should verify that with an ohm meter (not the PIC A/D) as you rotate the vane. Connect the meter between one end of the pot, and the wiper. It's been a long since I played with this item, so I don't remember the value of the pot. If you tie the low side of the pot to GND, and the high side to +5V, the wiper will vary from 0 to 5V as the vane rotates in the wind. You should be able to connect the wiper directly to the A/D input of the PIC, meaning that the A/D input impedance is high enough that it's not going to effect the operation of the pot.

The anemometer portion of the sensor is a reed switch that closes for each revolution of the wind speed cups. You can use the PIC to measure the closure frequency, and then you can calculate wind speed. I calibrated my unit by mounting the assembly to the roof of a car while moving at known speeds under no wind conditions.

Hope this helps!

Doug

ljbeng · Joined: 10 Feb 2004 Posts: 205

I can only connect to the wiper and one end of the pot in the anemometer so I used a pullup to 5v so I can convert between 0 and 4v dc. I graphed it out with better detail and the pot definately has an audio taper to it. I am using a lookup table for now.

Does anyone have a good program to enter in X,Y coordinates and let the PC derive a quadratic equation that will follow the data points? I used to have an old DOS program that did this.

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

ljbeng · Joined: 10 Feb 2004 Posts: 205

My screw up, I have access to high,low and wiper.... Sorry.

dyeatman · Joined: 06 Sep 2003 Posts: 1924 Location: Norman, OK

Here is some detail about how the Davis unit works:

http://www.emesystems.com/OL2wind.htm

inservi · Joined: 13 May 2007 Posts: 128

Hello,

dyeatman,

When i read the 'detail about how the Davis unit works', I'm surprised because you look to consider than the relation between wind speed and anemometer rotation speed is linear.

mph = 2.25 * Hertz
mps = 1.006 * Hertz

I'm sure that it's not true, it is a logarithm relation. You need either a table or a function to calculate the wind speed from the anemometer rotation speed .

(sorry for my bad English)

Best regards,
dro.
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dyeatman · Joined: 06 Sep 2003 Posts: 1924 Location: Norman, OK

With the Davis unit (of which I have two in operation) I found the non-linearity to be minimal. Both of mine were tested against the NOAA Severe Storms Lab equipment which is near where I live and found to be relatively accurate once I got them calibrated at a single speed (40 mph).

The max error I found at any tested speed was approx .2 m/s. The largest error was seen at the very low end (2-3 mph) and the high end (> 80 mph).

I created an offset error correction constant in the code, then tweaked that during testing to get my unit to match the readings of the NOAA unit. During testing I simply added that value to the multiplier then performed my calculations.

I also found the Wind Vane to be very linear and no table was required for that device either.

Link to a discussion of anemometer error:
www.otechwind.com/downloads/Anemometer%20Calibration%20Uncertainty,%20AWEA%20Windpower%202007.pps

inservi · Joined: 13 May 2007 Posts: 128

Hello dyeatman,

Thank you for precisions.

Best regards,
dro
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stijn023 · Joined: 28 Sep 2010 Posts: 49

ezflyr · Joined: 25 Oct 2010 Posts: 1019 Location: Tewksbury, MA

Hi,

This is an old thread so the original participants may no longer be here?

This is a relatively simple task to accomplish. Basically, you need code to measure the time between the pulses generated by the switch closure of the anemometer - a tachometer of sorts. The PIC CCP module is ideal for this. Once you know the time period or frequency you just need to make a simple conversion to wind speed using the know scale factor provided in the link shown in a previous reply. Search the forum for 'tachometer' to get yourself started. PCM posted some sample code a while back that with some slight modification (change 'rising edge' to 'falling edge' for example) will get you going!

John

dyeatman · Joined: 06 Sep 2003 Posts: 1924 Location: Norman, OK

To make it real simple.

The number of pulses in 2.25 seconds = the absolute wind speed.

So you set up a timer routine to keep track of 2.25 seconds and count the
pulses during that time interval to determine the speed.

Regardless of what inservi said earlier, the above originally came directly
from a Davis Engineer and has proved to be accurate on two different
systems of mine for many years.
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