# Correct way to scale a resistance input?

Newbie here…

For my first learning project, I’ve interfaced a Davis Instruments anemometer and wind vane to, respectively, a SNAP-AIRATE card, and a SNAP AIR40K card. Also A Hydreon RG11 rain sensor configured as a 0.001" per tip tipping bucket rain gauge emulator.

The anny is a magnetic sensor that “closes” once per rev of the wind cups. The direction is a 20K continuous turn pot.

Everything works and I can see the raw values change in a watch window in PAC Control. (The tach module might not be the best for this, as the threshold for the anny is 1 mph = 1 rev in 2.25 seconds. But the module came with the system I bought on ebay, so I thought I’d try it out.)

The question right now is, what’s the proper way to scale/calibrate the 0-20K pot to 0-360 degrees?

Thanks!

Chris

1 Like

There is an example strategy here that shows one way to do to the calibration, using a subroutine. Since you have a linear response, you don’t need to use the more complicated scaling methods that the example shows, and you can stick with just a simple linear calibration.

That example is really more useful when the scaling is non-linear. For something as simple as this, an easy solution would be to set the input scale for the AIR40k module to 0-20000 ohms, and then in one of your charts create a loop (with a delay) that converts the input value to degrees. If you can orient the anemometer so that 0 ohm = 0 deg and 19999 ohm = 359.98 deg, it would be nice, but if there is an offset because north is in a direction different than the anemometer is expecting, you could include that as well.

(output in deg) = [(input in ohm) * (360 / 20000)] + (offset in deg)

Note that when using a offset, a check is needed to keep the output under 360 deg

if ((output in deg) > 360) then
(output in deg) = (output in deg) - 360

If it turns out that there is a scaling problem (0 is not the minimum resistance or 20000 is not the maximum resistance), you could include that too:

(output in deg) = [(input in ohm - minimum resistance) * (360 / (maximum resistance - minimum resistance)] + (offset in deg)

Thanks! The non-linear scaling is useful to me as well. I have a thermistor connected to a SNAP AIR40K-4 card I could linearize, as well.

I finally got around to trying this. It works pretty well. I did, however, figure out that the pot isn’t exactly 20k. That was when my calculation showed the wind direction at 368 degrees.

Rather than hassle with disconnecting and measuring the wind vane pot, I added in a Maximum instruction which compared the current raw pot reading to the current maximum. Once the max stopped climbing, I had a new resistance value to use in the calculation.

I suppose I could make the algorithm auto-calibrating by using the max value in the calculation directly, rather than using it to create a correction factor I manually enter into the program.