hi Adafruit,
I've got a 12" eTape sensor for which I measure the resistance as 528 ohms when there's liquid at 30cm on the eTape, and 2200 ohms when there's no liquid on the eTape. It's wired with a 1.2K pullup resistor (measured at 1196 ohms). I used 1.2K because from my understanding of the estimate of resistance (res <- R1/((1023/ar)-1)), making R1 a little bigger might spread the analog readings a bit farther apart.
I've used the eTape with two different Arduinos. First with a Diavolino, and I got the following readings:
dp <- c(0, 10.3, 15.3, 20.3, 25.3, 30.0)
ar <- c(813, 742, 677, 603, 517, 396)
where dp is in cm and ar is the analog reading at the Arduino. Then with a stock Arduino Mega 2560:
dp2 <- c(0, 4.7, 9.3, 13.7, 18.2, 22.7, 27.1, 30)
ar2 <- c(702, 681, 648, 600, 553, 484, 389, 335)
These are pretty far apart. I have two questions:
a) is it possible that different Arduino boards will have substantially different analog readings from the same sensor? If so, does this have to do with unstable or wonky analog reference values? Is there anything I can do to stabilize this, or should I just resign myself to recalibrating for each board?
b) in terms of going from an analog reading to a depth estimate, you guys recommend linearizing the analog reading back to resistance, then using the proportion of measured resistance in the min, max range to find a proportional height, then multiplying by the sensor's length.
http://learn.adafruit.com/thermistor/using-a-thermistorI didn't find that to be a good fit (sorry, I'm a statistician in my day job). A somewhat better fit can be found by a 4th degree polynomial directly from the analog reading to depth. Details here:
http://wylbursinnergeek.net/2012/07/26/ ... el-sensor/ What do you guys think?
thanks -- wylbur.