@Will-Dickson Oh, silly us! The curve makes a u-shape, so we needed to drop the pH 4.74 value. Now the polynomial fitting works.

@Will-Dickson ... we had a discussion about this in the "Additional tests for colorimeter..." thread and I think I was able to get polynomial fits to work in May-June of 2017 after you packaged v05. But now @maddiehankard and our group at St Thomas have created another pH curve that we are playing with and v05 will only let us use a linear fit (not even a second or third order polynomial... and we need 4th order). Our x-y data are below.

# 2018-04-11 Hankard, Wolf, University of St.Thomas 
# Colorimeter Data
# LED white
# -----------------------------
# pH | Absorbance
4.74  0.26
5.45  0.21
5.69  0.22
6.36  0.25
7.00  0.34
7.43  0.44
8.16  0.55
9.17  0.62

@Will-Dickson Thank you! I very much appreciate your help!

Hi @Will-Dickson, I haven't noticed a bitbucket update in the downloads, and I'm afraid I don't have sufficient Python experience to update it myself. Keep me posted and I look forward using the updated version. Thanks!

@Will-Dickson

There isn't an error message. When I select 4th order and hit the plot button, nothing happens. But I choose linear, 2nd order, or 3rd order it works fine. Same data I posted, same that you used. I'm using that Mac OS software version 4, which appeared to be the current version when I looked yesterday.

(Thanks so much for your responsiveness!)

@Will-Dickson

Hmm... the colorimeter software won't let me fit beyond 3rd order.

pH curve screen shot

The address for the serial port always resets to /dev/tty.usbmodem upon opening the colorimeter software. I have looked through the arduino sketches, but do not see a place to designate the port name. Can someone direct me to this? Most of the time the device will be used on one computer, so I would like to change this default.

Thanks @Will-Dickson! I also just discovered that I have to select the fit type before plotting and it all works. Doing this should help @briansone get around the issue of the plot trying to intercept with (0,0).

@briansone

Are you willing to share the API results (bromothymol blue)?

I have pH calibration for API pH 6.0-7.6 and API High Range pH 7.4 to 8.8. The curves are non-linear (polynomial), especially for the high range. I am new to IO Rodeo Forum and I am wondering if there are discussions regarding non-linear calibrations (I looked and didn't find any).

Here's our calibration:
2016-04-27
Colorimeter Data
K.Jin, A.Aufdenkampe, B.Fisher
LED red
API pH 6.0-7.6 bromothymol blue
pH | Absorbance
5.8 0.00
6.0 0.02
6.4 0.07
6.8 0.24
7.2 0.40
7.6 0.56
7.8 0.65
8.0 0.69