Will,

I followed your instructions on checking the sensor board - and everything is fine now. I'm not sure if it had an impact, but I did switch to a new USB hub that may provide a bit more power than the ancient Mac Mini I was using before. I know that the USB connection can definitely cause issues with Arduinos - but I do not know that it was the cause of this problem.

Thanks,
Paul

Thank you for the suggestions. i'd tested the LEDs (they work), but not the sensor board. I'll give that a go this morning and report back.

Even with this (relatively minor) glitch, I think highly enough of the colorimeter that I've already bought a second for off-site use in water quality monitoring.

Now, if someone would help me figure out how to put the LEDs and sensors at a right angle to each other, rather than a direct pass-through, I could have a fluorometer for measuring chlorophyll A and phycocyanin!

About two weeks ago, I purchased and assembled the colorimeter. It was working beautifully -- up until today.

This afternoon, I started taking readings of ammonia using the API ammonia test kit. It looked like it calibrated fine, but the readings all came back 0.00. I rebooted my computer (Colorimeter software v.4 and v.5 tried on MacOS Catalina on a 2012 Mac Mini), but that didn't help.

Next step, I loaded the software onto a MacBook Air - same problem: after calibration, all readings are zero.

Just FYI, the samples definitely had plenty of color (the A620 values on a spectrophotometer were all above 0.78). Any suggestions on what to try?