As you suggested, I tested a duplicate of our set up in the lab and used a power supply source set at 1.5V in the aa0 and aa1 ports used to measure PAR on the mayfly. I get 93 mV and 470 umol for PAR. Clearly wrong. It appears the bits are recorded wrong or they are converted to mV incorrectly. Any thoughts on a next step to troubleshoot this?