Hi @woodsmn
Even though you can name your channel "Mg/Ca_mmol/mol", in most computing languages the forward slash is interpreted as a divide symbol. So you need to replace the forward slashes in your channel name with underscores so that iolite will find the correct channel. Similarly, the ÷ symbol needs to be replaced with forward slashes for the computer to understand it. And finally, the natural log operator is "log" in iolite (the base-10 log operator is "log10"). So the final expression you want is:
(log((Mg_Ca_mmol_mol) / 1.325)) / 0.065
If that doesn't work for you, please let me know.
Kind regards,
Bence