A pH sensor can output in millivolts (mV) - this is now encountered in Cape Cod. TADA does list both VOLTS and MV in the characteristic validation table for PH, but marks both as REJECTED. From what I can see 0 mV == pH 7.0. Need a good function for this, considering:

pH = 7 - mV/57.14

New function would go in convert module, convert.SiO2_to_NTU() is very similar.