How is the ORP sensor calibrated on the MP-6 and MP-6P portable meters?
Calibration of ORP on MP meters
The ORP electrodes rarely give false readings unless there is a problem in the reference electrode. For this reason, and because the calibration solutions for ORP are highly reactive and potentially hazardous, the MP meter has an electronic ORP calibration. This causes the zero point on the reference electrode to be set whenever the pH 7 calibration is done.
An ORP sensor uses a small platinum surface to accumulate charge without reacting chemically. That charge is measured relative to the solution, so the solution "ground" voltage comes from the reference junction. The same reference is used for both the pH and the ORP sensors. Both pH and ORP indicates 0 for a neutral solution. Calibration at zero corrects the error in the reference junction.
A zero calibration solution for ORP is not practical, so the MP series meters uses the offset value determined during calibrationto 7 in pH calibration (pH 7 =0 mV). Sensitivity of the ORP surface is fixed so there is no gain adjustment.