The SM500  orp meter is factory calibrated  calibrating it does not need to happen.  The MA9020 is a "validation" solution for positive ( + ) ORP.

A good  orp probe should read between 200mv and 275mv -- we consider a probe to be perfect at +235mv to + +245mv in the MA9020 "validation solution"  --

Not to confuse the issue but the SMS510 can be calibrated and the MA9020 is used to "calibrate" that unit.

Many of our customers use this for Kangen water testing as well