There is a small variance in the pH reading depending on the temperature of the water. Turning compensation on corrects for this variance. In a typical reef tank the temperature change is so small that the effect is negligible. For reef tanks I recommend just leaving it off.
pH and ORP are inversely correlated. For every one unit change in pH there is about a 58 mV change in ORP. Turning pH compensation on corrects for this variation. However, the industry norm is for compensation to be off. So if you want to compare your ORP readings to books, and others leave it off. I recommend leave this setting off as well.
Curt
