The acceptable slope range is 92% to 102%. Slopes below 92% indicate that the electrode may require cleaning or if cleaning does not help, the electrode should be replaced. Slopes above 102% indicate that the pH buffers are contaminated. General Note: Most Orion pH meters display the slope as a percentage of the theoretical slope (59.12 mV/pH unit). For example, a 98.5% slope is equivalent to a slope of 58.27 mV/pH unit. Back to Top • Do I need an Automatic Temperature Compensation (ATC) probe? The most common cause of error in pH measurements is temperature. The slope of a pH electrode is highly dependent on temperature, and pH buffer values and sample values change with temperature. For the most accurate results an ATC probe is always recommended. The Orion PerpHecT line of pH meters (Models 310, 320, 330, 350, 370) and electrodes allows for temperature compensation directly from the pH electrode. A separate ATC probe is not required. There are three advantages for using an ATC probe