PH MONITOR
pH Electrode Calibration
8-160 Aquastorm™ Series Options Guide Rev. 1
8
pH Electrode Calibration
Introduction To define proper electrode operation and maintain linearity during the electrode
life-span review the offset, slope and response time/stability checks.
Offset (STD) Check Offsets in a pH 7 buffer @ 25 °C (77 °F). Theoretical output is 0 mV. Any
deviation is the pH electrode offset (i.e. +008 mV). Electrode offsets greater
than ±50 mV indicate the electrode requires cleaning or replacement.
Slope (SLP) Electrode slope is the number of mV per pH unit. Temperature can have an effect
on electrode slope. Check the following graph that illustrates potential pH errors
when using a non-temperature compensated instrument.
Recommendations:
• Always calibrate solution temperature prior to electrode standard and slope
calibration.
• Electrode mV offset will track across the entire pH range.
• Coatings on the glass surface may affect sensor slopes.
• A constant output near 0 mV in pH 4, 7, and 10 typically indicates a shorted
electrode, requiring electrode replacement.
Theoretical mV Values @ 25
°C (77 °F)
pH mV
2 +295 mV
3 +236 mV
4 +177 mV
5 +118 mV
6+59 mV
70 mV
8 –59 mV
9 –118 mV
10 –177 mV
11 –236 mV
12 –295 mV
°C °F
pH Error
23456
7 8 9 10 11 12
15 59 0.15 0.12 0.09 0.06 0.03
0 0.03 0.06 0.09 0.012 0.15
25 77 0 0 0 0 0 0 0 0 0 0 0
35 95 0.15 0.12 0.09 0.06 0.03
0 0.03 0.06 0.09 0.12 0.15
45 113 0.3 0.24 0.18 0.12 0.06
0 0.06 0.12 0.18 0.24 0.3
55 131 0.45 0.36 0.27 0.18 0.09
0 0.09 0.18 0.27 0.36 0.45
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