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May 2023
Table 4-1: ORP of saturated quinhydrone solution (millivolts)
pH 4 solution
Temperat
68 (20)
ure: °F
(°C)
mV
168
potential
4. Remove the sensor from the buffer, rinse, and install in the
process.
4.4
When to service or replace a sensor
The usable life of a pH/ORP sensor depends on the application.
Common indicators that a pH/ORP sensor needs to be serviced or
has reached the end of its useful life include:
•
pH slope during a two-point buffer calibration is too low. Typically,
the lower limit is 40 to 50 mV/pH. If the slope is lower than that,
it indicates that the pH glass has reached the end of its useful life
and that the sensor needs to be replaced.
•
Zero offset during calibration is too high. The typical upper limit
is an absolute value of 60 mV. If the offset is higher than that,
it indicates that the reference system has reached the end of its
useful life and that the sensor needs to be replaced.
•
Sensor responds slowly to changes in process pH or has erratic
readings. This may be resolved by cleaning the sensor electrodes
or may indicate that the sensor needs to be replaced.
4.5
Clean pH/ORP sensor electrodes
Erratic readings or sluggish response may indicate that the sensor is
dirty and needs to be cleaned. The following are the recommended
cleaning procedures for pH/ORP sensors.
Procedure
Clean the electrodes.
•
To remove oil deposits, clean the electrodes with a mild, non-
abrasive detergent.
•
To remove scale deposits, soak the electrodes for one to five
minutes in a five percent hydrochloric acid solution.
Quick Start Guide
77 (25)
86 (30)
264
260
Quick Start Guide
pH 7 solution
68 (20 )
77 (25)
94
87
86 (30)
80
27
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