Measuring Reference Voltage; Checking For A Poisoned Reference Electrode - Emerson Rosemount Analytical HART 5081-A-HT Instruction Manual

MODEL 5081-A

16.13 MEASURING REFERENCE vOLTAGE

Some processes contain substances that poison or shift the
potential of the reference electrode. Sulfide is a good example.
Prolonged exposure to sulfide converts the reference electrode
from a silver/silver chloride electrode to a silver/silver sulfide
electrode. The change in reference voltage is several hundred
millivolts. A good way to check for poisoning is to compare the
voltage of the reference electrode with a silver/silver chloride
electrode known to be good. The reference electrode from a
new sensor is best. See Figure 16-6. If the reference electrode
is good, the voltage difference should be no more than about
20 mV. A poisoned reference electrode usually requires
replacement
94
FIGURE 16-6. Checking for a Poisoned
Reference Electrode.
Refer to the sensor wiring diagram to identify the
reference leads. A laboratory silver/silver chloride
electrode can be used in place of the second sensor.
SECTION 16.0
MAINTENANCE