Emerson Rosemount 3900 Quick Start Manual page 23

November 2020
2.3 Maintain pH electrodes
Electrodes should respond rapidly. Sluggishness, offsets, and erratic
readings are indicators that the electrodes may need cleaning or
replacement.
1. To remove oil deposits, clean the electrodes with a mild non-abrasive
detergent.
2. To remove scale deposits, soak electrodes for one to five minutes in a
5 percent hydrochloric acid solution.
3. Temperature effect on life expectancy: If glass electrode life
expectancy is 100 percent at 77 °F (25 °C), then it will be
approximately 25 percent at 176 °F (80 °C), and approximately 10
percent at 212 °F (100 °C).
WARNING
CORROSIVE SUBSTANCE
Hydrochloric acid is toxic and highly corrosive.
Avoid skin contact.
Wear protective gloves.
Use only in a well-ventilated area.
Do not inhale fumes.
In case of an accident, consult a doctor immediately.
2.4 Calibrate oxidation reduction potential (ORP)
Prerequisites
After making an electrical connection between the sensor and the
transmitter, obtain a standard solution of saturated quinhydrone
PN R508-8OZ (460 ± 10 mV). You can also make the solution by simply
adding a few crystals of quinhydrone to either pH 4 or pH 7 buffer.
Quinhydrone is only slightly soluble; therefore only a few crystals are
required.
Procedure
1. Immerse the sensor in the standard solution. Allow one to two
minutes for the ORP sensor to stabilize.
2. Standardize the transmitter to the solution value shown in
The resulting potentials, measured with a clean platinum electrode
and saturated KCl/AgCl reference electrode, should be within ±20
mV of the value shown in
ensure accurate interpretation of results. The ORP value of saturated
Quick Start Guide
Table 2-1. Note solution temperature to
Quick Start Guide
Table 2-1.
23