Maintenance
33
Instruction Manual
Maintenance
LIQ_MAN_6081-P
February 2014
Section 11: Maintenance
11.1
Overview
This section gives general procedures for routine maintenance of the 6081-P pH/ORP transmitter
and pH and ORP sensors. The transmitter needs almost no routine maintenance. Sensors require
periodic inspection and cleaning. The calibration of the transmitter-sensor combination should
be checked regularly, and the loop recalibrated if necessary.
11.2
Transmitter Maintenance
Periodically clean the transmitter window and housing as needed with a cloth dampened with
water. Do not use abrasive cleaning solutions. The O-rings and sealing surfaces must be kept
clean or moisture may enter the electronic enclosure.
11.3
pH Sensor Maintenance
11.3.1 Frequency of Cleaning
The frequency at which a sensor should be inspected and cleaned can be determined only by
experience. If the process liquid coats or fouls the sensor, frequent cleaning may be necessary. If
the process does not contain a high level of suspended solids, the need for regular cleaning will be
less. Often an increase in glass impedance indicates the electrode is becoming fouled and needs
cleaning. Refer to Section 12.4 for a description of the glass impedance diagnostic.
11.3.2 Cleaning Procedures
Problem
Cleaning Suggestions
Loose scale or debris
Use a stream of water from a wash bottle to rinse away solids
from the tip of the sensor. If water does not work, gently wipe
the glass bulb and liquid junction with a soft cloth, tissue, cot-
ton-tipped swab, or a soft bristle brush.
Oil and grease
Wash the glass bulb with mild detergent solution and rinse thor-
oughly with water.
Hard scale (carbonate
sulfate scales and corrosion
products)
If wiping the sensor tip with a tissue or cotton swab does not
remove the scale, soak the glass bulb ONLY in a solution of 5%
hydrochloric acid. To prepare the acid solution, add 15 mL of
concentrated hydrochloric acid to 85 mL of water. Keep the
acid away from the liquid junction and from any stainless steel
portions of the sensor. Rinse the sensor thoroughly with deion-
ized water. Some scales (for example, calcium sulfate) cannot be
removed easily with acid. Soaking the glass bulb in a 2% solution
of disodium EDTA may be helpful.
When using acid or alkaline solvents, be careful to keep the solvent away from the liquid junction.
If the cleaning solvent contacts the junction, hydrogen ions (acid solvent) or hydroxide ions
(alkaline solvent) will diffuse into the junction. Because hydrogen and hydroxide ions have much
greater mobility than other ions, they produce a large junction potential. When the electrode
goes back in service, the hydrogen or hydroxide ions slowly diffuse out of the junction, causing
the liquid junction potential and the pH reading to drift. It may take hours or days for the reading
to stabilize. For a discussion of the influence of ion mobility on liquid junction potentials, see
Section 13.4.
Consult the sensor instruction manual for additional information.
Always recalibrate the sensor after cleaning. If the sensor was cleaned with detergent or acid,
soak the sensor in pH 4 or pH 7 buffer for at least an hour before calibrating.
11.3.3 Checking the Reference Electrode.
Some processes contain substances, for example, sulfides, that poison the reference electrode.
