Campbell Scientific CSIM11 pH and ORP Sensors - Manual (Page 20)

CSIM11 pH and ORP Probes
8. Calibration/ORP Check
8.1 pH Sensor Calibration
Calibration should be carried out according to the detailed procedure later in
this document (see Appendix C, Detailed Calibration Procedure and
Manufacturer Tips). The following paragraphs are for general information.
The calibration should use two or more pH standards. It is recommended that
pH 7 buffer be used to check the zero point and at least an acid or alkaline
buffer, that brackets the sample’s pH value, be used to set the slope.
An electrode measuring many samples a day should be calibrated at least once
a day. The frequency of calibration will depend on the level of accuracy
required and the coating/fouling nature of the samples being measured.
Electrodes that are continuously monitoring a sample should be checked at
least once a week or whatever period experience dictates.
Grab Sample Calibration is a technique where the process electrode has been
calibrated and placed on line for some period of time. Its output is then
verified by measuring the pH of a sample with another electrode. The grab
sample should be measured as soon as possible to avoid errors caused by
changes in the sample’s temperature or changes in the samples pH due to
exposure to the atmosphere.
8.2 ORP Check
Check the ORP sensor when it’s initially deployed and after three months of
field service. To check the sensor, place it in a known millivolt solution. The
sensor manufacturer offers +230 mV and +470 mV solutions. If the sensor
reading is within ±20 mV of the millivolt value of the solution, the sensor is
operating properly.
9. Maintenance
Developing an effective maintenance schedule is incumbent on understanding
the process effects that are specific to your application. A pH sensor develops
a millivolt potential directly proportional to the free hydrogen ion
concentration in an aqueous solution. The sensor is composed of a reference
electrode and its gelled reference electrolyte, a measurement electrode exposed
to the process solution, and a porous junction that maintains electrical contact
between the two. Porous PTFE is the newest technology in reference
junctions. Wedgewood Analytical, Inc. offers a patented porous PTFE liquid
junction which is chemically inert; and is chemically compatible with virtually
all chemicals.
• High Temperature which causes 1) Faster Response / Lower Impedance;
2) Aging acceleration, Lithium Ions Leached from Membrane; 3) Short
Span
• Low Temperature which causes Slower Response / Higher Impedance
• Measurement > 10.0 pH causes Alkaline / Sodium Ion Error
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