5.8 Set up diagnostics
5.8.1 Purpose of diagnostic setup
Note
Diagnostic setup applies only to pH sensors. It appears only if you are using the
Rosemount
™
FCL-02..
This section describes how to do the following:
1. Turn pH sensor diagnostics on and off.
2. Set pH sensor diagnostic limits.
5.8.2
Definitions
Diagnostics
pH sensor diagnostics are useful in troubleshooting calibration
problems and in predicting when a pH sensor should be replaced.
Diagnostics can also alert you that the sensor is no longer submerged
in the process liquid.
Reference
offset
pH sensors are designed to have a potential of 0 mV in pH 7 buffer.
The reference offset is the actual potential (in mV) in pH 7 buffer. A
new sensor typically has a reference offset of a few mV. Old sensors
can have offsets of 60 mV or more.
Glass and
reference
impedance
During operation, the transmitter continuously measures the
impedance of the pH glass membrane. If the pH sensor has a solution
ground, the transmitter also continuously measures the impedance of
the reference junction. The Rosemount
™
3900VP pH sensor supplied
with the FCL-02 has a solution ground. The Rosemount 399VP sensor,
supplied with earlier versions of the FCL-02, did not have a solution
ground. If you are using a Rosemount 399VP sensor, reference
impedance diagnostics will not be available. Glass and reference
impedance measurements provide useful information about sensor
health and cleanliness.
Glass
impedance
temperature
correction
The impedance of a glass electrode is a strong function of
temperature. As temperature decreases, the impedance increases. For
glass impedance to be a useful indicator of sensor condition, the
impedance must be corrected to a reference temperature.
Glass fault high
A typical glass electrode has an impedance of about 100 MΩ. As the
sensor ages, glass impedance increases. Extremely high impedance
(greater than about 1000 MΩ) implies the sensor is nearing the end of
its life. High impedance may also mean that the sensor is not
submerged in the process liquid.
Programming the transmitter Reference Manual
May 2019 00809-0100-3412
60 Emerson.com/Rosemount
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