Introduction; Sensor Impedance Diagnostics; Ph Electrode Impedance Diagnostics - Emerson Rosemount 1066 Instruction Manual

1066 Transmitter Instruction Manual
LIQ_MAN_1066-P-FF
Section 9: Diagnostics
9.1

Introduction

There are a number of diagnostic messages to alert the user to issues with the transmitter, sensor,
or a failed calibration. Most of these require no configuration. Only the minimum/maximum
slope and reference offset calibrations, in the previous section can be configured, and the sensor
impedance diagnostics.
9.2

Sensor Impedance Diagnostics

9.2.1

pH Electrode Impedance Diagnostics

A glass pH electrode forms the pH measuring circuit with the reference electrode or solution
ground through the process solution. The glass electrode itself typically has an impedance of hun-
dreds of Mohm and this high impedance is the basis for pH electrode impedance diagnostics.
The measurement of glass impedance is complicated by the fact that it is highly temperature
dependant, decreasing by about half for every 8 C increase in temperature. The glass impedance is
compensated to 25 C, by an algorithm which provides compensation up to a temperature where
the raw glass impedance becomes too low to be accurately measured. Above this temperature the
glass impedance measurement is turned off. Glass impedance temperature correction (Z Temp
Correct'n) can be turned on or off in the 1066pH.
The impedance of a glass pH electrode is not a simple resistance, but also includes capacitance,
which makes a simple measurement of glass impedance time dependent. If the impedance is
measured over a short time, the measured impedance will not will not have time to reach its final
value, and the measured impedance will be less than the actual value.
The 1066pH glass impedance can use two types of glass impedance measurement, Basic and
Advanced. The Basic measurement type is fine for detecting low glass impedance, but when a more
accurate measurement of high glass impedance is desired, the Advanced type should be chosen.
9.2.1.1
Broken Glass Diagnostics
If the pH electrode is cracked or broken, the process solution penetrates the glass, creating a short
through the glass, and the impedance drops precipitously. Low glass electrode impedance can be
used to detect a broken or cracked electrode, which is no longer functional. This diagnostic will
also detect a short in the pH measuring circuit, which also causes the pH measurement to fail. The
low impedance limit is set to 1 Mohm.
High Glass Impedance Diagnostics
9.2.1.1
As a glass electrode ages, its impedance increases and causes sluggish electrode response. Severe
coating of the glass electrode can also have the same effect, as can a bad connection. A high glass
impedance fault alarm (GI Fault High) can be set at up to 2,000 Mohm and has a default value of
1,500 Mohm.
Diagnostics
Section 9: Diagnostics
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