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MODEL 1054B pH/ORP
SECTION 4.0
CONFIGURATION
4.6 pH ELECTRODE DIAGNOSTICS (1054B pH
only).
Display Mnemonic
InP
. Under this item are func-
tions associated with glass electrode diagnostics. These
diagnostics are possible through a continuous, temper-
ature compensated measurement of the sensor imped-
ance (resistance),
rES
, made from the preamp.
A soft-
ware selectable
on
setting will activate these diag-
nostics. If
oFF
is the setting (factory setting) these
diagnostics will all be disabled.
A new electrode has an impedance of approximately
200 megohms, and as it ages the impedance increas-
es because lithium ions (which carry current) in the
glass slowly get depleted by the process. If an elec-
trode cracks, the impedance drops sharply, usually to
below 70% of the normal value.
The following are typical impedance values for
new
Rosemount Analytical electrodes (Electrodes stored
over a period of time will have higher impedances). Upon
installation you can read impedance in megohms under
rES
.
Sensor/Glass Megohms
@
Type
30°C (86°F) when new
• General purpose,
30-100
HF, and high pH glass (GPLR)
• General Purpose
High Temp (GPHT)
50-90
• Ruggedized glass
200-300
For more information on these diagnostics and trouble-
shooting, refer to Section 8.0.
NOTE
Impedance diagnostic faults are not acti-
vated until the setpoint is continuously
exceeded for 30 seconds.
A. Calibration Warning.
Display Mnemonic
CAL
.
Under this mnemonic you can select the percent
increase in impedance before a calibration warning
fault appears. For example, if the impedance is 400
megohms and the setpoint is 20% (factory setting), a
CAL
warning will appear on the analyzer and it will go
into a fault mode when the electrode ages to 480
megohms. This diagnostic will reset after a buffer cali-
bration. Configurable range: 20-200%. A setting of
zero disables this fault.
NOTE
The recommended process temperature
range for the
CAL
diagnostic is 15-90°C
(15-194°F.) (For low impedance glass it is
15-80°C (59-176°F). If ruggedized glass is
used and the preamp is not in the sensor,
the minimum recommended temperature
is 35°C (95°F).
If you want to use this feature as a warning yet not
upset your process, use Alarm 2 as a fault alarm (Refer
to Section 4.2) and set the default current output to
non
(Refer to Section 4.8) to hold the output at the last
process value.
You may also get this fault if:
1.
The electrode or junction becomes excessively
coated.
2.
The electrode is not immersed in the process fluid.
If the electrode continues to remain out of solution
eventually the fault mnemonic
OId
will also appear
on the display. See part C.
B. Cracked Glass Diagnostic.
Display Mnemonic
SHt
.
One way to tell that you have a broken or cracked glass
electrode is that the analyzer will read a constant value
(usually between 5.0-7.0 pH) in any process or buffer.
The other way is to note the impedance value. When a
crack occurs the mnemonic
SHt
will appear on the ana-
lyzer to indicate that the circuit is shorted, and the ana-
lyzer will go into a fault mode. Directions for
SHt
value
determination:
SHt
~ 70% of normal
rES
Configurable range: 5-600 megohms. A setting of zero
disables this fault. Factory setting: 20 megohms.
NOTE
For low impedance glass, a broken elec-
trode may not be detectable above 70-
75°C (158-167°F).
C. Worn Out Electrode.
Display Mnemonic
OId
. This
mnemonic is used for programming the high imped-
ance limit of the electrode. For example, if the setpoint
is 1000 megohms (factory setting), and the impedance
rises above this value, the mnemonic
OId
will appear
on the analyzer and it will go into a fault mode. The
electrode is either worn out, severely coated, or not
immersed in the process fluid. Configurable range:
200-1999 megohms. A setting of zero disables this
fault.
Recommended setpoints:
1. 1000 megohms for all glass except ruggedized
2. 1300 meghoms for ruggedized glass
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