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4. REFERENCE IMPEDANCE. The majori-
ty of reference electrodes used in indus-
try are low impedance silver-silver chlo-
ride electrodes. Every pH and ORP sen-
sor manufactured by Rosemount
Analytical has a low impedance refer-
ence. However, there are applications
that call for either a high impedance sodi-
um or pH glass reference electrode. Both
high impedance and low impedance ref-
erence electrodes can be used with the
Model 3081 pH/ORP transmitter.
5. WARNING AND FAILURE LIMITS FOR
THE REFERENCE ELECTRODE.
Warning tells the user that the reference
electrode impedance is approaching the
failure limit. Low and high warning and
failure limits are programmable. For
conventional low impedance silver-sil-
ver chloride reference electrodes only
the high limits are useful. For high
impedance reference electrodes, both
low and high limits are used.
Figure 8-2 shows suggested limits for
low impedance reference electrodes.
Figure 8-3 shows suggested limits for
high impedance glass reference elec-
trodes.
MODEL 3081 pH/ORP SECTION 8.0
PROGRAMMING FOR pH MEASUREMENTS
FIGURE 8-2. Suggested Warning and Failure Limits for Low
Impedance Reference Electrodes
The impedance of a typical silver-silver chloride reference elec-
trode is less than 40 kilohms. If the impedance is greater than
about 140 kilohms the reference electrode has failed. Failure is
usually caused by a plugged or coated reference junction or a
depleted electrolyte fill solution (gel). The reference impedance
will also be high if the sensor is out of the process liquid.
FIGURE 8-3. Suggested Warning and Failure Limits for High
Impedance Glass Reference Electrodes.
The limits for a high impedance glass reference electrode are the
same as the limits for a high impedance glass measuring electrode.