Emerson Process Management 3081 pH/ORP instruction manual Section

MODEL 3081 pH/ORP

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.

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.

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