Honeywell UDA2182 manual Display Graph, 55V 80μA 240 160 ΜA 00 0.2 0.4 0.6 0.8

Input Calibration

before the test was initiated. During this voltage sweep, the probe current is monitored and the graph of current as a function of voltage is displayed.

If during the test the probe current rises above a factory-set upper limit, the bias voltage is returned to its pre-test value at 25mV/sec and the test is terminated without completing the full 1.0 Volt sweep. (The bias voltage test may also be terminated at any time by pressing the “EXIT” button.)

Display Graph

Under normal conditions, the completed display shows a graph of current as a function of voltage with the following features: from approximately 0 to 0.2 volts a fairly rapid increase in current is observed; from approximately 0.2 to 0.8 volts, the current exhibits a “flat” region where it is nearly independent of voltage and at some voltage above about 0.8 volts, the current rises quickly.

A typical current-voltage curve is shown below. The Sweep Bias millivoltage (along the bottom of the graph) is a voltage from 0 -1V that is applied to perform the test. The Operating Bias millivoltage is the current position of the cursor on the graph and represents the current bias voltage. The horizontal axis numerals are in hundreds of millivolts.

0.55V 80μA

240

160

80

μA 00 0.2 0.4 0.6 0.8 1V

Figure 8-5 Display of Probe Bias Test Done in Air

Note that the curve is quite flat at 0.55V. This means that even rather large changes in the probe current-voltage characteristic do not affect the current (and, thus, probe sensitivity) at 0.55V. In general, the curve formed by decreasing voltage is not identical to that formed by increasing voltage. This hysteresis is a function of the voltage scan rate and may be ignored.

The interpretation of figure shown above is as follows:

As the bias voltage of the oxygen-consuming electrode (relative to an internal reference electrode) is increased, there is an initial increase in current as more and more of the oxygen that approaches the electrode is reacted. However, at about 0.2V, the current stops rising and a flat region, independent of voltage, is observed. It is in this region that probe current is determined by oxygen mass transport limitation. Increasing the voltage cannot increase the current because oxygen movement is diffusion limited. Finally, at a voltage

January 2009 UDA2182 Universal Dual Analyzer Product Manual 173