Honeywell UDA2182 manual Faradaic Currents, Residual Currents, Electrode Conditioning Currents

Appendices

15.11Appendix J – Discussion on Chemical Interferences on Measured DO Currents

There are four contributors to measured current:

Faradaic Currents

Faradaic currents are those resulting from oxidation or reduction of chemical species. The reduction of oxygen to water, the oxidation of water to oxygen, and the oxidation of hydrogen, hydrazine or sulfur dioxide, are examples of Faradaic currents.

Residual Currents

Residual currents are unwanted Faradaic currents caused by impurities in the probe electrolyte. These impurities are metals (e.g. lead, zinc) in electrolyte reagents, which are capable of being reduced at the cathode and give rise to zero offset currents at “zero ppb oxygen”.

Electrode Conditioning Currents

The platinum cathode and anode materials are actually made up of conducting platinum oxides. These oxides exist at the molecular level. The actual platinum surface state strongly affects the observed Faradaic currents. Before methods of wire conditioning were established, upwards of 96 hours was needed to allow these conditioning currents to stabilize. Once wire-conditioning methods were established, it now takes approximately 24 hours for these conditioning currents to completely stabilize. Electrode conditioning currents occur on first probe power-ups, following power interruptions of more than 1 second (back-up power is provided for the probe to prevent this current during a power outage of 1 hour or less) and following a Probe Bias test.

Charging Currents

The Dissolved Oxygen (DO) probe consists of closely spaced bi-filar platinum windings separated by a high dielectric constant material. This is a description of a capacitor; the capacitance of a DO probe is in the hundreds if microFarads. When the probe is scanned during a Probe Bias Test(PBT) at 25mV/sec, an appreciable charging current is observed. This is equivalent to several hundred ppb dissolved oxygen.

The purpose of the PBT is to verify the optimum operating range of the current/voltage curve. It further allows one to determine if a reference shift has occurred. Most importantly, it allows one to select to identify a new bias point, if one is needed. To employ this diagnostic, you should be in air or air saturated water (ppm current is in uA range). A PBT should not be performed in a ppb application (ppb current is in nA range), due to charging and electrode currents being at a maximum value (µA range) during one of these scans. Furthermore, the final current rise during the PBT produces both hydrogen and oxygen gases within the probe. Time is needed before these gases can re- establish equilibrium with the outside sample. Therefore, the PBT should be limited to air level conditions and adequate time should be allowed for probe recovery following a PBT.

January 2009 UDA2182 Universal Dual Analyzer Product Manual 223