PRINCIPLE OF OXYGEN PROBE OPERATION

For low ppb levels of dissolved oxygen, three types of probe designs available: Clark-type, galvanic and luminescence. Each probe design has its particular advantage. Although the Clark-type probe is the predominant probe for power plant analysis due to the fact of its fast response, easy calibration/maintenance and long service life.

The Clark-type sensor has an oxygen permeable Teflon membrane that separates the sample from a gold cathode and silver anode immersed in an electrolyte.

Dissolved oxygen in the sample diffuses through the membrane to equilibrate with the sample concentration and at a rate proportional to temperature.

It reacts with the electrolyte producing a current proportional to sample concentration.

A polarizing voltage is applied to drive the reaction.

The current is generated from oxygen consumption it is extremely important that there are no other sources of oxygen but from the sample.

Sensors need equilibration time to achieve polarization after power off and to reduce oxygen from internal sources

CALIBRATION

Probe calibration is done in air. The partial pressure of oxygen in air saturated water is the same as in water saturated air. Therefore air makes a very convenient standard for calibration. These types of electrodes exhibit very linear, stable response from ambient air concentrations down to sub ppb levels.

Monthly calibration in air is adequate even for measurements at part per billion levels.

The absence of a current denotes true zero. This is achieved after running-in time and consumption of residual oxygen inside the system. There is no zero calibration.

The ambient air with 21% of oxygen is a perfect standard for gain calibration. Atmospheric pressure and temperature are factors for establishing the relation between percentage and quantity (ppm) during calibration.

TUBING MATERIAL SELECTION

Proper sample tubing material is essential for accurate and precise oxygen measurement. Loose fitting connectors and improperly installed lines can cause oxygen ingress which will adversely affect the oxygen results. Proper installation will insure accurate measurements.

316 Stainless Steel Tubing Flexible PVDF (Kynar) Braided Stainless Steel, flexible Nylon Polypropylene Tygon Polyethylene FEP (Teflon) PFA (Teflon)

Preferred Permanent Preferred Portable OK, Portable OK, Portable Occasionally NEVER NEVER NEVER NEVER

Accuracy Reproducibility Zero point

5% of end value 3% of end value Automatic Adjustment