Know the Difference
Oxygen analyzers use one of a several types of oxygen sensors. As industrial process applications call for improved measurement accuracy and repeatability, users are demanding analyzers that require a minimum of maintenance and calibration.
There is no one universal oxygen sensor type.
- Ambient temperature electrochemical oxygen sensors
- Paramagnetic oxygen sensors
- Polarographic oxygen sensors
- Zirconium oxide oxygen sensors
Below is a quick review of the various gas phase oxygen sensors. Use this information to help you select the right oxygen sensor type for your application:
AMBIENT TEMPERATURE ELECTROCHEMICAL SENSOR
- Often referred to as a galvanic sensor, is typically a small, partially sealed, cylindrical device (1-1/4” diameter by 0.75” height) that contains two dissimilar electrodes immersed in an aqueous electrolyte, commonly potassium hydroxide.
- Refinements in electrode materials, and enhanced electrolyte formulations, the galvanic oxygen sensor provides extended life over earlier versions and are recognized for their accuracy in both the percent and traces oxygen ranges.
- Response times have also been improved.
- They are easy to damage when used with samples containing acid gas species such as hydrogen sulfide, hydrogen chloride, sulfur dioxide, etc.
PARAMAGNETIC OXYGEN SENSORS
- This is the magnetodynamic or `dumbbell’ type of design and is the predominate sensor type.
- The paramagnetic oxygen sensor consists of a cylindrical shaped container inside of which is placed a small glass dumbbell. The dumbbell is filled with an inert gas such as nitrogen and suspended on a taut platinum wire within a non-uniform magnetic field.
- A precision optical system consisting of a light source, photodiode, and amplifier circuit is used to measure the degree of rotation of the dumbbell.
- Some paramagnetic oxygen sensor designs, have an opposing current is applied to restore the dumbbell to its normal position.
- In general, paramagnetic oxygen sensors offer very good response time characteristics and use no consumable parts, making sensor life, under normal conditions, quite good.
- Offers excellent precision over a range of 1% to 100% oxygen.
- They are quite delicate and sensitive to vibration and/or position.
- Due to the loss in measurement sensitivity, in general, the paramagnetic oxygen sensor is not recommended for trace oxygen measurements.
POLAROGRAPHIC OXYGEN SENSORS
- Often referred to as a Clark Cell [J. L. Clark (1822- 1898)].
- This sensor, both the anode (typically silver) and cathode (typically gold) are immersed in an aqueous electrolyte of potassium chloride.
- The electrodes are separated from the sample by a semi-permeable membrane that provides the mechanism to diffuse oxygen into the sensor.
- The current output generated from the sensor is measured and amplified electronically to provide a percent oxygen measurement.
- An advantage of the polarographic oxygen sensor is that while inoperative, there is no consumption of the electrode (anode).
- Storage times are almost indefinite. Similar to the galvanic oxygen sensor, they are not position sensitive.
- One major advantage of this sensor type is its ability to measure parts per billion levels of oxygen.
- The sensors are position sensitive and replacement costs are quite expensive, in some cases, paralleling that of an entire analyzer of another sensor type.
- Not recommended for applications where oxygen concentrations exceed 25%.
ZIRCONIUM OXIDE OXYGEN SENSORS
- This sensor is referred to as the “high temperature” electrochemical sensor and is based on the Nernst principle [W. H. Nernst (1864-1941)].
- Zirconium oxide sensors use a solid-state electrolyte typically fabricated from zirconium oxide stabilized with yttrium oxide. The zirconium oxide probe is plated on opposing sides with platinum which serves as the sensor electrodes.
- The zirconium oxide oxygen sensor has excellent response time characteristics.
- The same sensor can be used to measure 100% oxygen, as well as parts per billion concentrations.
- Due to the high temperatures of operation, the life of the sensor can be shortened by on/off operation.
- A major limitation is their unsuitability for trace oxygen measurements when reducing gases (hydrocarbons of any species, hydrogen, and carbon monoxide) are present in the sample gas. At operating temperatures of 650 degrees Centigrade, the reducing gases will react with the oxygen, consuming it prior to measurement thus producing a lower than actual oxygen reading.
- The magnitude of the error is proportional to the concentration of reducing gas.
- Zirconium oxide oxygen sensors are the “defacto standard” for in-situ combustion control applications.