Oxygen is the third-most abundant element on earth, after hydrogen and helium. It is also a highly reactive element and oxidizing agent that readily forms
compounds—notably oxides—with most other elements. These characteristics mean oxygen needs to be monitored across in a number of industrial applications and
processes to minimize its potential to corrode, contaminate or create hazardous conditions.
Oxygen is also integral to combustion. By measuring the amount of excess oxygen in flue gases, the efficiency of the system can be determined and tuned to
optimize heat rate efficiency which reduces energy and maintenance costs, lowers process variability, improves diagnostics and safety, reduces greenhouse gases,
and to meet regulatory requirements.
Some common applications for oxygen transmitters / analyzers include:
- Monitoring flue gases in boilers, process heaters, kilns and furnaces
- Monitoring of inert gas blanketing of tanks and vessels
- Oxygen measurement in process gases
- Monitoring potentially explosive atmospheres
- Natural gas transport
- Sewage wastewater digester gas
- Air separation and gas blending
- Heat treat and metallurgical processes
- Reactor feed gases
- Centrifuge gases
- Catalyst regeneration
- Inert gas purity
Oxygen Transmitter Technology
There are numerous technologies capable of measuring oxygen with each providing certain advantages and disadvantages. Understanding how a sensor works is the first
step in selecting what’s best for your particular application.
In general there are three sensor technologies that make up most of the industrial oxygen transmitter / analyzer market.
Zirconium oxide sensors
Zirconium oxide sensors rely on the principle that at high temperatures—above 650˚C—oxygen can pass through zirconium oxide where it can be compared to a reference gas
with a known oxygen concentration, usually dry air since it has a constant oxygen concentration of 20.95%.
Zirconium oxide sensors measure Nernst voltage by placing two zirconium dioxide discs which are coated with thin layers of platinum to serve as electrodes
at either end of a platinum ring to form a sealed chamber. An electric current is applied to one of the discs to create the high temperature needed for oxygen to pass
through. At the second disc, the oxygen that has passed through is compared to the reference gas and a small electrical signal is generated in proportion to the difference
between the two concentrations of oxygen ions.
Zirconium oxide sensors are used for oxygen measurements at any level between 0 and 100% in gases or gas mixtures. These sensors can become contaminated by combustible
gases which will greatly affect their accuracy so they should not be used in applications with such.
Paramagnetic sensors
Paramagnetic sensors operate on the principle that oxygen can be attracted into a magnetic field. In fact, oxygen is several hundred times more susceptible to magnetic
fields than most other gases. This physical property is ideal for the determination of the level of oxygen in a wide range of background gases.
There are three types of paramagnetic oxygen sensor commonly used in industry: magnetodynamic, thermomagnetic, and magnetopneumatic.
The magnetodynamic oxygen analyzer is the most popular of the three types, and consists of two nitrogen-filled glass spheres arranged into a dumbbell-shaped
body on a rotating suspension within a magnetic field. As oxygen is attracted into the magnetic field, it displaces the glass spheres, causing them to rotate. The movement
of the dumbbell is detected by a light beam reflected onto a photocell. The amount of movement detected is calibrated to read out the oxygen content in the test sample in percent.
Thermomagnetic oxygen analyzers rely on the fact that the magnetic susceptibility of oxygen decreases inversely with its temperature. Oxygen molecules
within a gas sample are attracted by using a magnetic field. By heating those molecules, their magnetic susceptibility decreases and they are displaced by cooler, more
magnetic molecules. The resulting flow or "magnetic wind" alters the equilibrium temperature between thermistor pairs resulting in a change of the electrical resistance
of the sensors and a signal that is proportional to the oxygen concentration in the measured gas.
Magnetopneumatic oxygen sensors use a nonhomogeneous magnetic field which attracts oxygen molecules within a sample to the pole with the greatest magnetic
strength. A reference gas with a known oxygen content is introduced and analyzed separately creating a differential pressure. The pressure differential is balanced by a
flow of reference gas which is measured and converted into an electrical signal proportional to the pressure differential.
In general, paramagnetic oxygen transmitters offer very good response time and use no consumable parts so that, under normal conditions, they offer an excellent senor life
and performance that doesn’t deteriorate. Paramagnetic technology also offers excellent precision over a range of 1 to 100% though, in general, they are not sensitive enough
to measure trace amounts of oxygen.
Galvanic fuel cell
Galvanic fuel cells are electrochemical devices in which oxygen in a sample chemically reacts with potassium hydroxide within the cell to create a current which is measured
by an external metering circuit connected to electrodes in the cell. The current is is proportional to the concentration (partial pressure) of oxygen present and is indicated
on a meter as a percentage or parts per million of oxygen in the sample.
Galvanic fuel cells are essentially a battery that produces energy when exposed to oxygen. Though they are rugged and insensitive to shock and vibration, they have short
relatively short lifespans since they are “consumed” by exposure to oxygen. As the cells age they also lose sensitivity and require frequent calibration to maintain accuracy.
Things to consider when selecting an oxygen transmitter / analyzer:
- What range is required? Is this to be expressed as a percentage or as part-per-million?
- What level of accuracy is needed?
- Will the transmitter be subject to heat? Vibration? Combustible gases?
- How will the transmitter be mounted?
- What output is required?
- Is the sample gas dirty or wet? Will a sample system be needed?
If you have any questions regarding oxygen transmitters/analyzers please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com
or calling 1-800-884-4967.
