The flue gas analyzer is basically a collection of different sensors and a central unit that collates all the results and provides a useful and understandable result. To carry out this task, a battery of different sensors are needed in the instrument and here are descriptions of some of them. Most of these sensors are designed to measure gas or gas concentration. Naturally, other sensors for pressure, temperature and humidity are also needed. The descriptions will be added in time.
The history of gas measurement has shown great changes since the first sensors allowing a continuous measurement of gas concentration were invented. These made it possible to combine modern electronics with gas measurement, which was not possible with the old chemical methods. Development has naturally been accelerated by the new interest in environmental matters and the realisation that changes must be made in this direction. There will be no discussion of the chemical measurement techniques used earlier and still used in some areas today. These cannot really be classified as sensors.
There are many different types of oxygen sensor available, depending on the application, interfering gases and a few other factors. These range from the expensive paramagnetic sensors to the standard electrochemical sensors with a limited lifetime. Oxygen sensors, in contrast to the sensors for toxic gases, operate as a current source, not a voltage source. Oxygen is not optically active, so it cannot be measured with infrared technology. Oxygen in this case is taken to be O2, ozone, O3, is a separate matter entirely. It is seldom encountered in classical flue gas applications.
The standard sensor for toxic gases is still the electrochemical sensor. This sensor acts as a battery, producing a voltage proportional to the concentration of the gas it is designed to measure. The great advantages of the electrochemical toxic sensor are the relatively low initial price and the small size. The disadvantages include a limited lifetime and cross-sensitivity problems. Stability can be ensured by regular calibration, but the sensor requires a minimum level of oxygen and humidity to operate correctly as well. Electrochemical technology will be with us for many years to come, and will probably never disappear entirely. The convenience of small and relatively robust sensors will always be of use in personal protection devices and the like.
Portable gas analysis equipment is starting to use infrared sensors for certain gases now. This started with the necessity of measuring certain components, notably carbon dioxide, which are difficult or impossible to measure in any other way. The other component commonly measured is methane, which is otherwise very difficult to evaluate. Most of these sensors are of the NDIR type, relying on absorption of the infrared wavelengths to measure the concentration of gas present. Infrared sensors have been used on continuous monitors for many years to measure the other gases as well, and this technology is now finding its way into portable flue gas analyzers. One of the major disadvantages of infrared sensors is the size required to provide good resolution to the signal. Longer sensors are needed for lower concentrations. This is sometimes solved by using mirrored chambers and multiple pass systems, but the mirrors are really only an extra surface that can collect dirt and reduce the signal strength.