Digital Zeus™ HVAC Tool & Instruments Journal › Edit — WordPress

Digital Zeus™ HVAC Tool & Instruments Journal › Edit — WordPress ‏

Oxygen Sensors

Oxygen SensorsThere are a number of possible methods to measure the concentration of oxygen in a gas sample. By far the most common is the electrochemical sensor to measure the concentration directly. The methods regularly used to measure oxygen are:

Electrochemical sensor

Partial pressure sensor

Zirconia sensor

Paramagnetic measurement.

These systems all have advantages and disadvantages for use in a flue gas analyzer, as will be discussed below.

Electrochemical Sensor:
The electrochemical sensor is, as stated above, by far the commonest type used in a flue gas analyzer. The basic functioning of an oxygen sensor is similar to a battery. It functions as a current source. The lifetime depends on the amount of electrolyte and material present for the reaction, as well as the exposure to oxygen, but lies generally in the region of one to two years. The link will lead to a more complete explanation. The major disadvantages of the standard electrochemical oxygen sensor are a cross-sensitivity to carbon dioxide and a tendency to form a carbonate layer on the internal lead electrode when high concentrations of carbon dioxide are encountered regularly. Added to the limited lifetime, this is a serious disadvantage. The great advantage is the simplicity of the sensor and measuring circuit, and a relative lack of sensitivity to pressure changes. This last point is particularly important for equipment such as flue gas analyzers that is used in all countries and at all levels.

The lifetime of the electrochemical sensor can be increased by leaving it open circuit when the instrument is switched off, but this has the disadvantage that the sensor takes about 20 minutes to settle down after reconnection. The commonest solution is to short the terminals of the sensor when not in use. This detracts from the lifetime of the sensor, but means that the sensor can be used immediately after switching on. One possible solution is to disconnect the sensor when the instrument will not be used for a longer period of time, but this is a somewhat unsatisfactory answer, and raises questions about warranty conditions.

Partial pressure sensor:
The partial pressure sensor is very similar in construction to the electrochemical sensor in many ways. This sensor is mainly used for medical or diving purposes, where the effect on the human body is the most important aspect of the measurement. Naturally, this sensor can also be used for other purposes, such as flue gas analyzers. It has the major advantage of not being sensitive to carbon dioxide, which is a major point when biogas measurements are considered. Since this sensor measures the partial pressure of oxygen directly, it is essential to compensate for ambient pressure if a concentration is needed as a result, which is the case with a flue gas analyzer, for instance. With an ambient pressure sensor, the partial pressure sensor can be used outside of the normal pressure range for an electrochemical oxygen sensor (typically atmospheric +/-10 %) and still measure accurately.

Zirconia sensor:
This has always been popular for fixed flue gas analyzers, so-called CEMS. It has advantages, in that it is not sensitive to carbon dioxide, and can also be used inside the stack, not requiring extractive technology. The most common use, however, is for installations where only oxygen is to be measured, for control of a burner system or chemical process, such as heat-treatment. It is also used in the medical sector, and produces very accurate results if used correctly. Below is a short description of the operating theory.

Principles of Operation
Pure zirconium oxide is a monoclinic crystalline material that transforms reversibly to a tetragonal form at 1000°C with a large change in volume. If placed in solid solution, however, with 4 % to 12 % MgO, CaO or Y2O3, it is held in the stable isometric (cubic) form which has no transformation in the range of normal flue temperatures. Due to the addition of these stabilizing oxides, oxygen ion vacancies are created in the crystal lattice. The mobility of O2- ions is greatly enhanced, and under specific conditions of temperature and composition, the conductivity is entirely due to oxygen ions. This condition coincides with the existence of the pure cubic crystalline phase, and is responsible for the oxygen sensing capability of stabilized zirconia.

A minimum quantity of the stabilizing oxides will ensure the existence of the pure cubic crystalline phase of zirconia. When this amount is present, the zirconia is said to be fully stabilized. The commercially available zirconia for oxygen sensors will have generally somewhat less than this minimum amount, resulting in a “partially stabilized” electrolyte with a better resistance to thermal fracture. The zirconia in average sensors contains about 6 mole % (10.5 weight %) of Y2O3. The cell construction demonstrates a characteristic typical of electrolytes having unity transference numbers for an ionic species; there is an electromotive force displayed at the terminals that can be precisely related to the corresponding molecular concentration at the two surfaces. In the case of cubic zirconia, the cell voltage is given by a form of the Nernst equation,

UC = -0.01528TK log10(p0/p1) millivolts

where TK is the absolute temperature in Kelvin, p0 and p1 are the oxygen concentrations at the inner and outer electrodes respectively.

The major disadvantages of the zirconia sensor are a strong cross-sensitivity to any combustible gases and a sensitivity to dirt in the gas. Whilst this is not a problem in medical applications, a flue gas analyzer is often required to measure dirty gases where combustibles may be found.

Paramagnetic sensor:
The paramagnetic oxygen sensor is a highly accurate measurement technique for oxygen concentration. The major disadvantage to this method is the price. Below is the theory of this type of measurement:

Magnetic properties of gases
All paramagnetic measuring instruments available on the market today utilize the paramagnetic properties of oxygen. Oxygen is one of very few gases with a strong magnetic susceptibility. The movement of the electrons within a molecule generates magnetic moments. A distinction must be made in this context between:

- orbital magnetic moment: movement of electrons around the nucleus, within the orbitals

- spin magnetic moment: the electron’s own rotation

External magnetic fields influence these magnetic moments, causing them to align. The orbital magnetic moment responds diamagnetically, in other words aligns in the opposite direction to the external field, thereby weakening it. In contrast, the spin magnetic moment responds paramagnetically, i.e. aligns parallel to the external field and hence strengthens it. Depending on the structure of a molecule, the orbital and spin magnetic moments will be more or less strongly marked, which in turn results in the different magnetic properties of gases. Oxygen has strong paramagnetic properties, while nitrogen responds diamagnetically.

Principle of measurement
There are various different principles of paramagnetic measurement, though in recent years the magnetomechanical or “dumb-bell” principle has come to be used in most measuring instruments. The principle of measurement is based on a sensor in which a dumb-bell comprising two nitrogen-filled spheres is arranged in rotational symmetry within a magnetic field. The gas to be measured passes through the sensor. If the sample gas contains oxygen, the oxygen is drawn into the magnetic field on account of its paramagnetic properties as described above, thereby strengthening the field. The nitrogen inside the glass spheres has the opposite magnetic polarization and is forced out of the field, causing the dumb-bell to rotate. The degree of rotation is directly proportional to the oxygen concentration. To reduce sensitivity to vibration, the dumb-bell’s rotation is no longer measured directly in modern sensors. Instead, a mirror is attached at the dumb-bell’s rotational axis and symmetrically reflects a beam of light onto a pair of photocells. When the dumb-bell starts to rotate, a potential difference is generated at the photocells. The resulting current is amplified and conducted around the dumb-bell through windings. The current flow generates an electromagnetic countermoment which causes the dumb-bell to return to its original position. The current needed to maintain the dumb-bell in its null position is directly proportional to the oxygen concentration.

Conclusion:
Despite years of using the standard electrochemical sensor, the partial pressure sensor is now becoming very attractive due to its lack of reaction to CO2. This is a cumulative action, where the lead electrode slowly reacts with carbon dioxide to form lead carbonate, and effectively shield the electrode from further reaction. There are appropriate ambient pressure sensors on the market at a reasonable price, which makes it quite possible to change the type without great difficulty. The Photon may well be fitted with the partial pressure sensor as standard, as will any flue gas analyzer intended for use with biogas or fitted with a CO2 sensor measuring in excess of 25 % carbon dioxide. There will in future be a conversion kit, allowing standard electrochemical oxygen sensors to be converted to the partial pressure sensor in most flue gas analyzers.

The zirconia sensor is well-known in most countries and is required in some of them. Despite its disadvantages and higher cost, it will remain popular for its simplicity of use, particularly where only oxygen is to be measured for control purposes. It will in future at some point be possible to connect a zirconia sensor to the stationary analyzers, to cover this need. Nevertheless, it will never be the mainstay of oxygen measuring technology for portable flue gas analyzers.

The paramagnetic sensor is a wonderfully accurate piece of technology, but both too expensive and too fragile for use in portable equipment. The paramagnetic sensor is used in many CEMS constructions, often as a standard requirement.

For portable use, the partial pressure sensor will possibly become the sensor of choice for many flue gas analyzer applications in the future, not least for its increased lifetime of about five years.

 

Electrochemical Sensors: Calibration

How often do electrochemical sensors need to be calibrated?

Zero calibration is carried out automatically every time the instrument is switched on, or, as is possible in some madur instruments, every time the auto-calibration of the oxygen sensor is performed (usually used for continuous measurements over a couple of hours – in connection with a gas cooler/drier ).

Span calibration, for toxic sensors, is recommended to be performed at least every six months. Basically, it is dependent on the expectations in regard to accuracy one has and the amount of use the analyser sees. Some companies perform a span calibration every other week. Therefore they are able to achieve accuracies around +/- 2%. An auto-calibration for the oxygen sensors to 20.9% O2 in air is performed every time the instrument is switched on. Or, as is possible in some madur instruments, at pre-set intervals to ensure good readings during continuous measurement sessions.

Calibration is an important factor in the accuracy of an analyser reading. The instrument refers to the calibration results as the standard value at every measurement and has no other way to compensate for changes in the characteristics of the sensors. These do remain reasonably stable over fairly long periods, but a certain drift and tendency to a reduced signal with time is unavoidable. The quality of the calibration gas is also important, as is its homogeneity.

Calibration should be carried out at a set temperature and pressure. The signal from the sensors is defined at 20°C, so a calibration at or near this temperature is to be recommended. Better instruments, including those from madur electronics, have a temperature sensor near the electrochemical gas sensors to allow these effects to be compensated electronically.

Electochemical sensors have a fairly wide range of tolerance on the output at a set concentration, so they cannot be used without calibration. These tolerances do not have anything to do with the quality of the sensors, they are rather a result of the complicated chemical processes involved. Having a rough idea of the calibration value to be expected will allow an experienced service technician to immediately see when a sensor is nearing the end of its useful operating life.

If an analyser is used for extended periods of time it will probably prove necessary to repeat the zero calibration at regular intervals to neutralise any changes that occur with time. Particularly the zero point will tend to drift to a certain degree with time. This can not always be compensated electronically since certain factors are random and a drift that produces a negative result will be automically treated as zero, since negative concentrations are obviously impossible. For this reason, stationary equipment will generally have a programme feature to do this automatically. This will show up as a sudden zero on the results, but such factors can easily be removed later by software.

Electrochemical Sensors: Life Expectancy

What is the expected lifetime of electrochemical sensors?

The expected lifetime of oxygen sensors used to be one year. New developments made possible an oxygen sensor with two years expected lifetime. There are some manufacturers who claim a much longer life-time for their oxygen sensors, but this can only be at the cost of sensitivity, since the signal is caused by consumption of the material of the sensor. To make an electrochemical sensor last longer, one must either reduce the rate of consumption and hence resolution or increase the size of the sensor.
For toxic sensors (CO, NO, NO2, SO2 etc.) two years lifetime is common. Experience shows that one can expect these electrochemical sensors to last about three years if the instruments are maintained properly, and the concentrations the sensors are exposed to are within the range regulations in most developed countries require.

There are, however, a number of factors that will affect he lifetime of electrochemical sensors. They will keep longer if stored at a lower temperature. It is advisable to keep them at a temperature of 20°C or less. Spare sensors are often stored in cellars (basements) or even in refrigerators. They must not be frozen, or the electrolyte will split the housing, but it is quite possible to keep them at about 4°C. Oxygen sensors will react wit hthe atmospheric air. Since this has a high concentration of oxygen, they will, perversely, wear out quicker in storage than in use. If an oxygen sensor is to be out of use for a longer period of time, then it can be removed from the instrument and stored in nitrogen. Theoretically, the whole instrument may be stored in nitrogen, but this is seldom practicable.

The toxic sensors will last longer if the system is thoroughly flushed with fresh air after measurements have been carried out. The time they last will be very dependent on the ranges they have to measure. If they are constantly at thelimit of range, then the lifetime will be reduced dramatically. In such cases it may be possible to use a sensor for a higher range, although this will necessarily reduce the sensitivity and hence the resolution of the system. An electrochemical sensor requires a certain level of humidity, otherwise the electrolyte will dry out and the membrane will be damaged. In flue gas applications this is not a problem, since there is sufficient water vapour present, but may be a factor in other applications. Here, it is probably possible to switch the air flow to ambient air at regular intervals and hence supply the essential water vapour pressure for a certain period of time. Failure to do so will lead to premature failure of the electrochemical sensor and unreliable results in the time beforehand. The sensor does not require much moisture, but completely dry gas is definitely not good.

Another factor that affects the lifetime of these sensors is cross sensitivity. If a component is present in high concentrations which causes a signal to be produced by the sensor, then this will consume the electrolyte of the sensor and reduce the lifetime accordingly. This may not be noticed if the instrument has a cross sensitivity compensation. It is nevertheless a much underestimated factor, as is the effect of extreme temperatures. The active part of a sensor is an electrode and electrolyte. If the electrolyte dries out, then the sensor will no longer function. For this reason, systems using electrochemical sensors should not be operated wit hcompletely dry gases. The level generally quoted is 5 % rH. This will keep the sensor in best condition and operating accurately.

Flue Gas Analyzers: Sensor Technology

Sensor Technology

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.

Oxygen 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.

Electrochemical sensors
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.

Infrared sensors
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.