15) What type of sensors are used in instruments?

Most of the sensors used in madur flue-gas analysers are of the electrochemical type.

The major elements of Toxic Gas electrochemical sensors are three coated electrodes (sensing, counter, and reference) and a small volume of an acidic or alkaline solution.

In use, the gases diffuse through an orifice on the sensing face of the sensor onto the electrode surface and cause a small electrical current. This current is amplified and measured by the electronics. The measured value is then displayed and available for printing, storing or downloading to a computer.

In its simplest form, a sensor operating on electrochemical principles requires two electrodes x a sensing and a counter x separated by a thin layer of electrolyte. Gas diffusing to the sensing electrode reacts at the surface of the electrode either by oxidation or reduction. This reaction causes the potential of the electrode to rise or fall with respect to the counter electrode. With a resistor connected across the electrodes, a current is generated which can be detected and used to determine the concentration of gas present.

One of the conditions required for the above sensor to work accurately is that the potential of the counter electrode should remain constant. In reality, however, the surface reactions at each electrode causes them to polarise. This effect may be small initially, but it increases with the level of reactant gas and effectively limits the concentration range the sensor can be used to measure. This effect can be counteracted by the introduction of a reference electrode of stable potential.

The reference electrode is shielded from any reaction, and so maintains a constant potential. Instead of the signal therefore being measured between the counter and sensing electrodes, it can now be more accurately measured between reference and sensing. With this arrangement, the change in potential of the sensing electrode is due solely to the current generated at the electrode by the reactant gas.

As the reference electrode must maintain a constant potential for correct operation, it is important that no current is drawn from this electrode. In order therefore to measure the potential difference between sensing and reference, it is not sufficient just to place a load resistor across them, as this would draw current. For this reason a potentiostatic feedback operating circuit is used.

The oxidation of carbon monoxide, for example, at the sensing electrode can be represented by the equation:

The counter electrode acts to balance out the reaction at the sensing electrode by reducing oxygen in air to water:

A similar equation can be given for other sensors depending on the reaction of the gas they are designed for on the sensing electrode:

Sulphur dioxide: SO₂ + 2H₂SO₄ ===> CO₂ + 2H⁺ + 2e^-

Nitric oxide: NO + 2H₂O ===> HNO₃ + 3H⁺ + 3e^-

Nitrogen dioxide: NO₂ + 2H⁺ + 2e^- ===> NO + H₂O

Oxygen sensors are slightly different. In use, oxygen diffuses through a membrane and the gas contacts the sensing electrode and the base solution and reacts at the wet surface of the electrode, this reaction consumes the counter electrode. The chemical change in the counter electrode allows a circuit in the instrument to measure a potential (voltage) between the electrodes. In reality, the oxygen sensor acts as a current source, so the voltage measurement must be carried out over a load resistor. This should not be large, otherwise the balance ofthe oxygen circuit will be upset.

All oxygen sensors used are of the self-powered, diffusion limited, metal-air battery type comprising an anode, electrolyte and an air cathode as shown below.

At the cathode oxygen is reduced to hydroxyl ions according to the equation:

The hydroxyl ions in turn oxidise the metal anode as follows:

Overall the cell reaction may be represented as:

The oxygen sensors used are current generators, and the current is proportional to the rate of oxygen consumption (Faraday's Law). This current can be measured by connecting a resistor across the output terminals to produce a voltage signal. If the passage of oxygen into the sensor is purely diffusion limited, this signal is a measure of oxygen concentration.

Other types of sensors may also be used, such as infrared sensors.