Gas sensing Panca Mudji Rahardjo, ST.MT Electrical Engineering - UB

Gas sensing and analysis is required in many applications. A primary role of gas sensing is in hazard monitoring to predict the onset of conditions where flammable gases are reaching dangerous concentrations. Danger is quantified in terms of the lower explosive level, which is usually reached when the concentration of gas in air is in the range of between 1% and 5%. Gas sensing also provides a fire detection and prevention function. When materials burn, a variety of gaseous products result. Most sensors that are used for fire detection measure carbon monoxide concentration, as this is the most common combustion product. Early fire detection enables fire extinguishing systems to be triggered, preventing serious damage from occurring in most cases. However, fire prevention is even better than early fire detection, and solid-state sensors, based on a sintered mass of polycrystalline tin oxide, can now detect the gaseous products (generally various types of hydrocarbon) that are generated when materials become hot but before they actually burn.

Gas sensing Catalytic (calorimetric) sensors Paper tape sensors Liquid electrolyte electrochemical cells Solid-state electrochemical cells (zirconia sensor) Catalytic gate FETs Semiconductor (metal oxide) sensors Organic sensors Piezoelectric devices Infra-red absorption Mass spectrometers Gas chromatography

Catalytic (calorimetric) sensors Catalytic sensors, otherwise known as calorimetric sensors, have widespread use for measuring the concentration of flammable gases. Their principle of operation is to measure the heat evolved during the catalytic oxidation of reducing gases. They are cheap and robust but are unsuitable for measuring either very low or very high gas concentrations. The catalysts that have been commonly used in these devices in the past are adversely affected by many common industrial substances such as lead, phosphorus, silicon and sulphur, and this catalyst poisoning has previously prevented this type of device being used in many applications. However, new types of poison-resistant catalyst are now becoming available that are greatly extending the applicability of this type of device.

Liquid electrolyte electrochemical cells These consist of two electrodes separated by electrolyte, to which the measured air supply is directed through a permeable membrane, as shown in Figure The gas in the air to which the cell is sensitive reacts at the electrodes to form ions in the solution. This produces a voltage output from the cell.

Electrochemical cells have stable characteristics and give good measurement sensitivity. However, they are expensive and their durability is relatively poor, with life being generally limited to about one or two years at most. A further restriction is that they cannot be used above temperatures of about 50°C, as their performance deteriorates rapidly at high temperatures because of interference from other atmospheric substances.

The main use of such cells is in measuring toxic gases in satisfaction of health and safety legislation. Versions of the cell for this purpose are currently available to measure carbon monoxide, chlorine, nitrous oxide, hydrogen sulphide and ammonia. Cells to measure other gases are currently under development.

Catalytic gate FETs These consist of field effect transistors with a catalytic, palladium gate that is sensitive to hydrogen ions in the environment. The gate voltage, and hence characteristics of the device, change according to the hydrogen concentration. They can be made sensitive to gases such as hydrogen sulphide, ammonia and hydrocarbons as well as hydrogen. They are cheap and find application in workplace monitoring, in satisfaction of health and safety legislation, and in fire detection (mainly detecting hydrocarbon products).

Detectors of Particular Molecules If a chemical sensing application requires detection of a particular molecule, several techniques are available. These techniques are based on the unique properties of particular molecules. One set of properties is associated with the vibrational and rotational modes of molecules. The exact energies of these modes are generally unique to a particular molecule, and may be used for identification purposes. Most vibrations and rotations are “optically active,” meaning that they may be excited by absorption of a photon, or may relax by emission of a photon. These photon absorptions are generally most likely to occur in the infrared, so infrared spectroscopy is a generally useful way to identify molecules.

Detectors of Particular Molecules CO is a very simple molecule, capable of oscillating at a single frequency (visualize them bouncing together and away) and rotating about two axes, both perpendicular to the line connecting the atoms. In quantum mechanics, a vibration is represented as a single frequency—the molecule may be in the ground state, or in any of a number of excited states, each of which is separated by the energy of the mode: hw/(2π). Quantum mechanics includes "Selection Rules" which strongly favor relaxation a single step of hw/(2π) at a time. This feature shows up in the infrared spectrum as a single absorption.

Besides the vibrations and rotations, molecules may also be recognized by their mass (or mass spectra).

Reference Alan S. Morris. “Measurement and Instrumentation Principles”. Butterworth Heinemann pp Jon S. Wilson. “Sensor Technology Handbook”. Newnes pp William C. Dunn. “Introduction to Instrumentation, sensors, and process control”. Artech House pp