Gas alert

A sensor that uses pressure waves to detect the concentration of gas could provide industry with a solution for tracking emissions

A sensor that uses pressure waves to detect the concentration of gas could provide government agencies and industry with a solution for tracking emissions such as carbon dioxide that is smaller and cheaper than alternatives.

The developers hope that in three years they will have a sensor that is less than 2cm long and at least twice as sensitive as other sensors of its size.

The MINIGAS project, a consortium led by VTT Technical Research Centre of Finland, is backed by €2.8m (£2.2m) from the European Union.

The sensor collects air samples inside a chamber and tests the concentration of gas, using the principle that different gases absorb different wavelengths of light.

The system uses a diode laser as a light source. At a specific wavelength the carbon dioxide absorbs light, heats up and the increase in temperature creates pressure inside the sample.

Pressure waves then move a silicon cantilever, which swings like a cat flap inside the device, and its movement is measured with a laser interferometer.

The amount of displacement depends on the intensity of the pressure wave, which is determined by the concentration of gas.

Other gas sensors use a similar photoacoustic principle, in which light waves are turned into pressure waves. But they rely on the waves producing a sound that can be detected with a microphone, measured and used to ascertain the concentration of any gases.

Pentti Karioja, one of the VTT researchers, said that method requires a longer optical path than that of MINIGAS.

He said theoretical analyses of their sensor, which replaces the microphone with a cantilever, show a greater potential for miniaturisation.

‘Typically a photoacoustic device is the size of a shoebox and we hope ours will be the size of a matchbox or smaller,’ he said.

The sensor will be small and versatile enough to be integrated into a wide range of applications.

In addition to detecting greenhouse gases, it will be able to sense explosive vapours and chemical agents such as nerve gases if used in anti-terrorist applications.

It may also be used in energy-efficient consumer products. Karioja said if these sensors are integrated into air conditioning systems, they could measure the amount of carbon dioxide present in buildings.

‘When the number of people in a room increases, the CO2 concentration increases,’ he said. ‘The sensors would detect the increased amount of exhaled CO2 from people and direct the heating, ventilation and air conditioning system to increase the amount of outside air used.’

As the number of people is reduced and CO2 levels decreased, the amount of outside air used would also fall. Such systems have the potential to significantly reduce the amount of energy needed to heat or cool a building.

The MINIGAS consortium includes a range of European institutes and multinationals. It draws on expertise from Finnish company Gasera, which developed the miniature cantilever, and UK defence and security defence technology specialist Qinetiq.

Siobhan Wagner