1 May 2005

Gas detection: So much at stake

Photoacoustic infrared technology is the newest method of gas detection.

It enables the detecting of gases at extremely low levels due to its inherent stability and reduced cross-sensitivity.

To understand how photoacoustic infrared technology works, it is important to understand how traditional infrared technology works.

Infrared detection uses infrared light to detect the presence of gas. When a gas is in the presence of infrared light, it absorbs some of the light's energy. Specific gases absorb light at specific wavelengths, allowing the identification of gases by measuring the absorption of light at these wavelengths. Optical filters pass only the particular band of wavelengths for the gas of interest.

Historically, one of the most commonly used forms of infrared detection has been absorptive infrared technology. In an absorptive infrared monitor, a sample of the gas in question goes into the measurement chamber of the monitor and contacts infrared light. Simultaneously, a sample of an inert gas (usually nitrogen) is present in a separate measurement chamber within the same monitor and is the reference gas. By using an inert gas, one ensures no absorption takes place and all the infrared light passes through the chamber. This provides a baseline from which to measure light absorption of the gas in question.

The detector compares the amount of light transmitted through the sample and the reference cells. The monitor can determine the concentration of gas present in the sample by the ratio of light that passes through the sample gas to the light that goes through the reference gas. For example, if the amount of light transmitted through both cells is equal, then the sample cell does not contain the gas of interest. Conversely, the difference between the amount of light transmitted through the sample and reference cells quantitatively determines the concentration of gas in the sample cell.

Building upon the success of basic infrared technology for percent level or high ppm detection, the latest innovation for ambient gas monitoring is photoacoustic infrared technology. This technology also exposes the gas sample to infrared light. However, unlike absorptive infrared, the reading depends upon what happens to the gas after it absorbs the infrared light. With this method, a comparison to a reference sample is not required, so a direct gas reading takes place.

Unlike absorptive infrared

In a photoacoustic infrared instrument, a gas sample passes into the measurement chamber of the monitor, where a specific wavelength of infrared light passes through it. If the sample contains the gas of interest, it will absorb an amount of infrared light proportional to the concentration of gas present in the sample.

However, photoacoustic infrared analysis extends beyond simply measuring how much infrared light is absorbed. Photoacoustic infrared technology observes what happens to the gas once it has absorbed the infrared light. The molecules of any gas are always in motion, and as they move around inside the measurement chamber, they generate pressure. When a gas absorbs infrared light, the molecules' temperatures rise, and the molecules begin to move more rapidly. As a result, the pressure inside the measurement chamber increases. This pressure creates an audible pulse that registers a signal on a microphone located inside the photoacoustic infrared monitor.

gas1

Because the optical filter will only pass the particular wavelength of light for the gas in question, a pressure pulse indicates that that gas is present. If no pressure pulse occurs, then no gas is present. Therefore, temperature or pressure changes will not change the zero reading on the unit. Additionally, this zero reading stands up to the aging of the IR source or microphone since the zero is based upon a true zero reading and not the difference between two readings as in absorptive IR.

The magnitude of the pressure pulse indicates the concentration of the gas present. The stronger the pressure pulse, the more gas that is present. The sensitive microphone inside the monitor can detect the smallest of pressure pulses, enabling it to detect even the lowest levels of gas.

Nicholas Sheble (nsheble@isa.org) edits the Sensors department. This column comes from a white paper that Allan Roczko (Allan.roczko@msanet.com) a manager at MSA (Mine Safety Appliance Company) wrote.

A hundred common industrial gases

For installations that require detection of a refrigerant gas at a very low level, particularly in an environment where cross-sensitivity is an issue, photoacoustic infrared monitors are the best choice.

Photoacoustic infrared monitors provide precise, low-cost, high-performance monitoring for a variety of gases. The monitors can currently detect more than 100 common industrial gases, including refrigerants, carbon monoxide, carbon dioxide, cleaning agents, heat transfer fluids, and a host of common industrial chemicals-with many other applications possible.

Photoacoustic infrared monitoring systems can expand to sample up to eight separate locations. Additional sensors can add within the same instrument enclosure to monitor non-infrared detectable gases.