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Photometric Gas Analysis

Some gases absorb unique wavelengths (colours) of light. This property can be used to detect the concentration of a selected gas in a mixture.






A light source (usually in the ultraviolet or infrared range) is focussed through a sample cell containing a continuously flowing sample of the gas mixture to be measured and onto a detector.



Depending on application, this basic concept is applied in three different ways:

SBSW (Single beam single wave)

Used where a miniature transducer is required. The light source is electronically modulated, removing the need for a motor and rotating filters.

Benefits:

Fast response - used where real-time measurements are required, faster compared to commonly used alternative solutions, such as electrochemical techniques.

High stability and accuracy - a specially designed source and temperature control sensor.

Low maintenance requirements - quality manufacture and design.

SBDW (Single beam double wave)

A pair of optical filters are mounted on a rotating disc. They pass through the beam of light alternately. One filter (the measure filter) is chosen to pass light only at a wavelength that the gas to be measured absorbs. The other filter (the reference filter) has a light passed through it at a wavelength unaffected by the gas to be measured.
The difference in absorbance is measured by the detector and provides a direct output of the gas concentration.

Benefits:

Fast response - used where real-time measurements are required, faster compared to commonly used alternative solutions, such as electrochemical techniques.

High stability and accuracy - the single-beam, dual wavelength technique and gas filter correlation (Gfc) are virtually immune to obscuration of the optics. This prevents drift hence reducing calibration frequency.
Unaffected by background gases - a result of Servomex application expertise enhanced by the inherent selectivity of gas filter correlation.

Low maintenance requirements - quality manufacture and design.

(Gfc) Gas filter correlation

Used where extremely accurate low level measurements are required or where background gases have the potential to interfere with the measurement.
The measure and reference filters are replaced with gas filled cuvettes. The reference cuvette is filled with a sample of the gas to be measured, the measure cuvette with (typically) nitrogen. This technique is known as 'Gas Filter Correlation'.

Benefits:

Fast response - used where real-time measurements are required, faster compared to commonly used alternative solutions, such as electrochemical techniques.

High stability and accuracy - the single-beam, dual wavelength technique and gas filter correlation (Gfc) are virtually immune to obscuration of the optics. This prevents drift hence reducing calibration frequency.

Unaffected by background gases - a result of Servomex application expertise enhanced by the inherent selectivity of gas filter correlation.

Low maintenance requirements - quality manufacture and design.