Some gases absorb unique light wavelengths. This property can be used to detect the concentration of a selected gas in a mixture. An Infrared (IR) light source is focused 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 two ways:
Single Beam Single Wave (SBSW)
Used where a miniature transducer is required. The IR light source is electronically modulated, removing the need for a motor and rotating filters. SBSW is ideal when real-time measurements are needed, being faster than comparable electrochemical techniques. Offering high stability and accuracy, SBSW has low maintenance requirements.
Single Beam Double Wave (SBDW)
A pair of optical filters are mounted on a rotating disc, which pass through a beam of IR light alternately. One filter (the measure filter) is chosen to pass light only at a wavelength that the gas to be measured absorbs, while 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. Offering a similar, fast, real-time measurement response as SBSW, Servomex’s SBDW technique is unaffected by background gases, while the technique is virtually immune to obscuration of the optics. This prevents sensor drift, reducing calibration frequency.