IS & Ex

Process safety transformed by TDL technology

January 2022 IS & Ex

Servomex has introduced a new advantage to safety systems with its unique line lock technology incorporated into the latest tunable diode laser (TDL) analysers. This innovative technology, which is included in all versions of the Servotough Laser 3 Plus series, is designed to avoid the possibility of drift i.e., the analyser losing its lock on the gas it is monitoring.

TDL analysers use a single-line ‘monochromatic’ spectroscopic technique that offers highly stable calibration, a continuous, fast, in-situ measurement and the avoidance of optical cross-interference from other gases.

The TDL system consists of a laser light source, transmitting optics, an optically accessible absorbing medium, receiving optics and detector(s). The signal information is held in the gas absorption line shape, which is obtained by scanning the laser wavelength over the specific absorption line. This causes a reduction of the measured signal intensity, which is detected by a photodiode and used to determine the gas concentration and other properties.

Servomex TDL analysers use a second harmonic detection modulation technique that delivers greater accuracy, sensitivity and reliability of measurement, especially in low ppm-level measurements.

Existing TDL instruments are set up to measure a given gas by measuring the unique absorption wavelength of each. However, if none of the gas is present in the measurement, the risk exists that the analyser can drift to an absorption line of a different gas spectra as it searches for a measurement.

For example, a carbon monoxide (CO) analyser, in the absence of CO, could end up measuring an adjacent water line and thus give a ‘wrong’ reading, creating a significant and potentially dangerous process error.

Servomex’s advanced line lock technology introduces a beam splitter, a cuvette with the gas of interest and a second detector. The beam splitter divides the laser light, sending a second beam towards the cuvette and second detector and providing the instrument with a reference.

The second detector always has a known quantity of gas to measure and therefore, knows where the measurement peak is and how strong it should be. This enables the main detector to be ‘locked’ in position and give the measurement of the gas in the sample – even if it is zero.

Aside from the added reliability gained from ensuring the measurement reading is correct, this technique also gives a clear advantage to safety systems, with the reassurance that the reading being delivered is correct. This gives all combustion, process oxygen and DeNOx/ammonia slip applications covered by the new analysers a vital extra level of process safety.


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