Toxic Gas Detection with Enhanced Laser Diode Spectroscopy

A common problem in the oil and gas industry is unreliable readings causing false alarms that cost millions of dollars in lost revenue annually. Safety systems triggered by the detection of potentially hazardous gases typically shut down processes and facilities to prevent potential catastrophe. But when a false alarm triggers the safety system, productivity is lost while the alarm origin is investigated, false alarm status is verified, and processes are slowly brought back on-line.

High false alarm rates, slow detection and poor reliability are problems that frequently plague work site operations when dealing with traditional NDIR or LDS-based open path gas detection systems and specifically in toxic gas leak detection applications.

Sensient Fence Line

A detector with a greater detection range is generally considered an advantage, since it means greater ability to detect the hazard either earlier or from greater distances. However, it is important that the detector does not lead to an increased incidence of false alarms.

Enhanced Laser Diode Spectroscopy (ELDSTM) is a newer class of laser diode spectroscopy for gas detection that significantly increases the sensitivity and reliability of traditional laser-diode-based gas detection and measurement, even in extreme environments.

ELDSTM  laser based gas detection is gas species specific and will only respond to the target gas.


This removes any cross interference and unwanted alarms. Currently ELDSTM products are available for Ammonia, Carbon Dioxide, Hydrogen Sulfide, Hydrogen Chloride, Hydrogen Fluoride and Hydrogen Sulfide.

There is also detector capable of detecting both Hydrogen Sulphide and Methane in one device. Each of the two detection channels is configured and specific to the target gas.

Three big benefits of ELDSTM  over conventional toxic detectors are:

  • Elimination of false alarms
  • Failsafe operation
  • Reduced maintenance


Elimination of false alarms

Key to the elimination of false alarms is a harmonic fingerprint detection method. A Harmonic Fingerprint is a specific set of harmonic components introduced by target gas absorption where the relative amplitudes and phases of the components are known and specific to the target gas absorption line that is being scanned. This technique eliminates the distortion and interference that typically affect conventional laser diode spectroscopy (LDS). The harmonic fingerprint means that you can reliably distinguish between genuine target gas and non-target-gas effects. In addition, this technology is less prone to water vapor interference, thus reducing the negative repercussions of false alarms while improving detection capability for general plant safety.  This is a link to a short Video showing how the technology is applied.

ELDS Harmonic Fingerprint

Failsafe operation

Most fixed gas detectors do not provide the end-user with  live information on the operational status of any device where-as the ELDS uses a daily auto-self test called SimuGas which is a unique auto-test feature available on all units. This test provides daily verification of the systems operation without the need for manual intervention or the need for externally applied test gas or optical filters. As well as providing an alarm in the event of a test failure, each test result is held in an internal event log for future retrieval.

For example; a common trait of solid state semiconductor sensors is their tendency to “go to sleep” when exposed to H2S free air for prolonged periods of time while ELDS will respond to H2S after long periods of H2S free exposure.

Electrochemcial sensors are effective in detecting H2S gas, yet they are not resilient in high heat and prolonged low or high humidity conditions.


Reduced Maintenance

The reduction in maintenance is achieved in at least two ways. One; ELDSTM does not have any consumable parts unlike other fixed point gas detectors, which need their sensing elements replaced on a regular basis. This saves the cost associated spares and service labor.

In addition; the SimuGas auto-self test provides integral, daily auto self-testing information, negating the need for regular manual testing using test gases and all associated costs. Gas detector functional testing is the bane of operators responsible for maintaining conventional fixed gas detector systems yet it is essential for ensuring the safety integrity of an entire system. The SimuGas technology provides operators with the means to perform remote, on-command functional testing of an ELDS gas detector more easily, safely and less expensively than with traditional laser diode spectroscopy gas detector technologies or point detectors.


Our conclusion is the Enhanced Laser Diode Spectroscopy can be utilized to improve performance and safety of toxic gas detection systems, eliminate lost production resulting from false alarms and reduce maintenance costs. Additional information on permanent gas monitoring is available in the  MSA Gas Detection Handbook.

Please let us know what your thoughts and comments?






Trace moisture measurements in gas applications often appear to be simple and straight forward because the theory is relatively well understood; however, when the analyzer system is in service and we see unexpected measurement results, the head scratching begins.

Electrically everything appears to be correct so we assume the probe/sensor must be out of specification, right???

Changing the sensor is not the best first action.  There are other key aspects that should be checked and confirmed first. Low dew point ranges make every detail critical. You are actually measuring molecules so attention to the fine points will be beneficial.


Pressure, temperature and flow rate as well as the analyzer configuration have a significant effect on the resulting measurement because you are measuring a very small component of the total sample.   The sample conditioning system design and operation are just as important as the analyzer and sensor. You might consider these nine common points before going to the expense of changing probes.

  • The sampling system should be allowed to equilibrate for up to (typically) 24 hours following exposure to ambient moisture (atmospheric air).
    • 80% of the dry-down can be reached in a few minutes yet the last 10%-20% of dry-down can take hours.
    • The lower the true dew point is, the longer equilibrium can take.


  • A purge‐type flowmeter aids in determining equilibration point and will aid in qualifying the integrity of seals.
    • If the hygrometer reading changes with a change of flow rate, one or more of the following is to blame:
      • saturation-indices (this is a subject we plan to discuss in a separate blog)
      • System is leaking
      • System is dirty
  • Measurements should be made at system pressure or the highest possible pressure to minimize outgassing of metal parts, wall effects, etc., which will result in higher dew points with increased sensitivity. Other things to remember in regards to pressure and temperature effects:
    • It’s necessary to convert readings taken at atmosphere to equivalent dew point at line pressure if line pressure dew point is what you want, or vice versa.
      • For example, ‐40F (-40C) dew point at 100‐psig line pressure is 16.2 PPMv and equal to ‐70.4F (-56.9C) dew point and  16.2 PPMv at atmosphere pressure.
      • We intend to discus “partial pressure measurements” and Dalton’s law of partial pressures  in an a future blog post.
    • Diffusion of atmospheric water vapor is always present. Even if a “pigtail” discharge system is fashioned, with a throttle valve at the end, the diffusion is only minimized. Results are influenced so much by ambient conditions that the precise measurement of low (less than ‐60C) moisture levels may be very difficult and require attention to good technique.
    • Be aware of the diurnal (day/night) effect. This effect produces a true change in moisture content due to heating/cooling of process surfaces.
  • Any cross-checking between two or more similar instruments should be done at identical time and conditions.
    • This is even more important when dissimilar measurement technologies are compared. It is not possible to use some makes of moisture meters under flowing conditions (in situ) on stream or at line pressures. Some are unstable or are affected by temperature changes and other variables.
  • Cleanliness is essential. Manufactures’ specifications on probe cleaning, sampling and measuring techniques, and interpretation of data should be followed to the letter.
    • Some contaminants can cause false readings or deactivate the probe. The system will have to dry out before data can be taken again. Remember sample must be chemically compatible with aluminum or the sensor will fail.
    • Cleaning an aluminum oxide sensor is relatively simple. All you need is reagent grade hexane or toluene and deionized water.


  • Recalibration should be performed every 12 months or sooner based on your experience in the specific application. There should always be spare probes available. This allows checking probes against another and avoids having the analyzer out of service when the probes are being recalibrated.
  • Engage the process specialist or other resources to assure a common understanding of the process conditions. Everyone involved should be as knowledgeable as possible on the difficulties of moisture measurement, the use of equipment and the meaning of data.
  • Define problems carefully. The right approach to equipment selection and measurement techniques will instill confidence in the end results. Remember you are dealing with a pressure gauge when interpreting sample system phenomenon. The dew point is effected by the pressure the sensor is experiencing. If the sample pressure goes down, the measured dew point of the sample will go down also.
  • Remain open to the possibility that an unexpected dew point is actually correct and something could be amiss with the process itself. A perfectly functioning analyzer system is unbiased. It simply reports what it experiences.

Everyone involved in testing the analyzer system – process engineers, operators and technicians – should be as knowledgeable as possible on the challenges of moisture measurement, the use of equipment and the meaning of the data.

Do not be afraid to ask questions or for support from your vendor, equipment manufacturer or to reach out to us here at goatnuggets. It is likely we have previously experienced something similar to your questions.