Tunable Diode Laser Spectrometers

In-Situ Measurement to Measuring Process Conditions at High Speed.

Tunable diode laser spectrometers (TDLS) allow for real-time gas analysis to increase efficiency, safety, throughput, quality, and environmental compliance. The non-contacting sensor allows measurement under severe conditions, such as high temperature, high pressure, corrosive/abrasive conditions, high dust concentration, etc. Maintenance can also be performed without taking the process offline because the TDLS is isolated from the process. The TDLS is a robust process analyzer that contributes to stable and efficient operation.
>>> Application Notes

Chemical Plants

TDLS Series Features

  • In-situ measurement in harsh environments
  • Measurement up to 1500℃
  • Up to 30m optical path lengths
  • Optimal for SIS integration via SIL 2 and 3 certification


Maximize uptime in aggressive processes

Various plant units and piping may contain pressurized, high-temperature, corrosive, combustible, and/or toxic gases which may be problematic for traditional technologies. The TDLS8000 can detect the concentration of process gas in real time without direct contact to allow for quick response to optimize process availability without sacrificing measurement availability.
>>> Principles

Improved OPEX and safety

The TDLS8000 helps reduce total OPEX and maximize output safely
Example: The TDLS8000 can measure up to 30m over the entire length of a furnace at the top of the radiant section to analyze for efficient combustion. Based on gas concentrations, operators can optimize the air-fuel ratio and analyze heat transfer efficiency while ensuring a safe environment.
>>> Solutions

Yokogawa TDLS technology can help to improve operational safety and emissions in a variety of industries

The TDLS8000 was designed to help users meet a variety of operational standards and is capable of being integrated into SIL2 and SIL3 applications.
>>> Operational Improvement

  • Yokogawa’s new TDLS™8000 houses all of the industry’s leading features in one robust device. The platform design is for in situ measurements which negate the need for sample extraction and conditioning.

  • Sampling unnecessary, installation flange is only on one side, installation cost is reduced by half while keeping high speed measurement / high speed response. It can be easily exchanged from existing equipment.

TDLS contributes to improving your work.

Features of TDLS Series

  • Measurement of oxygen, carbon monoxide, methane, ammonia, moisture, carbon dioxide, and other gases in seconds
  • No sampling equipment is required for the majority of measurements, eliminating a large portion of maintenance
  • The only process TDLS platform capable of measuring up to 30m
  • Measurements can be made in high temperature, high pressure, corrosive/abrasive, and high dust conditions because the sensor does not come in contact with process gas
  • Ideal for critical applications with a SIL-rated fast speed of response
  • Maximize efficiency, consider the environment, and reduce emissions of CO, NOx, etc.

Settling with legacy technologies

In a world which is continuously evolving in the direction of increased process optimization while maintaining safe operating state, it’s difficult to retain a competitive advantage without adopting new measurement techniques to stay cutting-edge.
>>> Principles

TDLS provides a safe, efficient and cost-effective solution.

Basic Concept and Measurements of Laser Gas analyzer

Measurement principle of a laser gas analyzer

The tunable diode laser absorption spectrometers (TDLAS) operate by measuring the amount of a laser light that is absorbed as it travels through the gas being measured. Because the sensor does not have contact with the process and there are no moving parts, maintenance is minimized, which decreases downtime and reduces the long-term cost of ownership (LTCO).

The attenuation due to IR absorption is determined by Lambert Beers'Law.

Lambert Beers'Law

Lambert Beer's Law


IR radiation will be attenuated


A highly reliable measurement in varying steam conditions is possible due to Yokogawa’s unique spectral analysis methodology

Yokogawa Electric's original spectral area method is hardly affected by interferences of other gases, and it can be measured with high accuracy by temperature and pressure-compensation.

Spectral Area Method

An integral reference cell retains measurement integrity, even in low absorption streams

The use of a reference cell keeps the peak position locked during trace measurement when the absorption signal is weak.

Up to 50 days of historical data, spectra, and all setting changes are accessible

This data is useful for remote troubleshooting of process issues long after upsets have occurred.


Increased measurement capabilities have allowed our users to safely increase operating efficiencies while reducing emissions.

  • In situ analysis
    Improves safety and process controlling capability due to near real time response
  • Tunable laser
    No moving part so that no consumable exists
  • Non-contact sensor
    Operates in a harsh environment and minimizes maintenance
  • Long-pass optical sensor
    Represents the entire process measured whereas inserted or extractive measurement reflects only a spot.

cearn and safetyThe TruePeak Tunable Diode Laser Spectrometers operate by measuring the amount of a laser light that is absorbed as it travels through the gas being measured. Involving no sensor contact with the process and no moving parts, it leads to a high mean time between failures (MTBF) and hence a low long term cost of ownership (LTCO).

Fired heaters are integral to industrial processes, including hydrocarbon processing and power generation. Specifically designed for the reaction of fuel and air to produce extremely high gas temperatures, heaters transfer this energy to potentially highly flammable process fluids via heat exchangers. They consume large quantities of fuel, produce large quantities of emissions and are a potential safety hazard to personnel and plant.

Yokogawa TDLS analyzers help control fired heater combustion with ever greater accuracy and reliability. There are measureable rewards for operating fired heaters at Low Excess Air (LEA) levels. In LEA combustion control, the lowest level of fuel is consumed and the products of combustion are cooled the least by unused excess air.

The cost benefits of these efficiencies are considerable, with just a single percentage saving in fuel enabling savings of tens or even hundreds of thousands of dollars per year. Controlling air levels just above the point at which incomplete combustion starts also enables the "cleanest burn," helping plants meet environmental emissions requirements. This in particular reduces the emission of NOx.



Maintaining operational efficiency in your furnaces to maximize throughput and minimize fuel consumption while ensuring safe operation is truly a difficult challenge.

Conventional analyzer technologies, such as zirconia and catalytic bead measurements, are point style measurements which can fail to completely capture what is happening in your furnace. In addition to being a potential ignition source, zirconia can have its reading lowered in the presence of combustibles. COE analysis has its own issues due to the minutes-long response time required for typical breakthrough levels in the thousands of ppm in the radiant section of a fired asset, and often require a separate sensor for CH4 measurement. Such issues can have a large impact on how safely your furnace is being operated.
The modern unit operator needs access to the most trustworthy data available to quickly mitigate the occurrence of an unsafe condition if they’re going to optimize asset performance.

The introduction of Yokogawa’s Tunable Diode Laser Spectrometer (TDLS) technology allows for the real-time, in-situ, interference free, reliable, and accurate measurement of oxygen and CO to maximize efficiencies.
It’s a major advance in supporting the entire facility in operating the asset safely and efficiently.


Features and advantages of Yokogawa's TDLS technologies

  • Real-time data for control
    The efficiency is maximized because and safely improves operation efficiency as well as emissions because the air/fuel ratio is always optimized
  • Laser analysis
    The new technology enables the adoption of industry best practices.
  • Non-contact sensor
    Operates in a harsh environment and minimizes maintenance
  • Long-path optical sensor
    Long path average measurement eliminates hot spots hence enables long lifetime of the heat exchange pipe (minimization of costs).


Safety / Process

When it comes to understanding what is happening in your furnace, a reliance on limited data outputs from conventional analyzer technology places a greater onus on the skills of the operator.

TDLS technology reduces the burden on operator competence and the potential for human error by delivering the most trustworthy data on the entire furnace as quickly as possible for optimal asset efficiency and safety.

It’s imperative you have the ability to detect, measure and control LOC (Limiting Oxygen Concentration) to ensure the safety of your facility and your workmates.

Safety / Process

Our customers' operating safety and operating efficiencies have enabled us to reduce operating costs and reduce CO2 and NOx emissions.


The ammonia (NH3) gas is injected to remove the NOx and thus reduce the NOx concentration in the stack flue gas. With conventional NH3 analyzers that perform measurements indirectly, NH3 concentrations are obtained through a sampling system. Therefore, there are problems with the maintenance and running costs of the sampling system, and time delays in measurement. The TDLS200 Laser Analyzer is the solution to all these problems.


Electrolysis plants create hydrogen and chlorine from a brine solution. Chlorine gas generated from the anolyte of the electrolysis tank generally contains between 0.5 to 2.0 vol% H2O. The sample is then cooled and filtered to remove brine, subsequently coming out as wet chlorine gas. The wet gas is sent to a drying tower where it is treated with sulfuric acid to get moisture down to the ppm level. 


In maintaining and managing industrial plants, monitoring waste water pH/ORP is both a legal obligation and an unavoidable necessity for protecting the environment. Monitoring without an attentive eye can lead to severe consequences.


Considering safety and environmental issues such as combustion efficiency and decreasing NOX and CO in exhaust gas, it has become important to control O2 concentration in garbage incineration processes.


H2S management of exhaust gas in black liquor recovery boilers is required to meet an environmental regulations.


Cement is made by heating calcareous and argillaceous materials to a temperature between 1100 and 1500 °C. As this process uses massive amounts of energy, various energy saving measures are taken, including the measurement of oxygen concentrations in exhaust gases to control combustion. 


Reduction of air pollutant emissions is essential to protecting the environment and this is an issue that concerns all industries. Efforts are being made to reduce the emission of harmful substances from municipal refuse incinerators by measuring concentrations of components such as NOx, SO2, CO2, and CO in their exhaust gases.


Spectrometric technology can assess many critical characteristics about products, but it has limits. It can be challenging to determine when the line has been crossed


SABIC is a global manufacturer of polymer resins, film and sheet products, special additives, and chemical intermediates. With operations in more than 50 countries, the company has an enormous variety of processes and plant designs to make its range of products. With so many plants, processes and products, there are frequent opportunities to make improvements with hardware and instrumentation.

A case in point is a reaction process in which oxygen gas is sparged into the reactor, and there is a resulting outlet gas stream. Proper and timely measurement of the oxygen content in this outlet stream is of key importance for reaction control and safety. Reaction progress, control of raw materials input, and reaction sequencing are all affected and dependent on the value of the oxygen concentration reading. The reactor contents and outlet stream can also be in the flammable range depending on conditions, so safety and process considerations call for continuous monitoring of the vent line contents.

TDLS system

A Two-Fold Measuring Function

The safety considerations of monitoring oxygen content in the vent line are very important. As long as the oxygen level remains below a limit, the mixture can be kept below the flammable range and will not undergo combustion. If the process allows the concentration to exceed this limit, it shuts off the oxygen flow to the sparging headers. But this safety consideration is only one of the reasons the measurement is important.

Secondly, the amount of oxygen bubbling through the liquid is an indicator of what is happening in the reaction. Oxygen consumption depends on reaction chemistry and it is a direct indication of the status of the process. Accurate reading of overhead oxygen content is especially important for control of reactant addition and temperature control.

The Challenges of Consistent Measurement

Technologies to measure oxygen in a gas stream are not new, and there are countless applications in chemical manufacturing and other industries where oxygen levels need to be monitored. Combustion processes of any size invariably use some type of oxygen sensor in the flue gas stream to maintain efficiency.

SABIC’s situation proved to be more challenging than most typical applications due to a mix of specific conditions. For many years operators struggled while working with paramagnetic and electrochemical cell sensors due to degradation of the cells, moisture and debris from the process. These sensors are both very common and used in a wide variety of oxygen measuring applications, but they have some key limitations that became apparent when reviewing this process.

Paramagnetic analyzers are sensitive to vibrations and cross-contamination from other gases. Although the application for these reactors does not call for measuring trace amounts of oxygen, there are also sensitivity issues at very low concentrations. Electrochemical cells should be replaced routinely and have sensitivity to different pressures, temperatures and cross-contamination.

Our sampling systems experienced high failure rates with electrochemical components including sampling lines being plugged from the process, filter element clogging, and failing pumps. Moreover, since an individual test during production took more than two minutes, a possibility existed that a climbing oxygen level may not be identified soon enough.

Paramagnetic and electrochemical cell oxygen analyzers have a three-month verification frequency, and the manufacturers recommended maintaining this regimen precisely. Although the testing does not take long, production was delayed in some situations while performing the verification. Delays and Emergency work due to the failures of these types of analyzers resulted in a significant amount of lost production. Due to these and other issues, a more robust oxygen analyzer technology was required.

Tuning in to Laser Technologies

One technology used commonly in combustion processes is tunable diode laser (TDL) spectroscopy, capable of detecting and measuring a variety of gasses, including oxygen, within many contexts. Theoretically, it has the capability to measure oxygen when mixed with toluene, but there was some concern about it being practical for this specific application.

A TDL analyzer sends a beam with a controlled wavelength range through the gas being analyzed to determine which products are present based on which specific wavelengths of light are absorbed. The problem in this case related to the duct size, because the transmitter and receiver should be a minimum distance apart to ensure adequate absorption.

The duct diameter here was less than half the normally recommended distance, so there was some concern as to whether it would deliver its full degree of accuracy, or even work at all. SABIC’s engineers felt the potential benefits to be gained were more than enough to justify installing one analyzer as a test. The performance would be easy to evaluate since the existing sensors were still fully operational and working in parallel.

After two weeks of operation, it was clear the Yokogawa TDL analyzers were performing very well (Figure 1). It was true that they were not delivering the full degree of precision they were capable of due to the short scanning distance, but the precision was high enough to satisfy the needs of the process.


Figure 1. While the duct size for this application was smaller than is usually recommended for TDL analyzers, the tunable diode lasers reliably provided readings with a high enough degree of accuracy for the application, while eliminating the maintenance problems associated with the earlier sensing technologies.

Once installed, the new analyzers proved very reliable and required far less validation and maintenance than the earlier technologies. One issue proved to be debris carried into the duct from the process blocking the light transmission path between the transmitter and receiver. Adjustments to a nitrogen purging system and better control of the process itself minimized this effect, leading to virtually trouble-free operation.

Facilitating the Safety Function

All of these TDL analyzers have been installed for over two years now, with no failures due to the TDLA’s to date. Some units were outfitted with the Yokogawa TDLS200 analyzer, while others were outfitted with the Yokogawa TDLS8000 models. There are other manufacturers of this technology but we chose Yokogawa for this application.

There have been occasional visibility blockage incidents, but these are rare after adjustments to the purge system. Overall, these TDL analyzers have supported higher levels of production, and added another layer of protection to the unit.


With fired heaters, users hope to get greater efficiency and reduced emissions but often are disappointed. Given the number of fired heaters operating every day and their importance in the process industries, any improvements realized across the board will have huge impacts. More units can reach their potential with some simple changes in work practices and technology upgrades.

Yokogawa Technical Report

In recent years, shale gas extraction technology has made rapid progress, inducing a shale gas revolution mainly in the USA. Thus, the need for analysis of hydrocarbon gases, including natural gas, is expected to grow rapidly. Traditionally gas chromatography has been used for the analysis of hydrocarbon gases; it can accurately measure the concentration of each hydrocarbon component in a sample of natural gas.


Combustion furnaces such as heating furnaces and boilers in plants include various sizes and types, and serve as energy sources, that is, they are cores in all production activities. Because a large amount of fuel such as gas or fuel oil is consumed in plants, their combustion efficiency directly affects the performance and running cost of the plants. 

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