A tunable diode laser reads one narrow absorption line of one gas, so it ignores almost everything else in a hot, dusty flue. That selectivity gives sub second, drift free readings of O2, moisture and ammonia where older analyzers struggle.
Tunable diode laser absorption spectroscopy shines a narrow laser line through the gas and measures how much light one molecule absorbs. This guide explains the principle, the installation styles and the field practice that keeps readings trustworthy.
What Is a TDLAS Analyzer?
A TDLAS analyzer is a gas analyzer that uses a tunable diode laser to scan across a single absorption line of the target molecule and calculates concentration from how much light is absorbed. It belongs to the optical family of gas analyzers and needs no chemical cell or consumable.
The letters stand for tunable diode laser absorption spectroscopy. Unlike a broadband NDIR gas analyzer, the laser linewidth is far narrower than the gas absorption line, so the instrument sees the fine shape of that one line.
AMETEK Process Instruments explains that its 5100 series splits the laser beam into two paths, one through a sealed reference cell and one through the process gas. The reference path keeps the TDLAS analyzer locked on the correct line, a feature often called line lock.
The laser in a typical near infrared analyzer is scanned by changing its drive current, which shifts its wavelength by a tiny amount many times each second. No grating, prism or moving mirror is needed to sweep the spectrum.
How Tunable Diode Laser Spectroscopy Works
Every gas molecule absorbs light only at particular wavelengths that match its vibration and rotation energy levels. Oxygen has a well known band near 760 nanometres, while water vapour, ammonia, CO and CO2 have usable lines in the 1.3 to 2.3 micrometre region.
The TDLAS analyzer ramps the laser wavelength across the chosen line and a photodiode records the transmitted intensity. Where the gas absorbs, a dip appears in the detector signal, and the depth and area of that dip give the concentration.
Endress+Hauser describes wavelength modulation spectroscopy, where the laser is also modulated at a high frequency such as 7.5 kHz and the detector reads the second harmonic, called 2f. This removes low frequency noise and allows parts per billion detection.
Endress+Hauser also notes that Herriott multipass cells can fold an optical path of up to 28 metres into a compact volume. A longer path means more absorption, so trace moisture and trace ammonia become measurable in extractive designs.
Beer Lambert Law and the Concentration Formula
The Beer Lambert law links absorbed light to concentration and path length. The measured absorbance grows in direct proportion to both, provided the absorption stays weak and temperature and pressure are known.
Absorbance A = ln(I0 ÷ I) = k × C × L
C = A ÷ (k × L)
I0 = detector signal with no absorption, I = signal at the line centre
k = effective absorption coefficient from calibration, per percent per metre
L = optical path length in metres
Example:
I0 = 1000 mV, I = 985 mV, k = 0.0025, L = 2 m
A = ln(1000 ÷ 985) = 0.01511
C = 0.01511 ÷ (0.0025 × 2) = 3.02 %
In real instruments the coefficient k depends on gas temperature and pressure, so the firmware applies compensation from live signals. A cross stack oxygen reading therefore usually needs a temperature input from a thermocouple or RTD in the same duct.
Always wire the process temperature and pressure signals to the TDLAS analyzer before commissioning. Running on fixed default values can shift the reading by several percent of value when the furnace load changes.
Absorbance to Concentration Calculator
In Situ Cross Stack vs Extractive Designs
Transmitter and receiver sit on opposite flanges of a duct and read the average gas across the full path.
A single probe with a folded path and reflector is inserted from one side.
Sample is drawn into a heated or multipass cell inside an analyzer house.
Yokogawa reports that its TDLS200 in situ design suits ducts up to about 20 metres wide and gas temperatures as high as 1500 °C. That lets a cross stack TDLAS analyzer read gas at the real process condition without any sample transport as in a gas chromatograph.
An extractive TDLAS analyzer trades speed for control of the gas. The sample can be filtered, heated and held at constant pressure, which suits clean hydrocarbon streams where parts per billion accuracy is needed.
Key Gases a TDLAS Analyzer Measures
| Gas | Typical Line Region | Common Use | Typical Style |
|---|---|---|---|
| O2 | Near 760 nm | Combustion control, safety | Cross stack |
| H2O | Near 1.4 µm | Natural gas dryness, drying | Extractive |
| NH3 | Near 1.5 µm | SCR ammonia slip | Cross stack |
| CO | Near 2.3 µm | Combustion, CO breakthrough | Cross stack |
| H2S | Near 1.6 µm | Sour gas, fuel gas | Extractive |
| CO2 | Near 2.0 µm | Synthesis gas, carbon capture | Extractive |
Yokogawa lists O2, NH3, H2O, CO and CO2 as target gases for its in situ analyzer. For stack reporting, the same components often feed a continuous emissions monitoring system.
TDLAS Analyzer for Combustion and Ammonia Slip
In fired heaters and boilers, the TDLAS analyzer reads average O2 across the whole radiant section, not one point. Many plants still keep a zirconia oxygen analyzer in the flue, but zirconia cells run hot and can be affected by combustibles.
In SCR denitration, excess ammonia that passes the catalyst is called ammonia slip. Yokogawa describes using the laser NH3 reading to trim ammonia injection, which saves reagent and limits ammonium bisulphate fouling of the air heater, a common problem around boiler back ends.
7 Proven TDLAS Analyzer Commissioning Steps
AMETEK stresses that stream composition, phase, temperature and pressure must be known before choosing an analyzer, because other species can absorb at nearby wavelengths. Skipping this first step is the most common cause of a disappointing installation.
Beam alignment of a TDLAS analyzer is done with the process hot, since duct walls move as they heat. Store the transmission figure after alignment, because a later drop in that value is the earliest warning of dust, window fouling or drift in the mounting, just as trend data helps in oxygen analyzer calibration.
Never switch off the window purge while the duct is hot. Flue gas then condenses on the windows within minutes and the transmission can fall below the working limit.
Advantages and Limitations
- Highly selective, one line of one gas.
- Sub second response suits control loops.
- No consumables or moving parts in the optics.
- In situ designs avoid sample transport delay.
- Path average reading across the whole duct.
- Each laser usually measures one or two gases.
- Heavy dust can block the beam.
- Needs accurate temperature and pressure input.
- Alignment and purge must be maintained.
- Higher first cost than a point analyzer.
Worked Example: Path Averaged Ammonia Slip
Many cross stack analyzers report an integrated value in ppm·m, which is concentration multiplied by path length. Suppose an SCR outlet duct has an optical path of 5 metres and the instrument shows 10 ppm·m for NH3.
The path average concentration is 10 ÷ 5 = 2 ppm. If the plant target is a slip of 2 to 3 ppm, the reading is on the edge, and the control system should trim injection slightly before the air heater fouls.
Troubleshooting Common Problems
- Check transmission percentage against the commissioning baseline.
- Inspect purge air flow and window cleanliness.
- Confirm live temperature and pressure signals are healthy.
- Verify line lock status and reference cell alarms.
- Look for beam misalignment after shutdowns or duct work.
- Check for new gases in the stream that may interfere.
- Compare against a portable reference analyzer or known gas.
A TDLAS analyzer reading that drops to zero with a transmission alarm usually means a blocked beam, not low gas. Good alarm design, as in alarm management to ISA 18.2, separates such diagnostic alarms from process alarms so operators respond correctly.
If the output is noisy while transmission is fine, check the 4 to 20 mA loop shielding and grounding before suspecting the optics. Most modern units also carry HART or Modbus diagnostics that show line position and signal strength.
Selection and Hazardous Area Notes
Many TDLAS installations sit in classified areas such as fuel gas lines or refinery heaters. The transmitter and receiver heads are then supplied as flameproof or purged designs, so check the zone and gas group using hazardous area classification before ordering.
For choosing between protection methods, compare explosion proof and intrinsically safe instruments. Also plan platform access at both flanges, since alignment and window cleaning need people at each side of the duct.
Before buying a TDLAS analyzer, ask the vendor for an interference study based on your actual gas composition. A written line selection report saves weeks of arguments if the reading later disagrees with lab results.
Endress+Hauser quotes repeatability better than plus or minus 50 ppb for NH3 in ethylene with its TDLAS analyzer. Such trace levels are important because ammonia poisons downstream polymerisation catalysts.
TDLAS Analyzer Reference Document
TDLAS Oxygen Sensor Video
TDLAS Analyzer FAQ
It is a gas analyzer that scans a tunable diode laser across one narrow absorption line of the target gas. The amount of light absorbed at that line gives the gas concentration directly.
Because the laser line is extremely narrow, other gases rarely interfere with the reading. It needs no chemical sensor and responds in well under a second.
Common targets are O2, H2O, NH3, CO, CO2, H2S and methane. Each gas has its own absorption lines, so the laser wavelength is chosen for that gas.
Yokogawa lists O2, NH3, H2O, CO and CO2 for its in situ model. Endress+Hauser adds trace H2S, CO2 and moisture in natural gas for its extractive TDLAS analyzer range.
An in situ unit sends the beam straight across the duct and reads the gas at process conditions. It is fast and avoids any sample transport delay.
An extractive unit draws a sample into a cell inside an analyzer house. It suits clean streams where trace accuracy and a controlled cell temperature matter more than speed.
The strength and width of an absorption line change with gas temperature and pressure. Without compensation, the same gas mixture would read differently every time the furnace load changes.
Most TDLAS analyzer models accept live 4 to 20 mA inputs for both values. Fixed defaults are acceptable only when the process conditions are truly steady.
Dust scatters light and lowers the overall transmission, but it does not absorb at one narrow line like the gas does. The analyzer normalises the absorption dip to the remaining light.
Readings stay valid until transmission falls below the vendor limit. Beyond that point the unit raises a diagnostic alarm instead of reporting a false value.
Ammonia slip is the unreacted NH3 that passes straight through an SCR catalyst bed. It wastes costly reagent and forms ammonium bisulphate that fouls the air heater downstream.
A cross stack TDLAS analyzer reading lets the control system trim injection in real time. Plants usually aim to keep the slip at only a few ppm.
Line lock and stable absorption physics keep TDLAS analyzer drift very low compared with many sensor types. Most sites simply do a periodic verification with a certified known gas or a flow through cell.
Follow the vendor interval and your plant quality procedure. Log the transmission and line position at each check to spot slow changes early.
Related Articles
- Zirconia Oxygen Analyzer Working Principle
- NDIR Gas Analyzer Working Principle
- CEMS Continuous Emissions Monitoring System
- Gas Chromatograph Basics for Process Analytics
- Oxygen Analyzer Calibration
External References
- TDLS200 Tunable Diode Laser Gas Analyzer and Its Applications, Yokogawa Technical Report
- Principles of Tunable Diode Laser Absorption Spectroscopy, Endress+Hauser
- Tunable Diode Laser Absorption Spectroscopy, Wikipedia
What We Learn Today
- A TDLAS analyzer scans a narrow laser across one absorption line, so it measures one gas selectively with sub second response and no consumables.
- The Beer Lambert law gives concentration as absorbance divided by the absorption coefficient times path length, with live temperature and pressure compensation.
- Cross stack designs read average O2, CO and NH3 slip across ducts up to about 20 metres, while extractive cells reach parts per billion levels.