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Tail Gas Analyzer

The TLG-837 continuously measures the H2S / SO2 ratio in Claus process tail gas to provide an always-reliable air demand signal.

We have release DEMISTER Probe v3 for the TLG-837, featuring the new isolation valve and redesigned cooling extensions. Read more.

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Industry's Leading Tail Gas Analyzer

  • Fast, continuous reading
    Total system response time under 10 seconds
  • Superior accuracy
    ±1% full scale
  • Excellent performance during upset / off-ratio conditions
    Accurate if 100:1 H2S:SO2 20:1
  • Additionally measure COS and CS2
    Using advanced multi-component analysis

Proven Reliability

  • Always online
    Allows for 99.6% uptime
  • Self-Maintaining DEMISTER Probe
    Automated steam cleaning and sulfur removal
  • Unattended operation
    Stabilized by Auto-Zero; remote operation optional

Engineered Safety

  • No process gas inside the analyzer
    Toxic sample gas contained within the probe Learn More
  • Service the analyzer without shutting process
    Ball valve provides easy process seal

Cost of Ownership

  • Proven low-maintenance
    Only requires monthly optics cleaning
  • Easy Installation
    The lightest, most compact tail gas analyzer
  • Solid state construction
    No moving parts or filters to replace
  • Light on utilities
    Low power/steam consumption


  • Suitable for modified Claus
  • Hazardous area certifications
    Class I Div 1 & 2; ATEX; GOST...
  • Customizable alarms & outputs
    4-20 mA analog outputs; MODBUS; Hart...
  • No shelter required
    Rugged instrument suitable for outdoor installation

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TLG-837 customers


The Claus Process

H2S is toxic at 10 ppm, entirely lethal at 800 ppm, highly corrosive to equipment, flammable when in excess of 4.3% by volume in air, and unpleasantly odorous at a threshold of less than 1 ppb.

Unfortunately, H2S occurs abundantly in the world’s fossil fuel reserves. The sulfur recovery unit (SRU) of a refinery is dedicated to processing the H2S stripped from the hydrocarbon fuel through a series of operations that convert it into water and harmless elemental sulfur, which can be sold and repurposed in fertilizer, gunpowder, and more.

The Claus process is the industry standard for treating the H2S-rich “sour” gas. In a furnace, H2S is combusted:

3H2S + 32O2 SO2 + H2O + 2H2S

A catalytic converter reacts the products of the combustion to create elemental sulfur in various crystalline forms:

2H2S + SO2 2H2O + 3XSX

As can be deduced from the second reaction above, the typical Claus reaction runs most efficiently when the stoichiometric ratio of H2S to SO2 is controlled at 2:1. The 1st reaction above demonstrates that this ratio is controlled by adjusting the amount of available oxygen.

Air Demand

As demonstrated above, the efficiency of sulfur recovery hinges on the ability to maintain a set H2S/SO2 ratio in the Claus reaction. This adjustment requires knowing the exact H2S/SO2 ratio in the tail gas at all times.

A tail gas analyzer measures H2S and SO2 in the stream and continuously outputs the “Air Demand” control signal, caclulated by multiplying the expression (2[SO2] - H2S) by a scaling factor. Additionally, operators sometimes require online measurement of COS and CS2 due to side reactions in the reactor.

Related Applications:


Principle of Operation

To analyze the chemical composition of the tail gas, the TLG-837 uses an analysis method known as ultraviolet-visible (UV-Vis) spectroscopy. The system measures a high-resolution absorbance spectrum from 200nm to 800nm, where H2S, SO2, COS, and CS2 all have strong absorbance curves.

Spectrophotometer Principle of Operation

The TLG-837 uses a long-life xenon light source to transmit a signal through the sample fluid in the flow cell within the probe head. The signal is carried by fiber optic cables from the analyzer to the probe, where the sampled tail gas has unique interactions with the light based on its current composition. Learn more

TLG-837 Optical Assembly

Tail Gas Absorbance Spectra

To analyze tail gas, the TLG-837 detects the distinctive absorbance curve of each chemical analyte and mathematically isolates this structure from the total sample absorbance. In accordance with Beer-Lambert Law, the TLG-837 correlates the height of each curve directly to the real-time concentration of its corresponding chemical.

Absorbance Curves: Tail Gas Analytes

Absorbance (AU)
Wavelength (nm)

User Interface

Our proprietary ECLIPSE software processes the raw spectral data to provide real-time concentration readings. The operator can easily navigate between views (trendgraph, spectrum, and more) using intuitive touch-screen navigation. You can also configure alarms, data logging, and outputs. Learn more

ECLIPSE software

Multi-Component Analysis

The TLG-837 simultaneously measures H2S and SO2 (and, optionally, COS and CS2) using a de-convolution algorithm which separates the absorbance curve of each analyte from the total sample absorbance by solving a regression matrix sourced from hundreds of data points. This approach is superior to the "chopper" filter wheel because it has no moving parts, no filters, and no cross-interference. Learn more

Tail Gas Multi-Component Analysis


The TLG-837 uses our patented in situ DEMISTER Probe for tail gas sampling. Learn more about this technology below.


The DEMISTER sampling probe was designed to be lightweight and compact, so it can easily be installed by a single technician. The probe is mounted on the process pipe via flange.

The actual interaction between the sample gas and the light signal occurs in the flow cell disk within the probe head, where fiber optic connections on opposite ends transmit light across the path length of the disk.

Automatic Sulfur Vapor Removal

Tail gas contains elemental sulfur which is quick to condense and plug mechanical cavities or obstruct optical signals. The DEMISTER Probe removes sulfur from the rising sample as an internalized function within the probe body. Recycling the steam generated by the Claus process, the probe controls the temperature along its body at a level where all sulfur vapor in the rising sample condenses and drips back down to the process pipe.

Inside the probe, an internal ‘demister’ chamber (concentric to the probe body) is fed with low pressure steam. Since the LP steam is much cooler than the tail gas, this chamber has a cooling effect on the rising sample. Elemental sulfur has the lowest condensation point of all of the components in the tail gas. Due to the internal probe temperature maintained by the LP steam, all of the elemental sulfur in the rising sample is selectively removed by condensation while a high-integrity sample continues upward for analysis in the probe head.

The point of interaction between the light signal and the sample gas occurs in the flow cell disk inside the probe head. The flow cell disk has a built-in HP steam channel to heat the cell and ensure that any present sulfur remains gaseous—eliminating the possibility of condensation on the optical windows.

An aspirator creates a Venturi effect which pulls the sample up the probe body intake path, through the flow cell for analysis, and down the return line. The used sample is released back into the process pipe.

System Overview

Hover over a component in the diagram below for details.

Utility Control Panel

TLG-837 Utility Control Panel

The Utility Control Panel (UCP) regulates utilities going to the DEMISTER sampling probe for the TLG-837. The UCP is optional; the customer can decide to purchase the UCP from Applied Analytics or build your own integration panel. View datasheet

  • Regulates LP steam pressure for demister chamber in probe body
  • Regulates HP steam pressure for flow cell steam tracing in probe head
  • Provides zero gas for Auto Zero sequence
  • Provides span gas in case Auto Span is desired
  • Controls aspirator flow rate
  • Provides steam failure blowback feature: in the event of faulty steam utilities, the flow cell disk is sealed from the sample and the cell is purged with nitrogen from the UCP

Ultra-Safe Design

TLG-837 safety diagram

The major safety flaw of other tail gas analyzers is that they bring the toxic sample fluid into the analyzer enclosure for analysis. Not only does this practice expose the system electronics to higher corrosion effects, it also poses a lethal threat: if there is any leak in the instrument — especially inside a shelter — the human operator is placed at enormous risk.

The key difference between the TLG-837 and other tail gas analyzers is the use of fiber optic cables: we bring the light to the sample instead of bringing the sample to the light. The toxic sample only needs to circulate through the probe, and never enters the analyzer electronics enclosure.

  • No danger of leaks inside the analyzer because the tail gas does not enter the analyzer enclosure
  • No need for a shelter — system designed for outdoor environment
  • Custom fiber length up to 6 meters allows for distance between analyzer and probe
  • User can safely perform service on the analyzer while process is running — no exposure to sample gas
  • Digital link (e.g. Modbus) provides additional process data during any upset conditions — personnel do not need to physically visit the analyzer during potentially dangerous situations
  • Full port 2” ball valve provides process seal, allowing isolation and removal of probe while process running


Note: All performance specifications are subject to the assumption that the sample conditioning system and unit installation are approved by Applied Analytics. For any other arrangement, please inquire directly with Sales.

Measurement PrincipleDispersive ultraviolet-visible (UV-Vis) absorbance spectrophotometry
Detectornova II™ UV-Vis diode array spectrophotometer
Spectral Range200-800 nm
Light SourcePulsed xenon lamp (average 5 year lifespan)
Signal Transmission600 μm core 1.8 meter fiber optic cables
Other lengths available
Sample IntroductionIn situ DEMISTER Probe
Analyzer CalibrationCalibrated with certified calibration fluids; no re-calibration required after initial calibration; measurement normalized by Auto Zero
Reading VerificationSimple verification with samples
Human Machine InterfaceIndustrial controller with touch-screen LCD display running ECLIPSE™ Software
Data StorageSolid State Drive
Analyzer EnvironmentIndoor/Outdoor (no shelter required)
Ambient TemperatureStandard: 0 to 35 °C (32 to 95 °F)
With optional temperature control: -20 to 55 °C (-4 to 131 °F)
To avoid radiational heating, use of a sunshade is recommended for systems installed in direct sunlight.
Electrical85 to 264 VAC 47 to 63 Hz
Power Consumption65 watts
Instrument Air70 psig (-40 °C dew point)
Steam Pressure70 psig for DEMISTER chamber
30-50 psig for probe blowback function
Standard Outputs1x galvanically isolated 4-20mA analog output per measured analyte
5x digital relay outputs for indication and control
1x K type ungrounded thermocouple input
Optional OutputsModbus TCP/IP; RS-232; RS-485; Fieldbus; Profibus; HART;
Wetted MaterialsStainless Steel 316/316L, Kalrez
Other materials available
Analyzer Enclosurewall-mounted NEMA 4X stainless steel type 304 Enclosure
Other enclosures available
Probe MaterialStainless Steel 316/316L
Other materials available
System DimensionsAnalyzer: 24” H x 20” W x 8” D (610mm H x 508mm W x 203mm D)
Probe (Average): 36” length x 12“ widest diameter (914mm x 305mm)
System WeightAnalyzer: 32 lbs. (15 kg)
Probe (Average): 29 lbs. (13 kg)
Accuracy/RepeatabilityAnalyteTypical RangeAccuracyRepeatability
H2S0-2% ± 1% full scale ± 0.4%
SO20-2% ± 1% full scale ± 0.4%
air demanduser-defined ± 1% full scale ± 0.4%
COS0-2,000 ppm ± 1% full scale ( ±5% under 500 ppm) ± 0.4%
CS20-2,000 ppm ± 1% full scale ( ±5% under 500 ppm) ± 0.4%
Off-Ratio Range100:1 H2S:SO2 20:1
Response Time1-5 seconds
Zero Drift ±0.1% after 1hr warm-up, measured over 24hrs at constant ambient temperature
Sensitivity ±0.1% full scale
Noise ±0.004 AU at 220 nm
Standard Design General Purpose
Available Options ATEX, IECEx, EAC
Please inquire with your sales representative for additional certifications (CSA, FM etc.).


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