Measurement Methods in Industrial Gas Analyzers: A Complete Guide to Technology Selection

Introduction

Industrial gas analysis is a field where the cost of error is measured not only in monetary terms but also in personnel safety, equipment integrity, and compliance with environmental regulations. When a potential customer faces the need to select a gas analytical system, they inevitably immerse themselves in a world of abbreviations: NDIR, UV, FTIR, TDLAS, FID, CLD... Behind each designation lies a specific physicochemical principle that determines which gases the instrument can measure and with what accuracy, under what conditions it will operate, how frequently maintenance will be required, and ultimately how successfully the system will solve the assigned task.

The key distinction between gas analysis methods can be reduced to a fundamental question: what physical or chemical process do we use to detect and quantify the gas? In some cases, we pass light of a specific wavelength through the gas mixture and measure how much energy has been absorbed (optical spectroscopy). In others, we force gas molecules to undergo a chemical reaction and measure the resulting electric current. In still others, we exploit the unique magnetic properties of individual gases.

In this article, we will thoroughly yet accessibly examine all the main measurement methods used in modern industrial gas analyzers. The goal is to provide the customer with a clear understanding of which method is suitable for their specific task, why, and what compromises are inevitable.

Optical Methods: How Gas "Absorbs" Light

Optical (spectroscopic) methods form the foundation of modern industrial gas analysis. Their general principle is simple: each gas absorbs light radiation at strictly defined wavelengths characteristic of its molecules. By passing light through a gas sample and measuring how much of it has been absorbed, the gas concentration can be calculated. However, different spectral regions and different signal processing approaches give rise to a whole family of methods, each with its own capabilities and limitations.

NDIR: Non-Dispersive Infrared Analysis

NDIR (Non-Dispersive Infrared) is the world's most widely used method of optical gas analysis. Its popularity stems from its simplicity, reliability, and relatively low cost. The operating principle is based on the selective absorption of infrared radiation by gas molecules: each molecule absorbs IR radiation at a specific wavelength, and gas concentration is determined by the degree of attenuation of the radiation passing through the measurement cell.

A key feature of the method is reflected in its name: "non-dispersive" means that the instrument does not use spectral decomposition (dispersion) with a prism or diffraction grating. Instead, narrow-band optical filters tuned to the characteristic wavelength of a specific gas and corresponding detectors are used. This simplifies the design and makes the instrument compact.

Which gases does NDIR measure? The method is effective for detecting virtually all gases whose molecules consist of atoms of different types (heteroatomic molecules): CO₂, CO, CH₄, SO₂, NO, hydrocarbons. However, homonuclear molecules (O₂, N₂, H₂, Cl₂) do not absorb IR radiation, so the NDIR method is inapplicable for their measurement — other technologies are required.

Application areas. NDIR gas analyzers are ubiquitous in industry: monitoring flue gases at power plants and boiler houses, monitoring emissions from cement plants, controlling atmospheres in greenhouses, measuring CO₂ in ventilation systems, as well as in numerous portable instruments for air quality testing.

Limitations. The NDIR method is relatively simple and inexpensive but has a limited capability for simultaneous measurement of many components (each gas requires its own optical filter and detector). At high dust and moisture concentrations, proper sample conditioning is required, and cross-sensitivity (influence of one gas on the measurement of another) may require compensation.

UV: Ultraviolet Spectroscopy

Ultraviolet gas analyzers operate on the same physical principle of light absorption as NDIR, but in the ultraviolet region of the spectrum. This is fundamentally important: some gases that weakly absorb in the IR region have very strong and characteristic absorption bands precisely in the UV range. Such gases include, above all, SO₂ and NO₂, as well as H₂S, Cl₂, O₃, and some others.

Several variations of UV gas analyzers exist. The simplest use the principle of direct UV photometry: a radiation source (e.g., a xenon lamp or UV LED) emits light in the required range, which, after passing through a gas cell, is partially absorbed by gas molecules, and a detector measures the degree of attenuation.

A more complex and sensitive method is UV fluorescence (UVF). Here, not the attenuation of transmitted light is measured, but the glow emitted by gas molecules after excitation by UV radiation. This method is widely used for continuous measurement of low sulfur dioxide (SO₂) concentrations in ambient air — many air quality monitoring stations are built upon it.

Another technology is Differential Optical Absorption Spectroscopy (DOAS). It allows analysis of a gaseous medium over an open optical path (without sample extraction), which is used in path gas analyzers for monitoring pollution over large areas.

Application areas. UV gas analyzers are indispensable where accurate measurement of SO₂ and NO₂ is required: environmental monitoring, emission control at thermal power plants, cement plants, metallurgical facilities, and in the chemical industry for controlling processes involving sulfur-containing gases.

FTIR: Fourier Transform Infrared Spectroscopy — Multi-Component Analysis

FTIR (Fourier Transform Infrared Spectroscopy) represents the pinnacle of optical gas analysis — a method capable of simultaneously measuring up to 15 or more gas components in a single instrument. The operating principle is based on infrared spectroscopy, but unlike NDIR, not a single wavelength but the entire IR spectrum is analyzed simultaneously. The heart of the instrument is a Michelson interferometer that modulates the IR radiation, and Fourier mathematical transformation converts the resulting interferometric signal into a full-fledged absorption spectrum. Dedicated software then "recognizes" the characteristic "fingerprints" of each gas within this spectrum and calculates their concentrations.

The main advantage of FTIR is the ability to measure virtually any IR-active gases (except homonuclear molecules) in a single instrument without prior knowledge of the mixture's composition. This is particularly valuable when analyzing complex multi-component emissions containing dozens of organic and inorganic compounds. FTIR analyzers successfully measure CO, CO₂, NO, NO₂, SO₂, CH₄, HCl, HF, NH₃, formaldehyde, and many other gases.

Application areas. FTIR is the choice for complex tasks: monitoring emissions from waste incineration plants (where the gas composition is extremely diverse), controlling technological processes in the chemical and petrochemical industries, analyzing flue gases from the combustion of alternative fuels, and measuring greenhouse gas emissions. FTIR analyzers are also available in portable versions for inspection control and in open-path systems for monitoring atmospheric pollution over large areas.

Limitations. FTIR analyzers are more complex and expensive than NDIR instruments, require qualified maintenance, and regular spectral calibration. However, their versatility often outweighs these disadvantages, especially when multiple components need to be measured at a single point.

TDLAS: Laser Precision and Selectivity

TDLAS (Tunable Diode Laser Absorption Spectroscopy) is a method that uses a tunable semiconductor laser as the radiation source. The laser generates light of an extremely narrow spectral range, which can be precisely tuned to a specific absorption line of the target gas. The beam passes through the analyzed medium, and a detector measures the attenuation of intensity at the resonant wavelength.

The main advantages of TDLAS are exceptionally high selectivity (the laser "sees" only the desired gas, practically unaffected by other components), fast response (response time can be fractions of a second), and high sensitivity. The method allows measurement of concentrations at the ppb (parts per billion) level. TDLAS analyzers determine concentration dynamics much faster than devices based on other technologies.

Another important advantage is the possibility of non-contact (in-situ) measurement: the laser beam can pass directly through the flue gas duct without requiring sample extraction and conditioning. This eliminates problems associated with gas adsorption in sampling lines and changes in sample composition during transport.

Application areas. TDLAS is sought after where high speed and selectivity are required: monitoring oxygen (O₂) in process gases, measuring ammonia (NH₃) in DeNOx systems, monitoring HCl and HF in emissions, analyzing moisture in natural gas, and measuring concentrations of methane and other hydrocarbons in the oil and gas industry.

Limitations. Each laser is tuned to a specific gas, so measuring multiple components requires either several laser modules or tuning the laser across the spectrum (which increases measurement time). The cost of TDLAS analyzers is higher than NDIR, but in applications where selectivity is critical, they are indispensable.

Non-Optical Methods: Electrochemistry, Flame, Magnetism

Optical methods cover the majority of gas analysis tasks, but not all. Some gases lack characteristic absorption spectra (e.g., oxygen and hydrogen), while others require methods based on entirely different physicochemical principles.

Electrochemical Sensors: Simplicity and Accessibility

Electrochemical gas analyzers are among the most common, especially in portable instruments and safety systems. Their operating principle is based on a chemical reaction between the target gas molecules and an electrolyte inside an electrochemical cell. The reaction generates an electric current whose magnitude is directly proportional to the gas concentration.

Structurally, an electrochemical cell consists of three electrodes (working, reference, and counter) immersed in an electrolyte and separated from the external environment by a gas-permeable membrane. Gas diffuses through the membrane and undergoes an oxidation or reduction reaction at the working electrode, generating a current that is measured by the electronic circuitry.

Advantages. Electrochemical sensors are compact, inexpensive, consume very little power, and can operate in diffusion mode (without forced sample extraction). They are available for a wide range of toxic gases (CO, H₂S, SO₂, NO₂, Cl₂, NH₃, etc.) and oxygen.

Application areas. Portable gas analyzers for personnel safety, stationary gas detectors in industrial premises, workplace air monitoring, and multi-component systems for flue gas monitoring at small boiler houses.

Limitations. Sensors have a limited service life (typically 2-3 years, after which replacement is required), are susceptible to "poisoning" by certain substances, exhibit cross-sensitivity, and have a limited measurement range. They are unsuitable for high concentrations and aggressive environments.

Paramagnetic Method: A Unique Solution for Oxygen

Oxygen is a special gas. Its molecule (O₂) possesses anomalously high magnetic susceptibility compared to the vast majority of other gases. The paramagnetic method for oxygen measurement is based on this unique property, providing exceptionally high selectivity and accuracy.

Modern paramagnetic analyzers employ two main designs. In magnetomechanical sensors, a dumbbell-shaped rotor consisting of two quartz spheres filled with nitrogen is suspended in an inhomogeneous magnetic field within the measurement chamber. When oxygen enters, the spheres are displaced from the field, creating a torque that is compensated by a feedback current — this current is proportional to the O₂ concentration.

In thermomagnetic sensors, the dependence of oxygen's magnetic susceptibility on temperature is exploited: in an inhomogeneous magnetic field and a temperature gradient, thermomagnetic convection arises, altering the resistance of a heated filament, which is measured by the electronics.

Paramagnetic analyzers have no consumable parts, a long service life, and high long-term stability. They are practically insensitive to "poisoning" and do not require frequent replacement of sensing elements.

Application areas. The paramagnetic method is the standard for accurate oxygen measurement in industry: combustion process control at thermal power plants, purity monitoring of process gases in the chemical and petrochemical industries, medical gas analysis, and inert atmosphere monitoring in metallurgy.

Flame Ionization Detector (FID): Champion for Hydrocarbons

FID (Flame Ionization Detector) is a method specifically developed for measuring the total content of hydrocarbons in gas mixtures. The operating principle is simple yet effective: a gas sample is injected into a hydrogen flame, where organic molecules are ionized. The resulting ions create an electric current between two electrodes placed in the flame, and the strength of this current is proportional to the number of carbon atoms in the sample.

FID is extremely sensitive to hydrocarbons (detection limit can be fractions of ppm), has a linear response over a wide concentration range, and practically does not respond to inorganic gases (except those that can extinguish the flame or produce a background signal). Proper operation of FID requires a supply of high-purity hydrogen and air, which complicates the infrastructure.

Application areas. FID is the standard method for measuring total hydrocarbons (THC) in industrial emissions, monitoring volatile organic compounds (VOCs) at chemical and petrochemical plants, detecting methane leaks in the gas industry, and in air quality monitoring systems.

Chemiluminescence Method (CLD): Standard for Nitrogen Oxides

The chemiluminescence method (Chemiluminescence Detection, CLD) is the globally recognized standard for measuring nitrogen oxides (NO and NO₂) in ambient air and industrial emissions. The principle is based on the reaction between nitrogen oxide (NO) molecules and ozone (O₃). This reaction produces an excited nitrogen dioxide molecule (NO₂*), which, upon transitioning to the ground state, emits a photon of light (chemiluminescence). The intensity of this glow is directly proportional to the NO concentration.

To measure NO₂, the sample is first passed through a converter that reduces NO₂ to NO, and then the total NOₓ (NO + NO₂) is measured. The NO₂ concentration is calculated as the difference between NOₓ and the original NO.

The chemiluminescence method provides exceptionally high sensitivity (down to fractions of ppb) and a wide dynamic range, making it indispensable for environmental monitoring. It is standardized internationally (e.g., ISO 10849) for controlling NOₓ emissions from stationary sources.

Application areas. The primary area is measuring nitrogen oxides in ambient air and industrial emissions, monitoring the efficiency of DeNOx systems at power plants and industrial facilities, and scientific research in atmospheric chemistry.

Limitations. CLD analyzers require an ozone source (usually a built-in ozonator) and regular maintenance of the NO₂→NO converter. They are more expensive than simple NDIR gas analyzers, but their accuracy and sensitivity justify the cost in applications where strict compliance with NOₓ limits is critical.

"Cold" and "Hot" Sampling and Measurement Methods

Choosing the measurement technology is only half the battle. The method of delivering the gas sample from the sampling point to the analyzer is equally important. In industrial gas analysis, there are two fundamentally different approaches: "cold-dry" and "hot-wet" (extractive method), as well as non-sampling (in-situ) analysis. The third approach — analysis directly in the flue gas duct without sample extraction — is often used in optical methods (NDIR, TDLAS, DOAS), where the instrument is mounted directly on the stack and the beam passes through the gas stream.

In extractive systems, sample extraction is performed using a probe, after which the gas is delivered to the analyzer via a heated or unheated line. Here, a key question arises: how to handle the moisture and temperature of the sample?

"Cold-Dry" Method

In the "cold" method, the gas sample after extraction is cooled to a temperature below the dew point, and moisture is removed from it (in a condenser or dryer). Dry gas at ambient temperature enters the analyzer. This method is inexpensive, simple to implement and maintain, and perfectly suited for natural gas combustion (where SO₂ and other acid-forming components are absent).

However, when burning coal or fuel oil, where the gas contains SO₂, HCl, and HF, the "cold" method creates a serious problem: upon cooling, these gases partially dissolve in the resulting condensate, and their concentration in the sample reaching the analyzer is underestimated. The result is "beautiful" numbers on the display and actual exceedance of permissible limits.

"Hot-Wet" Method

In the "hot" method, the entire sampling line — from the probe to the measurement cell — is maintained at a temperature above the dew point (usually 180–200 °C). This prevents condensate formation and ensures that all sample components, including SO₂ and HCl, reach the analyzer unchanged. The "hot-wet" method is mandatory for reliable emission measurement when burning coal, fuel oil, as well as at waste incineration plants and in the cement industry.

Non-Sampling (In-Situ) Analysis

An alternative to extractive methods is analysis directly in the flue gas duct. The sensor (optical or electrochemical) is placed directly in the gas stream, and measurement occurs without sample extraction. This approach eliminates sample conditioning issues but imposes increased demands on the sensor's resistance to temperature, dust, and aggressive components. It is often used for O₂ measurement (zirconia sensors) and in TDLAS and DOAS systems.

Comparative Table of Main Gas Analysis Methods

For ease of selection, the key characteristics of the methods are summarized in a table:

Method	Measured Gases	Selectivity	Cost	Maintenance
NDIR	CO, CO₂, CH₄, SO₂, NO, hydrocarbons	Medium (cross-sensitivity possible)	Low	Minimal
UV	SO₂, NO₂, H₂S, Cl₂, O₃	High	Medium	Moderate
FTIR	Virtually all IR-active gases	High	High	Requires expertise
TDLAS	O₂, NH₃, HCl, HF, H₂O, CH₄, etc.	Very high	High	Minimal
Electrochemical	CO, H₂S, SO₂, NO₂, Cl₂, NH₃, O₂, etc.	Medium	Low	Sensor replacement every 2–3 years
Paramagnetic	O₂	Very high	Medium	Minimal
FID	Total hydrocarbons (THC)	High for organics	Medium	Requires H₂ and air
Chemiluminescence	NO, NO₂, NOₓ	Very high	High	Requires O₃ and converter maintenance

How to Choose a Method for a Specific Task

Choosing a gas analysis method is always a search for the optimal compromise between required accuracy, the list of components to be measured, operating conditions, and budget. Here are several typical scenarios:

• Environmental monitoring of industrial emissions. If a standard set (CO, NOₓ, SO₂, O₂) needs to be measured at a gas-fired facility, NDIR + paramagnetic or electrochemical oxygen is optimal. If coal or fuel oil is burned, a "hot" method with SO₂ protection is required. For complex emissions (waste incineration, chemical industry) — FTIR.

• Cement industry. Here, conditions are challenging: high dust load, temperature, SO₂. For combustion optimization and emission control, "hot" extractive systems with NDIR or FTIR are used, as well as in-situ TDLAS for NH₃ in DeNOx systems.

• Oil and gas industry. Hydrocarbon measurement: FID or NDIR. H₂S monitoring: UV or electrochemical. Moisture analysis in natural gas: TDLAS. For portable inspections — electrochemical or optical instruments.

• Chemical industry. A wide range of gases is required: FTIR for multi-component analysis, TDLAS for highly selective measurements (HCl, HF, NH₃), paramagnetic for oxygen.

• Personnel safety. Electrochemical sensors in stationary and portable gas analyzers are the de facto standard due to low cost and compactness.

Conclusion

The diversity of gas analysis methods is not a complication but a flexibility that allows selecting the optimal solution for each specific task. NDIR provides reliability and cost-effectiveness for standard industrial measurements. FTIR and TDLAS enable solving complex multi-component tasks with the highest accuracy. Electrochemical sensors and FID fill the niches of portable monitoring and hydrocarbon measurement. The paramagnetic method remains unsurpassed for oxygen, and the chemiluminescence method for nitrogen oxides.

Understanding the physical principles underlying each method allows not just "buying an instrument," but selecting a tool that will reliably solve the assigned task for many years, with minimal operational costs and in full compliance with regulatory requirements.

Practical recommendations for selection:

1. Determine the complete list of gases to be measured and their expected concentrations. This determines the choice of method.

2. Clarify the conditions at the sampling point: temperature, humidity, dust load, presence of aggressive components. This will determine whether a "hot" method is needed and what type of sample conditioning.

3. Consider the regulatory requirements of your country (GOSTs, O'zMSt standards, ST RK) and international standards, if required.

4. Assess the availability of service support for the chosen method in your region. Even the most advanced instrument requires qualified maintenance.

5. Do not skimp on sample conditioning. Even the most accurate analyzer will yield incorrect results if the sample is improperly prepared.