In Brief
Selecting a gas analyzer starts with the measurement requirement, not the technology. Define the target gas, the concentration range, the required response time, the process matrix, and the installation geometry. Then match these requirements to the technology and configuration that satisfies them at the lowest lifecycle cost. TDLAS is the strongest fit for continuous, fast, selective monitoring of one to three specific infrared-active gases with minimal maintenance. FTIR, GC, NDIR, and electrochemical sensors each have conditions where they are the better choice.
Background
Gas analyzer procurement often starts with a product enquiry rather than a measurement specification. An engineer knows the gas, knows the process, and asks a vendor for a quotation. The vendor responds with a product recommendation. If the match is good, the installation works. If the match is poor, the result is a measurement point that under-performs, requires unexpected maintenance, or delivers data that the control system cannot use effectively.
A better approach is to write the measurement specification first, then evaluate technologies against it. This reverses the sequence: the requirement drives the selection, not the vendor’s product line. It also makes the evaluation transparent, because the specification defines the criteria by which competing technologies and products are compared.
This article provides a framework for writing that specification and mapping it to the available technology options, with specific guidance on where TDLAS fits and where other techniques are more appropriate.
Five specification anchors
A gas measurement specification can be organised around five questions. Answering them produces a document that any analyzer vendor can respond to, and that allows an objective comparison of responses.
What gas, at what concentration?
Identify the target species, the expected normal operating range, the minimum detection limit required by the application, and the maximum concentration the analyzer must handle without saturating. If multiple gases must be measured, note whether they are needed simultaneously from the same sample point or whether sequential measurement is acceptable.
This information determines the sensing principle. TDLAS measures one gas per laser module with high selectivity and low detection limits (sub-ppm to low-ppm for most species on the Beamonics platform). NDIR measures one or a few strong infrared absorbers (CO₂, CO, hydrocarbons) at moderate sensitivity. FTIR measures many species simultaneously from a single sample. GC provides definitive identification of complex mixtures in batch mode. Electrochemical sensors measure one gas per cell at low cost.
How fast must the measurement respond?
Define the T90 response time the application requires, considering the timescale of process changes, the bandwidth of the control loop, or the duration of events that must be captured. Note whether the requirement applies to the analyzer alone or to the complete measurement system including sample transport.
TDLAS provides sub-second analyzer response. In a cross-stack BeamStack installation, this is the system response: there is no sample transport delay. In an extractive BeamCell installation, sample transport through tubing adds seconds depending on line length and flow rate. FTIR cycle times are typically seconds to tens of seconds. GC cycles are minutes. Electrochemical T90 is 30 to 120 seconds. NDIR response is typically 1 to 10 seconds at the analyzer, plus sample transport if extractive.
For process control loops with time constants of seconds (combustion air-fuel ratio, SCR ammonia dosing), sub-second analyzer response is a functional requirement. For compliance monitoring reported as hourly or daily averages, the response time is less critical than long-term stability and data completeness.
What is the measurement environment?
Characterise the gas matrix (all species present, not just the target), the temperature, the pressure, the humidity, the dust or particulate loading, the presence of corrosive species, and the condensation risk. These conditions determine whether an in-situ, extractive, or remote measurement is appropriate, and what sample conditioning (if any) is needed.
TDLAS in cross-stack configuration (BeamStack) operates directly in the process gas with no sample conditioning, but requires optical line-of-sight and is affected by dust and condensation on the optical windows. The instrument is IP67-rated and operates from -10 °C to 55 °C, but the process gas temperature along the beam path can be much higher (the laser beam passes through the gas without contacting the instrument enclosure).
TDLAS in extractive configuration (BeamCell) isolates the measurement from the process environment. The flow chamber’s acid-resistant construction tolerates H₂S, HF, and sulfuric acid, but the sample transport path must manage condensation, particulate filtration, and pressure regulation upstream of the cell.
TDLAS in remote stand-off configuration (BeamSight) avoids contact with the gas entirely, measuring at distances up to 30 m (100 m with reflector). It is affected by atmospheric conditions between the instrument and the target: fog, rain, and dust reduce signal. The BeamSight is IP44-rated, suitable for sheltered outdoor or indoor deployment.
How does the measurement integrate with the plant?
Define the required data interfaces (4-20 mA for PLC analog inputs, RS-485/422 for Modbus, relay contacts for alarms), the power supply available at the measurement point, the communication link distance, and any synchronisation requirements between multiple analyzers.
The BeamStack and BeamCell provide 4-20 mA, RS-485/422, relay outputs, trigger inputs, and an expansion connector carrying I²C, SPI, UART, additional analog signals, and sync for multi-unit coordination. Supply voltage is 15 to 32 VDC at 5 W typical. Communication link length is rated to 30 m at up to 3 Mbit/s.
The BeamSight provides Mini USB, I²C, UART, and GPIO. Supply voltage is 9 to 24 VDC at under 5 W. The battery-powered version operates for approximately 5 hours.
If the installation requires 4-20 mA output for a legacy PLC input, Ethernet for a modern DCS, or HART protocol for smart transmitter integration, these must be specified upfront. A mismatch between analyzer output and control system input creates an integration gap that may require external converters or signal conditioners.
What are the calibration, maintenance, and compliance requirements?
Define the calibration regime: does the site require periodic span calibration with certified reference gas, or is verification (confirming that the existing calibration is correct) sufficient? What maintenance access frequency is practical: monthly, quarterly, annually? Are there regulatory method requirements (EN 14181, 40 CFR Part 60, EPA Method 21) that prescribe specific QA/QC procedures?
TDLAS is factory-calibrated against molecular absorption parameters and reference gases. Routine field span calibration is not required; verification with a reference gas can be performed at any desired interval. Routine maintenance consists of optical window inspection and cleaning, typically on a 6 to 12 month interval. This is a substantially lower lifecycle effort than electrochemical sensors (monthly bump tests, annual replacement) or extractive paramagnetic/NDIR systems (filter changes, sample-line maintenance, periodic span calibration).
However, regulatory compliance frameworks may impose their own QA requirements regardless of the analyzer’s inherent stability. If the measurement feeds a CEMS subject to EN 14181, the instrument must pass QAL2 and AST procedures even if it does not need recalibration for analytical reasons. The specification should distinguish between what the analyzer requires and what the regulatory framework requires.
Technology selection matrix
The following summary maps common measurement requirements to the technology most likely to satisfy them. It is a starting point for evaluation, not a deterministic rule.
Continuous measurement of one to three specific gases at low-ppm sensitivity with sub-second response and minimal maintenance: TDLAS. The Beamonics platform covers this in cross-stack (BeamStack), extractive (BeamCell), and remote (BeamSight) configurations.
Simultaneous quantification of many gas species from a single sample: FTIR. Requires extractive sampling with heated lines and conditioned gas, trained operators, and higher capital and maintenance cost.
Definitive compositional analysis of complex gas mixtures: GC. Batch operation with cycle times of minutes. Suited to laboratory, at-line, and periodic reference measurements rather than continuous process control.
Economical measurement of CO₂, CO, or total hydrocarbons at moderate sensitivity in clean, dry samples: NDIR. Lower cost than TDLAS, but susceptible to water vapour interference and limited in selectivity for complex matrices.
Low-cost, compact sensing for personal safety monitors or dense point-sensor networks: electrochemical sensors. Short sensor lifespan and drift between calibrations, but small, inexpensive, and available for many gas species.
These categories are not exclusive. A well-designed measurement architecture may combine TDLAS for fast, continuous process feedback with periodic GC reference analyses, or pair a TDLAS cross-stack measurement on the most critical duct with NDIR instruments on less critical points where the lower cost is justified.
Worked examples
Ammonia slip monitoring after SCR
The measurement point is downstream of a selective catalytic reduction system in a coal-fired boiler. The gas matrix is hot (150 to 350 °C), wet (8% to 15% H₂O), and contains CO₂, SO₂, NOₓ, and residual fly ash. The target is NH₃ at 0 to 20 ppm with sub-second response for dosing control.
TDLAS is the appropriate technology. The BeamStack across the duct provides in-situ NH₃ measurement at 0.2 ppm precision (1 m, 1 s, BM-H-3 TDS R1.7.1) without extracting or conditioning the gas. Purge air on the optical windows manages fly ash deposition. The NH₃ absorption line is selected to avoid interference from H₂O and CO₂. The 4-20 mA output feeds the SCR dosing controller directly.
An extractive NDIR or chemiluminescence NH₃ analyzer would require a heated sample line, a hot-wet converter or cold-dry dilution system, and more complex maintenance. An electrochemical sensor would not survive the gas matrix or achieve the required response time.
Fence-line methane surveillance at a gas storage terminal
The requirement is to detect and approximately localise CH₄ emissions along a 200 m perimeter, triggering investigation when sustained elevated readings are detected. No duct or enclosure is available; the measurement is in open air.
The BeamSight deployed as four or more fixed units covering overlapping sight lines of 30 to 50 m each provides continuous CH₄ detection at 15 ppm·m precision (10 m, 0.5 s, BM-V-2 TDS R1.2.1). The persistence-based alarm logic filters turbulent fluctuations and triggers a field investigation when readings remain elevated across multiple measurement cycles. Battery-powered portable BeamSight units support follow-up walking surveys to localise the source.
An FID or PID on a walking surveyor would provide component-level inspection but cannot maintain continuous unattended coverage. FTIR or GC would be impractical for open-air perimeter surveillance.
Biogas composition monitoring at a wastewater treatment plant
Two anaerobic digesters produce biogas that must be monitored for CH₄, CO₂, and H₂S to track digester health and manage the gas utilisation system. The biogas is saturated with moisture and contains corrosive H₂S at variable concentrations.
A single BeamCell connected to a valve manifold with sample lines from both digesters provides sequential measurement of all three gases from each digester within seconds. CH₄ precision is 1 ppm, CO₂ is 2.5 ppm, H₂S is 1.5 ppm at 0.2 m (BM-H-3-BC TDS R1.6.1). The acid-resistant flow chamber tolerates the H₂S and moisture, and a condensation trap upstream of the cell manages the high dew point.
An NDIR analyzer could measure CH₄ and CO₂ but not H₂S, and would require additional drying to avoid H₂O cross-interference. A GC could provide full biogas composition but operates in batch cycles that are too slow for continuous process feedback. Electrochemical H₂S sensors would require frequent replacement in this corrosive, wet environment.
Practical considerations
No specification is complete without stating the conditions under which the performance values apply. A detection limit quoted without path length, averaging time, pressure, and temperature is not a usable number. When comparing analyzer offerings from different vendors, normalise all performance claims to the same conditions, or request that each vendor state performance at conditions representative of the actual process.
Total cost of ownership should be evaluated over the expected installation life (typically 5 to 10 years), including calibration gas, sensor consumables, sample conditioning maintenance, and the labour cost of each maintenance intervention. The capital cost of the analyzer is often a minor fraction of the lifecycle total, particularly for technologies with high consumable or calibration burdens.
The cost of measurement failure (missing an event, reporting an incorrect value, triggering a false alarm) is real but often unquantified. In safety-critical applications, this cost can be expressed as the probability of a missed alarm multiplied by the consequence. In process-efficiency applications, it can be expressed as the fuel or yield penalty per unit of measurement error. Including these costs, even as estimates, changes the optimisation from a pure capital-cost comparison to a value-of-information comparison that better reflects the actual economics.
Closing remark
A measurement specification written before the technology search begins produces better purchasing decisions, clearer vendor comparisons, and fewer surprises during commissioning. The five anchors described here, gas and range, response time, environment, integration, and calibration model, capture the information needed to match the technology to the requirement. Starting from that match, rather than from a product brochure, is the most reliable path to a measurement point that works as intended over its full service life.
Related links
- How TDLAS works: measurement principle and engineering fundamentals
- Choosing between in-situ, extractive, and remote TDLAS configurations
- Where TDLAS fits among trace gas analysis methods
- Total cost of ownership for TDLAS gas analyzers
- When TDLAS justifies its cost, and when it does not
- BeamStack (BM-H-3) product page
- BeamCell (BM-H-3) product page
- BeamSight (BM-V-2) product page
