NABL 138 Air Quality Monitoring Equipment Calibration 2026: Gas Analyzers, PM Monitors and Traceability
NABL 138 Air Quality Monitoring Equipment Calibration 2026: Gas Analyzers, PM Monitors and Traceability
Updated: 13 September 2026
Air-quality data can influence public-health information, pollution-control decisions, industrial compliance and environmental policy.
But an air-quality monitoring station does not measure pollution simply because a number appears on a screen.
Every analyzer depends on a measurement chain.
Gas analyzers depend on known reference concentrations, dilution systems, gas flow and instrument response.
Particulate analyzers depend on flow, attenuation or other measurement principles, environmental corrections and instrument condition.
Calibration laboratories working on these systems therefore need more than general calibration capability. They need competence specific to environmental monitoring equipment.
What is NABL 138?
NABL 138 is titled:
Specific Criteria for Air Quality Monitoring Equipment Calibration Laboratories.
NABL's current Accreditation Documents register lists:
- Issue: 01
- Issue date: 22 January 2020
- Amendment: 02
- Amendment date: 3 November 2025
Although the latest amendment was issued in 2025, it is the current published NABL 138 version available in 2026.
NABL accreditation for calibration laboratories is based on ISO/IEC 17025, and NABL 138 adds sector-specific criteria for this type of environmental-monitoring equipment.
Which air-quality instruments are covered?
NABL 138 groups monitoring equipment into gas analyzers and particulate-matter analyzers.
| Group | Equipment examples in NABL 138 |
|---|---|
| Gas analyzers | NOx / NO2, CO, SO2, O3, ammonia and benzene analyzers |
| Particulate analyzers | PM2.5 and PM10 analyzers |
The current criteria allow laboratories to apply for additional devices under suitable reference standards where technically justified.
Permanent, on-site and mobile calibration
One important feature of NABL 138 is that air-quality calibration may be performed through different facility modes.
Depending on the approved scope and technical capability, calibration can be associated with:
- permanent laboratory facilities;
- on-site facilities;
- mobile facilities.
This matters because many ambient-air analyzers are installed inside continuous monitoring stations and may be difficult to remove without interrupting monitoring.
On-site calibration can reduce transport risks, but the calibration laboratory must still control the reference equipment, environmental conditions, setup and records sufficiently to produce valid results.
How is a gas analyzer calibrated?
A gas analyzer responds to a known concentration of the target gas.
For example, an SO2 analyzer may be challenged with a certified sulfur-dioxide reference gas at controlled concentrations.
The laboratory then compares the analyzer response with the known reference value.
A complete calibration may evaluate behaviour across multiple points rather than only checking zero and one span point.
Important elements can include:
- zero gas;
- certified reference gas;
- dilution system;
- flow control;
- stabilization time;
- analyzer indication;
- repeatability;
- measurement uncertainty.
What reference gases does NABL 138 require?
The current NABL 138 criteria state that certified span gas or span-gas mixtures from an accredited Reference Material Producer or National Metrology Institute can be used for calibration of gas analyzers.
The criteria also require the diluter to have metrological traceability in accordance with NABL 142.
The competence of the dilution process is to be verified during assessment.
This is important because the certified cylinder alone does not determine the concentration delivered to the analyzer when dilution is used.
The full system matters.
Why gas dilution is a metrology problem
Suppose a laboratory has a high-concentration certified gas but needs several lower concentration points.
A dilution system mixes the reference gas with zero gas to create those concentrations.
The resulting concentration depends on factors such as:
- reference-gas value and uncertainty;
- gas flow measurements;
- flow-controller performance;
- zero-gas purity;
- leakage;
- system stability;
- adsorption or material interaction where relevant.
If the dilution process is poorly characterized, every generated calibration point can be biased.
Reference standards for common gas analyzers
NABL 138 identifies reference approaches for several analyzers.
| Analyzer | Typical reference identified by NABL 138 |
|---|---|
| NOx analyzer | Certified reference NO gas |
| CO analyzer | Certified reference CO gas |
| SO2 analyzer | Certified reference SO2 gas |
| O3 analyzer | Reference ozone photometer |
| Ammonia analyzer | Certified reference gas appropriate to the method |
| Benzene analyzer | Certified reference benzene gas |
Laboratories should use the current NABL 138 and applicable method standards for the exact calibration configuration.
Why ozone calibration is different
Ozone is not handled in exactly the same way as a stable bottled reference gas.
NABL 138 identifies a reference ozone photometer for O3 analyzer calibration.
This reflects the practical need to generate and measure ozone through a controlled reference system.
The laboratory therefore needs competence not only in reading an analyzer but in operating and validating the ozone reference chain.
How are PM2.5 and PM10 analyzers calibrated?
Particulate analyzers present a different measurement problem from gas analyzers.
The current NABL 138 criteria include PM2.5 and PM10 analyzers based on the beta attenuation approach.
The criteria identify requirements including:
- traceable attenuation discs or a beta-attenuation calibration system;
- a reference flow meter;
- temperature and pressure sensors where practically applicable.
NABL 138 specifies a minimum of three traceable discs for attenuation-coefficient calibration in this context.
The reason is that the particulate result depends on more than one part of the analyzer.
Why flow calibration matters for particulate monitoring
PM concentration is related to sampled air volume.
If the analyzer's airflow is incorrect, the reported mass concentration can also be incorrect even if the particle-detection system itself is functioning.
A PM calibration programme should therefore consider the complete measurement chain, including:
- flow;
- pressure;
- temperature;
- attenuation response;
- sampling components;
- instrument software and corrections.
Why CPCB specifications make calibration important
CPCB technical specifications for continuous ambient air-quality monitoring stations include dedicated calibration requirements.
CPCB's CAAQM technical specifications describe a multipoint gas-calibration system capable of supporting calibration of gas analyzers such as SO2, NO, CO, NH3 and BTEX instruments, along with zero-air generation and ozone-generation capability.
CPCB also identifies calibration or calibration-check devices for meteorological parameters such as:
- temperature;
- wind speed;
- wind direction;
- relative humidity;
- solar radiation;
- rainfall.
These specifications show why continuous monitoring stations need measurement infrastructure, not just analyzers.
NABL accreditation and CPCB requirements are not the same thing
This distinction is important.
NABL accreditation demonstrates the competence of a calibration laboratory for activities listed in its approved scope.
CPCB technical requirements address environmental-monitoring equipment and monitoring-station needs.
A NABL-accredited calibration certificate does not automatically prove that an analyzer satisfies every CPCB equipment specification or regulatory requirement.
Likewise, equipment that meets a procurement specification still requires suitable calibration and quality control to maintain confidence in the data it produces.
What does metrological traceability mean here?
Traceability connects a measurement result to recognized references through a documented calibration chain.
For an air-quality calibration laboratory, traceability can involve:
- certified gas standards;
- reference photometers;
- flow standards;
- temperature standards;
- pressure standards;
- traceable attenuation references.
NABL 138 specifically points laboratories toward NABL 142 for metrological traceability of the dilution system.
Traceability does not mean "has a certificate"
A laboratory should evaluate whether the reference standard is suitable for the intended calibration.
Questions include:
- Is the reference within its valid calibration or certification period?
- Does the reference range cover the required calibration points?
- Is the uncertainty suitable?
- Is the reference material stable?
- Are storage conditions controlled?
- Is the dilution or transfer process validated?
- Can the calibration chain be reconstructed?
Measurement uncertainty for air-quality calibration
Air-quality analyzer calibration can involve several uncertainty contributors.
| Calibration area | Possible uncertainty contributors |
|---|---|
| Reference gas | Certified value, stability and reference-material uncertainty |
| Dilution | Flow ratio, controller performance, zero-gas quality and system repeatability |
| Analyzer response | Repeatability, resolution, zero drift and span behaviour |
| PM flow | Reference flow-meter uncertainty, temperature and pressure corrections |
| Attenuation system | Reference-disc value and analyzer response |
| On-site work | Environmental conditions, setup, transport and field stability |
The laboratory should build an uncertainty budget around its actual method rather than copy a generic value from another laboratory.
Why on-site calibration creates extra risk
Environmental monitoring equipment is often calibrated where it is installed.
On-site work can introduce additional influences such as:
- ambient temperature;
- humidity;
- station ventilation;
- power quality;
- gas-line length;
- transport of reference standards;
- restricted access or setup space.
The calibration laboratory should establish which conditions matter and record enough evidence to support the result.
Calibration vs routine zero/span checks
Continuous air-quality stations may perform routine zero and span checks.
These checks are useful for monitoring analyzer behaviour.
They are not automatically equivalent to a full traceable calibration.
| Routine check | Calibration |
|---|---|
| Detects whether instrument response has changed | Establishes response relative to suitable reference standards |
| May use an operational check standard | Uses controlled traceable reference standards and method |
| Supports ongoing station QC | Supports documented metrological traceability and measurement performance |
What should a laboratory prepare for NABL 138 accreditation?
- Define the analyzer scope. Decide which gas or PM analyzers the laboratory can calibrate competently.
- Study the current NABL 138. Use Amendment 02 dated 3 November 2025.
- Establish traceable references. Include certified gases, reference photometers, flow standards and PM references as applicable.
- Validate the calibration method. Include dilution-system competence where relevant.
- Evaluate measurement uncertainty. Base it on the actual calibration process.
- Demonstrate personnel competence. Include gas handling, analyzer operation and field work where applicable.
- Control environmental conditions.
- Establish on-site and mobile procedures. Where these are requested in scope.
- Monitor validity of results. Use suitable checks and comparison evidence.
- Use the current NABL 152 application form. NABL's document register lists Amendment 06 dated 18 August 2026.
Personnel competence needs instrument-specific evidence
A technician competent to calibrate a temperature indicator is not automatically competent to calibrate a chemiluminescence NOx analyzer or beta-attenuation PM monitor.
Competence can require knowledge of:
- analyzer measurement principle;
- gas safety;
- reference-gas handling;
- dilution systems;
- flow measurement;
- instrument diagnostics;
- data logging;
- measurement uncertainty;
- field calibration conditions.
Common mistakes in air-quality calibration
- Using a certified gas but an unverified dilution system.
- Confusing a zero/span check with full calibration.
- Ignoring flow calibration in PM analyzers.
- Using expired or unsuitable reference gases.
- Not accounting for transport effects during on-site work.
- Applying the same uncertainty to every analyzer and range.
- Assuming CPCB equipment compliance automatically demonstrates NABL calibration competence.
- Applying for a wide scope without sufficient reference equipment.
What should customers verify before ordering calibration?
- Is the calibration laboratory currently NABL accredited?
- Is the analyzer type included in the current scope?
- Is the required range covered?
- Is the calibration permanent-facility, on-site or mobile as required?
- Is the CMC suitable for the intended monitoring use?
- Does the certificate identify the reference standards and results clearly?
- Is the laboratory's current scope consistent with the service being offered?
Why this matters for Delhi-NCR and other high-monitoring regions
Cities and industrial regions with extensive ambient-air monitoring depend on a large network of analyzers and monitoring stations.
The value of the network depends on the comparability and reliability of its measurements.
If two stations respond differently to the same pollutant concentration because their analyzers are poorly calibrated, geographic comparisons can become misleading.
Calibration therefore supports data quality across the network, not only the performance of one instrument.
The key principle: air-quality data need a measurement chain
Air pollution is often discussed in terms of AQI values and daily concentrations.
Behind every reported concentration is a measurement chain containing:
- sampling;
- analyzer response;
- reference standards;
- flow;
- temperature and pressure corrections where relevant;
- software;
- quality-control checks;
- calibration.
NABL 138 addresses the competence of laboratories responsible for an important part of that chain.
Reliable environmental decisions start with reliable measurement.
Frequently asked questions
What is NABL 138?
NABL 138 is the Specific Criteria for Air Quality Monitoring Equipment Calibration Laboratories. The current NABL register lists Issue 01 with Amendment 02 dated 3 November 2025.
Which analyzers are covered by NABL 138?
The criteria include gas analyzers for pollutants such as NOx, CO, SO2, ozone, ammonia and benzene, plus PM2.5 and PM10 particulate analyzers. Laboratories may apply for other devices when suitable references and competence are available.
How are gas analyzers calibrated?
Calibration uses controlled known gas concentrations from certified reference gases or suitable reference systems, with traceable dilution and supporting flow controls where applicable.
How are PM2.5 and PM10 analyzers calibrated?
For beta-attenuation instruments, NABL 138 identifies traceable attenuation references or an appropriate calibration system, together with reference flow measurement and temperature or pressure sensing where applicable.
Does NABL calibration mean a monitor is CPCB approved?
No. NABL accreditation demonstrates calibration-laboratory competence for an approved scope. CPCB equipment specifications and environmental-regulatory requirements serve different purposes.
Is ISO/IEC 17025:2017 still current in 2026?
Yes. ISO states that ISO/IEC 17025:2017 was reviewed and confirmed in 2023 and remains the current edition.
Sources checked
- NABL: Accreditation Documents register . The current register lists NABL 138, Issue 01, Amendment 02 dated 3 November 2025 and NABL 152 Amendment 06 dated 18 August 2026.
- NABL 138: Specific Criteria for Air Quality Monitoring Equipment Calibration Laboratories .
- Central Pollution Control Board: Technical Specifications for Continuous Ambient Air Quality Monitoring Stations .
- International Organization for Standardization: ISO/IEC 17025:2017 . ISO confirms that the 2017 edition remains current.
Editorial note: Air-quality monitoring regulations, equipment specifications and accreditation documents can change. This article explains official public information available as of 13 September 2026. Calibration laboratories should use the current NABL 138, NABL 142, NABL 152, ISO/IEC 17025 and applicable CPCB or regulatory requirements when defining a calibration scope or service.
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