Calibration Gas Mixtures: A Complete Guide to Purpose, Standards, and Practical Application

Introduction

Calibration gas mixtures are not merely cylinders of gas. They are the foundation upon which the accuracy of all gas analytical measurements in industry, environmental monitoring, and scientific research rests. Without them, it is impossible to confirm that a gas analyzer on a factory stack is telling the truth, or to ensure that a methane leak detector will provide a timely warning of danger. Calibration gas mixtures are state reference materials of gas composition with precisely known concentrations of components, serving as a standard for adjusting and verifying measuring instruments. In this article, we will take a detailed look at what calibration gas mixtures are, the tasks for which they are used, the standards governing their production in different countries, the state of their manufacture in Central Asia, and how to properly select calibration gas mixtures for the calibration and verification of equipment.

Purpose of Calibration Gas Mixtures

The scope of application of calibration gas mixtures covers virtually all procedures related to controlling the accuracy of gas analytical equipment. Let us begin with a key terminological clarification: verification and calibration are not the same thing. Verification is a mandatory procedure during which the suitability of an instrument for use is established and its conformity of metrological characteristics to established norms is confirmed. Calibration is the establishment of correspondence between the instrument's readings and the actual values of gas concentration, essentially "tying" the instrument's scale to a standard.

Calibration gas mixtures are used to address a wide range of metrological tasks. Primarily, they are used for the graduation, calibration, and verification of gas analytical instruments, sensors, detectors, and measurement systems. Without calibration gas mixtures, it is impossible to establish and monitor the stability of the calibration characteristics of measuring instruments. The mixtures are also indispensable for the validation of measurement procedures — a process confirming that a specific method yields results with the required accuracy. Calibration gas mixtures serve as a means of accuracy control of measurement results performed according to standardized methods during their application.

A crucial role of calibration gas mixtures is the metrological certification of measuring instruments and control of metrological characteristics during testing, including for the purpose of type approval of a measuring instrument. This is a mandatory procedure for adding an instrument to the state register of measuring instruments. Furthermore, calibration gas mixtures are used in the control and certification of test equipment, certification of reference materials of subsequent grades, and for internal laboratory and external quality control of measurement results.

A special place is occupied by interlaboratory comparisons — comparative experiments in which several laboratories analyze the same calibration gas mixture to assess the consistency of the obtained results and confirm the competence of the participants. For such tests, mixtures with particularly stringent requirements for stability and homogeneity of composition are used.

Classification of Calibration Gas Mixtures

Calibration gas mixtures are classified according to several criteria.

By level of accuracy (grade), calibration gas mixtures are subdivided into zero, first, and second grades depending on the permissible error of the actual value of the calibration component content. Zero-grade mixtures possess the highest accuracy and serve for the certification of mixtures of lower grades. The first and second grades are used directly for the verification and calibration of working measuring instruments.

By composition, calibration gas mixtures are divided into single-component (pure gases), binary (two-component), and multi-component mixtures. The certified components can include a wide variety of gases: oxygen, hydrogen, methane, hexane, ethylene, propane, carbon monoxide, carbon dioxide, nitrogen oxides, and many others.

By balance gas, mixtures are prepared based on nitrogen, air, or other inert gases. The choice of balance gas depends on the type of instrument being verified and its operating conditions.

Special mention should be made of zero gas — a gas recommended by the manufacturer that contains no combustible gases or interfering substances, used for calibration and zero adjustment of the device. Without a quality zero gas, it is impossible to correctly establish the "reference point" on the gas analyzer's scale.

Standards Governing the Production of Calibration Gas Mixtures

The production of calibration gas mixtures is a strictly regulated activity, and in different regions of the world, it is governed by its own regulatory documents. Nevertheless, they are based on common metrological principles and international approaches.

European Union and International Standards

The fundamental international standard in the field of preparation of calibration gas mixtures is the ISO 6142 series. The first part of the standard, ISO 6142-1:2015, describes the gravimetric method for preparing Class I mixtures in cylinders with traceable values of the mole fraction of components. In 2024, the second part was published — ISO 6142-2:2024, which applies to Class II mixtures. This document describes a static gravimetric preparation method and the calculation of measurement uncertainty for each component. Unlike Class I mixtures, which undergo individual verification, Class II mixtures undergo validation at the batch level, which is accounted for in the uncertainty budget. European countries adopt ISO 6142 as national standards (e.g., BS EN ISO 6142-1 in the United Kingdom, SR EN ISO 6142-1 in Romania).

Russia

In the Russian Federation, the main document regulating the production of calibration gas mixtures is GOST R 8.976-2019 "State System for Ensuring the Uniformity of Measurements. Reference Materials of the Composition of Calibration Gas Mixtures. General Specifications." The standard came into effect on September 1, 2020, and applies to reference materials of the composition of calibration gas mixtures contained in cylinders under pressure. It is intended for all manufacturing enterprises that produce reference materials for use in the sphere of state regulation of ensuring the uniformity of measurements.

A key document defining the hierarchy of unit transfer is GOST 8.578, which establishes the state verification scheme for instruments measuring the content of components in gaseous media. It is in accordance with this scheme that calibration gas mixtures perform the function of working standards during the verification and calibration of instruments.

Kazakhstan

In the Republic of Kazakhstan, ST RK 2352-2019 "Calibration Gas Mixtures. Technical Specifications" is in force. This standard applies to state reference materials of the composition of gas mixtures, serially produced under the technical name "calibration gas mixtures — reference materials of composition," and performs the function of working standards in accordance with ST RK 2.118 and GOST 8.578.

It is important to note that the Kazakh standard was developed considering international norms: it is based on ISO 6141:2015 and ISO 6142-1:2015. This ensures harmonization of Kazakh requirements with global practices. Approval of state reference materials of the composition of calibration gas mixtures is carried out in accordance with ST RK 2.180 and GOST 8.315.

Central Asia: Current Status of Calibration Gas Mixture Production

The situation with the production of calibration gas mixtures in the countries of Central Asia is uneven, but the overall trend is as follows: domestic production is at an early stage of development, and the region is still heavily dependent on imports.

In Kazakhstan, small-scale production of reference materials of gas mixture composition for the needs of enterprises and organizations was first organized at the Karaganda branch of the Republican State Enterprise "Kazakhstan Institute of Metrology." However, production volumes do not cover the needs of the entire country, and Kazakh enterprises largely rely on imported mixtures, primarily from Russia. It is noteworthy that as early as 2016, scientists from Al-Farabi Kazakh National University proposed creating a specialized center in Kazakhstan for the production of calibration gas mixtures for the needs of the republic's oil and gas industry and the entire Central Asian region. The emergence of such an initiative precisely indicates a shortage of domestic capacity.

In Uzbekistan, the situation is similar. In 2022, the Russian company "Yugra-PGS" entered the markets of Uzbekistan and Kazakhstan as a partner for the supply of calibration gas mixtures. However, work is already underway to localize production: a project for a laboratory for the preparation of gas mixtures is being implemented at the Chirchik branch of the Uzbek National Institute of Metrology (UzNIM). The launch is scheduled for 2026, and at the first stage, based on industrial demand, it is planned to produce 8 types of gas mixtures. The laboratory is being equipped with high-precision measuring equipment and instruments, which will allow it to serve the needs of the oil and gas, mining, and metallurgical sectors, as well as the areas of industrial safety, environmental protection, and healthcare.

In Kyrgyzstan, Tajikistan, and Turkmenistan, domestic production of calibration gas mixtures is practically non-existent. Enterprises in these countries are entirely dependent on imports — mainly from Russia, but also from Kazakhstan, China, and European countries. This creates additional logistical difficulties and increases delivery times, which is especially critical for continuous production where equipment downtime due to the lack of a calibration mixture can result in serious financial losses.

Overall, the Central Asian region is on the threshold of qualitative changes in the field of metrological assurance for gas analysis. The launch of the Uzbek laboratory in 2026 and the development of Kazakh capacities may be the first step toward reducing import dependence and forming a domestic production base. However, fully covering the region's needs will still require years and significant investment.

Concentration Requirements for Calibration Gas Mixtures During Calibration and Verification

The correct choice of calibration gas mixture concentration is a critically important aspect on which the reliability of the instrument's verification or calibration directly depends.

The 80–100% of Upper Measurement Limit Principle

According to established metrological practice, verification of a gas analyzer should be conducted at several points within the measurement range. Typically, this includes three points: 0% (zero gas), 100% of a sub-range, and 90–100% of the final value of the range. For example, if a gas analyzer has a measurement range of 0 to 1000 mg/m³, the verification points might be chosen as 0 mg/m³ (pure nitrogen), 100 mg/m³, and 900–1000 mg/m³.

The main rule to be observed is: the concentration of the calibration gas mixture for calibration and verification must be 80–100% of the upper measurement limit of the instrument for the corresponding range. If an instrument has multiple sub-ranges (e.g., 0–100 mg/m³ and 100–1000 mg/m³), verification is performed for each sub-range separately, and a specific calibration gas mixture with a concentration close to the upper limit is selected for each.

Why exactly 80–100%? The reason is that verification must confirm the instrument's operability across the entire declared range, and the greatest errors typically manifest at the upper sections of the scale. Checking only in the middle of the range does not provide full confidence that the instrument maintains accuracy at the upper measurement boundary.

Gas Mixing Systems as an Alternative to Ready-Made Calibration Gas Mixtures

In some cases, gas mixing systems (gas mixture generators) are used instead of ready-made cylinders of calibration gas mixtures. This occurs when accredited mixtures of the required composition and concentration are unavailable on the market, or when it is necessary to quickly obtain a mixture with non-standard parameters. A gas mixing system mixes source pure gases in specified proportions and delivers the resulting mixture directly to the inlet of the gas analyzer.

This approach is convenient for laboratories with a high throughput of diverse instruments, but it requires the availability of the mixing system itself, high-purity source gases, and qualified personnel. Furthermore, a mixture prepared "on the fly" must be certified, or its composition must be confirmed using reference analytical equipment.

Additional Accuracy Requirements for Calibration Gas Mixtures

In the sphere of state regulation of ensuring the uniformity of measurements, special requirements are imposed on calibration gas mixtures. Mixtures must be prepared from source gases with a purity of the main component ranging from 99.9% to 99.95%. The relative expanded uncertainty of the calibration gas mixture composition must be strictly determined and documented. The production of calibration gas mixtures mandatorily includes incoming inspection of pure gases concerning the main component (balance gas) and impurities.

Recommended Cylinder Volumes for Various Types of Work

The volume of a calibration gas mixture cylinder is not a parameter that can be chosen arbitrarily. A cylinder that is too small will run out at the most inopportune moment, while one that is too large will unjustifiably increase purchase and logistics costs.

Calibration gas mixtures are supplied in small and medium-capacity steel cylinders — from 4 to 40 liters, equipped with diaphragm valves. The pressure of the mixture in the cylinder is usually 5 or 11 MPa (50 or 100 kgf/cm²). Cylinders with volumes of 1, 2, 5, and 10 liters are also available on the market.

What volume should be chosen? The answer depends on the purposes for which the calibration gas mixture is acquired:

• For laboratory verification and calibration of individual instruments, cylinders of 1–4 liters are usually sufficient. One such cylinder is typically enough for a full verification cycle of one or several similar gas analyzers (provided the procedure does not require prolonged purging).

• For verification of gas analytical channels of continuous emission monitoring systems (CEMS) at industrial facilities, larger volume cylinders — from 5 to 10 liters — are required. This is because the verification of stationary systems requires purging long gas lines, and the procedure itself takes considerable time.

• For validation of measurement procedures and interlaboratory comparison tests, where high stability of composition over a long period is required, it is recommended to use cylinders with a volume of 10 liters or more.

• For regular maintenance of a large fleet of instruments in accredited laboratories and at large enterprises, cylinders with a volume of 10–40 liters are the optimal choice — they provide an adequate supply of gas for multiple uses and reduce logistical expenses for frequent cylinder replacement.

It is important to remember that cylinders for calibration gas mixtures are manufactured in accordance with GOST 949 ("Small and Medium Volume Steel Cylinders for Gases at Pp <= 19.6 MPa") and the requirements of the Technical Regulation of the Customs Union TR CU 032/2013 "On Safety of Equipment Operating under Excessive Pressure." Cylinders are painted gray with the black inscription "CALIBRATION GAS MIXTURE."

Practical Recommendations for Working with Calibration Gas Mixtures

The acquisition and use of calibration gas mixtures require adherence to several rules, without which the entire verification procedure may be rendered meaningless:

1. Check certificates. Each batch of calibration gas mixtures must be accompanied by a passport (certificate) indicating the actual concentration values of the components, the error (uncertainty), and the expiration date. Without this document, the mixture cannot be considered a reference material.

2. Observe expiration dates. Calibration gas mixtures have a limited shelf life (usually from 6 to 24 months depending on the composition). Using an expired mixture can lead to gross measurement errors.

3. Monitor cylinder pressure. As gas is consumed, the composition of the mixture in the cylinder may change slightly. Most manufacturers guarantee composition stability at a residual pressure of no less than 0.5–1.0 MPa. It is not recommended to use the mixture after the pressure drops below the critical level.

4. Use quality regulators and gas lines. The regulator must be selected for the specific type of gas (for aggressive components such as SO₂ and NOₓ, special corrosion-resistant regulators are required). Gas lines should be of minimal length and made of materials that do not absorb the measured components (e.g., Teflon or steel tubing with inert coating).

5. Store cylinders correctly. Cylinders with calibration gas mixtures should be stored in a vertical position, away from heat sources, direct sunlight, and at a temperature recommended by the manufacturer (usually from –50°C to +60°C).

Conclusion

Calibration gas mixtures are an indispensable tool for ensuring the uniformity of measurements in gas analysis. Without them, neither verification, calibration, nor validation of gas analytical equipment is possible, and therefore, the reliability of any measurements — from industrial emission monitoring to ensuring safety at hazardous production facilities — is called into question.

The production of calibration gas mixtures worldwide is governed by strict standards: the international ISO 6142 series in Europe and globally, GOST R 8.976-2019 in Russia, and ST RK 2352-2019 in Kazakhstan. In the countries of Central Asia, domestic production is still poorly developed, and the region depends heavily on imports. However, the launch of a laboratory in Uzbekistan in 2026 and the development of capacities in Kazakhstan create prerequisites for a gradual reduction of this dependence.

When selecting calibration gas mixtures for calibration and verification, it is essential to strictly follow the rule: the mixture concentration must be 80–100% of the upper measurement limit of the instrument, and if there are multiple sub-ranges, verification must be performed for each of them separately. The cylinder volume is selected based on the type of work: from 1–4 liters for laboratory verification of individual instruments to 10–40 liters for servicing a large fleet of equipment.

Adhering to these simple yet critically important rules will ensure measurement accuracy, avoid penalties from regulatory authorities, and, most importantly, guarantee the safety of technological processes. Ultimately, saving on the quality of calibration gas mixtures is saving on one's own safety and reputation.