# Calibration gas

A calibration gas is a reference gas or gas mixture used as comparative standard in the calibration of analytical instruments, like gas analysers or gas detectors. Therefore, a calibration gas has to be of a precisely defined nature or composition, like zero gas or span gas, for example 500 ppm carbon monoxide in nitrogen.
Calibration gas is known by several names such as cal gas, span gas (used during span calibration), test gas (for general verification), reference gas (as it provides a known reference point), standard gas (due to standardized concentration), and certified NIST calibration gas (emphasizing NIST traceability).
Calibration gas requires traceability to a national or international standard. Traceability is the unbroken chain of comparisons to an acceptable international standard. The calibration gas standard establishes a known analyzer response to a certified chemical component concentration.

## National Institute of Standards and Technology (NIST)

In the USA, NIST (National Institute of Standards and Technology) serves as the United States' national metrology institute and plays a crucial role in providing calibration reference gases used across industries. NIST develops, certifies, and distributes Standard Reference Materials (SRMs) for gases. These SRMs are highly characterized, stable reference materials with certified chemical compositions that serve as the foundation for accurate measurements in analytical chemistry and calibration of instruments. NIST produces various types of reference gas mixtures, including:
* Primary gas standards with the highest accuracy
* Gas mixture SRMs for environmental monitoring
* Specialty gas standards for industrial applications
* Hydrocarbon gas mixtures
* Air quality monitoring standards
When a gas is "NIST-traceable," it means:
* Its composition has been verified against NIST standards
* The measurement process follows appropriate protocols
* Documentation exists showing the complete chain of custody and calibration
* Stated measurement uncertainties are provided
Traceability ensures measurement results are comparable regardless of when or where they are made, allowing for consistency across laboratories, regulatory compliance, and scientific reproducibility.

## Tolerance and Accuracy

In the calibration gas, preparation tolerance (PT) and certification/analytical accuracy (CA) are of utmost importance.
Preparation tolerance is the concentration range that includes minor component and concentration. PT is measured in range of concentration. It can also be referred to as blending tolerance. For instance, a calibration gas of 500 ppm CO balance nitrogen having PT +/- 10% contains between 450 ppm and 550 ppm. Preparation tolerance can be minimized by using latest technologies for manufacturing such gases. Preparation tolerance is decided on the basis of the manufacturer's experience and the customer's requirements.
Certification/analytical accuracy is defined as the agreement of measured values with its true value. Certification values are determined statistically, considering the uncertainties accompanied by the international standard (reference standard) to which it is traceable, instrument uncertainty, and instability (reactivity) of gas in the container.

## Calibration Gas and Gas Detectors

Gas detectors and calibration gases share an essential relationship that ensures accurate, reliable detection of potentially hazardous gases in various environments.
Gas detectors identify the presence and concentration of specific gases in the air. They typically use sensors that react when exposed to target gases, generating signals proportional to gas concentration. These signals are then processed to display readings and trigger alarms at predetermined thresholds.
Calibration gas serves several critical functions for gas detectors:
* Accuracy Verification: Ensures the detector accurately measures gas concentrations by comparing readings to known reference values.
* Sensor Response Confirmation: Verifies that sensors respond appropriately to the target gases they're designed to detect.
* Alarm Function Testing: Confirms alarms trigger at the correct threshold concentrations.
* Adjustment and Correction: Allows for adjustment of the detector to maintain accuracy as sensors age or drift over time.
Proper calibration using appropriate calibration gases ensures gas detectors perform reliably in safety-critical applications, helping prevent accidents, exposure to toxic substances, and potential regulatory violations.