A gas chromatograph, or GC, is the analytical instrument that tells you exactly what a natural gas stream is made of - how much methane, ethane, propane, nitrogen, carbon dioxide, and the rest. That composition drives the gas's heating value, and heating value is what a gas sale is priced on. This guide explains what a gas chromatograph is, how it separates and measures gas components, and where an online GC fits in oil and gas measurement.
Gas Chromatograph in one line: A gas chromatograph is an analytical instrument that separates a gas sample into its individual components and measures the concentration of each. In natural gas measurement, an online GC continuously reports the gas composition, which is used to calculate heating value (BTU), specific gravity, and the compressibility factors needed for accurate custody-transfer flow measurement.
A GC works by separation in time. A precise small volume of the gas sample is injected into a stream of inert carrier gas - usually helium or hydrogen - which sweeps it through a long, narrow column packed or coated with a material that interacts with each component to a different degree. Lighter, less-retained molecules travel through the column quickly; heavier or more strongly-retained ones lag behind. By the time the sample exits, its components have separated into a sequence of distinct peaks.
A detector at the column outlet - commonly a thermal conductivity detector (TCD) or flame ionization detector (FID) - senses each component as it elutes and produces a peak whose retention time identifies the component and whose area is proportional to its concentration. The GC's software integrates the peaks and reports a full composition breakdown, typically to methane through hexanes-plus, nitrogen, and CO2.
Natural gas is not sold purely by volume - it is sold by energy content, and energy content depends entirely on composition. Methane, ethane, propane, and the heavier hydrocarbons each contribute a known heating value, while inert components like nitrogen and CO2 dilute the mixture and lower its BTU per cubic foot. From the measured composition, the GC calculates the gross heating value (BTU), relative density (specific gravity), and the compressibility used to correct volume to standard conditions.
Those numbers feed directly into custody-transfer flow computation under AGA standards. A metered gas volume is meaningless for billing until it is converted to energy using the current heating value, and the flow computer needs the composition-derived compressibility to convert flowing volume to standard conditions accurately. An online GC updating every few minutes keeps those corrections current as the gas quality shifts.
On a custody-transfer meter run or at a gas plant inlet, an online (process) gas chromatograph runs unattended on a fixed cycle, drawing a fresh sample and reporting an updated composition every few minutes. It outputs the composition, BTU, and specific gravity to the associated flow computer, which combines them with the metered flow to compute energy and corrected volume.
For supervisory monitoring, those GC outputs are tags on the flow computer or controller. A cloud SCADA platform such as Merobix reads the composition, BTU, and specific-gravity values over Modbus, OPC UA, or a similar protocol and trends them - letting measurement technicians watch heating value and component percentages remotely, spot a drift or a failed analysis, and confirm the gas is on spec without traveling to the site.
It separates a natural gas sample into its individual components and measures the percentage of each - methane, ethane, propane, nitrogen, CO2, and so on. From that composition it calculates the gas's heating value (BTU) and specific gravity, which are used to price gas by energy and to correct custody-transfer flow measurement.
Natural gas is sold by energy, not raw volume, and energy content depends on composition. The composition sets the heating value used for billing and provides the compressibility needed to convert flowing volume to standard conditions. Without an accurate, current composition, the energy and corrected-volume figures on the sale are wrong.
A process GC on a meter run typically completes a full analysis on a fixed cycle of a few minutes, then repeats continuously and unattended. Each cycle draws a fresh sample and updates the composition, BTU, and specific gravity fed to the flow computer, keeping the measurement corrections current as gas quality changes.
Safety & engineering notice. This article is general educational information, not site-specific engineering, safety, or legal advice, and it does not reflect any particular facility. Standards and regulations (for example OSHA, API, IEC, ISO, NFPA, NIST, and NERC CIP requirements) change and vary by edition, jurisdiction, and application. SCADA and remote monitoring cannot verify physical isolation, atmosphere, lockout/tagout, permit status, or a safe go/no-go decision. Qualified personnel must perform site-specific engineering, hazard analysis, and safety review, and confirm current requirements with the authority having jurisdiction, before acting.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.