An online chromatograph does not report gas composition continuously; it runs a fixed analytical sequence, produces a result, and starts again. The length of that sequence, the analysis cycle time, sets how often the flow computer receives a fresh composition to update heating value and compressibility. This guide explains what the cycle time is, how a new composition streams to the flow computer between runs, and what happens to custody measurement when the chromatograph faults and the computer is left holding the last good composition.
GC Analysis Cycle Time in one line: GC analysis cycle time is how long an online chromatograph takes to complete one analysis and produce a fresh composition, typically on the order of a few minutes for a custody gas analyzer. Each new composition streams to the flow computer, which uses it to update heating value and compressibility until the next result arrives, holding the current values between cycles. If the chromatograph faults, the flow computer keeps applying the last good composition, which is safe only as long as the real gas has not changed and the fault is caught quickly.
A gas chromatograph separates a sample into its components by passing it through columns that make different molecules travel at different speeds, then detects each component as it emerges. That physical separation takes time: the sample has to be injected, carried through the columns, and detected component by component before the analyzer can report the full composition. The analysis cycle time is the total duration of that sequence, and for a custody natural-gas analyzer it commonly runs on the order of a few minutes per cycle, after which the analyzer resets and begins the next run.
The cycle time is a property of the method, not a delay to be eliminated. It is set by how many components must be resolved, how well they have to be separated, and the column and temperature program chosen to achieve that. A method that resolves the light components and lumps the heavy ends into a single peak can run faster than one that extends the analysis to separate more of the heavier hydrocarbons, so cycle time and analytical detail trade against each other.
Between the completion of one cycle and the next, the composition the analyzer reports is static, because a new number only appears when a run finishes. The gas keeps flowing during that interval, so the flow computer is always applying a composition that is at most one cycle old. As long as the real composition changes slowly relative to the cycle time, this discretization introduces negligible error, which is the normal case for pipeline gas.
When a cycle completes, the analyzer transmits the new composition to the flow computer, typically over a digital link. The flow computer uses that composition to recompute the gas properties it needs: the heating value that converts volume to energy, and the compressibility inputs that feed AGA 8 and the supercompressibility factor. From that update until the next cycle finishes, the flow computer holds those derived properties constant and applies them to the flow it measures continuously.
This division of labor is deliberate. The chromatograph is the source of truth for what the gas is made of, updated every few minutes, while the flow computer measures the fast-changing process variables, differential pressure, static pressure, and temperature, on a much shorter interval. Marrying a slowly updated composition to rapidly sampled process conditions is exactly how a custody point stays accurate without needing a continuous composition measurement, which no practical analyzer provides.
The link between the two is itself part of the measurement system and is monitored accordingly. The flow computer needs to know not just the composition but whether the composition it holds is current and valid, so the analyzer signals the status of each result and the health of the analysis. That status is what lets the flow computer decide whether to trust a new composition or fall back to its previous one, which becomes critical the moment the analyzer stops producing good results.
If the chromatograph faults, loses carrier gas, fails a validation, or otherwise stops producing valid results, it stops sending new compositions. A well-configured flow computer does not react by zeroing the composition or halting; instead it holds the last good composition and keeps computing heating value and compressibility from it, so measurement continues rather than collapsing. This last-good behavior is the correct default, because a brief analyzer outage on stable gas should not corrupt or stop the custody measurement.
The safety of that behavior depends entirely on two things: how long the fault lasts and how much the real gas drifts during it. If the composition is genuinely stable and the analyzer is repaired within a short window, holding the last good composition introduces little error. But if the gas composition shifts while the analyzer is down, the flow computer keeps applying an increasingly wrong composition, biasing heating value and compressibility for the entire outage without any indication in the flow numbers themselves. A held composition is a frozen assumption, and its risk grows the longer it is held.
This is where a cloud SCADA such as Merobix earns its place. The chromatograph's fault status and the flow computer's held-composition condition can be surfaced as alarms, so a measurement technician learns immediately that a GC is down and that a flow computer has fallen back to last-good, rather than discovering it at month-end reconciliation. Seeing the analyzer health, the age of the composition in use, and the affected meters together lets the team judge the risk of a prolonged outage and prioritize the repair, and the alarm history documents exactly which periods ran on a held composition, which is precisely the information an audit of that window will want.
It updates once per analysis cycle, which for a custody natural-gas analyzer is commonly on the order of a few minutes. The analyzer runs a fixed sequence to separate and detect each component, and only reports a new composition when that sequence finishes. Between cycles the flow computer holds the current composition, so it is always applying a value at most one cycle old.
A well-configured flow computer holds the last good composition and keeps measuring rather than stopping, so a brief analyzer outage on stable gas does not corrupt or halt custody measurement. The risk is that if the real gas composition drifts while the analyzer is down, the held composition becomes increasingly wrong, biasing heating value and compressibility for the whole outage until the fault is fixed.
Not necessarily. A faster cycle updates composition more often, which helps only if the gas is changing fast enough for that to matter, and faster methods often resolve fewer components or lump the heavy ends more coarsely. There is a trade between speed and analytical detail, so the right cycle time balances how quickly the gas changes against the resolution the custody application needs.
This page references the standards, specifications, and official documentation 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.
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