Relative density is one of the properties a custody calculation depends on, and there is more than one way to get it. A gas chromatograph computes it from the full composition, but a gravitometer measures it directly and continuously as a dedicated instrument. On a custody site, having both means you have two independent witnesses to the same property, and when they disagree, that disagreement is a warning that one of them is drifting. This page focuses on the gravitometer as a backup and cross-check rather than on the physics of the measurement alone.
Gas Gravitometer in one line: A gas gravitometer, also called a recording gravitometer or gas relative density meter, is an instrument that measures the relative density of a gas directly and continuously, independent of any composition analysis. In custody service it serves as a backup or cross-check to the relative density that a chromatograph computes from composition. When a SCADA host compares the two, a growing gap between the gravitometer and the GC-derived gravity flags an analyzer that is drifting.
A gravitometer answers a narrower question than a chromatograph, and that narrowness is its strength. It does not care what the gas is made of; it measures how dense the gas is relative to air, continuously, without separating the stream into components. Because it produces a live, uninterrupted reading rather than a fresh answer every few minutes at the end of an analysis cycle, it can catch a change in the gas the moment it happens, filling the gaps between the discrete analyses a GC produces.
This is a fundamentally different route to relative density than the one the chromatograph takes. The GC first separates and quantifies every component, then computes density from that composition. That route is powerful because the same analysis also yields heating value and the full composition, but it is only as good as the composition it derived, and it delivers its answer in steps rather than continuously. The gravitometer gets to density by a direct physical measurement that does not pass through composition at all, which is precisely why it makes a useful independent check.
Because it is a dedicated instrument with a single job, a recording gravitometer is also comparatively robust and simple to keep running. It does not depend on carrier gas, column condition, or valve timing, so the ways it can fail have little overlap with the ways a chromatograph can fail. Two instruments that measure the same property but fail for unrelated reasons are exactly what you want when the goal is to trust the number, because a fault in one is unlikely to be mirrored by the same fault in the other.
The value of running a gravitometer alongside a chromatograph comes from comparing their two answers. Both report a relative density for the same gas at roughly the same time, one measured directly and one computed from composition, and in a healthy system the two should agree closely and stay in agreement. The comparison is a form of redundancy that does not require either instrument to be perfect, only that they normally track each other, so that a departure between them is informative.
When the two begin to disagree, the disagreement itself is the diagnostic, even before you know which instrument is at fault. A chromatograph that is drifting, perhaps from a response-factor shift or a backflush timing problem, will compute a relative density that pulls away from the physically measured value, and that divergence appears in the comparison well before it might be obvious in the composition alone. Equally, a gravitometer that is fouling or drifting will pull away from a GC that is still accurate. Either way the operator learns that one of two normally-agreeing instruments has moved.
Deciding which instrument is wrong is the next step, and the comparison narrows it. If other GC diagnostics such as the unnormalized total or the response-factor trends are also shifting, the chromatograph is the likely culprit and the gravitometer is the reliable witness. If the GC diagnostics look clean while the gravitometer wanders, suspicion falls the other way. Having an independent, continuously measured gravity to hold the composition-derived gravity against converts a silent analyzer drift into a visible discrepancy that points toward its own cause.
The comparison only becomes an operational tool when a SCADA host brings both readings together and watches their difference over time. Trending the gravitometer relative density and the GC-derived relative density on the same axis, and trending the gap between them explicitly, turns two separate instrument outputs into a single reconciliation signal. A gap that stays small and steady confirms both instruments are healthy; a gap that grows or steps is the alarm that one of them has started to drift.
Alarming on the difference rather than on either instrument alone is what makes this powerful, because a slow drift can hide inside each instrument's individually plausible range while still showing up clearly as a widening gap. A host can set a tolerance on the divergence and fire when it is exceeded, and it can trend the divergence so a gradual separation over days is caught long before it becomes large enough to affect a custody calculation. The reconciliation catches the fault at the moment the two witnesses stop agreeing, not at the moment either one finally reads obviously wrong.
Across a fleet, this reconciliation scales the same way other analyzer diagnostics do. A cloud host overseeing many meter runs can watch the gravitometer-versus-GC gap at each one, so the single site where the two are diverging stands out against the sites where they still agree. When the gravitometer is available as an independent input, the host can also fall back on it to sustain a custody calculation while a suspect chromatograph is investigated, so the divergence not only flags the problem but helps keep the measurement defensible while it is being resolved.
A gravitometer measures relative density directly and continuously by a physical effect, without knowing the gas composition. A chromatograph instead separates and quantifies every component, then computes relative density from that composition, delivering the answer in steps at the end of each analysis. Because the two reach the same property by unrelated routes and fail for unrelated reasons, they make a strong independent cross-check.
For independence and continuity. The gravitometer provides a continuous reading between the GC's discrete analyses and a measurement that does not depend on the chromatograph being right. When the two are compared, a growing disagreement flags a drifting analyzer before the composition alone would reveal it, and the gravitometer can also serve as a backup input while a suspect GC is investigated.
It trends the gravitometer relative density, the GC-derived relative density, and the gap between them, then alarms on the gap rather than on either instrument alone. A small steady gap confirms both are healthy; a widening gap signals one has started to drift. Watching the difference catches a slow drift that would stay hidden inside each instrument's individually plausible range.
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