A chromatograph computes gas relative density from composition, but that is not the only way to get it, and it is not always the fastest or the most robust. A gravitometer is a dedicated instrument that measures the relative density of gas directly and continuously, comparing the gas against air by a physical effect rather than deriving the number from a chemical analysis. Because it produces a relative density reading on its own, independent of the GC, it can serve as a fast input to a flow calculation and as a cross-check that catches a chromatograph reading the wrong density. This guide explains how a gravitometer measures relative density, whether by a spinning-impeller mechanical method or a vibrating-element method, why its speed and independence make it useful as a backup to AGA-3 flow computation, how it is calibrated against reference gas, and how it fits a SCADA scheme built for redundancy.
Gravitometer in one line: A gravitometer, also called a gas relative density meter or specific gravity meter, is an instrument that continuously measures the relative density of gas directly, by a physical effect that compares the gas against air rather than by computing it from composition. Mechanical types often use a spinning impeller whose effect depends on gas density, while other types use a vibrating element whose frequency shifts with density. Because it produces relative density fast and independently of the chromatograph, a gravitometer serves both as a direct input to AGA-3 flow computation and as a cross-check on the density the GC derives, and it is calibrated against a reference gas of known relative density.
A gravitometer measures relative density by exploiting a physical property that varies with how dense the gas is, and comparing the gas to air. In a classic mechanical spinning-impeller design, the instrument imparts motion to the gas and to air and measures a difference that depends on the density of each; because the effect a gas has scales with its density, the ratio of the gas effect to the air effect gives the relative density directly. The measurement is continuous, because the mechanism runs constantly rather than analysing discrete samples, and it yields the relative density as a live reading rather than as a value computed after a cycle. This directness is the defining feature of the instrument.
Other gravitometers use a vibrating-element approach, in which a mechanical element such as a tube or a cylinder is driven to vibrate and its resonant frequency depends on the density of the gas surrounding or filling it. Denser gas loads the element more and shifts its frequency, so measuring the frequency yields the gas density, and comparing against the value for air gives the relative density. Like the mechanical impeller type, this is a continuous, direct measurement that does not depend on knowing the gas composition. Whichever physical principle is used, the common thread is that the gravitometer senses density itself rather than inferring it from what the gas is made of.
This is fundamentally different from how a chromatograph arrives at relative density. The GC first separates and quantifies every component, then computes the density from that composition and the appropriate compressibility. That route is powerful because it also yields heating value and full composition, but it is indirect and it takes a full analysis cycle to produce a number. The gravitometer skips the composition entirely and reads density as a physical quantity, which makes it faster and simpler for that single purpose but blind to what the gas actually contains. The two instruments reach relative density by completely different paths, which is exactly why one can check the other.
The speed and independence of a gravitometer make it valuable as an input to orifice flow computation under AGA-3, which needs the relative density of the flowing gas to turn a measured differential pressure into a flow. A chromatograph provides that density but only every analysis cycle, and if the GC is down or between cycles the flow computer has no fresh density from it. A gravitometer supplies a continuous relative density that the flow computer can use directly, so the flow calculation keeps running on a live density even when the GC is unavailable. As a fast, always-on source of the density input, it fills the gaps and the outages that a cycle-based analyzer leaves.
Beyond serving as a backup input, a gravitometer is a genuine cross-check on the density the chromatograph derives, and this is one of its most useful roles. Because the gravitometer reaches relative density by a completely independent physical route, a disagreement between the gravitometer and the GC-derived density is a strong signal that something is wrong with one of them. If the two normally track closely and then diverge, the operator knows to investigate, because a healthy chromatograph and a healthy gravitometer measuring the same gas should agree. A GC that has drifted, developed a carryover problem, or is misreporting a component will often show up first as a growing gap against the gravitometer, which the GC's own internal checks might not catch.
Using the two together gives a flow measurement both accuracy and resilience. The chromatograph provides the full composition and the heating value that only a compositional analysis can give, along with a composition-based density, while the gravitometer provides a continuous, independent density that keeps the flow calculation fed and continuously tests the GC. Some installations run the flow calculation on the GC density under normal conditions and fall back to the gravitometer when the GC is unavailable, using the gravitometer both as the redundancy and as the ongoing sanity check. That arrangement means neither a GC outage nor a quiet GC error leaves the flow measurement without a defensible density.
A gravitometer is calibrated against gas of known relative density, so that its physical reading is anchored to a true value. Typically the instrument is referenced using air, whose relative density is one by definition, and a reference gas or gases of certified relative density, so its response is set to read those knowns correctly. Because the instrument measures a physical effect that can be influenced by mechanical wear, contamination, or drift over time, this calibration has to be maintained, and the gravitometer's reading should itself be checked periodically against a reference to confirm it has not wandered. An uncalibrated gravitometer that has drifted would not only feed a wrong density but could raise false alarms against a perfectly good GC, so keeping it in calibration is essential to its value as a cross-check.
For the cross-check to work, the gravitometer's relative density and the chromatograph's derived relative density need to be brought together where they can be compared continuously, and this is where SCADA integration matters. A cloud SCADA platform such as Merobix can carry both density values as parallel trends and compute the difference between them in real time, so the normal close agreement is visible and any divergence stands out immediately. Rather than the two instruments living in separate systems where no one notices they have drifted apart, the platform puts them side by side and can alarm when the gap between the gravitometer and the GC exceeds a chosen threshold, turning the two independent measurements into an active, monitored redundancy.
That integration also lets the platform manage the failover cleanly and keep the operator informed. If the GC goes into alarm or stops delivering fresh analyses, the platform can make clear that the flow calculation is now relying on the gravitometer, so no one mistakes a backup condition for normal operation, and it can flag the affected period for the measurement record. Trending both densities over time additionally builds a history that shows how well the two agree, which is useful evidence that the density feeding the flow calculation has been continuously cross-checked. By holding the gravitometer and the GC together, alarming on their divergence, and tracking which one is driving the flow calculation, a monitoring platform turns a second density instrument from a passive spare into a working part of a redundant, self-checking measurement system.
It measures relative density directly through a physical effect that varies with gas density and compares the gas against air, rather than computing density from composition. Mechanical spinning-impeller types measure a density-dependent effect on the gas versus air, while vibrating-element types sense the shift in a vibrating element's resonant frequency as the surrounding gas density changes. Either way the measurement is continuous and does not require knowing what the gas is made of.
Because the gravitometer reaches relative density by a completely independent physical route, so it both fills the gaps the cycle-based GC leaves and acts as a cross-check. It gives a continuous density that keeps AGA-3 flow computation running when the chromatograph is down or between cycles, and because it and the GC should agree when both are healthy, a divergence between them flags a problem the GC's own checks might miss. Together they make the flow measurement both more accurate and more resilient.
It is calibrated against gas of known relative density, commonly referencing air, whose relative density is one by definition, along with a certified reference gas, so its response is set to read those known values correctly. Because it senses a physical effect that can drift with wear or contamination, the calibration must be maintained and the reading checked periodically against a reference. A drifted gravitometer would feed a wrong density and could raise false alarms against a good chromatograph, so keeping it calibrated is essential to its role as a cross-check.
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