Gas measurement leans heavily on a single number that summarizes how heavy the gas is compared with air, and that number goes by several names that are not quite interchangeable. The ideal relative density is a pure ratio of molar masses; the real relative density accounts for the fact that neither the gas nor air is a perfect ideal gas; and the loosely used phrase specific gravity can mean either depending on who is speaking. The distinction becomes consequential the moment this number is plugged into a gross-method flow calculation, because the gross methods are built to expect one particular basis and will bias the result if fed the other. This guide focuses on which basis the AGA 8 gross methods expect, how a gravitometer and a chromatograph each arrive at the value, and why supplying the wrong relative density skews the supercompressibility and the volume.
Relative Density vs Specific Gravity in one line: For gas, the ideal relative density is the ratio of the gas's molar mass to air's molar mass, while the real relative density is the ratio of the actual densities of gas and air at the same base conditions, which includes compressibility; the term specific gravity is used loosely for either. The AGA 8 gross methods are written to expect a specific relative density basis as their input, and supplying the wrong one biases the supercompressibility factor and therefore the reported volume. A gravitometer measures relative density directly as a real density ratio, whereas a chromatograph derives it from composition, so knowing which basis a source provides matters.
Which page do you need? This page focuses on how the number is used in flow measurement. For the definition and how relative density and specific gravity relate, see Relative Density vs Specific Gravity of Gas.
The ideal relative density is the cleaner of the two definitions and the easier to compute. It is nothing more than the molar mass of the gas divided by the molar mass of air, a ratio that follows straight from the composition once you sum each component's molar mass by its mole fraction. It is called ideal because it treats both gas and air as if they obeyed the ideal gas law exactly, ignoring any real-gas deviation, which makes it a pure compositional property with no dependence on pressure or temperature. This is often the number people mean when they say gas gravity, and it is the value a composition alone can produce with no further physics.
The real relative density, sometimes written Gr, is the ratio of the actual density of the gas to the actual density of air at the same base conditions. Because real gases do not obey the ideal law even at base conditions, this ratio differs from the ideal one by the extent to which the gas and air deviate from ideal behavior, which is captured through their compressibility factors. For lean, mostly-methane gas the two bases are nearly identical because the deviation is small, but for heavy or carbon-dioxide-rich gas they pull apart enough to matter. The real relative density is a physically richer quantity because it reflects how the gas actually behaves, not merely what it is made of.
The confusion arises because the phrase specific gravity is used loosely for both, and a value labeled only as gravity does not tell you which basis it represents. In some contexts specific gravity means the ideal molar-mass ratio; in others it is used for a measured real-density ratio; and a number carried through a system as gravity may have started as one and been treated as the other. Because the two bases are close for light gas, this looseness usually causes no visible harm, which is exactly why it survives, but it becomes a real trap on heavier or inert-rich gas where the bases diverge and a calculation quietly expects a particular one.
The AGA 8 gross methods exist to compute compressibility from a small set of summary inputs rather than a full composition, and relative density is one of those inputs. The methods are calibrated and defined around a particular relative density basis, so the input must be supplied on the basis the method expects for the compressibility it returns to be correct. This is the crux of why the distinction is not academic: a gross method does not know what basis you handed it; it simply uses the number as defined, and if that number is on the wrong basis the compressibility it computes is off, and the supercompressibility factor and volume derived from it are off with it.
Because the gross methods work from relative density plus the inert content rather than a detailed analysis, the relative density input carries a large share of the information about the gas, which makes getting its basis right especially important. On lean gas the two bases are so close that an inadvertent swap barely moves the answer, so a gross method fed a slightly-wrong-basis gravity on light gas may look fine. On heavy or carbon-dioxide-rich gas the same swap moves the relative density by a meaningful amount, and since the gross method leans on that input, the error propagates into the compressibility and the volume. This is precisely the gas where the gross method is already working hardest and where the basis error compounds.
The practical rule is to know what basis the flow computer's gross-method configuration expects and to ensure the relative density it receives is on that basis. This is a configuration and data-provenance question as much as a measurement one: it requires knowing whether the number coming from the analyzer or gravitometer is an ideal molar-mass ratio or a real-density ratio, and whether that matches what the gross method wants. When the two agree the gross method returns the compressibility it was designed to, and when they do not the method silently produces a biased result that looks entirely normal until it is reconciled against a detail-method or independently measured value.
The two common sources of relative density arrive at it by different routes, and each naturally produces a particular basis. A gravitometer measures relative density directly by a physical effect that compares the gas to air, so what it reports is inherently a real-density ratio at the conditions it operates, a real relative density. A chromatograph, by contrast, derives relative density from the composition: it produces the ideal molar-mass ratio directly and can compute the real relative density by additionally applying the base compressibility from an equation of state. So the same site can have two sources of gravity that are subtly on different bases unless the chromatograph is configured to output the basis the calculation needs.
This is where making the value and its basis explicit in a cloud SCADA platform such as Merobix pays off. When the platform carries the composition, the ideal relative density, and the real relative density as distinct trended values rather than collapsing them into a single ambiguous gravity, a measurement engineer can see which basis is feeding the gross-method flow computer and confirm it is the one the method expects. Trending both bases alongside the composition also makes it visible when a stream is heavy or inert-rich enough that the two diverge, which is exactly the condition under which a basis mismatch would bias the volume, so the platform highlights the meters where the distinction actually matters.
Keeping relative density and its basis visible also lets the platform cross-check a gravitometer against a chromatograph-derived value in real time, since the two independent sources should agree once they are compared on the same basis. A persistent gap between them, after accounting for basis, points to drift or a fault in one of the instruments, while a healthy pair agrees closely. By holding the relative density with its basis explicit, alarming on unexpected divergence, and confirming the gross-method input matches the expected basis, a monitoring platform turns a subtle definitional distinction into an operational safeguard against a bias that would otherwise sit undetected on heavy or inert-rich gas.
Ideal relative density is the ratio of the gas's molar mass to air's molar mass, a pure compositional number that treats both as ideal gases. Real relative density is the ratio of the actual densities of gas and air at the same base conditions, which includes the compressibility that makes real gases deviate from ideal behavior. The two are nearly equal for lean methane-rich gas but diverge for heavy or carbon-dioxide-rich gas, where the real-gas deviation is larger.
The AGA 8 gross methods are defined around a particular relative density basis and expect the input on that basis, because they use the number as defined to compute compressibility from summary inputs rather than a full composition. Supplying the wrong basis biases the compressibility, the supercompressibility factor, and the volume. The error is small on lean gas where the ideal and real bases nearly coincide, but grows on heavy or inert-rich gas where they diverge and the gross method leans heavily on the relative density input.
They arrive at it differently and naturally produce different bases unless configured to agree. A gravitometer measures relative density directly as a real-density ratio by comparing the gas to air physically, so it inherently reports a real relative density. A chromatograph derives it from composition, producing the ideal molar-mass ratio directly and the real value only by additionally applying the base compressibility. Comparing the two requires putting them on the same basis, after which they should agree closely for a healthy pair.
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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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