In gas measurement the terms specific gravity and relative density are often used as if they meant the same thing, and in casual conversation they usually do. But there is a real distinction that matters when the numbers feed a custody flow calculation. The ideal-gas specific gravity of a gas is simply the ratio of its molar mass to the molar mass of air, a clean number that ignores how real gases deviate from ideal behaviour. The real-gas relative density is the ratio of the actual density of the gas to the actual density of air at the same base conditions, which accounts for that non-ideal behaviour through compressibility. For most lean natural gas the two are almost identical, but for heavy or carbon-dioxide-rich gas they diverge enough to matter. This guide explains how each is defined, how a chromatograph computes them, why they differ, and where each belongs in AGA-3 and AGA-8 flow work.
Relative density vs SG (gas) in one line: Ideal-gas specific gravity is the ratio of a gas's molar mass to the molar mass of air, treating both as ideal gases, while real-gas relative density is the ratio of the actual density of the gas to the actual density of air at the same base conditions, which includes the compressibility that makes real gases deviate from ideal behaviour. The two are nearly equal for lean natural gas but differ noticeably for heavy or CO2-rich gas, where non-ideal behaviour is larger. A chromatograph computes the ideal-gas value directly from composition and molar masses, and the real-gas value by also applying compressibility, and flow standards specify which one a given calculation requires.
The ideal-gas specific gravity of a gas is one of the simplest properties to define. It is the molar mass of the gas divided by the molar mass of air. If you know the composition, you know the molar mass, because it is just the sum of each component's molar mass weighted by its mole fraction, and dividing that by the molar mass of air gives the ideal-gas specific gravity directly. It is called an ideal-gas quantity because it rests only on molar masses and treats the gas and air as if they obeyed the ideal gas law exactly, with no correction for how the actual gas behaves under pressure. This makes it a purely compositional number, independent of pressure and temperature.
Because it depends only on composition, the ideal-gas specific gravity is easy for a chromatograph to produce and is unambiguous once the composition is known. A gas that is mostly methane has a molar mass close to that of methane, well below that of air, so its ideal-gas specific gravity is well under one, meaning it is lighter than air. As the gas picks up heavier components such as propane and butane, or carbon dioxide, its molar mass rises and its specific gravity rises with it, and a sufficiently heavy or CO2-rich gas can approach or exceed the density of air. All of that follows straight from the molar masses, with no thermodynamic subtlety involved.
The virtue of the ideal-gas specific gravity is its simplicity and its exactness as a molar mass ratio; the limitation is that real gases do not obey the ideal gas law. At the pressures and compositions of real pipeline gas, the actual number of moles occupying a given volume is not quite what the ideal law predicts, and the deviation is captured by the compressibility factor. Ideal-gas specific gravity ignores that deviation by construction, which is fine when the deviation is small but becomes a source of error when it is not. That is exactly the gap the real-gas relative density fills.
Real-gas relative density is defined as the actual density of the gas divided by the actual density of air, both taken at the same specified base conditions of pressure and temperature. Unlike the ideal-gas value, this is a ratio of real densities, so it accounts for the fact that neither the gas nor air behaves exactly ideally. The link between the two is compressibility: the real density of a gas equals what the ideal law would give, adjusted by its compressibility factor at the base conditions. So the real-gas relative density is essentially the ideal-gas specific gravity multiplied by the ratio of air's compressibility to the gas's compressibility at those conditions. When the two compressibilities are nearly equal, the two density measures are nearly equal too.
The reason the two diverge for heavy or CO2-rich gas is that such gases deviate from ideal behaviour more than lean methane does, so their compressibility factor differs more from air's. A gas rich in propane, butane, or carbon dioxide is more non-ideal at base conditions, which means its real density differs from the ideal prediction by a larger margin, and the relative density pulls away from the specific gravity by a correspondingly larger amount. For lean, mostly-methane pipeline gas the deviation is small and the two numbers agree to well within the precision anyone cares about, which is why the terms get used interchangeably in that setting. It is on the heavy or acid-gas streams that the distinction becomes real and using the wrong one introduces error.
A chromatograph computes both because it has what it needs for each. The ideal-gas specific gravity comes straight from the composition and molar masses. The real-gas relative density is obtained by additionally computing the compressibility of the gas at the base conditions, typically through an equation of state such as the one embodied in the AGA-8 method, and applying it to convert the ideal ratio into a real-density ratio. So the same analysis that yields the composition yields both density measures, and a well-configured analyzer reports the one, or both, that the downstream calculation needs. The important thing is to know which number a given output represents, because on a heavy gas they are not the same.
The distinction is not academic because the flow calculation standards call for a specific density basis, and using the wrong one biases the computed flow. AGA-8 is the method for computing the compressibility and density of natural gas from its composition and conditions, and it is precisely the machinery that produces the real-gas density and hence the real-gas relative density. When a flow calculation needs the actual density of the gas, for example to convert a measured flow into mass or into standard volume with proper accounting for compressibility, it is the real-gas quantity from AGA-8 that is correct, because it reflects how the gas actually behaves rather than an ideal approximation. Substituting an ideal-gas value there introduces the very compressibility error the standard exists to avoid.
AGA-3, the orifice metering standard, computes flow through an orifice and depends on the density of the flowing gas, again the real density, because the mass flowing through the orifice depends on the actual density at flowing conditions, not on an idealised one. The composition-derived properties feed this calculation, and the relative density or density used must be the real-gas value consistent with AGA-8 for the result to be accurate, especially on heavier or CO2-bearing gas where the ideal and real values differ. Where a relative density input is called for as a shortcut or a backup, it is meant to be the real-gas relative density, so that it is consistent with the physics the flow equation assumes.
For a measurement and monitoring operation the practical point is that the density number flowing into the flow calculation must be the right kind, and a cloud SCADA platform such as Merobix helps by carrying the composition, the ideal-gas specific gravity, and the real-gas relative density explicitly so the distinction is visible rather than hidden inside a single ambiguous gravity figure. Trending both alongside the composition lets a measurement team see when a stream is heavy or CO2-rich enough that the two diverge meaningfully, which is exactly the condition under which using the wrong one would bias the flow. Keeping the density basis explicit in the record, and confirming that the flow computation is fed the real-gas value consistent with AGA-8, is part of ensuring the measurement is right on the gases where the difference actually matters, and monitoring both together makes any inconsistency easy to catch before it distorts the volumes.
Ideal-gas specific gravity is the ratio of the gas's molar mass to the molar mass of air, treating both as ideal gases, so it depends only on composition. Real-gas relative density is the ratio of the actual density of the gas to the actual density of air at the same base conditions, which includes the compressibility that makes real gases deviate from ideal behaviour. They are nearly equal for lean natural gas but differ for heavy or CO2-rich gas.
The two differ by the extent to which the gas deviates from ideal behaviour, captured by its compressibility factor relative to air's. Lean, mostly-methane gas is close to ideal at base conditions, so its real density is close to the ideal prediction and the two numbers agree. Heavy gas rich in propane, butane, or carbon dioxide is more non-ideal, so its real density departs more from the ideal value and the relative density pulls away from the specific gravity.
AGA-8 computes the compressibility and real density of natural gas from its composition and conditions, so it produces the real-gas density and the real-gas relative density. That real-gas value is the one flow calculations such as AGA-3 orifice metering require, because the flow through the meter depends on the actual density of the gas rather than an idealised one. Using an ideal-gas specific gravity in place of it would introduce the compressibility error the standards are designed to avoid, especially on heavy or CO2-bearing gas.
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