The same gas can be reported with different heating values depending on how much water vapor you assume it contains, because water vapor displaces combustible gas and carries no heating value of its own. That assumption is captured in the heating value basis: dry, water-saturated, or as-delivered. This guide explains the three bases, how the water-content assumption shifts the Btu per cubic foot by a small but non-negligible amount, and why a gas contract must specify the basis or two parties will disagree about the delivered energy from identical gas.
Dry vs Saturated Heating Value Basis in one line: The heating value basis states what water content is assumed when reporting Btu per cubic foot: dry assumes no water vapor, saturated assumes the gas is fully saturated with water at a stated condition, and as-delivered uses the gas's actual water content in the pipeline. Because water vapor displaces combustible gas without adding heating value, moving between dry and saturated shifts the reported heating value by roughly a couple of percent for the same gas. A contract must specify the basis, or the parties will compute different delivered energy from identical gas.
Heating value is expressed per unit volume of gas, and a volume of real pipeline gas contains some water vapor along with the combustible components. Water vapor does not burn and contributes no heating value, but it occupies space, so it dilutes the combustible content of each cubic foot. The three bases are three different assumptions about how much water vapor is in that cubic foot. The dry basis assumes the gas contains no water vapor at all, so every part of the volume is dry gas and the heating value is at its highest.
The water-saturated basis assumes the opposite extreme: the gas is fully saturated with water vapor at a defined temperature and pressure, holding all the water it can. Because that water vapor displaces the maximum amount of combustible gas, the saturated heating value is the lowest of the three for the same underlying gas. The as-delivered, or as-is, basis sits between them, using the actual water content of the gas as it flows in the pipeline, which is typically partially saturated and therefore gives a heating value between the dry and saturated figures.
Crucially, none of these describes a different gas; they describe the same combustible mixture reported under different water assumptions. The dry heating value is a property of the gas that is convenient to measure and report because it removes the variable of water content, while the saturated and as-delivered values fold a water assumption back in. Converting between them is a defined correction, not a re-measurement, which is why an analyzer or flow computer can report the same gas on whichever basis is required.
The gap between the bases comes entirely from the volume that water vapor occupies. Fully saturating a volume of gas with water at typical conditions replaces a small fraction of the combustible gas with vapor, and since that vapor carries no heating value, the per-cubic-foot heating value drops by that fraction. For natural gas at common reference conditions the difference between the dry and water-saturated bases is on the order of a couple of percent, roughly in the region of one and a half to two percent, a small number that is nonetheless far too large to ignore in custody accounting.
The as-delivered basis lands somewhere in that band depending on the actual moisture in the pipeline. Dry, sales-quality gas that has been dehydrated carries little water and sits close to the dry basis, while wetter gas carries more and moves toward the saturated figure. Because the actual water content varies, the as-delivered basis is the least fixed of the three, which is one reason contracts often specify the dry or the saturated basis, whose assumptions are unambiguous, rather than an as-delivered value that depends on conditions.
What makes this a measurement issue rather than a curiosity is that the shift is systematic and one-directional. Reporting a dry heating value where a saturated one is expected overstates the energy per cubic foot by that couple of percent, every cubic foot, in the same direction. It does not average out, so on billed volumes the couple-of-percent basis difference is a persistent bias, not random noise, and it applies to the identical gas regardless of how accurately everything else was measured.
Because the same gas yields different heating values on different bases, a gas contract has to state which basis its heating value and energy accounting use. If it does not, one party can compute energy on a dry basis and the other on a saturated basis and both be internally correct, yet their delivered-energy figures will differ by the couple-of-percent water shift for identical gas. That is a disagreement no calibration can resolve, because it stems from an unstated assumption rather than a measurement error, and it can only be settled by fixing the basis in the contract.
The specification also has to reach the flow computer and the analyzer, not just the contract document. The devices computing energy must be configured to report and use heating value on the contractually specified basis, applying the water correction, or not, accordingly. A contract that says saturated while a flow computer computes dry produces energy figures that are consistently high, and the mismatch is invisible in the numbers themselves because a dry heating value is a perfectly valid number, just not the one the contract calls for. Aligning the configured basis with the contracted basis is therefore part of setting up a custody point correctly.
A cloud SCADA such as Merobix helps by surfacing the heating value basis each analyzer and flow computer is using alongside the reported heating value and energy. When the basis in use is visible across a field, a measurement team can confirm every custody point computes energy on the contracted basis and catch a meter configured on the wrong one before it produces a persistent, couple-of-percent energy discrepancy. Because the water-basis error is silent and systematic, making the basis observable is precisely what turns it from a dispute discovered at reconciliation into a configuration check done up front.
Dry assumes the gas contains no water vapor, giving the highest heating value. Saturated assumes the gas is fully saturated with water at a stated condition, giving the lowest, because water vapor displaces combustible gas without adding heat. As-delivered uses the gas's actual pipeline water content and falls between the two. All three describe the same combustible gas under different water assumptions.
For natural gas at common reference conditions, the difference between the dry and water-saturated bases is on the order of a couple of percent, roughly one and a half to two percent, because saturating the gas with water vapor displaces that fraction of combustible gas. The shift is small but systematic and one-directional, so on billed volumes it is a persistent bias rather than random scatter.
Because identical gas yields different heating values on different bases, so without a stated basis one party can compute energy on a dry basis and the other on a saturated basis and both be internally correct while disagreeing by the water shift. Fixing the basis in the contract, and configuring the flow computer and analyzer to match, is the only way to ensure both parties compute the same delivered energy.
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