Natural gas does not obey the ideal gas law at pipeline pressure, and the supercompressibility factor Fpv is the correction that accounts for that deviation when converting a measured volume to base conditions. Before the AGA 8 equation of state became the standard way to compute compressibility, the industry used an earlier correlation known as NX-19 to estimate Fpv from a handful of easily obtained inputs. NX-19 is still found in older flow computers and is still written into some contracts, so it remains part of the measurement landscape even though newer methods have largely superseded it. This guide explains how NX-19 estimates the supercompressibility factor from gas gravity and inert content, where it diverges from AGA 8 at high pressure, and why a SCADA migration is the right moment to find and quantify the meters still running on it.
NX-19 Supercompressibility in one line: NX-19 is a legacy correlation for the supercompressibility factor Fpv of natural gas, developed before AGA 8 and still present in older flow computers and some contracts. It estimates Fpv from a few gross inputs, primarily the gas relative density along with the carbon dioxide and nitrogen content, rather than from a full compositional analysis. It is simpler than AGA 8 but less accurate, and it diverges from AGA 8 most at high pressure and for gas that is heavy or rich in inerts, so meters still using it carry a compressibility bias relative to a modern basis.
The supercompressibility factor Fpv corrects for the fact that real gas is more compressible than an ideal gas, which means a given volume of real gas at pressure contains more mass, and therefore more standard volume, than the ideal law would predict. In a flow calculation Fpv appears as part of the correction from flowing conditions to base conditions, and getting it right is essential to reporting the correct volume. NX-19 is a method for computing Fpv, and its defining characteristic is that it is a gross method: it does not require a full breakdown of every component but works instead from a small set of summary properties of the gas.
The inputs NX-19 uses are the relative density of the gas, which is a proxy for its average molar mass, and the mole fractions of the two principal inert diluents, carbon dioxide and nitrogen. From those, together with the flowing pressure and temperature, the correlation estimates the compressibility and hence the supercompressibility factor. The appeal of this approach in its era is obvious: measuring or estimating the gravity and the inert content is far easier than running a full chromatographic analysis, so NX-19 let a flow computer produce a defensible Fpv from modest instrumentation. Much of the installed base of older gas measurement was built on exactly this economy.
The tradeoff is that a gross correlation built from a few summary inputs cannot capture the full effect of composition on compressibility. Two gases with the same gravity and the same inert content can still differ in their detailed makeup, and those differences affect the real compressibility in ways that gravity and inerts alone do not fully describe. NX-19 was fitted to represent typical gas of its time reasonably well within a certain range of conditions, but it is an empirical fit rather than a physically complete equation of state, so its accuracy degrades as the gas or the conditions move away from what it was fitted to represent.
AGA 8 replaced the gross-correlation approach with an equation of state that computes compressibility from either a detailed composition or, in its gross form, from summary properties similar to those NX-19 uses. The detail method takes the full molar composition and models the real-gas behavior far more completely, and even the AGA 8 gross method is generally regarded as an improvement over NX-19. The consequence is that for the same gas at the same conditions, NX-19 and AGA 8 will in general return slightly different values of Fpv, and the difference between them is the bias that a meter still running NX-19 carries relative to the modern basis.
The divergence is not uniform; it is small where NX-19 was well fitted and larger where it was not. At lower pressures and for lean, low-inert gas close to the conditions NX-19 was built around, the two methods agree closely and the choice matters little. The gap tends to widen at high pressure, where real-gas deviation is strongest and the empirical fit is under more strain, and for gas that is heavy or rich in carbon dioxide or nitrogen, where composition effects that a gross method handles only approximately become more significant. A high-pressure transmission meter on rich or inert-laden gas is exactly where NX-19 and AGA 8 are most likely to disagree by an amount that matters for custody.
Because the difference is a systematic bias rather than random noise, it does not wash out over time. If NX-19 returns a slightly low Fpv for a particular stream and pressure, every volume that meter reports is biased in the same direction, and over a month or a year that consistent offset accumulates into a real discrepancy against a counterparty measuring the same gas on an AGA 8 basis. This is the kind of difference that shows up in imbalance and reconciliation between parties, and tracing it back to a legacy compressibility method is far easier if you know which meters are still using one.
A migration to a cloud SCADA platform is a natural point to inventory the compressibility method each meter is using, because migration already involves cataloguing every flow computer, its configuration, and its calculation basis. During that inventory, meters still configured for NX-19 can be identified and recorded as an attribute of the station, so the measurement team ends up with an explicit list of which sites are on the legacy method rather than a vague sense that some old ones probably are. A cloud platform such as Merobix can hold that calculation-method attribute alongside the live measurement, making the compressibility basis a visible, queryable property of the fleet.
Knowing which meters run NX-19 lets the team quantify the bias rather than guess at it. For a given station's typical gravity, inert content, pressure, and temperature, the difference between the NX-19 Fpv and an AGA 8 Fpv can be estimated, which turns an abstract worry about a legacy method into a numerical exposure per meter. That estimate tells the team which NX-19 stations are diverging enough to matter, the high-pressure and rich or inert-heavy ones, and which are close enough that the method choice is immaterial. The migration then produces not just a modern platform but a prioritized picture of where the legacy method is actually costing accuracy.
That picture drives a defensible remediation plan. The NX-19 stations whose bias is significant become candidates to move to an AGA 8 basis, whether by reconfiguring the flow computer, replacing it, or recomputing volumes on the new basis where a contract permits, while the stations whose bias is negligible can be left alone with the difference documented. Because the platform records which basis each meter used and when it changed, the transition itself is auditable, and any discrepancy that predates the change can be attributed to the legacy method rather than left unexplained. In this way a SCADA migration converts a scattered legacy of old compressibility methods into a managed, quantified, and gradually retired liability.
NX-19 is a gross method that computes the supercompressibility factor from a few summary properties rather than a full composition. Its principal inputs are the relative density of the gas, which stands in for its average molar mass, and the mole fractions of the two main inert diluents, carbon dioxide and nitrogen, along with the flowing pressure and temperature. From those it estimates the compressibility and the supercompressibility factor without requiring a chromatographic analysis of every component.
NX-19 is an older empirical correlation, while AGA 8 is a more complete equation of state that can use either a detailed composition or a gross set of summary properties. For the same gas at the same conditions they generally return slightly different values of Fpv, with the difference smallest for lean low-pressure gas close to what NX-19 was fitted for and largest at high pressure and for heavy or inert-rich gas. AGA 8 is considered the more accurate modern basis, so a meter on NX-19 carries a bias relative to it.
Because NX-19 introduces a systematic compressibility bias relative to AGA 8 that does not average out and can accumulate into real imbalance against a counterparty measuring on the modern basis. A migration already inventories every flow computer, so it is the natural time to record which ones use the legacy method, estimate the bias for each from its gravity, inert content, and pressure, and prioritize the high-pressure or inert-rich stations for conversion. Documenting the basis also makes any resulting discrepancy traceable rather than unexplained.
This page references the standards, specifications, and official documentation published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
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.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.