Flow Modeling the molbloc-L-2 with the Fluke molbox2 Flow Terminal: From the Legacy Approach to the NIST Physics‑Based Model

Prepared by: Casey Rombouts, Senior Flow Metrologist — December 10, 2025

Abstract

The legacy Hagen–Poiseuille–based method used gas‑ and range‑specific coefficients that were historically effective for achieving good performance in each gas, but these coefficients were not directly linked to underlying physical models (geometries, gas properties etc), which limited the ability to fully document uncertainty and traceability. The new NIST physics‑based model implemented in the Fluke molbox2, builds on that foundation by establishing global coefficients through a single N₂ calibration and applying them consistently across gases, with transparent uncertainty contributions from geometry, instrumentation, and gas‑property data.

This note outlines the considerations behind transitioning from the legacy approach, describes the structure and benefits of the NIST model, and discusses accreditation and traceability aspects for practical use with the Fluke molbox2 Flow Terminal and laminar molbloc‑L‑2. For sonic molbloc‑S‑2, while NIST RefProp10 gas properties are used within the molbox2, a comparable NIST‑based flow model for sonic flow has not yet been implemented.

Legacy Method: Limitations

• Based on Hagen–Poiseuille flow but implemented with hidden parameters (CG, β, γ, δ) inconsistent across gases/ranges.
• Functioned as per‑gas corrections: a molbloc met specification only in the gas used for calibration.
• Did not comprehensively model molbloc physics; uncertainties became non‑defensible across gases and pressure ranges, necessitating separate calibration chains.

NIST Physics‑Based Model: Overview

The NIST model addresses the full laminar‑flow physics of molbloc‑L. A single N₂ calibration sets coefficients valid for all supported gases. Uncertainties are reasonable across gases without gas‑specific recalibration; results are close to gas‑specific calibrations when property data—especially viscosity—carry low uncertainty.

Three Pillars of the Approach

1) Geometry of molbloc-L-2 / Physical Parameters

An accredited N₂ calibration through the use of the Fluke molbox2 Flow Terminal finalizes the laminar molbloc-L-2 geometry/parameters with standard uncertainty (e.g., 0.1% at k=1). It includes all relevant terms from Technical Note 2011TN06B (molbloc‑L L1–L9). Outcome: geometry is reasonable, functional, flow‑calibrated, and documented for uncertainty/traceability/accreditation.

2) Instrument Measurements and Uncertainties

The model explicitly incorporates P₁ (upstream), P₂ (downstream), ΔP (P₁–P₂), and temperature T, each with its calibration chain, accreditation, and documented uncertainty/traceability.

3) Gas Properties and Uncertainties

Density and viscosity values are taken from NIST RefProp 10 using worst‑case values where appropriate. These are treated as documented measurement uncertainties within the model.

Metrological Traceability (VIM / ILAC)

Per VIM (BIPM JCGM 200:2008) and ILAC guidance, metrological traceability requires a documented, unbroken chain of calibrations to recognized standards, with documented measurement uncertainty, procedures, accredited technical competence, SI traceability, and appropriate calibration intervals. Our approach addresses these elements through (1) accredited N₂ geometry calibration, (2) calibrated measurement inputs, and (3) documented gas property data (RefProp 10).

Accreditation and Application

A. Accredited Gases

For gases within Fluke’s accreditation, data show the NIST model works for all laminar molbloc geometries within the stated uncertainty, delivering accredited and traceable results. We are working with A2LA to include ±(0.5% rdg or 0.05% FS) as the uncertainty for uncalibrated gases (within our accredited list) on the scope.

B. Non‑Accredited Gases

For gases outside our accreditation, the only missing element is direct accreditation for the property data (pillar 3). Documented values from RefProp 10 are still used. With (1) and (2) established and (3) documented, we meet the VIM definition of metrological traceability.

Mitigating Gas‑Property Uncertainty (GFS Referencing)

Some gases exhibit high property uncertainties that constrain model performance. GFS gas referencing reduces sensitivity to uncertain property coefficients, enabling the ±0.5% specification. For gases that cannot be corrected via GFS, the model includes viscosity/density uncertainties explicitly—still superior to legacy‑method outcomes.

Practical Takeaway

Utilizing the capabilities of the Fluke molbox2 Flow Terminal, the N₂ calibration step is performed to determine laminar molbloc-L-2 physical parameters and calibration coefficients required by the NIST RefProp10 model, forming the traceable basis for gas independent application, with uncertainties evaluated and documented in accordance with ISO 17025. We use N₂ because it offers the best uncertainty, but other gases could be used to determine the model if configured accordingly. In short, coefficients determined in N₂ apply to all gases in a physically consistent way, with documented uncertainties for geometry, measurements, and gas properties—yielding traceable and accredited results for accredited gases, and traceable results (via documented uncertainty) for non‑accredited gases.

Appendix: Key Terms

• Legacy coefficients (CG, β, γ, δ): Hidden correction terms used per gas/range; not physically comprehensive.
• RefProp 10: NIST database for thermophysical properties (density, viscosity) with documented uncertainties.
• L1–L9 (molbloc‑L): Uncertainty components per Technical Note 2011TN06B.
• Standard Uncertainty (k=1): One‑sigma uncertainty; e.g., 0.1% at k=1 for geometry calibration.
• GFS gas referencing: Procedure to reduce influence of uncertain gas‑property coefficients and meet ±0.5% specification.

Uncertainty Sources → Evidence Mapping