ASME SI-9-1981 — Guide for Metrication of Codes and Standards (SI Metric Units)

📐 ASME SI-9-1981, the first edition of the Guideline for Metrication of Codes and Standards — SI (Metric) Units, was issued on March 15, 1981, by the ASME Metric Study Committee. Although it is built on the metrication experience of the ASME Boiler and Pressure Vessel Code (BPVC), the methods it codified — soft versus hard conversion, treatment of temperature points, dimensional reconstitution of equations, and rounding of values — are directly useful to industry at large and to any standards-writing organization planning to introduce SI (metric) units into its documents. The Guide records the policies developed by the Task Group for the Metrication of the Code and explains how specific problems were resolved through the hard conversion of data.

1. Scope and Overview 📌

The Guide states plainly that far more editorial and mathematical work was required to metricate the complex ASME Boiler and Pressure Vessel Code than a simple application of conversion factors. It presents the guidelines and policies developed by the Task Group and describes how problems were resolved, so that other committees contemplating metrication can benefit from the lessons learned. Committees are advised to begin their work well ahead of the required date and to prepare for the associated costs.

SI-9 is intended for use with the 1980 Edition of the ASME Boiler and Pressure Vessel Code. It contains eleven sections: Introduction; Introducing SI Units in ASME Publications; Publication of a Metric Edition; Definitions — What Do We Mean?; Prefixes of SI Units to Be Used; Conversion of Equations for Use with SI; Temperature Points; Tables of Data; Format; Rounding Values; and Modified ASME Code Equations for Use with SI Units (covering Section I, Section III Division 1 Subsection NB, Section IV, Section VIII Divisions 1 and 2, and Section X).

2. Metrication Strategy: Soft Conversion, Hard Conversion, and Separate Equivalence 📐

The Guide describes a deliberate, three-stage path: first, soft conversions were inserted in the narrative of the Code as parenthetical SI equivalents, gently familiarizing users with SI. As originally planned, later editions were to extend the dual system progressively to nomenclature, figures, tables of data, and equations, eventually achieving a hard conversion to the SI system under which the end results are the same whether either system of measurement is used.

Two decisions shaped the 1980 Edition. First, temperature-related material mechanical property tables, which are numerous and lengthy, were to be converted only once: they were hard-converted directly for convenient temperature points in degrees Celsius and included as separate but equal tables, because interpolation-based conversions could introduce large errors. Second, because some equations contain factors or numerical constants that are not dimensionless, entirely new formulas had to be developed for SI use, and the values of terms had to be expressed with specific multiples or prefixes of SI units. When ASME staff estimated that a dual system would enlarge the Code by at least 30% with a similar price increase, publication in separate but equivalent SI and U.S. customary editions was studied.

In the 1980 Edition itself, soft conversions appear only in the narrative; SI equivalents were removed from the nomenclature of all equations and formulas. This seeming regression was deliberate: it avoids the very real possibility that unmodified equations could be incorrectly and unknowingly applied with SI values, with potentially disastrous results.

3. SI Units, Prefixes, and Temperature Conversion 🔢

Metrication in the Guide is based on ANSI Z210.1-1976, Metric Practice. Each ASME committee may give preference to U.S. customary or to SI units: when SI is preferred, customary units may be omitted or given in parentheses; when customary units are preferred, the SI equivalent is given in parentheses or in a supplementary table. Every standard must state, in a note, which system governs referee decisions. Table 1 summarizes the SI prefixes recommended for use, with prefix selection intended to keep numerical values between 0.1 and 1000.

Temperature conversion is one of the easiest places to make an error, and the Guide devotes a dedicated section to temperature points. Thermodynamic temperature is expressed in kelvin (K) and engineering temperature in degrees Celsius (°C), with strict conversion relationships (Table 2). Temperature points are chosen at convenient whole-degree Celsius values for the hard conversion of material property tables, and temperature tolerances are converted through the difference-of-temperature coefficient — never by converting one end-point temperature and treating it as a tolerance.

4. Equation Conversion and Significant Digits ⚗️

The most engineering-relevant part of the Guide is the methodology of Section 6 (Conversion of Equations for Use with SI) together with Section 11 (Modified ASME Code Equations for Use with SI Units). The principles are: identify dimensional constants in equations and rebuild them for SI; prescribe the specific multiples or prefixes in which each term must be expressed; and provide the converted equations volume by volume, as summarized in Table 3. Section 10 then sets out rounding and significant-digit conventions (Table 8, Recommended Significant Digits) so that converted values are neither over-precise nor under-accurate relative to the original data.

5. Engineering Practice Essentials 🛠️

• 🔍 Fix the referee unit system first: each standard should state by note which system governs design, inspection, or referee decisions; where both systems coexist, no drawing, calculation, or purchase order may mix values from the two unit systems.
• ⚗️ Rebuild equations, do not substitute coefficients: for formulas containing non-dimensionless constants, derive the SI form (fold psi, lb/in.² and similar constants into new coefficients) before substituting SI values; check that dimensional analysis closes.
• 🌡️ Separate temperature points from temperature differences: hard-convert material property tables at whole-degree Celsius points; convert differences and tolerances with the 1 °C = 1.8 °F relation.
• 📊 Convert data tables only once: generate “separate but equal” tables at convenient temperature points to avoid cumulative interpolation error.
• 🔢 Control rounding: follow the recommended significant digits; for limits and tolerances, round so the converted tolerance band is never wider than the original requirement.

6. Official Errata 📝

An official errata sheet was issued for this First Edition covering typographical and transcription errors in Table 1, Example 3, Table 5, and Table 9. Users of SI-9-1981 should incorporate the corrections, which include:

• Page 8, Table 1 (space): correct the conversion-factor entries for dimension and surface finish, and the entries for volume.
• Page 9, Table 1 (heat — temperature): correct the comment to read T_K = T_c + 273.15 and the customary unit(s) entry to read °R = T_F + 459.67.
• Page 10, Table 1: correct Note (6) on volume units (liter L = 0.001 m³ or cm³; m³ for large volumes; spelling per ANSI Z210.1) and add Note (7): “Multiply customary quantity by conversion factor to obtain SI quantity.”
• Page 14, Example 3 (by dimensional analysis): correct to W (in.) = t A (in.) / [G (lb/in.²)].
• Page 26, Table 5: extend the radical in the SI equation for PFT-24.3.1 to include −0.608; correct the customary equation for A-22.6.2.4.2.1 to read P = 0.5H [S/(S + 5,000)] psi.
• Page 31, Table 9: correct the third equation under SI (Metric) Units for AR-523 (10–100) to read W₁ = 0.076 C A (with the stated units).

7. Frequently Asked Questions ❓