Methodology

Each tool carries a visible classification on its own page, and the classification is the contract: a planning estimate is arithmetic on your figures; an entered-rating comparison compares your requirement against ratings you copied from a manufacturer’s plate; a control-route planning aid orders options for a competent person to assess. No classification anywhere on this desk is a certification, an assessment of your workplace, or an instruction. Where a result depends on a nameplate, a data plate or a marked cylinder, the tool asks you to enter that figure and never invents it; wherever entered data and manufacturer data disagree, the manufacturer’s data governs.

Inputs are validated before any arithmetic runs. A missing value, a non-numeric or non-finite value, a zero or negative where the formula requires a positive, or an unresolved manufacturer-dependent field puts the record into HOLD — INPUT REQUIRED: the card lists the offending fields and shows no numbers at all. Conditions that no calculation should survive — a combustion generator indoors, in an enclosed or restricted space or in a trench; an enclosed or unresolved location for fume planning — replace the record body with a STOP CONDITION and the route to a competent person or the manufacturer’s instructions.

Editing any calculation-bearing input immediately voids the current record: the figures and stamp are struck through and stay void until you recalculate. A stale result is never left presentable. Records are stamped only with the four stamps of this design — PLANNING ESTIMATE, CHECK NAMEPLATE, HOLD — INPUT REQUIRED and STOP CONDITION — and every issued record repeats the line “Planning support only — verify against nameplate/manufacturer data.”

The complete arithmetic of the desk, tool by tool. Symbols are defined where used; units are stated with every quantity; each tool’s own page carries the same method with a worked example whose figures are locked into the automated test suite.

T-01 · Welding fume extraction planner

No formula and no score. The planner reads nine selections against a fixed decision table drawn from HSE welding-fume guidance (see the source register below): every welding selection carries a control requirement; enclosed, restricted or unresolved locations stop the route; outdoor work is never offered LEV as effective capture; regular indoor work routes to capture at source; occasional short-duration work surfaces HSE’s RPE-plus-general-ventilation route as something to assess; unknowns are echoed as unresolved inputs, never resolved optimistically.

T-02 · Welding generator size check

load kVA        = entered kVA, or entered kW ÷ PF   (0 < PF ≤ 1)
running kVA     = Σ load kVA
start peak_i    = load kVA_i × multiplier_i + Σ other running kVA
peak kVA        = max(running kVA, every start peak_i)
planned kVA     = peak kVA × (1 + margin% ÷ 100)

Starting multipliers (≥ 1) are optional per load, treated strictly as entered data, and modelled one sequential start at a time. Stop conditions: combustion generator indoors, enclosed/restricted or in a trench. Hold conditions: empty load list, invalid values, or a rating basis other than nameplate/manufacturer data.

T-03 · Gas cylinder duration

usable free gas L ≈ water capacity L × (start bar − reserve bar)   [idealised mode]
gas-on minutes    = usable free gas L ÷ flow L/min
elapsed minutes   = gas-on minutes ÷ arc-on duty fraction   (0 < duty ≤ 1)

The idealised line is an ideal-gas planning approximation; a marked free-gas volume, where the cylinder states one, is entered directly instead. Regulator behaviour, temperature, residual gas, leaks and delivered flow all move real duration; the cylinder supplier’s data outranks the estimate.

T-04 · Weld volume & filler metal

fillet area mm²        = z² ÷ 2                (equal legs z)
square-groove area mm² = width × depth
volume cm³             = area mm² × length mm ÷ 1000
deposited mass g       = volume cm³ × density g/cm³
input filler mass g    = deposited mass ÷ deposition efficiency   (0 < e ≤ 1)

Theoretical geometry only: profile, penetration, reinforcement and procedure move real consumption. Density and efficiency are always user-entered; 7.85 g/cm³ and 0.85 appear only as labelled worked-example values.

T-05 · Duty-cycle planner

operating minutes = window minutes × duty% ÷ 100
rest minutes      = window minutes − operating minutes

Valid only at the current the rating was stated for. The planner never extrapolates a duty cycle to another current — that needs the manufacturer’s curve or a rating for that current.

T-06 · MIG wire runtime estimator

wire cross-section m² = π × (diameter m)² ÷ 4
wire volume m³        = mass kg ÷ density kg/m³
wire length m         = volume ÷ cross-section
runtime min           = length ÷ feed speed m/min   (continuous feed)

A continuous-feed figure, not arc-on time, and no voltage, current or setting recommendation — TWI notes that wire diameter and feed speed influence welding current, and the manufacturer’s data stays authoritative.

T-07 · Steel weight calculator

plate / rectangular bar V = L × W × T
round bar V               = π × (D² ÷ 4) × L
round tube V              = π × ((OD² − ID²) ÷ 4) × L      (OD > ID ≥ 0)
RHS V                     = (oW × oH − iW × iH) × L        (inner dims > 0)
mass kg                   = V m³ × density kg/m³

Dimensions are entered in millimetres and converted to metres before the volume line. Nominal geometry with user-entered density; supplier section data governs ordering and lifting decisions.

T-08 · Plasma cut thickness checker

A three-way comparison of the required thickness against the clean-cut and severance ratings you entered from the manufacturer’s data plate — within clean-cut, within severance, or exceeding both — with any unconfirmed power, supply or air prerequisite suppressing the positive comparison. The checker holds no amperage or thickness tables of its own and makes no universal amps-versus-thickness claim.

T-09 · Welding screen work-zone planner

perimeter P  = 2 × (length + width)
screen run R = max(0, P − opening)
panels N     = ⌈R ÷ usable panel width⌉

A quantity estimate only. Screen material, conformity, stability, overlap and site layout are outside the count; HSE describes the purpose of screens as protecting passers-by and nearby workers from arc light.

All arithmetic runs at full floating-point precision; rounding happens only at display. The shared formatter shows a value to at most four decimal places unless a tool states a coarser display for a given line (headlines commonly show one or two), trims trailing zeros, and groups thousands with narrow spaces. It refuses non-finite values outright — a NaN or Infinity reaching the formatter is treated as a bug and the record fails closed rather than displaying it. Panel counts round up (never to nearest); no other quantity is silently rounded in your favour or against it.

Where a tool embodies guidance rather than arithmetic — the fume planner above all — the rules are a fixed decision table paraphrasing the HSE documents in the source register, and the tool’s output is an ordered list of actions to assess, never a score, a rating or a verdict. Unknown answers never improve a route. The desk-wide rule follows the design contract: no number without a unit, no claim without a source, and the guidance documents themselves — listed with publisher, update and retrieval dates on the sources sheet — outrank every paraphrase of them here.

The method version (M.2026-08-20.1) names this exact set of formulas, validations and decision rules; it changes whenever any of them changes, and every tool page and printed worksheet carries it. Page REV dates are real last-substantive-edit dates — never decorative. The formulas are implemented as pure modules locked by automated tests: golden worked examples (the same figures shown on each tool page), invalid and boundary inputs, and checks that no banned verdict language appears in any built page. Corrections follow the same discipline: a change to any formula or rule bumps the method version and the revision date together.

Limits that apply to everything — the competent-person boundary, the nameplate boundary and the stop conditions — are collected on the safety sheet.