Methods, formulas and review rules

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. The manufacturer boundary applying to every record is set out on the safety sheet.

• Hold the record. A missing or unusable number, a zero or negative where a positive value is required, or an unresolved manufacturer-dependent field produces HOLD — INPUT REQUIRED. No planning figures are shown.
• Stop the calculation. Two situations replace the normal result with a STOP CONDITION: a petrol generator indoors, in an enclosed space or trench; and a fume-planning task in an enclosed, restricted or unresolved location. Each record gives the route to competent assessment.
• Void changed records. Editing a calculation-bearing input strikes through the current figures and stamp until the record is recalculated.

An issued record carries its tool class — PLANNING ESTIMATE, ENTERED-RATING COMPARISON or CONTROL-ROUTE PLANNING AID. Unconfirmed, unusable or stopped work instead carries CHECK NAMEPLATE, HOLD — INPUT REQUIRED or STOP CONDITION.

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^[S1][S2]. Selection, commissioning and maintenance of any extraction system remain subject to HSE’s LEV guidance^[S3].

• Every welding selection carries a control requirement.
• Enclosed, restricted or unresolved locations stop the route.
• Outdoor work is not offered LEV as effective capture.
• Regular indoor work routes to capture at source.
• Occasional short-duration work surfaces the RPE-plus-general-ventilation route for assessment.
• Unknowns remain unresolved; they never improve the route.

T-02 · Welding generator size calculator

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. Petrol-powered equipment indoors, in enclosed spaces or in trenches is the carbon-monoxide stop condition^[S4].

T-03 · Welding gas cylinder duration

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

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 · Welding duty-cycle planner

Applies only at the current for which the nameplate duty-cycle rating is stated. 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

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^[S5].

T-07 · Steel weight calculator

Plate, flat-bar and round-bar dimensions are entered in millimetres and converted before the area line. UB, UC, PFC, RSA, SHS, RHS and CHS use an exact designation and kg/m copied from current section or supplier data. No nominal section table is embedded: the SCI Blue Book is the reference route, and supplier data governs ordering, handling and lifting^[S6].

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

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^[S2].

Arithmetic runs on the unrounded values; rounding happens only when a record is displayed. Practical planning figures normally show no more than two decimal places, with coarser headlines where the inputs do not justify extra precision. Very small cubic-metre volumes may show up to six places so they do not collapse to zero. Trailing zeros are removed and thousands use narrow spaces. Panel counts round up; comparisons always use the unrounded value.

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 accessed dates on the sources sheet — outrank every paraphrase of them here.

The method version (M.2026-09-02.1) 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. A page REV date changes only when its substance changes. The worked examples on each tool page are recalculated against the same formulas used by the live tool, including invalid entries and boundary cases. A change to any formula or decision rule bumps the method version and 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.