High-Altitude Suit Hypobaric Moisture Transport

The formula

This is the expression the tool evaluates. Every term is named underneath, with the unit it must be supplied in.

How the result is derived

Step by step, from the values you type to the figure on screen.

1. The 9 inputs are read from the form on every keystroke: Altitude, Ambient Temperature, Ambient Relative Humidity, Skin Temperature, Evaporative Resistance, Air-Layer Share of Resistance, Body Surface Area, Metabolic Heat Output and Heat Lost by Evaporation.
2. Each value is checked against the accepted range in the input table below. A value outside its range stops the calculation rather than producing a misleading figure — the results blank out and a message appears.
3. The validated values are substituted into the expression above, which resolves Evaporative Capacity together with every supporting figure in one pass — no value is carried over from a previous entry.
4. The supporting outputs — Ambient Pressure, Vapour Pressure Difference, Pressure-Corrected Ret, Evaporative Flux, Required Sweat Rate and Capacity vs Requirement — come from the same pass, so they always describe the same case as the headline figure.
5. Results are rounded for display only. The full-precision value is used throughout the chain, so reading a rounded intermediate figure back into the tool by hand can shift the last digit.

What each input means

Where to read each value on the floor, the unit it must be in, and the range the tool accepts.

What the tool returns

The headline figure and every supporting value it is built from.

Worked example

The tool loads with this case already solved — the Evaporative Capacity shown above is its answer. Change one value and the difference from this baseline is the sensitivity of the result to that variable.

How to use it

1. Work through the input groups in order — Environment and Garment & Climber. The defaults are a realistic case, so you can change one value at a time and watch what moves.
2. There is no calculate button. Every figure recalculates as you type or drag, which is what makes this usable for a what-if sweep rather than a single answer.
3. Read Evaporative Capacity in the dark results panel — that is the headline figure, expressed in g/h.
4. Check the supporting rows underneath (Ambient Pressure, Vapour Pressure Difference, Pressure-Corrected Ret, Evaporative Flux, Required Sweat Rate and Capacity vs Requirement) before acting on the headline — they are where an implausible input usually shows itself first.
5. Reset to defaults returns every field to the reference case, which is the quickest way to check whether a surprising result came from the tool or from an input you had changed earlier.

Where this is used

• Process planning — establishing Evaporative Capacity before a trial is booked, so machine time and material in High-Performance Sports & Extreme Environments are committed against a calculated figure rather than an estimate.
• Costing and quotation — Evaporative Capacity is an input to the cost sheet, and quoting from a worked number rather than a remembered one is what keeps a margin intact.
• Troubleshooting — when the floor result drifts from plan, entering the measured values (starting with Altitude) shows how much of the gap in Evaporative Capacity each variable explains.
• Teaching and study — the accepted ranges bracket normal High-Performance Sports & Extreme Environments practice, so moving one variable at a time shows the shape of the relationship rather than a single answer.

Assumptions and limits

• A capacity ratio comfortably above one is not good news here — it means the climber is losing water at the full metabolic rate into air that will take all of it, and respiratory loss at altitude adds substantially more on top. The pressure correction applies only to the air-layer share of Ret; a membrane's own resistance is a material property and does not change with altitude, so that split has to be right or the answer is not. The model also assumes a fully wetted skin at steady state and ignores condensation and freezing within the insulation, which is the actual failure mode of multi-day cold-weather clothing. High-altitude physiology is medical territory and this is a clothing calculation, not physiological guidance.
• Every input is bounded to the range normal practice occupies (Altitude 0 to 9000 m, Ambient Temperature -60 to 30 °C and Ambient Relative Humidity 1 to 100 %, and so on for the rest). Those bounds are guard rails against typing errors, not a claim that the formula fails one unit outside them.
• The calculation is deterministic: the same inputs always give the same result. It carries no allowance for machine condition, operator skill, ambient conditions or lot-to-lot material variation unless an input above explicitly represents one.
• Nothing is sent anywhere. The maths runs in your browser, so the numbers you type never leave the page.

Questions people ask