Pressure Converter

Pressure Converter works from a table of thirteen units: pascal, hectopascal, kilopascal and megapascal, bar and millibar, the standard atmosphere, psi and ksi, torr, millimetre and inch of mercury, and the kilogram-force per square centimetre. Every conversion routes through the pascal — the value is multiplied by its own unit's factor and divided by the target's — so no pair of units needs a rule of its own, and bar to kilopascal comes out at exactly 100. Most of the factors are exact by definition — 100,000 Pa to the bar, 101,325 to the standard atmosphere, 133.322387415 to the millimetre of mercury, 98,066.5 to the kilogram-force per square centimetre — and psi is held to thirteen digits, three more than the automatic display shows. The one short factor is the inch of mercury, whose conventional 3,386.389 Pa runs out after seven digits, so an inHg conversion cannot be more exact than that.

Each line of the input box is converted separately. A bare number uses the From and To dropdowns, which open on bar and psi; a line that writes its own units, such as 32 psi to bar or 1013.25 hPa to atm, overrides both for that line alone, so a pasted column of readings in mixed units converts without being tidied up first. One unreadable line stops the whole run, though: the unit is quoted back but the line number is not, and no results come back at all, not even for the lines that had already converted. Precision is either automatic — ten significant digits with trailing zeros trimmed, falling back to exponential notation at eight digits once a result reaches 10¹⁵ or drops below a millionth, as one pascal in ksi does — or a fixed 0, 1, 2, 3, 4, 6 or 8 decimals. Setting the target to All units prints the reading in all thirteen at once, and a single converted pair is footnoted with its factor, as in 1 bar = 14.50377377 psi.

The limit that matters is the one a unit table cannot fix: this scales a pressure, it never moves the datum. A tyre pressure of 32 psi means 32 psi above the air around the tyre, while a figure such as 1013 hPa from a weather station is measured from vacuum. Convert that 32 psi and you get 2.206322334 bar, which is still a gauge figure — it is not 2.206322334 bar absolute. Crossing between the two is arithmetic you do yourself, adding or subtracting one atmosphere: 14.69594878 psi, 1.01325 bar, 101.325 kPa. Nothing in a typed line says which kind of reading it is, so the page does not guess, and the suffixes psig and psia are not recognised as units.

Two smaller things are worth knowing. Torr and millimetres of mercury are separate entries because they are defined differently, and they part company around the seventh significant figure — invisible on a forecourt gauge, not invisible in a vacuum log. Both mercury-column entries also carry conventional values rather than measured ones, so a real barometer at another column temperature or latitude will not agree to the last digit. Water-column units such as cmH₂O and inH₂O, the ones ventilation and duct work are quoted in, are not in the table at all. All of this happens in the browser: the readings you paste stay in the tab, nothing is transmitted, and the page carries on converting with the network switched off.

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