Bar to Torr Converter
Common Conversions
| bar | torr |
|---|---|
| 0.01 | 7.501 |
| 0.05 | 37.503 |
| 0.1 | 75.006 |
| 0.25 | 187.516 |
| 0.5 | 375.031 |
| 1 | 750.062 |
| 1.01325 | 760 |
| 2 | 1500.124 |
| 5 | 3750.31 |
| 10 | 7500.62 |
| 20 | 15001.24 |
Why this conversion matters in chemistry
High-vacuum and surface-science work has historically run in torr; modern instrumentation increasingly carries bar-based alarms and setpoints. The factor between them is 750.062 torr per bar — a number that falls out of 1 bar = 100,000 Pa and 1 torr = 133.322 Pa. An ultrahigh-vacuum chamber at 10⁻¹⁰ bar sits at 7.5 × 10⁻⁸ torr, the working pressure where residual-gas adsorption onto a clean single-crystal surface starts to compromise an XPS or LEED measurement. The same factor lets a rotary-evaporator solvent table (in torr) meet a modern instrument controller setpoint (in bar).
Formula
Where the factor comes from
Nothing defines a bar in terms of a torr, so this factor has to be assembled from two separate definitions that meet at the pascal. The torr is exactly one seven-hundred-and-sixtieth of a standard atmosphere, and the atmosphere is exactly 101325 Pa, which fixes a torr at 101325/760 Pa. The bar is exactly 10⁵ Pa. Dividing one into the other gives 10⁵ × 760 ÷ 101325, or exactly 3040000/4053, which comes to 750.061682704… torr per bar. The result is pure accounting. The bar was never sized against a mercury column and the modern torr no longer depends on mercury at all, so 750.0617 records nothing physical about either unit — only the distance between two independently chosen reference points.
Precision and significant figures
The exact fraction is 3040000/4053, which never terminates, so every printed form of this factor — 750.062, 750.0617, 750.06168 — truncates it somewhere. Six figures cost nothing to carry, and 750 on its own is off by only 0.008 percent, fine for checking a number in your head. The more useful precision observation concerns range. Bar is a poor unit at the vacuum end, where pressures fall to 10⁻⁶ bar and below and leading zeros accumulate, while torr keeps its digits near the decimal point across the working span of a rotary pump. Let significant figures follow the gauge, which at low pressure rarely justifies more than two.
Worked Examples
The IUPAC reference pressure expressed in torr — slightly below the 760 torr standard atmosphere.
One standard atmosphere — the calibration anchor that links the two scales through the mmHg history.
A working pressure in the rotary-evaporator range for many common solvents.
A modest reduced pressure — the kind a rotavap might pull on a higher-boiling solvent.
Common mistakes
Millibar and torr are not interchangeable
One millibar is 0.750 torr, so the two scales run about 33 percent apart. That gap is the dangerous size: large enough to matter, small enough that a millibar number and a torr number look like the same pressure on neighboring gauges. Converting bar to torr and then comparing against a millibar readout without the second step goes wrong quietly.
Correction charts are denominated in torr
Boiling-point nomographs and pressure-correction charts are almost all drawn against mmHg or torr, inherited from the mercury era. A controller holding a setpoint in bar has to be converted before its value can be read onto one of those scales, and dropping the 750 factor puts the working point somewhere off the chart entirely.
System total pressure is not vapor pressure
A converted torr value from a distillation controller is the total pressure in the apparatus: solvent vapor plus whatever air has leaked in plus any inert bleed. Reading it directly against a pure-component vapor-pressure table treats the total as though it were the solvent's contribution alone, which overstates how readily the solvent should be coming over.