Atmospheres to Torr

Sizing a Vacuum System in Torr Instead of Atmospheres

Specifying a vacuum system means working downwards through decades, and atmospheres are a poor ruler for that. Below about a hundredth of an atmosphere the interesting numbers all become leading zeros, while the same pressures written in Torr stay short and comparable — which is why pump curves, gauge ranges and process windows are almost always published in Torr rather than as a fraction of ambient air.

Conversion factor: 1 atm = 760 Torr, exact by definition — the Torr was created as one seven-hundred-and-sixtieth of a standard atmosphere. So a chamber that a rough pump has taken down to 0.05 atm is sitting at 38 Torr, still in the viscous-flow region and nowhere near a turbo pump's working range.

What Changes as the Pressure Falls

Progress is measured in decades

Halving the pressure means little; dropping a factor of ten is what changes the physics. A specification that reads "10⁻⁶ Torr" is naming a decade, not a precise target.

Gas stops behaving like a fluid

In the first decades molecules mostly collide with each other and the gas flows. A few decades further down they meet only the walls, and flow calculations have to be rewritten.

The chamber becomes the gas source

Once the original air is gone, what remains comes off the walls, the seals and the fixtures. Below roughly 10⁻⁶ Torr the material choice matters more than the pump size.

Pumps are staged, not stretched

Each pumping principle works over a limited span, so systems are built as a chain: one machine hands the chamber over to the next at a pressure both can manage.

Working Down the Decades from One Atmosphere

Both fields are live, so a whole pump-down can be walked through in one go, forwards or backwards.

Enter the fraction of an atmosphere on the left

Type 1, 0.1, 0.01 or 0.001 and read the Torr value straight off. Exponent shorthand is not needed — a plain decimal such as 0,000001 works, with the comma read as a decimal point and spaces ignored.

Let the notation change with the decade

Down to a millionth the result is written out in full with up to eight decimals; below 10⁻⁶ it switches to scientific notation, which is the form pump and gauge datasheets use anyway.

Flip it to judge an ultimate pressure

Press ↔ for Torr → atm when a datasheet quotes an ultimate of 2 × 10⁻³ Torr and you want that as a share of ambient air. The hand version is a division by 760, or a multiplication by 0.001 315 789 5.

Copy the value into the pump-down log

Each field carries its own copy button and hands over the number with no unit and no spacing, ready for a log sheet or a plot of pressure against time. Ctrl + C inside a field does the same job.

Vacuum figures are absolute: a compound gauge that reads "29 inHg of vacuum" is describing how far below ambient it has got, so it cannot be entered here as though it were an absolute pressure. Convert the reading to absolute first, then bring it in.

Vacuum Regimes, Their Torr Ranges and the Pumps That Reach Them

The four working regions of vacuum practice, with each boundary shown in Torr, as a share of one atmosphere, and against the distance an air molecule travels between collisions. Sources differ by a decade at the edges, so treat the limits as conventions rather than physical walls.

Regime	Range (Torr)	Share of 1 atm	Mean free path in air	Typical pumping
Rough (low) vacuum	760 – 1	1 – 1.3 × 10⁻³ atm	70 nm – 50 µm	Rotary vane, dry scroll, diaphragm, liquid ring
Medium vacuum	1 – 10⁻³	1.3 × 10⁻³ – 1.3 × 10⁻⁶ atm	50 µm – 5 cm	Two-stage rotary vane, Roots booster on a backing pump
High vacuum	10⁻³ – 10⁻⁸	1.3 × 10⁻⁶ – 1.3 × 10⁻¹¹ atm	5 cm – 5 km	Turbomolecular or diffusion pump on a rough pump
Ultra-high vacuum	10⁻⁸ – 10⁻¹²	1.3 × 10⁻¹¹ – 1.3 × 10⁻¹⁵ atm	5 km – 50 000 km	Ion, titanium sublimation or cryogenic pumping after a bakeout
Extreme high vacuum	below 10⁻¹²	below 1.3 × 10⁻¹⁵ atm	over 50 000 km	Combined ion and getter or cryogenic pumping, sustained bakeout

The whole span covers fifteen orders of magnitude. Getting from 760 to 1 Torr removes 99.87 % of the air and takes minutes; getting from 10⁻⁸ to 10⁻¹² Torr removes almost nothing by mass and can take days of baking.

What Helps When Reading Pump Curves

Small numbers stay legible

Values below a millionth are shown as powers of ten instead of a row of zeros, so a high-vacuum figure can be read at a glance and compared with a catalogue entry.

Millibar and pascal one search away

European suppliers quote in millibar and standards bodies in pascal; the searchable dropdowns hold all 26 units, so a datasheet in any of them can be brought onto the same scale.

Ultimate pressures back in context

Reversing the pair turns a quoted ultimate pressure into the fraction of ambient air it represents, which is often the clearest way to explain a specification to someone outside the lab.

Straight into the pump-down record

The copy button returns bare digits with no unit and no thousands spacing, which is what a log spreadsheet or a plotting script expects to receive.

Pump and Gauge Selection Questions

Where does one vacuum regime end and the next begin?

The common convention runs rough vacuum from 760 down to 1 Torr, medium vacuum from 1 to 10⁻³ Torr, high vacuum from 10⁻³ to about 10⁻⁸ Torr and ultra-high vacuum below that. The boundaries are not physical constants — different textbooks and standards shift them by a decade, and ISO expresses the same divisions in pascal. What the names really describe is how the gas behaves and therefore which pumping principle still works.

Why can't a single pump take a chamber from atmosphere to high vacuum?

Because the two ends of the range need opposite things. Near 760 Torr the job is shifting a large volume of dense gas, which suits a positive-displacement machine that traps and expels it. Below roughly 10⁻³ Torr there is almost nothing to displace; the pump has to give individual molecules momentum towards the exhaust, which is what a turbomolecular or diffusion pump does — and neither will start against atmospheric pressure. The rough pump therefore runs first and then stays on as the backing stage.

How far does a molecule travel between collisions at a given pressure?

For air at room temperature a workable rule is that the mean free path in centimetres is about 0.005 divided by the pressure in Torr. That gives roughly 70 nanometres at atmospheric pressure, half a millimetre at 0.1 Torr, 5 centimetres at 10⁻³ Torr and about 5 kilometres at 10⁻⁸ Torr. Once that distance exceeds the chamber's own dimensions the gas is in molecular flow, and conductance rather than pump speed sets how fast the chamber empties.

Why does pump-down stall at a certain pressure even though the pump is healthy?

Because a steady pressure is a balance, not an end point: the chamber settles where the gas load equals what the pump can remove. Once the original fill is gone the load is water desorbing from the walls, hydrogen diffusing out of stainless steel, elastomer seals permeating and any real leak that is present. A bigger pump moves that balance only slightly; cleaning the surfaces, switching to metal seals and baking the chamber move it by decades.

Which gauge type reads which part of the range?

No single sensor spans it, which is why systems carry two or three. Capacitance diaphragm heads are accurate near atmosphere and cover about four decades each, down to roughly 10⁻⁵ Torr. Pirani and thermocouple gauges cover the rough and medium region, from 760 to somewhere near 10⁻⁴ Torr, and lose sensitivity at both ends. Below 10⁻³ Torr the work passes to ionisation gauges: a hot-cathode Bayard-Alpert head reaches into the 10⁻¹¹ decade, while a cold-cathode gauge trades some accuracy for robustness. Each also responds differently to different gases, so a reading in a nitrogen-calibrated scale is not the whole story.