Torr to Millibar

Reading a Microscope Column Gauge in the Other Lab's Units

Electron microscopes and surface-analysis rigs are assembled from parts made on both sides of the Atlantic. The instrument controller may report the column at 2 × 10⁻⁷ Torr while the turbo-pump datasheet promises a base pressure in millibar, and the ion-pump manual quotes yet another figure. Nothing is wrong — the two scales differ by a fixed factor, and the number only has to be moved once.

Conversion factor: 1 Torr = 1.33322368 mbar (both trace back to 133.322368 Pa). A Schottky field-emission gun held at 1 × 10⁻⁹ Torr therefore sits at 1.333 × 10⁻⁹ mbar — the figure a European gun specification prints for that same vacuum.

Why the Decade Matters So Much Here

Emitter stability

A field-emission tip only gives steady current while its apex stays clean; residual gas adsorbing on the emitter turns up as flicker and drift in the beam, which is why gun chambers are pumped decades harder than the specimen space.

Specimen contamination

Hydrocarbons arriving from fingerprints, tape or vacuum grease crack under the beam and leave a dark rectangle exactly where you were imaging — a vacuum-quality symptom rather than an optics one.

Surface-sensitive analysis

XPS and Auger sample only the outermost few nanometres, so whatever lands on the specimen during the measurement becomes part of the spectrum. That constraint, not the electronics, fixes the base pressure of the chamber.

The deliberately poor vacuum

Variable-pressure and environmental modes hold the specimen chamber at a few Torr of water vapour on purpose, so wet or insulating samples can be imaged uncoated while the gun stays isolated behind apertures.

Moving a Gauge Reading Between the Two Scales

Whether the number came off the console, a pump-down chart or a supplier's quotation, the routine is short.

Put the Torr figure in the left box

Type it exactly as displayed. Decimals are accepted with either a dot or a comma, and spaces inside the number are ignored, so a pasted value rarely needs cleaning up first.

For deep vacuum, convert the mantissa only

The factor is identical in every decade, so entering 5 for a chamber at 5 × 10⁻¹⁰ Torr returns 6.666 118 — read it as 6.666 × 10⁻¹⁰ mbar. Typing the full decimal works as well; anything under one millionth comes back in exponent form.

Lift the number into the instrument log

Each field carries its own copy button, and it hands over the digits alone — no unit, no thousands spacing — which is what a maintenance spreadsheet or a service report expects. Pressing Ctrl + C with the cursor in the field produces the same result.

Turn it around for a Torr-labelled console

The ↔ button runs the pair the other way, mbar → Torr, when a European pump specification has to be judged against an American controller. By hand that direction is a multiplication by 0.750061683.

Gauge type limits the reading: a Pirani head stops being meaningful somewhere near 10⁻⁴ Torr and a cold-cathode head does not begin until well below that, so two numbers on one rig can legitimately disagree if either is read outside its own range.

Working Pressures from Backing Line to Ultra-High Vacuum

Typical operating values across an electron-microscopy and surface-analysis suite, in both scales. Every instrument differs, but the decades are stable enough that you can spot a reading which has drifted where it should not be.

Stage or instrument	Typical pressure (Torr)	Same pressure (mbar)
Environmental / variable-pressure SEM chamber	5 Torr (water vapour)	6.666 mbar
Backing line behind the turbomolecular pump	1 × 10⁻² Torr	1.333 × 10⁻² mbar
Load-lock at the moment of transfer	1 × 10⁻⁴ Torr	1.333 × 10⁻⁴ mbar
Tungsten-filament SEM specimen chamber	1 × 10⁻⁵ Torr	1.333 × 10⁻⁵ mbar
Conventional TEM column	1 × 10⁻⁷ Torr	1.333 × 10⁻⁷ mbar
Schottky field-emission gun chamber	1 × 10⁻⁹ Torr	1.333 × 10⁻⁹ mbar
XPS / Auger analysis chamber	5 × 10⁻¹⁰ Torr	6.666 × 10⁻¹⁰ mbar
Cold field emitter or UHV scanning-probe stage	1 × 10⁻¹¹ Torr	1.333 × 10⁻¹¹ mbar

Eleven decades separate the first row from the last, and no single pump spans them. What the millibar column buys you is a like-for-like comparison with the pump, gauge and chamber literature written in Europe, where identical hardware is specified on the other scale.

What Helps When the Numbers Are This Small

Exponent form appears on its own

Below one millionth the result switches to scientific notation instead of a run of zeros, so a base-pressure figure stays legible at a glance.

Both directions of a spec sheet

One tap on ↔ turns a European base-pressure claim back into the scale your console prints, the direction you want while comparing quotations.

Pascal is one dropdown away

Both selectors are searchable and hold all 26 pressure units, so the same page also produces the SI value a journal method section will ask for.

Log-sheet numbers without retyping

Results carry up to eight decimals, and copying strips everything except the digits, so nothing is transcribed wrongly into the record.

Questions from the Microscope Room

Why does surface analysis insist on 10⁻⁹ Torr and lower?

Because of how quickly a fresh surface gets buried. As a rough rule, at 10⁻⁶ Torr (1.333 × 10⁻⁶ mbar) enough gas strikes the specimen to build about one atomic layer per second if every molecule sticks. Drop three decades to 10⁻⁹ Torr and that becomes roughly a quarter of an hour; at 10⁻¹¹ Torr it stretches beyond a day. A technique that reads only the top few nanometres needs the surface to survive the length of the measurement, and that requirement — not the detector — decides the base pressure.

What does baking a chamber actually remove?

Mostly water. Once a chamber has been open to room air, its walls hold adsorbed water layers that desorb far too slowly at room temperature to reach the 10⁻¹⁰ Torr region in any useful time. Wrapping the vessel and holding it around 120–250 °C for a day or two, pumps running, drives that water off while it can still be carried away; pressure climbs during the bake and settles a decade or more lower than before once everything has cooled. Anything inside that cannot take the heat — some detectors, cabling, plastics, certain samples — has to come out first, which is why bakeable UHV systems are built quite differently from a routine SEM.

Which pump holds which part of the column?

They are staged. An oil-free scroll or a rotary-vane pump takes the vessel down from atmosphere and then sits on the backing line near 10⁻² Torr, because a turbomolecular pump cannot exhaust straight to air. The turbo carries the specimen chamber into the 10⁻⁶–10⁻⁷ Torr region. Gun chambers on field-emission instruments are usually held instead by a sputter-ion pump, often with a titanium sublimation pump beside it: no moving parts, no vibration reaching the column, and strong pumping for the light gases that dominate down there. Cryopumps show up where a large throughput of water vapour must be captured fast, such as on a deposition chamber bolted to an analysis system.

How can an environmental SEM image a sample at several Torr?

By splitting the instrument into zones. Small pressure-limiting apertures, each with its own pumping stage, sit between the specimen chamber and the column, so the gun stays in high vacuum while the sample sits in a few Torr — about 6.7 mbar at 5 Torr — of water vapour. The gas is not merely tolerated: electrons leaving the specimen ionise it, the resulting cascade amplifies the signal, and positive ions drifting back to the surface neutralise charge build-up. That is what makes uncoated, insulating or moist specimens imageable at all.

Why does the reading jump after I load a new specimen?

Everything you put in brings a surface load of water and hydrocarbons with it, and that load outgasses into the chamber. Porous, powdered and biological samples are the worst offenders, along with ordinary adhesive tape, bare fingers and any trace of vacuum grease. Pressure normally recovers over minutes as the pumps clear it; a spike that will not settle points to a genuinely wet specimen, a badly seated seal or a leak. Pumping the load-lock properly before transfer, storing mounted samples in a dry box and plasma-cleaning holders all shorten that recovery — and cut down the contamination rectangles that appear later in the images.