mmHg to Torr

Pressure Set Points on a Vacuum Heat-Treating Cycle

A vacuum furnace recipe is mostly a list of pressures. Pump down to a crossover point, hold while the load outgasses, drop another decade before the filler metal melts, then backfill to a partial pressure so the alloying elements stay in the steel. Some furnace controllers print those lines in Torr and others in mmHg, and the operator has to move between them without stopping the cycle.

Conversion factor: 1 mmHg = 1 Torr — the numbers carry across untouched. A nickel brazing hold specified at 5 × 10⁻⁵ Torr is 0.00005 mmHg, and a 0.5 mmHg argon backfill is 0.5 Torr, or 500 microns on a gauge scaled in microns of mercury.

Where the Number Comes From on the Shop Floor

Brazing holds

Nickel and copper filler metals wet a joint only once the oxide skin has been reduced away, which is why brazing recipes sit in the 10⁻⁴ to 10⁻⁵ decade rather than at rough vacuum.

Bright annealing

Solution annealing stainless or Inconel without scale means keeping residual oxygen and water vapour far below the level that would tint the surface — the reason parts come out with the finish they went in with.

Sintering and debind

Metal-injection-moulded and carbide loads give off binder for hours. The controller holds a modest vacuum through debind so the vapour is swept away, then pulls deeper for the sintering soak.

Partial-pressure backfill

Chromium, manganese, zinc and copper evaporate out of an alloy in a hard vacuum at soak temperature. Bleeding nitrogen or argon back to a fraction of a Torr holds that loss down while oxygen stays out.

Working Through a Furnace Recipe Line by Line

Whether the figure came off a customer specification, an aerospace procedure or the controller's own batch log, the routine is the same.

Enter the set point exactly as written

Put the recipe figure into the mmHg field — 0.5, 0.001, 0.00005. Torr appears alongside it while you type. A decimal comma is read the same as a dot, and any spaces you leave in are dropped.

Line it up against the gauge in front of you

Compare the converted figure with what the chamber is actually holding before letting the cycle step forward. A hold that will not settle below 2 × 10⁻³ is telling you something about the load or the seals.

Lift the plain number into the travel sheet

Each field carries its own copy button, and it hands over the digits with no unit and no spacing attached — which is what a recipe editor or a batch-record spreadsheet will accept. Ctrl + C inside a field does the same thing.

Turn it around for a spec written in Torr

The swap arrows (↔) run the pair the other way, Torr → mmHg. The reverse multiplier is also 1, so a 1 × 10⁻⁴ Torr requirement is a 1 × 10⁻⁴ mmHg requirement on a mercury-scaled gauge.

Microns are not Torr: a furnace gauge marked in microns of mercury reads a thousand times finer. A recipe line saying "pump to 100" means 100 microns — that is 0.1 Torr, not 100 Torr, and mixing the two puts the crossover three decades off.

Operating Pressures by Heat-Treatment Process

Working ranges for the processes a vacuum furnace runs, each value shown three ways: Torr, mmHg, and microns of mercury — the trade unit most shop gauges are labelled in.

Process	Pressure (Torr)	Pressure (mmHg)	Microns
Low-pressure carburizing, hydrocarbon pulse	5 Torr	5 mmHg	5 000
Nitrogen or argon partial-pressure backfill	0.5 Torr	0.5 mmHg	500
Crossover from the mechanical stage to the diffusion pump	0.05 Torr	0.05 mmHg	50
Tool-steel hardening, graphite hot zone	1 × 10⁻² Torr	0.01 mmHg	10
Sintering of MIM steel and cemented carbide	1 × 10⁻³ Torr	0.001 mmHg	1
Nickel brazing of stainless assemblies	1 × 10⁻⁴ Torr	0.0001 mmHg	0.1
Titanium work in an all-metal hot zone	1 × 10⁻⁵ Torr	0.00001 mmHg	0.01
Clean, empty chamber at blank-off	1 × 10⁻⁶ Torr	0.000001 mmHg	0.001

Six decades separate a carburizing pulse from an empty chamber at blank-off, and the load itself moves the reading: a furnace that blanks off at 10⁻⁶ may struggle to reach 10⁻⁴ with a rack of freshly machined, oily parts inside it.

What Helps While the Cycle Is Running

Either box accepts the set point

Both fields stay live, so you can walk the whole recipe — crossover, brazing hold, backfill — one figure after another without clearing anything between entries.

Follow whichever unit the specification prints

One press of the swap arrows flips the pair to Torr → mmHg, so the page lines up with the customer's procedure rather than the other way round.

Reach for mbar or Pa when an imported controller asks for it

Searchable dropdowns on both sides list all 26 pressure units across eight groups, so one set point can be restated for a differently scaled gauge without leaving the page.

Deep vacuum still reads cleanly

Figures carry up to eight decimals and roll over to scientific notation below a millionth of a unit, which is exactly where high-vacuum brazing values live.

Vacuum Furnace Operator Questions

My gauge reads 200 microns — what is that in Torr and mmHg?

A micron of mercury is one thousandth of a millimetre of mercury, so 200 microns is 0.2 Torr and 0.2 mmHg. Divide microns by 1 000 to reach either one. Shop gauges use microns because the interesting rough-vacuum band then falls between about 1 and 1 000 instead of trailing off into four leading zeros.

Why backfill nitrogen or argon instead of simply pumping deeper?

Several alloying elements have vapour pressures high enough to evaporate at soak temperature. Chromium, manganese, zinc and copper leave the surface, plate onto the cold wall and leave the part depleted, and the deposits eventually track across insulation and feedthroughs. Holding a fraction of a Torr of inert gas over the load suppresses that loss while oxygen and moisture stay out. Backfills usually land between roughly 0.1 and 2 Torr, chosen for the element being protected.

What leak-up rate should a furnace pass before a cycle starts?

Pump the empty, hot chamber down, isolate it from the pumping system and watch how fast the reading climbs — the rate-of-rise test, quoted in microns per hour. Aerospace practice commonly caps it around 20 microns per hour, and a well-maintained chamber does considerably better than that. A rate that has doubled since the last check normally points at a door seal, a thermocouple feedthrough or a water-cooled joint rather than at the pumps.

Why does chamber pressure climb as soon as the load starts heating?

Everything that went in with the parts comes back off: adsorbed moisture, cutting-fluid residue, forming oils, plating salts, and gas trapped inside porous or previously brazed assemblies. The reading usually walks upward through the first ramp, peaks somewhere in the mid-hundreds of degrees, then falls again. That is why recipes place an isothermal hold partway up the ramp — to let the load outgas at a temperature the pumps can still cope with, rather than at brazing temperature.

Does it matter whether the partial-pressure gas is nitrogen or argon?

It matters for some materials. Nitrogen costs less and is perfectly acceptable for many steels, but it is not truly inert at temperature: titanium, zirconium and certain high-chromium and nickel grades pick up nitrogen and form surface nitrides that change hardness and corrosion behaviour. Those loads get argon instead. The pressure written in the recipe stays the same either way — only the gas on the manifold changes.