PSI to mmHg for Chromatography Inlet and Vacuum Specs

Carrier Gas in psi, Column Outlet and Vacuum in mmHg

A gas chromatograph asks for its inlet pressure in psi: the method file, the electronic pneumatic control screen and the regulator on the helium cylinder all speak that language. The other end of the column does not. Outlet conditions, detector specifications and every mass-spectrometer vacuum figure are published in millimetres of mercury, so working out what a method is really doing means putting both ends on the same scale.

Conversion factor: 1 psi = 51.7149326 mmHg. A column head pressure of 15 psi is 775.72 mmHg above ambient — slightly more than the whole atmosphere the outlet of an FID column is pushing against.

What Head Pressure Is Actually Controlling

Linear velocity

Pressure drop along a narrow capillary is what moves the carrier gas. Set the head pressure and you have set the average linear velocity, which is what the van Deemter curve is drawn against.

Oven temperature

Gas viscosity rises as the oven ramps, so holding a constant flow means the instrument must raise the inlet pressure steadily through the run — often by several psi from start to finish.

Outlet conditions

An FID vents to the room at roughly 760 mmHg. A mass spectrometer holds the column exit near vacuum, so the same column sees a much larger pressure drop for the same inlet setting.

Gauge against absolute

Inlet pressures are quoted as gauge, above the room; vacuum specifications are absolute, measured down from zero. Add about 14.7 psi to a gauge reading before comparing it with an absolute one.

Taking a Method Setting Through the Converter

Two directions come up during method development: turning a familiar inlet pressure into mercury units, and turning a published vacuum or outlet figure back into something the instrument screen understands.

Enter the head pressure from the method

Type the inlet setting — 8, 12.5, 20 — and the mercury figure appears as you go. If your regional keyboard produces a comma for the decimal mark, that is accepted just as readily as a dot.

Compare it against the outlet

Sea-level ambient is close to 760 mmHg, so a 12 psi head pressure (620.58 mmHg) means the column outlet is still at higher absolute pressure than the drop across the column itself.

Move the value into the notebook

The copy control beside each field puts the number on the clipboard on its own — unit stripped, spacing removed — so it lands tidily in a method record or a validation report. Ctrl + C with the cursor in a field does exactly the same.

Turn a vacuum spec back into psi

Press the swap arrows (↔) to work mmHg → psi, the direction you want when a detector datasheet quotes mercury. By hand it is a multiply by 0.019336775, and a source pressure of 1×10⁻⁵ mmHg comes out as 1.933677e-7 psi, shown in scientific notation because it is below one millionth.

Altitude moves the outlet, not the inlet. A laboratory well above sea level has a lower ambient pressure, so a method transferred unchanged gives a different linear velocity even though the psi setting on the screen is identical. Retune the velocity rather than copying the pressure.

Column, Inlet and Detector Conditions Side by Side

Indicative operating points across a capillary GC system, with the gauge pressures shown in both units and the absolute conditions at the far end of the column for context. Real values depend on column dimensions, carrier gas, oven programme and how the instrument was calibrated.

Condition	Setup	Pressure (psi)	Pressure (mmHg)
Head pressure, 30 m × 0.32 mm	Helium, wide bore, low oven temperature	4 psi	206.86 mmHg
Head pressure, 30 m × 0.25 mm, hydrogen	Lower viscosity carrier at similar velocity	5 psi	258.57 mmHg
Head pressure, 30 m × 0.25 mm into an MS	Vacuum outlet, constant flow near 1 mL/min	6 psi	310.29 mmHg
Head pressure, 30 m × 0.25 mm, helium	FID outlet, oven at initial temperature	8 psi	413.72 mmHg
Same column at the end of the ramp	Constant flow held as viscosity rises	15 psi	775.72 mmHg
Head pressure, 60 m × 0.25 mm	Long column, same target velocity	20 psi	1 034.30 mmHg
Inlet leak-test pressurisation	Split vent closed, pressure held and watched	25 psi	1 292.87 mmHg
Carrier supply at the regulator	Cylinder second stage feeding the instrument	60–80 psi	3 102.90–4 137.19 mmHg
Column outlet at an FID (absolute)	Venting to the room at sea level	14.696 psia	760.00 mmHg

That last pair is the anchor worth remembering: one standard atmosphere is 14.696 psi absolute and 760 mmHg, so any gauge pressure above about 14.7 psi has already exceeded a whole atmosphere. Against it, an ion source held near 1×10⁻⁵ mmHg is close to eight orders of magnitude lower — the reason a GC-MS transfer line, not the column, defines where the vacuum begins.

Built for Method-Development Arithmetic

Values that follow you while you retune

Both boxes accept typing and each updates the other immediately, so stepping a head pressure up in half-psi increments never means clearing a field.

Start from the mercury figure instead

The swap control reverses the pair to mmHg → psi, which is how a detector or vacuum specification usually arrives on the datasheet.

Torr and mbar in the same lists

Search either dropdown for the unit a pump or gauge manufacturer prefers; every pressure unit the app supports is offered on both sides of the pair.

Small numbers keep their digits

Results run to eight decimals and change to scientific notation below one millionth, which is exactly where ion-source and manifold pressures live.

Carrier Gas and Vacuum Questions

What head pressure gives roughly 30 cm/s of helium on a 30 m × 0.25 mm column?

On a cool oven venting to an FID it usually lands in single figures — around 8 psi, or 413.72 mmHg — but treat that as a starting point rather than a specification. Column length, internal diameter, film thickness, oven temperature and outlet conditions all move it, so confirm with an unretained marker or the instrument's own velocity calculation.

In constant flow mode, why does the inlet pressure climb during a run?

Because carrier-gas viscosity increases with temperature. To keep the same mass flow through the column as the oven ramps, electronic pneumatic control has to push harder — a method starting near 8 psi (413.72 mmHg) can finish close to 15 psi (775.72 mmHg). In constant pressure mode the pressure is held instead and the flow falls away as the oven heats, which is why retention times behave differently between the two modes.

Why does the same column need less head pressure on a GC-MS than on an FID?

The outlet is the difference. An FID exit sits at ambient, near 760 mmHg absolute, so the inlet must overcome that plus the column's own drop. A mass spectrometer holds the column exit at high vacuum, effectively zero on this scale, so a lower inlet pressure produces the same flow. Move a method between the two detectors without adjusting for vacuum outlet and every retention time shifts.

What delivery pressure should the carrier-gas regulator be set to?

Comfortably above the highest inlet pressure the method will ever call for, so the instrument's own control always has authority — commonly somewhere in the 60 to 80 psi region, which is 3 102.90 to 4 137.19 mmHg. Follow the instrument manual rather than a rule of thumb, and remember that supply pressure has no effect on chromatography beyond keeping the electronics in range: gas purity, traps and leak-free fittings decide the baseline.

How does a pressure-hold leak check on an inlet work?

The instrument seals the split and purge paths, pressurises the inlet to a set value — around 25 psi, or 1 292.87 mmHg, on many systems — then watches the decay for a minute or so. A tight inlet holds within the tolerance the manufacturer publishes, typically a fraction of a psi; a septum past its life or a loose column nut shows up as a steady fall. Half a psi lost is 25.86 mmHg, small on a gauge but plenty to spoil a baseline.

PSI to mmHg