PSI to inHg

Bag Vacuum in inHg, Compaction Pressure in PSI

A composite shop lives with two vacuum vocabularies at once. The pump, the gauge on the bag and the resin-trap manifold are almost always graduated in inches of mercury, while the number that decides whether a laminate consolidates — the pressure actually squeezing the plies together — is a psi figure. Both describe the same pull; only the scale differs, and moving between them is a daily habit once you start writing process sheets.

The factor: 1 psi = 2.03602097 inHg. So a process sheet asking for 12 psi of compaction on the laminate is 12 × 2.03602097 = 24.43 inHg on the bag gauge — comfortably inside what a decent venturi generator can hold.

What Sets the Ceiling in a Vacuum Bag

One atmosphere is the entire budget

A bag does not push; the outside air does. Evacuate perfectly at sea level and the atmosphere presses with 14.7 psi, equal to 29.92 inHg on the gauge. Nothing you do to the pump raises that number.

Your shop's elevation is a hard limit

At 5 000 ft (1 525 m) the standard atmosphere is only 24.9 inHg — about 12.23 psi. A gauge that never passes 25 inHg in a mountain workshop is reading correctly, not failing.

Bag, tape and fittings decide the rest

Sealant tape joints, pleats around corners and through-bag fittings are where the last few inches of mercury leak away. The difference between 26 and 29 inHg is nearly 1.5 psi of consolidation on every square inch.

Resin boils long before the bag empties

Styrene, solvents and dissolved gas come out of solution as pressure drops, so an infusion pulled too hard can foam and leave voids. Some of the vacuum is deliberately given back once resin is moving.

Writing a Process Sheet in Both Scales

Layup instructions circulate as psi, hardware reads inHg. Convert once when you write the traveller, print both figures on it, and the person at the bench never has to guess.

Enter the compaction pressure you need

Type the psi figure from the resin or prepreg documentation into the left field — 7, 10, 13.75, whatever the spec calls for. The inHg equivalent builds as you type, and the box treats a comma and a dot the same way.

Compare it with what your setup can hold

Check the target against the steady reading your pump reaches on a sealed, empty bag. If the spec needs 27 inHg and the rig plateaus at 24, the shortfall is a leak or an altitude limit — not something to be talked around.

Copy the value into the traveller

The copy button on either field lifts the number by itself, with no unit or padding attached, so it pastes straight into a process-sheet cell or a quality record. The keyboard shortcut Ctrl + C inside a field does the same job.

Read the gauge back the other way

Press the swap arrows (↔) to run inHg → psi, which is what you want mid-cure: the dial shows 28 inHg, and the reverse multiplier of 0.49115415 tells you the laminate is seeing 13.75 psi.

Gauge placement changes the story: a gauge teed in at the pump reads better than the bag ever does. Put the measuring point at the far end of the part, opposite the vacuum port, or you will record a vacuum the laminate never felt.

Vacuum Levels Through a Bagging and Infusion Cycle

Typical set points at each stage of a wet layup, prepreg or infusion job, with the pressure the atmosphere applies to the laminate at that vacuum level. Read the psi column as the pressure difference below ambient, not an absolute pressure.

Stage	Gauge vacuum (inHg)	Pressure on the laminate (psi)
Perfect vacuum at sea level (theoretical ceiling)	29.92 inHg	14.70 psi
Bag leak-down test, starting point	29 inHg	14.24 psi
Resin infusion, initial pull before resin release	28 inHg	13.75 psi
Prepreg bag under autoclave pressure	25 inHg	12.28 psi
Ceiling in a workshop at 5 000 ft (1 525 m)	24.9 inHg	12.23 psi
Prepreg intermediate debulk	22 inHg	10.81 psi
Infusion during resin flow, bled back	20 inHg	9.82 psi
Wet layup bagging with a venturi generator	15 inHg	7.37 psi

The useful lesson in that column is how little the last stretch buys you. Going from 20 to 28 inHg adds under 4 psi of consolidation, while going from 28 to a theoretically perfect 29.92 adds under 1 psi — which is why chasing the final inch of mercury rarely pays compared with fixing a fibre-volume or bleeder problem.

How This Page Fits Into Bench Work

Both boxes are live while you plan

Type into either side and the other follows immediately, so you can sweep a set of candidate set points and see the consolidation they buy before committing a process sheet.

Reverse it while the bag is under pull

One tap on the swap arrows turns the pair around to inHg → psi, the direction you use when reading a live dial rather than writing a target.

Torr, mbar and kPa in the same list

The searchable dropdowns hold all 26 pressure units, so a European pump datasheet in mbar or an ultimate-vacuum spec in Torr can be checked without leaving the page.

Resolution for a leak-down log

Answers run to eight decimals, enough to record the small drop measured over a five-minute hold test, and the copy button drops that clean value into a record.

Vacuum Bagging Questions From the Shop Floor

Why can't I squeeze the laminate harder than about 14.7 psi?

Because the force comes from the air outside the bag, and there is only one atmosphere of it. Removing air from inside cannot create more pressure outside — at best you remove all of it and the full 14.7 psi (29.92 inHg) presses down. Anything beyond that requires a second source of pressure, which is precisely what an autoclave or a press adds on top of the bag.

My pump tops out at 24 inHg — is the shop's elevation to blame?

It might be. A gauge reads the difference from ambient, and ambient falls with height: around 24.9 inHg at 5 000 ft, near 27.4 inHg at 2 500 ft. If you are at altitude, 24 inHg may be close to everything available. Rule it in or out by running the pump on a blanked-off hose with no bag attached — if that reaches the pump's rated ultimate vacuum, the shortfall on the part is a leak, not the sky.

What leak-down rate is acceptable on a bagged part?

The usual bench criterion is to pull down, close the valve to isolate the bag from the pump, and watch for several minutes. A widely used pass mark for infusion work is losing no more than about 1 inHg — roughly 0.49 psi — in five minutes, and tighter shops halve that. Anything faster and resin will meet air somewhere it should not. Chase joints with sealant tape, the pleats over corners and every through-bag fitting first.

Should an infusion run at full vacuum from start to finish?

Common practice is to pull hard first and ease off later. A strong initial pull near 28 inHg compacts the stack and proves the bag before resin is released. Once flow starts, many fabricators bleed back towards 20 inHg because a hard vacuum lowers the pressure over the resin enough to drive dissolved gas and volatiles out of solution, feeding voids at the flow front. Resin systems differ, so the supplier's data sheet outranks any rule of thumb.

My compressor makes 100 psi but the venturi only pulls 26 inHg — why?

Those numbers measure different things. The 100 psi is compressed air above atmosphere; the 26 inHg is how far below atmosphere the venturi can drag the bag, and no vacuum generator can go past 29.92 inHg at sea level however much supply pressure you feed it. Venturi units also depend on getting their rated flow — a long, narrow hose or a small quick-coupler starves them, and the reading sags well below the catalogue figure.