inHg to PSI

What the Vacuum Side of a Pump Gauge Is Telling You

Commission a centrifugal pump that has to draw water up from a sump, a river intake or a buried tank and the first instrument you look at is the compound gauge on the suction flange. Its vacuum half is graduated in inches of mercury, while the pump curve, the NPSH figures and every head calculation on the job sheet are in psi or feet. Converting the reading is how a needle position becomes a number you can put into a sum.

Conversion factor: 1 inHg = 0.49115415 psi, so halve the inches figure and add a whisker. A suction gauge sitting at 15 inHg of vacuum is 7.367 psi below atmosphere — which is what it takes to hold a static column of cold water about 17 ft up the suction pipe.

What Actually Does the Lifting

Atmosphere pushes, the pump only gets out of the way

The impeller lowers the pressure at the eye; the 14.7 psi resting on the sump surface does the pushing. That is the whole budget — nothing you specify on the pump can add to it.

Theory says 33.9 ft, the field says far less

A perfect vacuum of 29.92 inHg would balance 33.9 ft of cold water. Pipe friction, fittings, entrance losses and the pump's own NPSH requirement usually cap real installations around 20 to 25 ft.

Warm liquid spends your margin

Water at 80 °C already has a vapour pressure near 6.9 psi — close to 14 inHg — so nearly half the available suction is gone before the pipework takes its share.

Checking a Lift Before You Sign Off the Installation

Three or four numbers decide whether a suction arrangement will run for years or cavitate its impeller away in a season.

Put the running vacuum reading in

Type the inHg figure the gauge holds with the pump at duty flow into the left field. The psi equivalent builds up character by character, and a comma decimal or stray space in the number is handled without complaint.

Set it against the atmosphere you have

Subtract the result from the local atmospheric pressure to get the absolute pressure at the suction flange. That absolute figure — not the vacuum reading — is what goes into the NPSH available comparison.

Carry the value into the head sheet

The copy control on either field puts the number on the clipboard without its unit or any spacing, which is what a spreadsheet cell or a pump-selection form expects. Ctrl + C while a field has focus does the same job.

Reverse it when the spec is in psi

Datasheets that quote a maximum suction of, say, 12 psi below atmosphere need the opposite direction: the swap button (↔) switches to psi → inHg, where the multiplier is 2.03602097. Twelve psi is 24.43 inHg on the dial.

Read it running, not at rest: a gauge on a stopped pump shows static lift only. The number that decides cavitation is the one measured at full flow, with friction and entrance losses already in it.

Static Lift, Vacuum and the Pressure Behind It

Cold water, sea-level atmosphere, no friction: this is the pressure a suction gauge must hold simply to keep a given column standing in the pipe. Everything the pipework costs you comes on top of these figures.

Static lift (ft)	Static lift (m)	Gauge vacuum (inHg)	Below atmosphere (psi)	Comment
5 ft	1.52 m	4.41 inHg	2.17 psi	Comfortable for almost any end-suction pump
10 ft	3.05 m	8.83 inHg	4.34 psi	Routine sump or tank draw
15 ft	4.57 m	13.24 inHg	6.50 psi	Watch pipe size and fitting count from here up
20 ft	6.10 m	17.65 inHg	8.67 psi	Self-priming territory; friction budget gets tight
25 ft	7.62 m	22.07 inHg	10.84 psi	About the practical ceiling for a good installation
28 ft	8.53 m	24.71 inHg	12.14 psi	Only with short, oversized, leak-tight suction pipe
33.9 ft	10.33 m	29.92 inHg	14.70 psi	The theoretical limit — unreachable in practice

The pattern is easy to carry in your head: each foot of cold-water lift asks for about 0.88 inHg, which is roughly 0.43 psi. Turn it around and every inch of mercury on the dial is worth about 1.13 ft of column.

Handy Details on the Suction Side

Walk a whole lift survey without clearing

Both boxes accept input, so you can step through readings taken at different flows and different valve positions and watch the psi column build as you go.

Match whichever way the datasheet is written

Some manufacturers state a suction limit in inHg and others in psi below atmosphere; the swap control lines the page up with the document in front of you.

Feet and metres of water are in the list

The searchable dropdowns hold all 26 pressure units across eight groups, including the water-column group — so a vacuum reading can go straight to ftH₂O or mH₂O without a second step.

Clean numbers for the commissioning record

Results carry up to eight decimals with thousands split by a space, and the copy control hands over the bare value for a report or a handover sheet.

Suction-Side Questions from the Field

How high can a pump really draw water before it stops working?

Sea-level atmosphere is 14.70 psi, or 29.92 inHg, which balances 33.9 ft of cold water — that is the arithmetic limit and nothing beats it. Once you subtract friction in the suction line, losses through the foot valve and strainer, and the pump's own NPSH requirement, a well-built installation lands somewhere between 20 and 25 ft. Anything advertised beyond that is either a very short pipe run or a vacuum-assisted package.

The gauge reads 22 inHg and the pump is rattling — is that cavitation?

Very likely. 22 inHg is 10.81 psi of vacuum, leaving under 4 psi absolute at the flange. Vapour bubbles form when the local pressure falls to the liquid's vapour pressure and collapse violently at the impeller, which sounds like gravel in the casing and pits the vane tips. The cure is on the suction side: shorten and enlarge the pipe, remove elbows, clean the strainer, or lower the pump — never throttle the suction valve.

Why does the same pump lose lift when we move it up into the hills?

Because the pushing force is atmospheric and there is less of it up there. At 5 000 ft the ambient pressure is near 24.90 inHg, which is 12.23 psi — enough for only about 28 ft of cold-water column instead of 33.9 ft. Roughly a foot of theoretical lift disappears per 1 000 ft of elevation, so a marginal design at the coast becomes an unreliable one on a plateau.

The pump loses its prime overnight — can the gauge diagnose it?

It can. Pull the suction line down to a steady vacuum, shut the pump off, and watch the needle. A line that holds its inHg reading is tight, so the loss is at the foot valve seat; a reading that decays means air is being drawn in — each inch of mercury lost is 0.49 psi of leak — and threaded joints, gasket faces and shaft seals above the liquid level are the usual culprits. Suction-side leaks never weep, which is exactly why they get missed.

Our duty is hot condensate — how much lift does that cost us?

A great deal. Cold water at 20 °C has a vapour pressure around 0.34 psi, which is barely 0.7 inHg off the budget. At 80 °C that rises to about 6.9 psi, or nearly 14 inHg, so more than half the atmospheric allowance is consumed before any pipework is considered. Hot-liquid duties are normally arranged with a flooded suction — liquid level above the pump — precisely so no lift is needed at all.