The gauge needle climbs, stalls at the red sector, drops every time a tool starts and climbs again, cycle after cycle. Bottom left the tank fills and air hisses out of a fitting, on the right an impact wrench hammers, and a blue marker travels the measuring scale from edge to edge. The red sector does not pulse for looks: behind it sits the relief-valve setting.
Unit passport
American pneumatics sheet 1/1 · rev. 2026-09Pound per square inch
psi · non-SI unit · pressure · 6894.757 293 Pa
The unit you can hear
In an American shop pressure is heard more than it is read: the compressor in the corner climbs to one hundred thirty-five and falls silent, the hose under the ceiling whistles at a coupler, the impact wrench takes its ninety and knocks the needle down twenty, and the motor wakes up again. The whole choreography is marked out in pounds per square inch, and no system of units has managed to rewrite it yet.
The unit is built with no tricks: one pound of force applied to an area of one square inch. A whole technical culture grew out of that simplicity — tires, hoses, fittings, air tools, hydraulics, pipeline tests — and every item in it carries a number a mechanic keeps in his head the way a musician keeps pitch.
There is one trap here, and it is worth remembering for good: a gauge measures the excess over the atmosphere. Ninety on the dial is one hundred five absolute, and every calculation of stored air, of flow through a nozzle or of compression work needs the absolute figure — which is why the unit picked up its suffixes: g for gauge, a for absolute, from vacuum.
Written in lowercase with no period, and the suffix goes flush against it: psig — gauge, psia — absolute, psid — differential. In languages where the full name is clumsy, translated documentation keeps the Latin psi, while spelling it as the Greek letter looks odd and suits only conversation.
A gauge shows the excess over the atmosphere, not the full pressure, so ninety psi on the dial means one hundred five absolute: when you set a regulator the difference does not matter, but in a calculation of stored air or nozzle flow it is essential. And second: psi is pressure while cfm is flow, and a compressor is chosen by flow, even though the shop always argues about pressure.
01 · Definition
pound per inch · tank · flow versus pressureThe pound per square inch is the pressure created by a force of one pound spread over an area of one square inch. Since the pound-force is defined through the international pound of mass and standard gravity, and the inch is exactly twenty-five point four millimetres, the whole unit is expressed in pascals by an exact number and has no standard of its own.
Practical pneumatics lives on two numbers at once, and they must not be confused. The first is pressure, the very psi without which a tool simply will not run. The second is flow, the cubic feet per minute a tool eats while it works. An impact wrench and a jackhammer ask for the same ninety psi, but the second one's flow is four times larger, so a compressor that spins nuts beautifully will not cope with the hammer at any regulator setting.
The tank smooths out the difference, but not for long. The stock of free air in it equals its volume multiplied by the pressure drop in atmospheres, and a thirty-gallon tank charged from one hundred thirty-five down to ninety-five psi holds about eleven cubic feet — roughly half a minute of hammer work. After that everything depends on the compressor's delivery, and if it is below the tool's demand, the needle will inevitably go down.
Hence the shop rule: a compressor is sized by flow with a margin of about one and a half times, pressure is set by the most demanding tool and lowered locally with regulators. The red sector on the gauge is no decoration either — behind it sits the relief-valve setting, fixed by pressure-vessel codes, not by the hose maker.
Will the compressor keep up
On the left, gauge and tank: the tool is running, air is leaving, and the needle falls from one hundred thirty-five psi towards the threshold below which work is no longer possible, while the motor sends back only what it can. On the right the same process over time — the yellow pressure curve against the red threshold line and two flow bars. Move the compressor delivery: as long as it is below the tool's demand, the curve will reach the line sooner or later, and the only question is how many seconds; once delivery exceeds demand, the curve levels off and the tool runs without stopping.
02 · Conversion
pressure · gauge and vacuum · shopOne pressure in three conversations
The first tab spreads the entered pressure across units. The second converts it between the gauge and absolute scales and shows what changes with altitude. The third computes what the unit is kept in the shop for: force on a piston, stored air and tightening torque.
A rule to remember: one bar is about fourteen and a half psi, and a hundred psi is just under seven bar.
Tool air consumption is given differently in catalogues: some makers state average consumption in normal use, others the instantaneous figure in continuous work, and the two can differ by a factor of two. Comparing a compressor with a tool makes sense only when both numbers are brought to the same inlet conditions.
| Quantity | Value | Note |
|---|---|---|
| {k} | {v} | {note} |
{text}
04 · Measuring instruments
Bourdon tube · pressure switch · tire gauge · deadweight testerFour ways to catch pressure
The arc that straightens
Inside almost any gauge lies a brass tube, flattened in section and bent into an arc: under pressure it tries to straighten, and this movement, through a link and a toothed sector, turns the needle. The design has survived a century and a half unchanged because it needs no power and works honestly in shop grime, and the price is an accuracy of about two percent of full scale, which is why a three-hundred-psi gauge is no good for checking thirty.
An instrument that makes a decision
A pressure switch does not show the quantity: it compares it with two setpoints and commands the motor, so the tank pressure always runs in a sawtooth between the lower and upper threshold. The gap between setpoints is not chosen at random: too narrow makes the motor cycle every few seconds and overheat, too wide drops the pressure below what the tool needs. Hence the typical pair — one hundred five and one hundred thirty-five.
Thirty-two on a cold tire
Tire pressure is specified for a cold tire, and this is no nitpicking: after an hour of driving the air inside warms up and the pressure rises by about four psi, so "letting out the extra" at a gas station after a trip guarantees an underinflated wheel. The figure on the door jamb belongs to the car, not the tire, while the number molded on the sidewall is the maximum the tire can withstand, and it must not be used as the working pressure.
The definition, assembled on a bench
A deadweight tester reproduces the definition of the unit literally: certified weights are placed on a piston of known area, and the pressure in the oil beneath equals their weight divided by that area. The uncertainty of such a setup lies in hundredths of a percent, so it serves as the reference for all shop gauges, and there is no electronics in it — only mass, area and gravity.
05 · Writing rules
psi · psig · psia · suffix flushThe letter after the unit decides everything
Formally the suffixes g and a are not part of the unit symbol and metrologists frown on them, but pneumatics cannot do without them, because otherwise it is unclear from which zero the pressure is counted.
The first error leaves the reader guessing about the zero point, and in a compression calculation that is a miss of fifteen units at once. The second mangles the symbol, which is written in lowercase with no period. The third and fourth are one and the same trouble: a high pressure on the nameplate says nothing about the ability to feed a tool, because flow does the feeding. The fifth puts a tire's maximum pressure in as its working pressure, while the cold working figure is on the car's door jamb. The sixth rounds the factor in the dangerous direction: in a pipeline test the lost half percent adds to the gauge tolerance, and the safety margin quietly melts.
06 · Neighbouring units
bar · pascal · inch of mercuryA family of pressure units has grown up around psi, each serving its own industry: the bar settled in European pneumatics, the pascal stayed in calculations, the inch of mercury lives in vacuum engineering and aviation. Converting between them takes simple factors, and the pain starts where two scales meet on one drawing.
Europe's answer to the same question: a round hundred thousand pascals, almost an atmosphere. Old World pneumatics is marked in bar, so one and the same tool carries two numbers on its housing.
The system unit to which psi is tied by an exact number. For the shop it is too small: a tool's working pressure comes out as six hundred twenty kilopascals, and the number does not stick.
The unit of the vacuum part of the scale and of American aviation reports: "twenty-nine ninety-two" is standard atmospheric pressure expressed as the height of a mercury column in inches.
Standing nearby among the Simetrium passports are millimetre of mercury and horsepower: both of them, like psi, hold on not by the rigor of their definition but because instruments, catalogues and the habits of whole industries are ruled for them.
07 · Historical section
archive · from steam boilers to the air shopA unit that grew out of explosions
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The first three bars nearly merge at the axis, and that is an honest picture: all of shop pneumatics occupies a narrow strip at the bottom of the scale, while a breathing cylinder goes up to three thousand.
Why the shop did not switch to pascals
The United States officially adopted the metric system as preferred back in the seventies, but converting pneumatics means not new tables but new hardware: gauges, regulators, fittings, tool catalogues and training materials are marked in psi, and as long as a single hose in the shop carries the old scale, mixed units are more dangerous than inconvenient ones.
Tellingly, America itself is changing, but industry by industry: medical gases, aerospace and science labs moved to pascals long ago, while auto repair, construction and woodworking hold on to their ninety psi. The line runs where the document outlives the tool: where a drawing lives for decades the unit is changed, and where the habit of the hand lives, it is not.
Metrologically the unit is flawless and quite modern: it is expressed through the pascal by an exact number, reproduced by a deadweight tester to hundredths of a percent, and needs neither mercury nor water nor special conditions. The argument is not about physics but about what it costs to retrain hands, and so far habit keeps winning.
Catalogue of quantities
pressure, flow and pneumaticsSI base units
highlighted: kilogram, metre and second — the pascal is built from themWhat needs how much
bar and run time are computed from the first columns| Consumer | psi | bar | cfm | Remark | |
|---|---|---|---|---|---|
| {name} | {psi} | {bar} | {cfm} | {run} | {note} |
The next-to-last column computes how many seconds a tool will run on one tank charge without help from the compressor, and it explains the shop hierarchy: any tank will serve a stapler for half a day, a spray gun needs a steady stream, and a jackhammer empties thirty gallons faster than you can take aim. The pressure is nearly the same for all of them; the difference is in the flow.