Pressure Measuring Instruments: Types, Uses and Prices

Which instrument measures which pressure

The quiz answer, if that is what brought you here

“Which instrument is used to measure pressure?” is a stock question in middle-school science reviews, ASVAB study sets and HVAC licensing prep. Graders want manometer in general, barometer when the question says atmospheric or air, and sphygmomanometer when it says blood. If “pressure gauge” and “manometer” appear as separate choices, pick manometer for a liquid-column instrument and pressure gauge for a dial.^[1]

What pressure is, and the units used to state it

Pressure is force divided by the area it acts on. The SI unit is the pascal — one newton per square meter — so small that everyday values are quoted in kilopascals or in whatever unit each trade grew up with. In the United States that unit is almost always psi.

Two anchors are worth memorizing. Standard atmospheric pressure at sea level is 14.7 psi = 29.92 inHg = 1013.25 mbar, which is why a barometer in decent weather sits near 30 inches.^[2] And water produces about 0.433 psi per foot of height, so roughly 34 feet of water equals one atmosphere and 1 bar equals about 33.5 feet — which is why every story of a building costs you 4 to 5 psi of static pressure.

The UK and most of Europe run on bar and millibar instead. A British boiler gauge reads 1 to 1.5 bar where an American hydronic boiler reads 12 to 15 psi, and a UK tire pressure of 2.4 bar is the same 35 psi an American door placard lists.

Absolute, gauge, differential and vacuum

Nearly every mistake in pressure measurement comes from confusing these four.

Gauge pressure (psig) is measured against whatever the atmosphere is doing right now — your tire gauge reads zero in open air because the atmosphere presses equally on both sides of its sensing element. Absolute pressure (psia) is measured against a hard vacuum, so it never goes below zero. Differential pressure (psid, or inches of water column) is the difference between two ports, neither of which needs to be atmosphere. Vacuum is gauge pressure below zero, quoted in inches of mercury or, for deep vacuum work, in microns.

Mechanical instruments, one by one

1. 1
   U-tube manometer. A U-shaped tube half filled with water, oil or mercury; pressure on one leg pushes the liquid up the other and the height difference *is* the reading. Its accuracy comes from gravity and fluid density rather than calibration, which is why manometers still serve as reference standards.^{[[3]](https://control.com/textbook/continuous-pressure-measurement/manometers/)}
2. 2
   Inclined manometer. The reading leg laid at a shallow angle, stretching a tiny vertical rise into long, readable travel. Standard for duct static and filter tests under one inch of water column.
3. 3
   Piezometer tube. A plain vertical tube tapped into a pipe or channel; water climbs to the height matching static pressure there. Unbeatable for low-head water, useless for gases or vacuum.
4. 4
   Bourdon tube gauge. The round dial on every air compressor, well tank and sprinkler riser: a flattened curved tube tries to straighten as pressure rises and a linkage turns that into needle rotation. Invented around 1850, still the default from a few psi to tens of thousands.^{[[1]](https://www.britannica.com/technology/pressure-gauge)}
5. 5
   Diaphragm gauge. A thin corrugated disc that moves noticeably under a small load, so it reads low pressures — inches of water up to a few psi — far better than a Bourdon tube, and tolerates corrosive media behind a chemical seal.
6. 6
   Bellows gauge. An accordion-pleated capsule that stretches with pressure, giving enough travel to drive a switch or recorder pen directly — the mechanism inside pressure switches and draft gauges.
7. 7
   Mercury barometer. An inverted glass tube in a mercury reservoir; the column height in inches is the reading, and it is the origin of the 29.92 inHg on aviation weather reports. Retired from field use over handling, still the conceptual reference.
8. 8
   Aneroid barometer and micromanometer. The aneroid uses a sealed, partly evacuated capsule that flexes as outside air changes — no liquid, so it travels, and it is the movement inside wall barometers and altimeters. The micromanometer is the precision end of the family, resolving hundredths of an inch of water column.

Electronic instruments

Piezoresistive and strain-gauge transmitters dominate industry: a strain gauge bonded to a diaphragm changes resistance as the diaphragm flexes, and the bridge output becomes a 4–20 mA loop, a 0–10 V signal or a digital protocol. The 4–20 mA convention exists because a broken wire reads 0 mA — outside the live range, so a fault can never be mistaken for a zero reading.^[4]

Piezoelectric sensors put out a charge when a crystal is squeezed. They respond in microseconds — the choice for combustion pressure, blast waves and hydraulic spikes — but the charge bleeds away, so they cannot hold a static reading.

Capacitive sensors track the changing gap between a diaphragm and a fixed plate: sensitive at low pressure and very stable, which is why they appear in high-accuracy differential transmitters and barometric references.

Vacuum gauges run on different physics, because below a point there is not enough gas left to push on a diaphragm. A Pirani gauge measures how fast a heated wire loses heat to the surrounding gas, from roughly atmospheric down to a millitorr; below that, ionization gauges count ionized molecules. The HVAC pocket micron gauge works the same way, pulling a system to about 500 microns to prove it is dry before charging.

MEMS barometers are the ones most people own without knowing it — a few millimeters square inside a phone or watch, fixing altitude and counting flights of stairs.

Comparison at a glance

Where you already use these instruments

Tire gauge. Check tires cold — parked three hours or more — against the driver’s door placard, not the higher maximum molded into the sidewall.^[5] Pressure falls about 1 psi per 10 °F, which is why the TPMS light appears on the first cold morning of fall.

HVAC manifold gauge set. Blue reads the low side, red the high side, and both are compound so blue can swing into inches of mercury during evacuation. An R-410A high side routinely passes 400 psi.

Gas appliance manometer. Natural gas arrives near 7 in w.c. and is set to about 3.5 in w.c. at the burner manifold, propane near 11 in w.c. — far too small for a psi dial, which is why furnace startup calls for a manometer.

Well pump pressure switch. A bellows switch cuts the pump in at 30 or 40 psi and out at 50 or 60, with the tank gauge beside it. A short-cycling pump usually means a waterlogged tank, not a bad switch.

Home weather station. The trend beats the number: a drop of more than about 0.06 inHg per hour means a system moving in, a steady climb means clearing.

Blood pressure monitor. The cuff inflates past systolic and reads the oscillations as it deflates, in mmHg because the original instrument was a mercury column.

Instant Pot. High pressure runs about 10.2 to 11.6 psi above atmosphere, raising water’s boiling point to roughly 240 °F; the float valve is the indicator and a spring-and-weight regulator holds the setpoint.

Real instruments and what they cost

Indicative US prices as of September 2026; retail pricing moves constantly, especially on Amazon and at Harbor Freight.

How to choose one

1. Size the range so normal pressure lands between 25% and 75% of full scale. A 0–100 psi gauge on a 90 psi line will drift and fatigue; a 0–1000 psi gauge cannot resolve a 5 psi change. ASME B40.100 keeps steady pressure under 75% of span, pulsating under 65%.
2. Match accuracy grade to consequence. Common commercial grades are B (±3-2-3% of span) and A (±2-1-2%); test gauges run 1A (±1%), 2A (±0.5%) and finer.^[6] A ±3% shop gauge is fine on an air line and useless in a lab.
3. Use a liquid-filled case wherever there is vibration. Glycerin damps needle flutter, spares the movement and stops condensation inside the lens — on a piston compressor it can triple service life.
4. Confirm the connection. US gauges are overwhelmingly 1/4 in NPT, 1/8 in on small dials and 1/2 in on large industrial ones; lower mount and back mount are not interchangeable.
5. Check the wetted materials. Brass is the cheap default for air and clean water, stainless is needed for most chemicals, potable water requires a lead-free (LF) part number.
6. Decide absolute or gauge before you order. Barometric, altitude and vacuum-process work needs absolute; tanks, tires and lines need gauge.
7. Analog or digital. A dial reads at a glance, needs no battery and shows pulsation and trend better than any number; digital wins on resolution, unit switching and logging.

Calibration and where readings go wrong

Zero shift is the first check: open to atmosphere, the instrument should read exactly zero. A gauge sitting a few psi high at rest was overpressured or dropped.

Temperature affects the elastic element and the electronics alike. Most gauges are specified near 70 °F; a reading taken on a 20 °F morning or against a hot pipe can drift a percent or more of span.

Meniscus and parallax errors belong to liquid columns and dials — read a manometer at the bottom of the curve for water and the top for mercury, eye level to it, and read a dial straight on, which is what the mirrored arc on precision gauges is for.

Mounting height shifts a water reading about 0.43 psi per foot above or below the tap: trivial on a 500 psi line, significant on a 10 psi one.

Calibration interval follows consequence: annual for shop and process gauges, more often for safety-critical and lab instruments, immediately after any overpressure event.

The reference standard behind all of it is the deadweight tester, which creates pressure rather than measuring it — calibrated masses on a piston of known area produce a value you can calculate from first principles, and the instrument under test is adjusted to match.

Six mistakes worth avoiding

1. Reading warm tires. A couple of miles adds 3 to 5 psi; set pressure cold or you run chronically underinflated.
2. Trusting the sidewall number. That is the tire’s maximum permissible pressure, not the vehicle’s recommendation.^[5]
3. Running a gauge at the top of its span. The fastest route to a permanently shifted zero.
4. Using a psi gauge for gas manifold pressure. 3.5 in w.c. is 0.13 psi — it does not register.
5. Specifying a piezoelectric sensor for static pressure. It reads correctly for a moment, then decays toward zero.
6. Confusing absolute and gauge on a spec sheet. A “15 psi” rating means very different things as psia and psig, and the gap is exactly one atmosphere.

Frequently asked questions

What instrument is used to measure pressure? A manometer, called a pressure gauge in US shop usage; the specialized versions are the barometer, vacuum gauge, differential pressure gauge and sphygmomanometer.

What is the difference between a manometer and a barometer? A manometer measures gas or liquid in a container, usually against the atmosphere; a barometer measures the atmosphere itself and is sealed against a reference vacuum.

What instrument measures atmospheric pressure? A barometer, mercury column or aneroid capsule, read in the US in inches of mercury and on weather maps in millibars.

What instrument measures blood pressure? A sphygmomanometer — cuff plus gauge — reading in millimeters of mercury, whether an aneroid dial with a stethoscope or an automatic monitor.

Is a pressure gauge the same thing as a manometer? Functionally yes, and catalogs use both words; strictly, manometer means a liquid-column instrument and pressure gauge a mechanical dial.

What does psi stand for? Pounds per square inch. Written psig it is measured against atmosphere, psia against a vacuum, psid as a difference between two ports.

How do I convert psi to bar? Divide by 14.5, so 35 psi is 2.41 bar; multiply the other way, so 2 bar is 29 psi.

What is the difference between gauge pressure and absolute pressure? Absolute equals gauge plus atmospheric, about 14.7 psi at sea level: a tire reading 35 psig holds air at roughly 49.7 psia.

What instrument measures vacuum? A compound gauge for rough vacuum in inches of mercury, a Pirani or HVAC micron gauge down to a few millitorr, an ionization gauge below that.

Do tire pressure gauges need calibration? They are not field-calibrated, but check yours against a known-good gauge every year or two; digital models calibrated to ANSI B40.7 survive being dropped better than cheap pencil types.

Why does my tire pressure drop in cold weather? Roughly 1 psi per 10 °F, so a tire set to 35 psi on an 80 °F afternoon reads about 30 psi on a 30 °F morning and trips the TPMS light.

What device measures pressure inside a pipe? A Bourdon gauge on a threaded tap for general service, a piezometer tube for low-head water, and a strain-gauge transmitter with 4–20 mA output where the reading must be logged.

Sources

1. Pressure gauge — Encyclopaedia Britannica
2. Station Pressure from Elevation and Altimeter Setting — NOAA National Weather Service
3. Using Manometers for Measuring Pressure — Control.com textbook
4. How to Measure Pressure: A Complete Guide to Instruments and Methods — Keyence America
5. Tires — NHTSA TireWise
6. What Are ASME Accuracy Grades for Mechanical Pressure Gauges? — Ashcroft