By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed July 28, 2026
Vacuum pressure measurement means reading a pressure below the surrounding atmosphere. Industry does it two ways: as a negative gauge pressure referenced to the atmosphere, on scales such as 0 to −1 bar or 0 to −14.7 psi, or as a small absolute pressure referenced to a perfect vacuum, in millibar, torr or microns. The reference and the unit you pick decide which instrument can do the job.
The span is enormous: process vacuum lives between atmosphere and about 1 mbar, while coating and research plants work down to 10−7 mbar and below. No single technology covers it. This guide connects the two worlds: what vacuum pressure is, the units and their conversions, the standard vacuum ranges, the gauge vs absolute trap, and which measurement technology covers each range.
Contents
- Vacuum pressure explained
- Units and conversions
- Vacuum ranges
- Gauge or absolute reference
- Measurement technologies
- Selecting the range
- Choosing the instrumentation
- FAQ
Vacuum pressure explained
Vacuum pressure is negative gauge pressure. A vented instrument reads zero at atmospheric pressure; pull gas out of the system and the reading goes negative. The floor is the atmosphere itself: at sea level the surrounding air pushes with 101.325 kPa (14.7 psi), so no pump can pull more than −101.325 kPa of gauge vacuum. At that point the absolute pressure is zero, which is why full vacuum reads −14.7 psig and 0 psia on the same connection.
The same state can therefore be written two ways. The relationship is one subtraction:
vacuum (gauge) = absolute pressure − atmospheric pressure
A suction line at 41.3 kPa absolute sits 60 kPa below a 101.3 kPa atmosphere, so a vacuum gauge on it reads −60 kPa. Which notation you use is not cosmetic: it decides which sensor reference you specify.
Units and conversions
Vacuum work mixes more units than any other pressure measurement. Process plants use kPa, mbar and inHg; vacuum technology uses torr and mbar absolute; refrigeration technicians evacuate in microns. They all anchor to the same standard atmosphere.
| Unit | One standard atmosphere | Where it is used |
|---|---|---|
| kPa | 101.325 kPa | SI process scales, often as negative gauge (−60 kPa) |
| mbar | 1,013.25 mbar | Vacuum technology, absolute; 1 mbar = 100 Pa |
| psi | 14.696 psi | US scales; full vacuum = −14.7 psig = 0 psia |
| torr | 760 torr | Vacuum technology; 1 torr = 1 mmHg = 133.32 Pa |
| micron (mTorr) | 760,000 microns | Refrigeration evacuation; 1,000 microns = 1 torr |
| inHg (vacuum) | 29.92 inHg | North American rough vacuum, counted down from 0 |
| percent vacuum | 100% = perfect vacuum | Vacuum lifting and packaging; depends on local atmosphere |
Handy exact factor: 1 mbar = 0.7501 torr. The common shortcut of 0.76 overstates it by 1.3 percent.
One worked example ties the table together. A vacuum gauge on a sea-level receiver reads −60 kPa. That is 41.3 kPa absolute = 413 mbar = 310 torr = 17.7 inHg of vacuum = 59.2 percent vacuum. Our absolute and gauge pressure calculator does the arithmetic in both directions, and the pressure units page covers the positive-pressure conversions.
Vacuum ranges
Vacuum technology divides the span below atmosphere into standard bands. The boundaries shift slightly between sources, but the mbar bands below follow the vacuum-technology textbook convention and are the ones you will meet on gauge datasheets.
| Band | Absolute pressure | Typical service |
|---|---|---|
| Rough (low) vacuum | 1,000 to 1 mbar | Suction lines, filtration, vacuum conveying, packaging, evaporators |
| Medium vacuum | 1 to 10−3 mbar | Freeze drying, vacuum distillation, furnace degassing |
| High vacuum | 10−3 to 10−7 mbar | Coating, electron microscopy, accelerators |
| Ultra-high vacuum | Below 10−7 mbar | Semiconductor research, surface science, space simulation |
Some trade sources put the rough-to-medium break at 25 torr instead of 1 mbar; the bands are conventions, not physics.
Almost every process-industry job sits in the rough band, and that band is where negative gauge scales, dial gauges and standard transmitters work. Everything from medium vacuum down is measured on an absolute reference, with its own instrument families.
Gauge or absolute reference
A vented gauge instrument compares the process to today’s atmosphere, and the atmosphere is not a fixed line. Weather alone swings a barometer between roughly 27 and 33 inHg, and altitude shifts it far more. At 1,600 m, the altitude of Denver, the standard atmosphere is 83.5 kPa, so the deepest vacuum any pump can show on a gauge scale there is −83.5 kPa, not −101.3 kPa. A percent-vacuum reading moves with it.
That drift does not matter when you watch a suction line for blockage. It matters a great deal when the process cares about the true molecular state of the gas: a freeze dryer at 0.5 mbar absolute is at 0.5 mbar whatever the weather outside. The rule follows: gauge reference for rough process vacuum, absolute reference from medium vacuum down, and absolute whenever readings must compare across sites or seasons. The reference logic is covered in depth in absolute vs gauge pressure.
Measurement technologies
No single sensor spans 10 orders of magnitude. Coverage comes in stages: each technology covers a band, and the next one continues below it.
| Technology | Usable range (abs) | Notes |
|---|---|---|
| Bourdon vacuum gauge | Atmosphere to about 10 mbar | Dial reads negative gauge; cheap, no power |
| Capsule and diaphragm gauges | Atmosphere to about 1 mbar | Linear scale; sealed-reference types read absolute |
| Piezoresistive transmitter | Atmosphere to about 1 mbar | 4-20 mA output; negative gauge or absolute cells |
| Capacitance diaphragm gauge | About 10−5 to 1,000 torr (per-sensor spans of 3 to 4 decades) | Gas-independent; the standard choice for medium vacuum |
| Pirani (thermal) | About 10−4 to 10 torr | Gas-dependent; convection types reach atmosphere |
| Ionization gauge | About 10−11 to 10−2 torr | High and ultra-high vacuum; hot or cold cathode |
Ranges are representative manufacturer figures; confirm per datasheet.
The first three rows are ordinary pressure instrumentation working below zero; the bourdon mechanism behind the dial version is covered in our bourdon tube pressure gauge guide. One warning on the indirect types: a Pirani gauge infers pressure from heat loss, which depends on the gas. A nitrogen-calibrated Pirani reading 10 torr of argon can be standing at 1,000 torr of true pressure, an over-pressure risk that gauge manufacturers document explicitly. A capacitance diaphragm gauge responds to pressure alone, whatever the gas, which is why it anchors medium-vacuum work.
Selecting the range
For rough vacuum the choice is a scale, not a technology. Three patterns cover almost every specification. A full-vacuum range, 0 to −1 bar or 0 to −100 kPa, covers anything a pump can do and is the default. A short negative range buys resolution when you only work near atmosphere: on a 0 to −250 mbar scale a 1.6 percent instrument is within ±4 mbar, against ±16 mbar for the same class on a full-vacuum scale.
A compound range covers services that swing across atmospheric, which answers a common exam question: the instrument that measures both vacuum and positive pressure is a compound gauge.
Scaling example: a compound transmitter ranged −1 to +3 bar on a 4-20 mA loop spans 4 bar over 16 mA. Atmospheric (0 bar gauge) lands at 4 + 16 x 1/4 = 8.0 mA, and a half-vacuum reading of −0.5 bar at 6.0 mA.
Overpressure is easy to overlook. A vacuum-only sensor on a line that also sees positive pressure spikes needs the compound range. And keep the fitting small and short: dead volume and long impulse lines slow every vacuum reading.
Choosing the instrumentation
Match the instrument to what the reading has to do. A dial vacuum or compound gauge gives a local check with no power. Where a falling vacuum must trip an alarm or stop a pump, a vacuum pressure switch with a relay output is the simplest reliable answer. For a signal into the control system, a negative-gauge transmitter covers 0 to −100 kPa with a MEMS piezoresistive cell; see the low pressure transducer line and the wider pressure transmitter range.
Where the reference must not move with the weather, an absolute pressure transmitter reads against a sealed vacuum cell. From medium vacuum down, capacitance, Pirani and ionization instruments take over; those are vacuum-technology equipment rather than process instrumentation, and we will say so rather than force a transmitter where it cannot read.
Application example
Refrigeration plant, United States. The customer’s Teledyne VAS-2000 vacuum transmitter had failed and the model was no longer easy to source. They asked for a like-for-like replacement, and we proposed a vacuum transmitter matched to the original’s range and output for a drop-in swap. Replacing a vacuum instrument starts with three numbers from the old nameplate: the range, the reference and the output signal.
FAQ
How do you measure vacuum pressure?
With a vented instrument that reads negative gauge pressure (a vacuum dial gauge, switch or transmitter) for rough vacuum, or an absolute-reference instrument for anything deeper. From medium vacuum down, capacitance diaphragm, Pirani and ionization gauges take over in sequence.
How to calculate the vacuum pressure?
Subtract atmospheric pressure from the absolute pressure: vacuum (gauge) = absolute − atmospheric. A tank at 41.3 kPa absolute under a 101.3 kPa atmosphere reads −60 kPa. Watch the sign convention: inHg vacuum and percent vacuum scales count the same number as positive.
What is a normal vacuum pressure?
There is no single normal; it is set by the process. Pump suction and filtration jobs run tens of kPa below atmosphere, vacuum packaging around −60 to −90 kPa, freeze drying near 0.1 to 1 mbar absolute, and coating processes far below that. Atmosphere itself is 101.325 kPa at sea level.
What is the unit of vacuum pressure?
Rough process vacuum is quoted in negative kPa, mbar or psi, or in inHg of vacuum. Vacuum technology uses absolute millibar or torr, and refrigeration technicians use microns, where 1,000 microns equal 1 torr. All tie back to the 101.325 kPa standard atmosphere.
Request a quote
Tell us the range, the reference and the signal you need, and we will match a vacuum gauge, switch or transmitter to the service. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.