By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed September 30, 2026
A vacuum pressure gauge is a dial instrument that reads pressure below the surrounding atmosphere, on a scale such as 30–0 inHg or 0 to −100 kPa. A compound pressure gauge puts vacuum and positive pressure on the same dial, in catalog steps from 30″-0-15 psi up to 30″-0-300 psi. Both are ordinary bourdon-tube instruments working below zero.
The buying decisions sit in three places the dial face does not explain: scale direction, accuracy across a compound span, and how deep a dial stays readable. This guide covers the scale conventions and the catalog range steps, with the acceptance error worked out in psi. It also shows how to read the needle and when to step up to a switch or transmitter.
Contents
- Three vacuum gauge worlds
- How the dial works
- Scales and conventions
- Catalog range steps
- Accuracy on compound dials
- Reading the dial
- Selecting the gauge
- FAQ
Three vacuum gauge worlds
Search results for this instrument mix three separate markets, and the first job is to pick the right one. This page covers the industrial kind: a 2.5 or 4 inch dial on a process line, suction pipe, filter housing or condenser. It threads 1/4 NPT into a tap and reads in inHg, kPa or bar.
The second market is automotive. A 52 mm engine gauge scaled −30 to 0 inHg reads intake-manifold vacuum for tuning and carburetor diagnosis. It sells for under a hundred dollars and has nothing to do with process piping.
The third is electronic. Refrigeration technicians and vacuum-technology labs evacuate systems with micron gauges, which are Pirani or thermocouple sensors reading thousandths of a torr. A dial does not reach that far down; the boundary is quantified in the accuracy section below. The electronic instrument types are mapped in our vacuum pressure measurement guide.
How the dial works
Inside the case is the same C-shaped bourdon tube as a positive gauge. Positive pressure makes the flattened tube straighten. Vacuum inside the tube flattens the cross section further and curls the tip inward, so the pointer swings the opposite way.
On a standard 270-degree dial arc, the zero of a vacuum-only scale sits near the end of the arc, where a positive gauge prints its top number. Deeper vacuum drives the needle counterclockwise from there. The mechanism itself, link, sector and pinion, is explained in our bourdon tube pressure gauge guide.
A published disagreement needs settling. Some suppliers warn that pulling vacuum on a positive-pressure gauge damages it; others state the opposite. Both are right about different instruments.
For a standard bourdon gauge, full vacuum is an excursion of only about −14.7 psi. Gauge manufacturers state that a standard bourdon element generally survives it.
Low-range capsule and diaphragm instruments, and any gauge with an internal pointer stop, are a different matter and can be harmed. The practical reason to buy a compound pressure gauge is not survival but information. A positive-only dial pressed below zero pushes its pointer against the stop and reads nothing useful.
Scales and conventions
Vacuum dials do not agree on which way to count, and the convention decides how you read every number. US dials scaled in inches of mercury count upward as pressure falls: 0 inHg at atmosphere, 30 inHg at full vacuum. Metric dials count downward into negative numbers: 0 to −100 kPa, or 0 to −1 bar.
The same physical state, a perfect vacuum at sea level, is 29.92 inHg of vacuum, or −101.3 kPa; printed dials round those endpoints to 30 inHg and −100 kPa. On a psi scale that state is −14.7 psig, or 0 psia.
Compound dials in North America pair the two systems on one face: inches of mercury on the vacuum side of zero, psi on the pressure side. The two sides carry different graduation spacing, so the value of one small division changes as the needle crosses zero. Metric compound dials stay in one unit, for example −100 kPa to +300 kPa.
Weather and altitude move the gauge zero, because a vented dial always compares against the local atmosphere. The reference logic lives in absolute vs gauge pressure. For conversions between all of these units, use our pressure unit converter.
Catalog range steps
The vacuum sweep is the one span you never have to size. The deepest any process can pull is one atmosphere down. A single full-sweep scale, 30–0 inHg or −100 kPa to 0, therefore covers every vacuum-only service. Sizing effort belongs entirely to the positive side of a compound range, and US catalogs step it in a fixed series.
| Catalog range | Total span | Class 1.6 error | Fits lines running |
|---|---|---|---|
| 30″-0-15 psi | 29.7 psi | ±0.48 psi | 4 to 11 psi |
| 30″-0-30 psi | 44.7 psi | ±0.72 psi | 8 to 22 psi |
| 30″-0-60 psi | 74.7 psi | ±1.20 psi | 15 to 45 psi |
| 30″-0-100 psi | 114.7 psi | ±1.84 psi | 25 to 75 psi |
| 30″-0-160 psi | 174.7 psi | ±2.80 psi | 40 to 120 psi |
| 30″-0-300 psi | 314.7 psi | ±5.04 psi | 75 to 225 psi |
Span = 14.73 psi of vacuum sweep plus the positive scale. The last column keeps the normal operating point inside the middle half of the positive scale, the placement rule from our pressure gauge reading guide. Metric catalogs step the same idea as −100 kPa to +60, +150, +300, +500 or +900 kPa and on to +1.5 and +2.4 MPa.
The table also shows what a tall positive scale costs on the vacuum side. On a 30″-0-160 psi dial, class 1.6 acceptance is ±2.80 psi, which is ±5.7 inHg, about a fifth of the vacuum sweep. A range like that confirms a line is under vacuum and does no more. When both sides matter, keep the positive top at 60 psi or below, or fit a full-sweep vacuum dial and a separate pressure gauge.
Accuracy on compound dials
Accuracy classes on dial gauges are a percentage of span, per EN 837-1. On a compound dial the span runs from full vacuum to the top of the positive scale, not over the positive numbers alone.
A class 2.5 instrument ranged 30″-0-15 psi may deviate by 2.5 percent of 29.7 psi. That allowance is ±0.74 psi anywhere on the face, including the vacuum side, where it equals about ±1.5 inHg. ASME B40.100 grades work the same way on span, with tighter tolerance in the middle half of the scale for the lettered grades. The grade arithmetic for positive dials is worked through in our reading guide and in pressure gauge calibration.
The same arithmetic sets the floor for dial instruments. A class 1.6 full-sweep vacuum dial may be off by 1.6 kPa, which is 16 mbar.
A process holding 50 mbar absolute carries an error band equal to a third of the reading, so the dial means little there. At the 1 mbar of a freeze dryer it means nothing. Below roughly 50 mbar absolute, move to capsule instruments or electronic gauges; the technology map by range is in the vacuum measurement guide.
Reading the dial
How to read a vacuum pressure gauge comes down to three habits: two carry over from positive dials, and one reverses. Work out the value of one graduation before trusting any reading, and tap the case lightly to free a movement held by friction. Both steps are covered in how to read a pressure gauge.
The habit that does not carry over is direction. On an inHg scale the numbers grow as the needle swings counterclockwise, so a bigger number means less pressure in the pipe.
A suction dial marked every 5 inHg with 1 inHg graduations, needle three marks past 20 = 23 inHg of vacuum, which is −77.9 kPa gauge, or 3.4 psia absolute.
On a compound face the needle rests at zero with the line open, swings clockwise into psi under pressure, and counterclockwise into inHg under suction. Remember the two sides are scaled differently: one division on the vacuum arc is not worth the same as one division on the pressure arc. A pointer sitting below zero on an open line is a zero error, not a vacuum.
Selecting the gauge
The first decision is whether the line ever carries positive pressure. A compound pressure gauge suits pump suctions that flood on shutdown, filter housings that get blown back, and vessels that cycle between pressure and vacuum. A line that only pulls below atmosphere takes the full-sweep vacuum scale.
The rest follows the same choices as any dial instrument. Pulsation from a vacuum pump is a case for a liquid fill; the fluid and temperature windows are covered in our liquid filled pressure gauge guide. Away from a 20 °C reference, expect roughly ±0.4 percent of span of added error per 10 K.
Common builds are 2.5 and 4 inch dials on a 1/4 NPT lower mount in brass or 316 stainless steel. A digital vacuum pressure gauge puts a battery-powered display on the same port and adds min and max capture. That helps when a leak test has to record how far the vacuum decayed.
When the reading has to act rather than inform, a dial is no longer the right instrument. A falling vacuum that must stop a pump or raise an alarm calls for a vacuum pressure switch with a relay output.
A control system input calls for a transmitter, negative-gauge for rough vacuum. Where the reference must not move with the weather, specify an absolute pressure transmitter. Dial gauge models are on our pressure gauges page.
Application note
Across the 330 inquiries in our case log, vacuum work appears twice, and both requests were for transmitters. One was a US refrigeration plant replacing a failed unit; the other, a vacuum-equipment builder ordering sensors and flow meters.
Not one entry is a dial vacuum gauge on its own. These gauges usually arrive already fitted to the pump or the package. The selection above therefore matters most at replacement time, when the range and connection on the old dial are the specification.
FAQ
What is a vacuum pressure gauge?
A vacuum pressure gauge is a dial instrument that reads pressure below the local atmosphere, typically on a 30–0 inHg or 0 to −100 kPa scale. It uses the same bourdon-tube mechanism as a positive-pressure gauge, deflected in the opposite direction, and fits suction lines, filters and condensers.
How does a vacuum pressure gauge work?
A curved, flattened bourdon tube sits inside the case. Vacuum in the tube flattens the cross section further and curls the tip inward. A link and geared movement turn that motion into a counterclockwise pointer swing. The deeper the vacuum, the further the needle travels from the zero at atmospheric pressure.
What is the difference between a compound gauge and a pressure gauge?
A pressure gauge reads only above atmospheric pressure. A compound gauge covers both sides of zero, with a vacuum scale in inHg or negative kPa and a positive scale in psi. Compound dials suit lines that swing across atmospheric, such as pump suctions and filter housings.
How many psi is a perfect vacuum?
A perfect vacuum at sea level reads −14.7 psi on a gauge scale. On a US vacuum dial that is 29.92 inHg, and in absolute terms it is 0 psia. No pump can pass that point, because the reading is limited by the weight of the atmosphere itself, about 101.325 kPa at sea level.
Request a quote
Tell us the scale you want, whether the line swings above atmospheric, and the connection and fill. We will match a vacuum or compound pressure 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.