Vacuum Gauges and Digital Vacuum Gauges: Scales, Units and How to Choose

Quick answer: a vacuum gauge measures pressure below atmospheric pressure. On an ordinary gauge the vacuum scale runs from 0 down to -100 kPa (about -1 bar), where -100 kPa is close to a perfect vacuum. If your system swings between vacuum and positive pressure, you need a compound gauge. For low vacuum on pumps, filters and suction lines, a mechanical or digital vacuum gauge is enough. For very deep vacuum, such as refrigeration evacuation measured in microns, you need a specialist vacuum gauge that is a different instrument. This guide explains the scales and units, the types, how to choose, and shows which of our gauges cover vacuum.

What is vacuum?

Vacuum is simply pressure lower than the atmosphere around you. Atmospheric pressure at sea level is about 101.3 kPa, 1.013 bar or 14.7 psi. When a pump removes air from a space, the pressure inside falls below that. A perfect vacuum has no pressure at all, which is 0 kPa absolute.

That gives two ways to describe the same condition.

• Gauge (relative) vacuum: measured against the atmosphere. Atmospheric pressure is 0, and vacuum is shown as a negative number, such as -60 kPa.
• Absolute pressure: measured against a perfect vacuum. The same condition is 101.3 minus 60, which is 41.3 kPa absolute.

Always check which scale a datasheet uses, because the numbers are different. Our guide to gauge vs absolute vs vacuum pressure goes deeper.

Vacuum units and conversions

A micron here is a thousandth of a Torr, so 1,000 microns equals 1 Torr, which is about 0.133 kPa absolute. Deep vacuum is measured in microns because the numbers become tiny in kPa.

Types of vacuum gauge

Bourdon vacuum and compound gauges

The same curved tube used for pressure can be set up to read vacuum. A vacuum gauge reads from 0 to -100 kPa. A compound gauge reads vacuum on one side of zero and positive pressure on the other, for example -100 to 0 to +100 kPa. Compound gauges are the usual choice on pump suction lines, which may see a vacuum when starting and pressure when running. See Bourdon tube pressure gauges explained for the principle.

Capsule and diaphragm gauges

For small vacuum levels, such as a few kilopascals in a ventilation duct, a capsule gauge is more sensitive than a Bourdon tube. Our KPCh capsule gauge has compound ranges starting at -60/0 kPa and reads for air and gas only.

Digital vacuum and compound gauges

A digital gauge shows vacuum as a number and can switch units. The PDK PDR100 has a compound range of -1 to 2 bar, and its units include bar, kPa, mmHg, inHg and psi, which makes it useful when different manuals use different units.

Specialist vacuum gauges

For high and ultra-high vacuum, gauges use different physics, such as heat loss from a wire (Pirani and thermocouple gauges). These measure far below what a Bourdon or strain gauge sensor can. Refrigeration technicians use digital micron gauges of this type to confirm that a system is evacuated and dry. These are specialist instruments, and we do not suggest that a general-purpose pressure gauge replaces them. If you need micron-level measurement, contact us to confirm what we can supply.

Which of our gauges cover vacuum?

How to choose a vacuum gauge

1. How deep a vacuum? Down to about -100 kPa is the territory of ordinary vacuum gauges. Deeper than that needs a specialist instrument.
2. Does pressure ever go positive? If so, choose a compound range.
3. What media? Air, water and oil suit brass. Corrosive media needs stainless steel.
4. How steady is it? Pulsation or vibration favours a liquid filled gauge. See liquid filled vs dry.
5. What do you need to do with the reading? A person reading it needs a gauge. A control system needs a sensor.
6. What connection? See connection types.

Worked examples

Example 1: a pump suction line

A pump draws water from a tank 3 metres below it. Suction lift creates a vacuum of roughly 3 × 9.81 = 29 kPa, plus friction losses. During priming it may reach -50 kPa. Choose a compound gauge with at least -100 to 0 kPa, with a positive section if the line is ever pressurised. A liquid filled gauge steadies the pointer against pump pulses.

Example 2: a vacuum filter in a packaging line

A packaging machine uses vacuum cups that need at least -40 kPa to lift a carton. A gauge with a range of -100 to 0 kPa shows the vacuum level, and a mark at -40 kPa shows the minimum. If the reading drifts toward -30 kPa, the operator knows to check for leaks or a blocked filter.

Example 3: a laboratory vacuum oven

A lab oven runs at an absolute pressure of 20 kPa, which is a gauge vacuum of about -81 kPa. A digital gauge with a compound range, switched to the unit used in the method, makes this easy to read and record. For traceable work, see pressure gauge calibration.

Accuracy and the range problem

Accuracy percentages apply to the full span of the gauge. On a compound range of -100 to +100 kPa, the span is 200 kPa, so a class 1.6 gauge may be wrong by 3.2 kPa. At a vacuum of -40 kPa that is 8% of the reading. For a precise vacuum reading, choose a range close to what you need, or a digital gauge with better accuracy. Our accuracy guide shows the maths.

Installation tips

• Fit the gauge above the point of measurement where possible, so liquid does not collect in the gauge line.
• Use proper thread seals, because even a small leak spoils a vacuum reading.
• Use a trap or filter if the media can carry dirt or liquid into the gauge.
• Do not overpressure a vacuum-only gauge. Positive pressure can damage it.
• Check that the pointer rests at zero when open to the atmosphere.

Common mistakes

• Choosing a pressure gauge with no vacuum range for a suction line.
• Reading absolute and gauge scales as the same.
• Expecting a standard gauge to measure the deep vacuum of refrigeration work.
• Putting a vacuum-only gauge on a line that can go positive.
• Ignoring leaks at the thread, which makes the vacuum look worse than it is.

Electronic vacuum signals

If a controller needs the vacuum level, use a pressure sensor with a vacuum or compound range. The pressure type matters: a gauge-pressure sensor with a range of -100 to 0 kPa reads vacuum relative to the atmosphere, while an absolute sensor reads from a perfect vacuum. The WIKA Model A-10 is offered as gauge, absolute and compound, so state which one you need. For a vacuum switch that simply turns something on or off at a set level, a pressure switch is a simpler alternative, though it does not show the actual value.

Maintenance and safety

• Check vacuum gauges for a pointer that no longer returns to zero, which suggests the tube has been overstressed.
• Keep moisture and oil out of the gauge line with a trap, because liquid in a vacuum line can give false readings.
• Replace seals when you replace the gauge. A single leaking thread can stop a system reaching its target vacuum.
• Remember that a vacuum can be hazardous. Vessels not designed for external pressure can collapse, so never apply vacuum to a tank unless it is rated for it.

Quick conversion reminder

A vacuum of -100 kPa gauge is almost a perfect vacuum, -50 kPa is about half an atmosphere of vacuum, and -10 kPa is a light suction. When you meet a figure in inHg or mmHg, convert it first so that you compare like with like.

Frequently asked questions

What is a compound gauge?

One that reads both vacuum and positive pressure on the same dial.

What does -100 kPa mean?

It is almost a perfect vacuum, 100 kPa below atmospheric pressure.

Can I use a digital pressure gauge for vacuum?

Yes, if it has a compound or vacuum range, such as the PDR100 at -1 to 2 bar.

What gauge measures microns?

A specialist digital vacuum gauge, such as a Pirani type. Ordinary pressure gauges do not.

How do I convert inHg to kPa?

Multiply by about 3.386. For example, 20 inHg of vacuum is about 68 kPa below atmospheric pressure.

Next steps

Start with what is a pressure gauge, compare the sensor, transducer and transmitter options for electronic vacuum signals, and browse the pressure gauges and digital pressure gauges. For help, contact us.