# Vacuum pressure gauges - How they work

Example of a vacuum pressure gauge

A vacuum gauge measures pressure below the atmospheric pressure. Normally the atmospheric pressure is set as zero and the vacuum pressure is given in negative values, so -1 barg (-15 psig) means complete vacuum.

## What is vacuum pressure?

The atmospheric pressure is the standard pressure at sea level and is 101.325 kPa. It is more common to express pressure in bar or psi. Vacuum pressure (Pvac) is expressed in a negative value with respect to the atmospheric pressure.

It is not the same as absolute pressure (Pabs), which is measured with respect to the absolute zero point. The drawing below shows clearly the relationship between the different pressure definitions.

The relation between gauge pressure, vacuum pressure and absolute pressure.

The different pressure definitions explained:

• Absolute pressure Pabs: relative to the absolute zero point
• Atmospheric pressure Patm: the pressure at sea level (1.01325 kPa)
• Gauge pressure Pgauge: relative to the atmospheric pressure
• Vacuum pressure Pvac: measured relative to atmospheric pressure.

## How does a vacuum pressure gauge work?

Most pressure gauges make use of a bended tube. This tube is called a Bourdon tube. The tube deflects slightly under a pressure differential between the medium and the atmosphere. The tube deflection is converted via a mechanism to a needle position on the dial.

Most gauges are available with bottom connection or back connection (panel mount). Gauges are available with plastic casing or stainless steel. Plastic is cheaper and corrosion-resistant, but is more sensitive to impact. They can be analog or digital pressure gauges.

## What are typical applications for vacuum gauges?

Practically all industries make use of vacuum, from pick and place in the automotive industry, food & beverage, manufacturing, pharmaceutical, chemical, nautical, etc. The vacuum gauges are important devices to ensure optimal performance and safety of the system. They may be used for pick-and-place applications, keeping the workplace clean from pollutants, transport materials, etc.

## Accuracy of vacuum gauges

The accuracy is the difference between the real value and the value that is displayed on the dial. This is usually expressed as a percentage of the span.

The EN 837 standard provides accuracy requirements for pressure gauges expressed in accuracy classes. An accuracy class of 2.5 means that the gauge may deviate up to 2.5% of the span from the real value. For a vacuum gauge the span is 1 bar (15 psi), this means 1 bar * 2.5% = 0.025 bar deviation. The deviation can both be positive or negative.

Note that temperature fluctuations or heavy vibrations can impact the performance of the gauge. Even the observer has an effect on the accuracy: reading the dial can lead to small deviations.

## Pressure gauges for absolute pressure

It is important to note that vacuum pressure gauges and absolute pressure gauges are not the same thing. Normal vacuum pressure gauges measure against the atmospheric pressure, while absolute pressure gauges measure with reference to the absolute zero point. In order to do so, these gauges contain a reference chamber with a near full vacuum. These are high end products that are for example used to monitor vacuum pumps or vacuum packaging machines.

### How is vacuum pressure measured?

The most common device to measure vacuum pressure is a bourdon style manometer.

### What is the difference between gauge pressure and vacuum pressure?

Gauge pressure and vacuum pressure are both measured relative to the atmospheric pressure. The difference is that gauge pressure is higher than atmospheric pressure, while vacuum pressure is lower than the atmospheric pressure.

### Is vacuum positive or negative pressure?

Vacuum pressure is usually expressed with a negative number. It is measured with respect to the atmospheric pressure.