# Vacuum Pressure Gauge Guide

Figure 1: A vacuum pressure gauge.

A vacuum pressure gauge measures vacuum pressure, which is lower than atmospheric pressure. A vacuum gauge is found in many industrial, laboratory research, and automotive applications. This article covers how vacuum pressure gauges work, their typical applications, and what to pay attention to when selecting them.

## What is vacuum pressure?

Vacuum pressure (Figure 2 labeled C) is a measure of the pressure of a gas in a space where the pressure is lower than atmospheric pressure. It is expressed as a negative gauge pressure, often in units of millimeters of mercury (mmHg) or Pascals (Pa). Other pressures displayed in Figure 2 are:

• Absolute pressure (A): The pressure measured relative to a perfect vacuum.
• Gauge pressure (B): The pressure measured relative to the atmospheric pressure.
• Atmospheric pressure (D): The pressure exerted by the weight of the atmosphere on a given point.
• Absolute zero pressure (E): Absolute zero pressure is a theoretically perfect vacuum.

Figure 2: Visualization of pressure types: absolute pressure (A), gauge pressure (B), vacuum pressure (C), atmospheric pressure (D), and absolute zero pressure (E).

## How does a vacuum pressure gauge work?

The following list explains the various components of a vacuum pressure gauge and how they work together to provide accurate pressure readings.

1. A vacuum pressure gauge uses a sensor to measure the difference in pressure between the device and the surrounding atmosphere.
2. The sensor, typically a bourdon tube or diaphragm, will change position or shape in response to changes in pressure.
3. A mechanical linkage connects the sensor to a pointer on a gauge dial, allowing the pressure reading to be easily read by the user.
4. The gauge may also have electronic components that convert the mechanical movement of the sensor into an electrical signal. The signal can display the vacuum gauge pressure reading on a digital display or send the data to a remote monitoring system.

## How to calibrate a vacuum pressure gauge

Like other pressure gauges, calibrating a vacuum gauge requires a calibrator that meets the requirements set by ISO/IEC 17025. ISO 3567 details the necessary technical, physical, and metrological criteria. Read our article on pressure gauge calibration for an overview of how to calibrate a vacuum pressure gauge.

## What are typical applications for vacuum gauges?

• Industrial use: To control pressure in industrial processes like vacuum drying or distillation.
• Pick and place: Vacuum pick and place applications are spread throughout many industries, such as the automotive, food & beverage, manufacturing, pharmaceutical, chemical, and nautical industries.
• Medical equipment: In suction devices and vacuum-assisted delivery systems.
• HVAC systems: When pulling a vacuum during installation or repair of a refrigerant circuit. Pulling a vacuum means evacuating air and matter from the system.
• Research: To measure and control pressure in laboratory experiments. For example, measuring the pressure in a vacuum to see how materials behave at low pressures.
• Automotive: To monitor vacuum in brake systems that have a brake booster, which controls proper power breaking.
• Food Processing: To monitor pressure levels in vacuum packaging machines.
• Pharmaceuticals: To monitor pressure levels in vacuum drying and granulation processes.
• Fire service water pump system: The systems that fire services use to pump water are typically fitted with special vacuum gauges called compound gauges. Compound gauges can measure positive and negative pressure.

Figure 3: A vacuum pressure gauge on a vacuum chamber in a laboratory.

## Accuracy of vacuum gauges

The accuracy of a pressure gauge is defined as the difference between the true pressure value and the value indicated on the dial. The EN 837 standard establishes accuracy requirements for pressure gauges, which are expressed as accuracy classes. For example, an accuracy class of 2.5 indicates that the gauge may deviate up to 2.5% of the pressure range from the true value. In the case of a vacuum gauge with a pressure range of 1 bar (15 psi), this would result in a deviation of 0.025 bar. It is important to note that the deviation can occur in either direction. Additionally, it is worth noting that factors such as temperature fluctuations and vibrations can impact the performance of the gauge, and even the act of reading the dial can introduce small deviations in the accuracy.

## Vacuum gauge selection criteria

There is not a single vacuum gauge that is suitable for all vacuum measuring applications. Therefore, it’s necessary to consider the following when determining which vacuum gauge to select for any given application:

1. Connection: Choose the correct connection size and type, e.g., NPT or flanged. Concerning pressure gauges, flanged connections are often best for viscous media. Also the connection location can be on the side, back, or bottom of the pressure gauge’s cabinet/dial.
2. Measuring range: Choose a range that is the closest to the range that’s important for the given application.
3. Cabinet material: Choose a gauge with cabinet material that will not be adversely affected by the system’s media.
4. Dry vs liquid damped: Choose a liquid filled pressure gauge if the application produces heavy vibrations or shocks that can damage the pressure gauge. If heavy vibrations are not expected, a dry pressure gauge is suitable.

## FAQs

### 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. A negative pressure gauge (a vacuum gauge) is used to measure vacuum pressure.

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

A vacuum pressure gauge measures pressure below atmospheric pressure, while a compound pressure gauge measures pressure in both positive and negative ranges (high to low pressure).