By:
Paul Francoletti
Director of Product Sales – Mechanical
Published on:
February 5th, 2024
Last updated on:
May 20th, 2026
Topics:
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By:
Paul Francoletti
Director of Product Sales – Mechanical
Published on:
February 5th, 2024
Last updated on:
May 20th, 2026
Topics:
Vacuum pressure, absolute pressure, and compound pressure gauges each measure pressure differently based on their reference point. Selecting the correct type of pressure gauge is important because the wrong reference can lead to inaccurate readings, poor scale readability, and unreliable process measurements.
Different systems, environments, and process conditions require different measurement approaches, and using the wrong gauge can affect both performance and accuracy.
Read this article to learn the differences between vacuum pressure, absolute pressure, and compound pressure gauges, how each type works, and when to use them in industrial applications.
What is the difference between absolute, gauge, and vacuum pressure?
Different industrial applications require different pressure references depending on the process conditions and measurement requirements. Mechanical pressure gauges, for example, can be used to measure absolute pressure (relative to a perfect vacuum), vacuum pressure (below atmospheric pressure), and compound pressure (relative to atmospheric and positive pressure). Here's a closer look at each:
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Absolute pressure is measured referenced to absolute vacuum, or zero pressure. Absolute vacuum occurs when air is completely absent. Because it uses absolute zero as the reference point, there is no negative absolute pressure.
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Gauge pressure is pressure measured relative to atmospheric pressure, using the surrounding ambient air pressure as the reference point.
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Vacuum pressure is a negative pressure less than atmospheric or barometric pressure, using ambient pressure as the reference.
Some applications may also require gauges capable of measuring both positive and negative pressure. These are commonly referred to as compound pressure gauges.
Figure 1. Types of pressure
What is barometric or atmospheric pressure?
Barometric or atmospheric pressure is the ambient air pressure surrounding the earth and is commonly used as the calibration reference for mechanical pressure gauges.
Typically, mechanical pressure gauges are measured and calibrated at atmospheric pressure. At sea level, this is approximately 30 inches of mercury or 14.7 psi.
It changes based on elevation and weather conditions. On rainy days, barometric pressure falls, while pleasant weather generally causes it to rise. Extreme conditions can significantly affect atmospheric pressure readings.
For example, a sealed-case gauge calibrated at sea level may be affected if installed where atmospheric pressure is substantially lower. Because this pressure changes over time and location, gauges calibrated at one atmospheric condition may experience slight accuracy shifts when installed in another environment.
What instruments are used to measure absolute pressure?
Absolute pressure can be measured using mechanical pressure gauges, digital pressure gauges or pressure transducers designed for absolute pressure applications.
Mechanical pressure gauges can provide an economical absolute pressure option
Mechanical gauges are typically calibrated at atmospheric pressure. When absolute pressure measurement is required, one common approach is to calibrate the gauge at atmospheric pressure and then offset the pointer 14.7 psi upscale at zero pressure.
Although this is not considered a true absolute gauge with an isolated pressure chamber, it is often an acceptable and economical solution for many industrial applications. True absolute mechanical gauges with isolated chambers are available but are generally more costly and less commonly used.
Digital pressure gauges and transducers provide true absolute pressure measurement
Digital pressure gauges and pressure transducers use sensor technology that allows for true absolute pressure measurement. These instruments are used in many applications, including semiconductor, off-highway vehicle, and natural gas applications.
Fig. 2: With standard 0/100 psi gauge pressure
Fig. 3: With 0/100psia absolute pressure
When to use a vacuum pressure gauge?
Use a vacuum pressure gauge when you need to measure pressure below atmospheric pressure. These gauges are calibrated at atmospheric pressure and read only negative pressure or vacuum conditions.
Vacuum pressure gauge dials typically read counterclockwise. As the vacuum increases, the gauge pointer rotates counterclockwise. On a standard 270-degree dial arc gauge, zero pressure is positioned near the 270-degree location on the dial.
Fig. 4: Vacuum pressure gauge
What is a compound pressure gauge?
A compound pressure gauge measures both positive pressure and vacuum pressure on the same dial. These gauges are commonly used in low-pressure systems and applications where operators need to monitor both vacuum and positive pressure conditions with a single instrument.
Standard positive-pressure gauges are generally not damaged if a full vacuum is applied. However, when an application requires the dial to display both positive and negative pressure readings, a compound pressure gauge is typically used.
The pointer moves clockwise when measuring positive pressure and counterclockwise when measuring vacuum or negative pressure.
Fig. 5: Compound pressure gauge
When should you use absolute, vacuum, or compound pressure gauges?
The correct pressure gauge depends on whether the application requires measurement relative to atmospheric pressure, below atmospheric pressure, or referenced to absolute zero.
1. Use absolute pressure instruments when you need measurements that are completely independent of changes in atmospheric pressure.
Absolute pressure instruments with isolated sensors are commonly used in weather stations and applications where air pressure is used to calculate elevation.
Absolute pressure gauges with non-isolated sensors, such as bourdon tube designs, are often used in refineries, petrochemical plants, and chemical processing facilities.
2. Vacuum pressure gauges are used in systems that operate below atmospheric pressure
Vacuum gauges are used in systems that rely on vacuum pumps to remove pressure. Common applications include many pump systems, refrigeration systems, leak detection equipment, and air conditioning systems.
3. Compound pressure gauges are used when both positive and negative pressure must be monitored
Compound pressure gauges are commonly used to measure leaks in pressurized lines, HVAC systems, laboratory test chambers, pump systems, and other low-pressure applications where both positive and negative pressures need to be displayed.
Ready to learn more?
Now that you have learned the different types of pressure to measure, let us help you find the right pressure gauge range for your application. Check out the related resources below or contact us to discuss your process and your measurement needs.
In the meantime, feel free to download our guide to learn how to prevent pressure instrument failure.
