Pressure can be defined as the force applied to an area. Among the various systems of pressure measurement, absolute pressure and gauge pressure are two of the most common. Understanding the differences between these measurements is crucial, as they significantly affect their application and accuracy. Choosing the correct reference pressure is just as important as selecting the appropriate pressure range, especially for low-pressure applications. An incorrect choice can lead to significant measurement errors.

Key Differences

The simplest way to explain the difference is that absolute pressure uses absolute zero as its zero point, while gauge pressure uses atmospheric pressure as its zero point. Due to varying atmospheric conditions, gauge pressure measurements may lack precision, whereas absolute pressure measurements are always definitive.

Gauge Pressure

• Definition: Gauge pressure is the most common reference for pressure measurement, indicated by a ‘g’ after the pressure unit (e.g., 30 psig). It is measured relative to ambient atmospheric pressure.

• Influence of Atmospheric Pressure: Changes in atmospheric pressure due to weather or altitude directly affect the output of a gauge pressure sensor. A gauge pressure higher than ambient pressure is termed positive pressure, while a pressure below atmospheric pressure is referred to as negative pressure or vacuum gauge pressure.

• Sensor Design: Gauge pressure sensors typically have one pressure port. Ambient air pressure is directed through a vent hole or vent tube to the back of the sensing element. A vented gauge pressure transmitter allows the outside air pressure to be applied to the negative side of the pressure sensing diaphragm, ensuring it measures with reference to the ambient barometric pressure. Thus, a vented gauge pressure sensor reads zero when the process pressure connection is open to atmospheric air.

• Sealed Gauge Pressure: In contrast, a sealed gauge sensor has atmospheric pressure sealed on the negative side of the diaphragm. This design is often used in high-pressure applications, such as hydraulic systems, where variations in atmospheric pressure have minimal effects on sensor accuracy. The zero point for sealed gauge pressure is set based on the pressure within the device at the time of sealing.

Absolute Pressure

• Definition: Absolute pressure is defined as the pressure in a perfect vacuum, meaning no matter exists inside the space. Measurements of absolute pressure reference this absolute zero point. A common example is the measurement of barometric pressure.

• Sensor Design: To create an absolute pressure sensor, manufacturers seal a high vacuum behind the sensing diaphragm. Therefore, if the process pressure connection of an absolute pressure transmitter is open to air, it will read the actual barometric pressure.

When to Use Each Measurement

Determining when to measure absolute pressure versus gauge pressure can be nuanced:

• Gauge Pressure: If you need to measure or control pressure influenced by atmospheric changes—such as the liquid level in an open tank—you should use vented gauge pressure. This allows you to focus on the pressure reading minus the atmospheric component.

• Absolute Pressure: For pressures not affected by atmospheric variations, such as leak testing a completely sealed, non-flexible container, an absolute pressure sensor is the appropriate choice. If you were to use a gauge pressure sensor in this scenario, any change in barometric pressure would alter the sensor reading, even if the pressure within the container remained constant.