Difference and conversion between gauge pressure and absolute pressure

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In industrial production and daily life, pressure is everywhere. In industry, there is \"gauge pressure\" and \"absolute pressure\", and the same applies in daily life as well. Understand different types of stress and use various pressures to my advantage. Gauge pressure, also known as indicated pressure, generally refers to the pressure measured by a pressure gauge or pressure transmitter, with atmospheric pressure as its zero point. Absolute Pressure, abbreviated as AP, is the actual pressure exerted on a substance, with absolute vacuum serving as its zero point. The simplest way to distinguish between gauge pressure and absolute pressure is the different reference points (zero points). In practical engineering applications, the vast majority of pressure gauges or pressure transmitters used select gauge pressure as the measurement type. Gauge pressure is a type of relative pressure; it is a way of expressing pressure with atmospheric pressure as the reference point. In other words, when the pressure gauge is placed in air or when the pressure in the system being measured equals the local atmospheric pressure, its reading is 0. Gauge pressure gauges also find certain applications in fields such as industrial process control and scientific experiments. Gauge pressure is a method of expressing pressure with absolute vacuum as the reference, and a gauge pressure gauge is a pressure gauge that directly measures absolute pressure. When a gauge pressure meter is placed in air, its reading indicates the local atmospheric pressure; it reads 0 only when the system being measured by the gauge pressure meter is in a complete vacuum. In summary, the difference between gauge pressure and absolute pressure is one unit of local atmospheric pressure; the relationship between them is as follows: Gauge pressure = Absolute pressure – Atmospheric pressure. Diagram illustrating the relationship between gauge pressure, absolute pressure, and negative pressure. Definition of pressure: In physics, pressure refers to the vertical force per unit area. In the manufacturing industry, pressure not only affects the shape and structure of objects but also has a direct impact on the stability and safety of the manufacturing processes. This seemingly simple definition encompasses a wealth of potential applications. From supporting skyscrapers to designing efficient airplane wings, the concept of pressure is everywhere. A force that acts perpendicularly on the surface of an object is called pressure. 2. Units of pressure: There are various units for measuring pressure, reflecting its wide range of applications in different fields: • Pascal (Pa): The standard unit of pressure in the International System of Units (SI); it represents a pressure of 1 N (newton) per m2 of area. In other words, 1 Pa equals 1 N/m2. • Kilopascal (kPa): 1 kPa equals 1000 Pa and is commonly used in industrial measurements. • Megapascal (MPa): 1 MPa equals 1000 kPa, or 1,000,000 Pa, and is also used in industrial applications. • Bar: 1 bar equals 100,000 Pa and is frequently used in engineering fields. • psi (pounds per square inch): 1 psi is approximately equal to 6894.76 Pa and is mainly used in the British and American systems. • Standard atmospheric pressure (atm): Equal to 101,325 Pa, it is often used in meteorology to define gauge pressure. Gauge pressure, as the name suggests, refers to a pressure measurement relative to the local atmospheric pressure. It tells us by how much the pressure being measured is higher or lower than the surrounding air pressure (usually atmospheric pressure). 2. Working principle of gauge pressure: Gauge pressure is measured by comparing it with atmospheric pressure to determine the magnitude of pressure. If the gauge pressure is positive, it indicates that the pressure is higher than atmospheric pressure ； If it is a negative value, it indicates that the pressure is lower than atmospheric pressure. III. Absolute Pressure 1. Definition of Absolute Pressure Absolute pressure is a pressure measurement relative to a complete vacuum. It directly reflects the absolute value of pressure, including all pressures such as atmospheric pressure. 2. Working principle of gauge pressure: Gauge pressure measurement ignores atmospheric pressure and focuses on the pressure exerted on an object itself. -IV. Comparison between gauge pressure and absolute pressure 1. Differences in measurement principles The main difference between gauge pressure and absolute pressure lies in the reference pressure used. Gauge pressure is measured relative to atmospheric pressure, while absolute pressure is measured relative to a complete vacuum. This leads to differences in their application and interpretation. 2. Conversion relationship: To convert gauge pressure to absolute pressure, the current atmospheric pressure value must be taken into account; this is usually done by adding or subtracting the value of one standard atmosphere. The conversion formulas are as follows: • Absolute pressure = Gauge pressure + Atmospheric pressure • Gauge pressure = Absolute pressure – Atmospheric pressure 5. Pressure measurement techniques 1. Types of pressure sensors: Pressure sensors include mechanical, resistive, piezoelectric, and other types, with each type having its own specific working principle and application areas. The Jingcheng Engineering breathing blaster uses Texas Instruments pressure sensors. 2. Accuracy of pressure measurement: Factors that affect the accuracy of pressure measurement include temperature changes, sensor precision, and environmental conditions; these need to be taken into consideration and controlled during design and use. 3. Calibration of pressure measurements Regular calibration of pressure measurement equipment is an important step to ensure its accuracy and reliability, especially in precision manufacturing and safety-critical applications. -For example: if the gauge pressure is 0.5 MPaG, then using the formula above, the absolute pressure = (0.5 + 0.101325) MPaA = 0.601325 MPaA. Note 1: Strictly speaking, atmospheric pressure needs to be determined based on the operating conditions, and it is related to the local altitude and weather conditions. Quite often, for the sake of convenience, the standard atmospheric pressure is used directly: 1 atm = 101.325 kPa. Note 2: Based on the abbreviations of the corresponding English words Gauge and Absolute, the suffixes “G” or “A” are usually added after standard units to denote gauge pressure and absolute pressure respectively; this format is generally used in the materials provided by design institutes. Note 3: If a gauge pressure sensor is used in an application while the actual measurement task requires the measurement of absolute pressure, the following additional errors must be taken into account: ① ±30 mbar due to weather changes; ② up to 200 mbar when the location changes (for example, rising from sea level to 2000 m). Depending on the measurement range, these errors can be significant (such as in pneumatic systems with a measurement range of 1 bar) or negligible (such as in hydraulic systems with a pressure of 400 bar). Remember? The value of standard atmospheric pressure (atm) was proposed by the Italian physicist Evangelista Torricelli in 1644; in the same year, he also co-invented the mercury barometer. It is a pity that fate was against such talent; in 1647, at the age of 39, Torricelli fell suddenly ill and passed away. In engineering practice, it is very important to distinguish between gauge pressure and absolute pressure. For example, many calculations related to gases, such as converting the volumetric flow rate under normal operating conditions to the volumetric flow rate under standard conditions, involve pressure. Do you specify whether gauge pressure or absolute pressure should be used in these calculations? To make it easier to remember, let’s recall that gauge pressure is measured relative to atmospheric pressure, and changes in atmospheric pressure caused by altitude or weather conditions directly affect the accuracy of gauge pressure measurements. In other words, the measurement of gauge pressure is not precise, whereas absolute pressure is always definite. Since absolute pressure is fixed, the pressure used in various formulas and calculations is generally absolute pressure. Finally, to ensure the accuracy and reliability of the measurement results, it is necessary to perform regular calibration and verification based on factors such as the accuracy class of the pressure gauge and pressure transmitter, as well as the operating environment.