Pressure measurement is an essential part of many industries and processes. It plays a crucial role in ensuring the safety, efficiency, and accuracy of various systems and equipment. Before we go into the details of pressure measurement, let us first understand what pressure? It is the force applied per unit area.
It is typically denoted by the symbol 'P' and is measured in units of force per unit area, such as pounds per square inch (psi) or newtons per square meter (N/m2). In the field of physics, pressure is a crucial quantity that describes the state of matter and how it behaves under different conditions. In the context of fluid mechanics, Pressure is a fundamental concept that governs the flow of fluids and their interaction with solid object.
Pressure
Pressure exists in both liquids and gases, but its effects and behavior differ in these two states of matter. Let us explore how pressure affects liquids and then move on to gases:
Pressure and Liquids
Unlike gases, liquids are in compressible, which means they cannot be squeezed into smaller volumes. Therefore, any change in pressure on a liquid would result in a change in its volume.
This principle is known as Pascal's Law, which states that the pressure exerted by a fluid on an object is transmitted equally in all directions. This property of liquids makes them ideal for hydraulic systems, where pressure is used to transmit and amplify forces over long distances.
Pressure and Gases
In contrast to liquids, gases are compressible, which means their volume can change significantly in response to pressure. This is why gases are used in different types of pressure measuring devices, as we will discuss later in this topic.
Another important concept in gas pressure is the ideal gas law, which states that the pressure of a gas is directly proportional to its temperature and the number of gas particles present. When temperature of gas increase, the kinetic energy of their particles increase, result in an increase in pressure.
Pressure Units
There are several units used to measure pressure, and their choice depends on the application and the system of measurement. Some common pressure units include psi, bar, pascal, and atmosphere.
In engineering and industrial applications, psi (pounds per square inch) and bar (1 bar = 14.5038 psi) are commonly used, while pascal (1 Pa = 1 N/m2) is used in scientific and academic settings. When measuring vacuum pressure, the unit torr (1 torr = 1/760 atm) is often used.
Absolute, Gauge and Atmospheric Pressure
It is essential to understand the different types of pressure when it comes to their measurement.The whole pressure that a fluid exerts, including pressure in the atmosphere, is referred to as absolute pressure. It is often used as a reference point when measuring pressure. And Gauge pressure, is the pressure measured relative to atmospheric pressure.
For example, if the atmospheric pressure is 14.7 psi, a gauge reading of 10 psi would mean an absolute pressure of 24.7 psi. Atmospheric pressure, also referred to as barometric pressure, is the pressure exerted by the Earth's atmosphere at a given location.
Pressure Measuring Devices
Now that we have a basic understanding of pressure and its units, let us go into the different devices used for measuring pressure:
The Manometer
One of the simplest pressure measuring devices is the manometer, which consists of a U-shaped tube filled with a liquid, such as mercury or water. It utilizes the principle of Pascal's Law, where pressure at a point is transmitted equally in all directions. One end of the tube is connected to the source of pressure, while the other is open to the atmosphere.
As the pressure at the source increases, the liquid in the tube will be displaced, with the difference in height of the liquid columns indicating the pressure difference. The manometer is commonly used to measure small pressure differences and is often used in pressure gauges to amplify small changes in pressure.
The Well Manometer
A well manometer is a modified version of the U-shaped manometer and is commonly used to measure small pressure differences. It consists of a U-shaped tube with one end connected to the source of pressure and the other end sealed.
The tube contains a liquid, and a well located at the bottom allows for accurate measurement of small pressure differences. The height difference between the liquid levels in the U-tube and the well indicates the pressure difference.
The Inclined Limb Manometer
The inclined limb manometer is another type of manometer that utilizes the U-shaped tube. In this device, one side of the U-tube is inclined, with the other side connected to the source of pressure.
The inclined limb allows for a larger range of pressure measurement compared to the traditional manometer. The pressure difference is measured by the height difference between the liquid levels in the two limbs.
The Bourdon Tube Pressure Gauge
The Bourdon tube pressure gauge is a common type of mechanical pressure gauge used in various industrial and commercial applications. It consists of a curved tube that is closed at one end and connected to the source of pressure at the other end.
As the pressure increases, the curvature of the tube changes, and this movement is translated into a needle reading on a calibrated scale. The Bourdon tube pressure gauge is highly accurate and can measure a wide range of pressures.
Special Adaptation of the Bourdon Tube
The Bourdon tube has undergone many adaptations to make it suitable for different types of pressure measurement. For example, the diaphragm pressure gauge uses a thin, flexible diaphragm instead of a curved tube. The diaphragm deforms under pressure, and this movement is converted into a reading on a calibrated scale.
This type of pressure gauge is commonly used for measuring small pressure differences and for applications where the pressure needs to be contained within the gauge.
In conclusion, pressure measurement is a crucial aspect of many industries and processes, and its accurate measurement is critical for ensuring safety, efficiency, and accuracy. With various devices and techniques available for measuring pressure, engineers and scientists can choose the most suitable method for their specific needs.
Understanding the fundamentals of pressure and its measurement is essential for anyone involved in fluid mechanics, process control, and other related fields.
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