 # Atmospheric Pressure and Gauge Pressure  View Notes

The Relation Between Atmospheric Pressure and Gauge Pressure

Pressure, the measure of force in a given area, is a simple concept. However, depending on the query, there can be several different ways of understanding the strain. Here are several guidelines that will help distinguish forms and units of pressure measurement when explaining when and why such pressure measurements are being used.

What is Gauge Pressure?

The gauge pressure is the pressure measured relative to the ambient atmospheric pressure. The gauge pressure can be measured using a diaphragm sensor, where one side of the diaphragm is exposed to the pressure media to be measured, while the other side is exposed to ambient atmospheric pressure.

When measuring the gage pressure, it should be noted that the measured gage pressure changes with changes in barometric pressure due to changes in weather patterns, unless the measurement point is itself exposed to ambient atmospheric pressure. Sensor for calculating the pressure gage In vacuum systems, measuring and monitoring the pressure gage may be used to accurately ventilate the vacuum chamber to ambient pressure and avoid contamination of the particles when opening up to the atmosphere. This practice is also used in semiconductor load lock applications.

Different Types of Pressure

There are several ways in which pressure can be referenced. The application must be considered in order to accurately identify and relay pressure measurements. Pressure sensors shall use the following references -

Gauge Pressure uses a reference to the atmosphere around the sensor. Since the sensing device has a deflection due to a change in pressure, a reference point is required to determine exactly what pressure is being measured. Pressure sensors that use gauge pressure — usually seen in PSIG, BARG, and kPaG — have some kind of vent. This vent can be installed in the sensor or even in the electrical connection by means of a tube. The vent is positioned to use atmospheric pressure as a reference point for the media measurement sensor. One common reason for the use of gauge pressure is to ensure that, with any location in the world, the sensor will always refer to the location at which it is installed.

Absolute Pressure as its reference to the perfect vacuum. This type of pressure reference is the pressure gage of the media plus the pressure of the atmosphere. As locations change, particularly when managing elevation changes, the reference point can change due to variations in atmospheric pressure. The use of an absolute pressure sensor removes the relation to changing atmospheric pressure and relies on a common reference pressure range.

Differential Pressure can be a little more complex than the gage or the absolute, but it basically measures the difference between the two sources. While most gage pressures are theoretically a differential pressure sensor — measuring the difference between the media and the ambient pressure — a true differential pressure sensor is used to measure the difference between the two distinct physical areas. For example, the differential pressure is used to regulate the decrease or loss of pressure from one side of the body to the other.

Sealed Pressure is less popular than the previous three, but it still has a place in the pressure world. Sealed pressure uses a default reference point, not just a vacuum. It makes it possible to measure pressure in areas that can vary based on atmospheric changes. According to the predetermined reference point, no ventilation of the sensor is necessary.

Units of Pressure

Many units are widely used when measuring strain. Some of these units of measurement can be used for the universal unit system, such as kilo, mega, etc. These units shall be defined as follows:-

PSI (Pounds Per Square Inch): is the unit of measure for one pound of force applied to one square inch of area. PSI is a typical pressure unit in the United States.

BAR: One bar is equivalent to Earth's air pressure at sea level. The BAR device has been developed in Europe and is still widely used there.

PA (Pascal): One Pascal is equal to one Newton of pressure per square meter.

InHg (Inches of Mercury): that's the pressure exerted by a one-inch circular column of mercury, one-inch thick, at gravity, and by 0 ° C (32 ° F). Usually, barometric pressure is used inHg.

Torr: This is the pressure exerted by a one-millimeter-high circle column of mercury. It was also known to have a millimeter of mercury (mmHG). It's equal to 1/760 atmospheres.

InH₂O (Inches of Water): this is the unit of measurement for a one inch circular column of water, one inch wide, at gravity, and 4 ° C (39.2 ° F). It is typically used for differential pressure measurements or for low-pressure water applications.

Absolute pressure is defined in many applications without any specific need for it. There is a misunderstanding that all pressure measurements must be absolute. Although absolute pressure measurement is definitely required, most applications require only a gage pressure or an alternative. By knowing the application specifics, it can be simple to pick an acceptable pressure sensor. The correct pressure sensor allows for more precise processes and the most efficient and economical performance.

Q1. What Does Gauge Pressure Mean?

Ans. A gauge is also used to calculate the difference in pressure between the system and the surrounding atmosphere. This pressure is the force of the gauge and can be expressed as -

pg = ps – patm

Where:

pg = gauge pressure

ps = system pressure

patm = atmospheric pressure

The pressure of the gage is the pressure equivalent to the ambient pressure. This is thus positive for the pressure above the ambient pressure and negative for the pressure below it.

Q2. What is the Difference Between Gauge and Absolute Pressure Measurement

Ans - Definition of absolute pressure

Absolute Pressure is the pressure relative to zero pressure in the empty, air-free space of the universe. The reference pressure is the ideal or absolute vacuum. The subscript "abs" is denoted as - Pabs.

Definition of gauge pressure

The gage pressure is defined as the difference between the absolute pressure  (Pabs) and the prevailing atmospheric pressure (Pamb). It is denoted by subscription "e": Pe and is calculated as follows:  Pe = Pabs – Pamb

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