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.
FAQs on Atmospheric Pressure and Gauge Pressure
1. What is atmospheric pressure and what causes it?
Atmospheric pressure is the pressure exerted by the weight of the air in the Earth's atmosphere. It is caused by the gravitational pull of the Earth on the column of air directly above any given point. At sea level, the standard atmospheric pressure is approximately 101,325 Pascals (Pa) or 1 atmosphere (atm).
2. How is gauge pressure defined in Physics?
Gauge pressure is the pressure of a system measured relative to the surrounding atmospheric pressure. It is the amount by which the system's pressure is greater than the local atmospheric pressure. Instruments like a tyre pressure gauge or a blood pressure monitor measure gauge pressure, not the total or absolute pressure.
3. What is the fundamental difference between absolute, atmospheric, and gauge pressure?
The key difference lies in their reference point or 'zero' value.
- Absolute Pressure: Measured relative to a perfect vacuum (zero pressure). It represents the total pressure being exerted.
- Atmospheric Pressure: A specific type of absolute pressure caused by the weight of the atmosphere.
- Gauge Pressure: Measured relative to the current atmospheric pressure. It is the difference between absolute pressure and atmospheric pressure.
4. What is the formula relating absolute pressure, gauge pressure, and atmospheric pressure?
The relationship between these three pressures is described by a simple and important formula:
Pabsolute = Pgauge + Patmospheric
Where:
- Pabsolute is the absolute or total pressure.
- Pgauge is the gauge pressure.
- Patmospheric is the local atmospheric pressure.
5. Why is gauge pressure sometimes negative, and what does a negative value mean?
A negative gauge pressure indicates that the pressure of the system is below the local atmospheric pressure. This is commonly known as a vacuum pressure. For example, when you sip a drink through a straw, you lower the pressure in your mouth to be less than the atmospheric pressure, creating a partial vacuum or a negative gauge pressure that allows the liquid to be pushed up the straw.
6. Can you use a real-world example, like a car tyre, to explain gauge and absolute pressure?
Certainly. Imagine a car tyre.
- When you use a gauge and it reads 32 psi (pounds per square inch), this is the gauge pressure — the pressure inside the tyre that is above the surrounding atmospheric pressure.
- The local atmospheric pressure is also pressing on the tyre from the outside (approximately 14.7 psi at sea level).
- The absolute pressure inside the tyre is the sum of both: 32 psi (gauge) + 14.7 psi (atmospheric) = 46.7 psi (absolute). This is the total pressure the tyre structure must contain.
7. How does atmospheric pressure change with altitude, and why is this concept important?
Atmospheric pressure decreases as altitude increases. This happens because at higher elevations, there is a shorter column of air above, resulting in less weight and therefore less pressure. This principle is crucial in several fields, including:
- Aviation: Aircraft altimeters are essentially barometers that use pressure changes to determine altitude.
- Meteorology: Changes in barometric pressure help forecast weather patterns.
- Human Physiology: It explains why it's harder to breathe and why water boils at a lower temperature on high mountains.
8. Why is atmospheric pressure considered the 'zero' reference for gauge pressure?
Setting atmospheric pressure as the zero point for gauge measurements provides a practical and intuitive frame of reference for most engineering and everyday applications. We are often more interested in the pressure difference relative to our immediate environment. For instance, a flat tyre has zero gauge pressure because its internal pressure equals the external atmospheric pressure, even though its absolute pressure is not zero. This simplification makes it easier to assess whether a system is pressurised or under a vacuum relative to its surroundings.
