Everyone who has been in the industry must have come across these two types of pressure countless of times. I guess most might have ignored it but for some it can be quite confusing and so we are going back to the basics and discuss: Absolute vs Gauge Pressure.
In this article:
- You will see the general types of pressures used in the industry
- We will discuss the differences of Gauge pressure and Absolute pressure
- We will show the relationship between the types of pressures
- You will see practical applications of each pressure types and how a gauge pressure sensor is different from absolute pressure sensor
Let us quickly discuss the fundamentals.
What is Pressure?
Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed.
What are the general types of pressure?
Gauge Pressure (Pgag) â€“ Gauge/gage is the pressure relative to the atmospheric pressure. It is calculated by subtracting atmospheric pressure from absolute pressure. It measures how much above or below (vacuum) the atmospheric pressure is the point of measurement.
Absolute Pressure (Pabs) – is measured relative to perfect vacuum (absolute zero pressure) as its reference. It is the sum of the gauge pressure and atmospheric pressure.
Atmospheric Pressure (Barometric Pressure) (Patm) – the atmospheric pressure is the pressure that an area experiences due to the force exerted by the atmosphere. For engineering calculations typically the pressure used is the pressure at sea level. Why sea level? Because in most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point, which in this case is the sea level. At higher elevation, the atmospheric pressure decreases. Typically, the quantity used for engineering calculations is 1 atm or 101 kPa. Atmospheric pressure varies depending on location.
Units of Measurement
Pressure is measured by these commonly used units. These are defined as follows:
- PSI (lb/in2): Short for (pounds per square inch). This is the unit of measure for one pound of force applied to one square inch of area.
- BAR (bar): One bar equals the atmospheric pressure on the Earth at sea level. This unit was created in Europe and is still commonly used there.
- Pascal (Pa): One Pascal is equal to one Newton of pressure per square meter.
- Inches of mercury (inHg): This is the pressure exerted by a one inch circular column of mercury, one inch tall, at gravity and 0Â°C (32Â°F). inHg are typically used for barometric pressure.
- Torr (torr): It is equal to 1/760 atmospheres. Historically, one torr was intended to be the same as one “millimetre of mercury”. However, subsequent redefinitions of the two units made them slightly different (by less than 0.000015%).
- Inches of Water (inH2O): This is the unit of measure for a one inch circular column of water, one inch tall, at gravity and 4Â°C (39.2Â°F). It is typically used for a differential pressure measurement or in low pressure water applications.
Relationship of each pressure type:
Applications of Gauge Pressure
In some instances, absolute pressure is specified without any real need for it. There might be a misconception that all pressure measurement needs to be absolute. While there is certainly a need for absolute pressure measurement, most applications only need gauge pressure or an alternative.
Generally, if you want to measure or control a pressure that is influenced by changes in atmospheric pressure, like the level of liquid in an open tank or the air pressure in an automobile tire; you would choose vented gauge pressure as you are interested in the pressure reading minus the atmospheric pressure component.
Gauge pressure devices uses a reference to the atmosphere around the sensor. On this type of pressure measurement, the sensing element has a deflection due to a pressure change, a reference point is needed to know exactly what pressure is being measured. Pressure sensors that use gauge pressureâ€”typically seen in Psig, Barg, and kPagâ€”have some type of vent. The vent is positioned to use atmospheric pressure as a reference point for the sensor to measure the media. One common reason for using gauge pressure is to ensure that with any location throughout the world, the sensor will always reference the atmospheric pressure at that particular location in which it is installed.
Most pressure-measuring devices, however, are calibrated to read zero in the atmosphere, and so they indicate the difference between the absolute pressure and the local atmospheric pressure, which is the gauge pressure.
So when you see the gauge reading of 32 psi in the pressure gauge used to measure your automobile tire, it indicates a pressure of 32 psi above the atmospheric pressure.
Applications of Absolute Pressure
You use absolute pressure sensor if you want to measure pressures that are not influenced by changes in atmospheric pressure (e.g. leak testing a completely sealed vessel container or barometer). Unlike gauge pressure sensor, this is referenced to a perfect vacuum. If a gauge pressure sensor was used instead to measure the pressure inside a closed vessel, and the atmospheric pressure changed, then the sensorâ€™s reading would change even though the pressure in the vessel remains the same.
As location and elevation changes, the reference point of this sensor can change because of atmospheric pressure differences.
We hope this article has helped you understand the difference and practical uses of absolute and gauge pressures. So next time someone asks you why the pressure measuring device shows/uses gauge pressure, you can easily explain it to them.
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