11.1: Two Ways to Measure Pressure - Absolute Pressure and Gage Pressure
- Page ID
- 116667
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)How Pressure can be Represented in Two Methods with Absolute and Gage Pressures
Pressure can be represented using two methods: absolute pressure and gauge pressure. Understanding these representations is crucial for accurate measurement and application in various fluid power systems.
- Absolute Pressure: Absolute pressure is measured relative to a perfect vacuum. It includes the atmospheric pressure as well. The absolute pressure at sea level is typically around 101.3 kilopascals (kPa), which is equivalent to 1 atmosphere (atm) or 14.7psi. That is how much the atmosphere (above) weighs onto what is measured. In absolute pressure measurements, the value starts from zero, indicating a perfect vacuum, and increases as pressure increases. Absolute pressure is commonly used in scientific and engineering applications where the exact pressure relative to a vacuum is crucial.
- Gage Pressure: Gage pressure is measured relative to atmospheric pressure. It does not include atmospheric pressure; rather, it measures the pressure above or below atmospheric pressure. For instance, if the atmospheric pressure is 14.7psi and the gauge pressure reads 150 psi, it means the pressure is 150 psi above atmospheric pressure. This would result in an absolute pressure of 164.7 psi. Conversely, if the absolute pressure reads 50 psi, it indicates the pressure is 14.7 psi above gage pressure. This would result in a gage pressure of 35.3 psi. Gage pressure is often used in everyday applications like tire pressure gages, where what's important is the pressure relative to the surrounding atmosphere.
Absolute Pressure (psia) = Gage Pressure (psig)+ 14.7 psi (Atmospheric Pressure at sea level)
Gage Pressure (psig) = Absolute Pressure (psia) - 14.7 psi (Atmospheric Pressure at sea level)
To summarize, while absolute pressure measures pressure relative to a perfect vacuum, gauge pressure measures pressure relative to atmospheric pressure. Both methods have their specific applications depending on the context and requirements of the measurement. Understanding this correlation is crucial for accurate pressure measurements and calculations in various fields, including engineering, meteorology, and fluid dynamics.
Vacuum Pressure Scale
The vacuum pressure scale measures pressure lower than atmospheric pressure, decreasing from atmospheric pressure. In the U.S., vacuum is often reported in inches of mercury (in. Hg), mirroring the gage scale but working in reverse. At sea level, a vacuum can reach 29.92 in. Hg, equivalent to near-zero absolute pressure.
A common method of determining vacuum pressure uses a barometer setup connected to a flask. If a vacuum is created within the flask, atmospheric pressure will push mercury up a tube proportionate to the vacuum strength. If the flask pressure decreases by 10 in. Hg, for example, atmospheric pressure supports a 10 in. Hg mercury column, indicating a 10 in. Hg vacuum. A full vacuum would equate to 29.92 in. Hg, or nearly 0 absolute pressure.
Vacuum pressure readings align with absolute pressure values; for example, a 12 in. Hg vacuum is equivalent to an 18 in. Hg absolute pressure. This correlation facilitates conversions between vacuum and absolute pressure scales, aiding accurate calibration and system design.
In a pneumatic system, air pressure is generated by increasing the density of air within a closed container, which is achieved through a process called compression. The machine that performs this function in a pneumatic system is known as an air compressor. This compressed air, which has a pressure several times greater than atmospheric pressure, becomes the primary source of power for various pneumatic operations.
Air pressure in a pneumatic system is created by compressing ambient air using a compressor and then distributing the compressed air to various components to perform work. This process allows pneumatic systems to power a wide range of applications in industries such as manufacturing, automation, and transportation.