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Introduction to Marine Engineering

Massachusetts Maritime Academy

Subsection 3.2.6 Pressure Conversions

In interpreting pressure measurements, a great deal of confusion arises because the zero point on most pressure gages represents atmospheric pressure rather than zero absolute pressure, and both pressure and head scales are used. Thus it is important to specify the kind of pressure being measured under given conditions. To clarify the numerous meanings of the word pressure, the relationship among gage pressure, atmospheric pressure, vacuum, and absolute pressure is illustrated in Figure 3.2.1.
Figure 3.2.1. Pressure Relationship
If you can remember the following facts, it is relatively easy to perform unit conversions between the various pressure scales.
Atmospheric pressure equivalents:
  • \begin{align*} \text{Atmospheric Pressure} \amp= 14.7 \text{ psi} \\ \amp = 29.92 \textrm{ in Hg} \\ \amp = 760 \textrm{ mm Hg} \\ \amp = 34 \textrm{ ft H}_2\textrm{O} \\ \amp= 101.3 \textrm{ kPa} \\ \amp= 1.01 \textrm{ bar} \end{align*}
  • Absolute scales measure up from zero
  • Gage scales measure up or down from atmospheric pressure
  • Vacuum scales measure down from atmospheric pressure

Example 3.2.2. psia to vacuum conversion.

Convert from 10.4 psia to mm Hg Vacuum
Answer.
10.4 psia = 222.3 mm Hg (vacuum)
Solution.
  1. Set up a ratio to convert psia to mm Hg Absolute
    \begin{align*} \frac{10.4 \textrm{ psia}}{14.7 \textrm{ psia}} \amp= \frac{p}{760 \textrm{ mm Hg} }\\ \amp = 537.7 \textrm{ mm Hg (absolute)} \end{align*}
  2. Subtract from atmospheric pressure to get vacuum
    \begin{equation*} V = 760 - p = 222.3 \textrm{ mm Hg (vacuum)} \end{equation*}
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