Recognizing pressure units:\r Which of the following is <em>not</em> a unit of pressure used in engineering or science?

Introduction:
Pressure is expressed in many units: SI (pascal), derived (N/m^2), industrial (bar, ata), and head-based (mm H2O, mmHg). Recognizing legitimate pressure units prevents dimensional mistakes when reading datasheets or performing conversions.

Given Data / Assumptions:

• We distinguish pressure units from those of length, time, or other quantities.
• Common pressure units include absolute and gauge variants.
• “Ata” denotes absolute atmospheres.

Concept / Approach:
Torr (mmHg) measures pressure; Newton per square meter is the pascal (SI). Bar and pascal (Pa), as well as “ata,” are established pressure units. Head units like mm H2O express pressure via equivalent fluid column. A parsec, however, is an astronomical unit of length (approximately 3.09 × 10^16 m) and is unrelated to pressure.

Step-by-Step Solution:
List pressure units: Pa (N/m^2), bar, ata, torr, mm H2O.Identify the outlier: parsec is a length unit.Select parsec as the only option that is not a pressure unit.

Verification / Alternative check:
Dimensional analysis confirms N/m^2 is M L^-1 T^-2, the correct pressure dimension; a parsec has dimension of length.

Why Other Options Are Wrong:

• Torr and mm H2O: Traditional pressure measures (mercury and water columns).
• Newton/m^2: SI pressure unit (pascal).
• Ata, bar, pascal: All standard pressure units.

Common Pitfalls:
Confusing gauge vs absolute labeling; “bara” and “barg” indicate absolute and gauge, respectively, but both are still pressure units.

Final Answer:
Parsec