# ME301: Measurement & Experimentation Laboratory

Unit 7: Force, Torque, and Pressure Measurements   Force, torque, and pressure measurements can be related by temporal and geometric coupling.  Consider the schematic of a see-saw balance.  The relative masses of objects M1 and M2 can be determined by the torques they exert about point P at different distances (L1 and L2) from that point under the acceleration of gravity g.  Many more sophisticated geometries and sensing arrangements can be coupled to allow measurements of related quantities.  In this unit, you will review some of the common configurations for such measurements.

This unit should take you 8 hours to complete.

☐    Subunit 7.1: 2 hours

☐    Subunit 7.2: 2 hours

☐    Subunit 7.3: 1 hour

☐    Assignment: 2 hours

☐    End of Unit Self-Assessment: 1 hour

Unit7 Learning Outcomes
Upon successful completion of this unit, the student will be able to:

• Demonstrate proficiency with use and conversion of units of pressure, force, and torque.
• Demonstrate understanding of absolute, gauge, static and dynamic pressures.
• Research, understand, and communicate the capabilities and operation of modern torque measurements.

7.1 Force Measurements   7.1.1 Units and Standards   - Reading: National Physics Laboratory (UK): “SI Unit of Force” Link: National Physics Laboratory (UK): “SI Unit of Force” (HTML)

Instructions: Read the linked section above and familiarize yourself with commonly used units of force.  For example, how is a dyne related to an ounce of force?

7.1.2 Inference of Mass from Weight   - Reading: Wikibooks: The Free High School Science Texts: A Textbook for High School Students Studying Physics: “Newtonian Gravitation/Mass and Weight” Link: Wikibooks: The Free High School Science Texts: A Textbook for High School Students Studying Physics: “Newtonian Gravitation/Mass and Weight” (PDF)

Instructions: Read the linked section above.  Calculate your mass and weight in kg and lbm and Newtons and lbf on Earth.

7.1.3 Strain or Deflection Measurements   - Reading: All About Circuits: “Volume 1, Chapter 9: Strain Gauges” Link: All About Circuits: “Volume 1, Chapter 9: Strain Gauges” (PDF)

Instructions: Read this section and consider the following issues: Why does the resistance of the strain gauge depicted in the resource cartoon increase under tension?  How might strain measurements be confounded by changes in temperature?  Might you design a strain gauge to work by measuring changes in capacitance?  To view as a PDF, click the PDF link in the top right corner.

7.2 Pressure Measurements   7.2.1 Units and Standards   - Reading: Simon-Fraser University: Stephen Lower’s Chem1 General Chemistry Virtual Textbook: “Observable Properties of Gases” Link: Simon-Fraser University: Stephen Lower’s Chem1 General Chemistry Virtual Textbook: “Observable Properties of Gases” (PDF)

Instructions: Review this first chapter.  Calculate atmospheric pressure in units of atm, bar, mm Hg, and ft. of water.  What might be meant by the term “negative pressure?”

7.2.2 Static Versus Dynamic Pressure   Note: These concepts arise from Bernoulli’s equation.  They are not to be confused with Gauge and Absolute pressures.  Gauge pressure is the system pressure minus some reference (atmospheric pressure).

• Reading: NASA Glenn Research Center’s “Bernoulli’s Equation” and “Pitot-Static Tube” Link: NASA Glenn Research Center’s “Bernoulli’s Equation” (PDF) and “Pitot-Static Tube” (PDF)

Instructions: Read the two web pages linked above and consider the following issues: Do static and dynamic pressures have the same units?  What is the origin of the terminology?  How would you use measurements of both to determine the speed of an airplane?

7.2.3 Barometers and Manometers   - Reading: Georgia State University Hyperphysics Pages: “Fluid Pressure Measurement” Link: Georgia State University Hyperphysics Pages: “Fluid Pressure Measurement” (HTML)

Instructions: Read the linked page above and those following as interested.  Consider the following questions during your reading: What is the difference between a barometer and a manometer?  Why is mercury often used as the fluid in a manometer?  Why might one use another fluid?  You may wish to play with the applet to consider the effects of fluid properties on the observed measurements.

7.2.4 Pressure Transducers   - Reading: National Instruments’ Guide for Pressure Measurements: “Measuring Pressure with Pressure Sensors” Link: National Instruments’ Guide for Pressure Measurements: “Measuring Pressure with Pressure Sensors”  (HTML)

Instructions: Read the first three sections in the linked material above, entitled “What is Pressure?”, “The Pressure Sensor,” and “Pressure Measurement.”  What factors influence the time-response of a pressure transducer?

Also available in:
iTunes U

Instructions: This video should be a review of concepts you learned in physics coursework.  You may refer to previous or subsequent videos in the series if you need additional exposure.  Make sure you understand appropriate units for torque.

• Activity: The Saylor Foundation’s Dynamometers Activity Link: The Saylor Foundation’s Dynamometers Activity

Instructions: Perform the same exercise on YouTube for dynamometers as you did for length measurement devices in Section 5.4 of this course.

Unit 7 Assessment   - Assessment: The Saylor Foundation’s “ME301: Unit 7 Quiz” Link: The Saylor Foundation’s “ME301: Unit 7 Quiz