# Creating a Mass Scale

Creating a Mass Scale
with Elastic Materials
This project is worth 9% of your overall grade for this course. Be sure to read all the instructions and assemble all the necessary materials before you begin. You will record your data and insert your answers in the Project Write-up section of this project. When you have completed this project you may submit it electronically through the online course management system. Check the instructions in the online course for more information.
Objective
Construct and calibrate a mass scale with simple materials and test its accuracy, precision, and range.
Instructions
Since you have completed the Tension Lab Activity in Lesson 7, you should have a good understanding of the way rubber bands respond under various amounts of tension.
Step 1:    Use the knowledge you gained from the Tension Lab Activity in Lesson 7 to create and calibrate a scale capable of accurately measuring the mass of a variety of objects.
Step 2:    You may use rubber bands or any other elastic material but you may not use any components made for use as part of a commercially available scale or balance (one that you buy already constructed) in your scale construction.
Step 3:     Decide what range of masses you intend to measure. Suggestions include:
1 – 10 g
5 – 50 g
10 – 100 g
1 – 100 g
5 – 500 g
Note that if you are building a scale that will function over a large range of masses, you may need to build separate components (one part to measure masses between 1 and 10 grams and a separate part to measure masses between 10 and 100 grams.
Step 4:     Decide what accuracy your scale should have (accuracy is ability to measure the CORRECT value). No matter what mass you are measuring, you should not be more than 5% off.  Calculate the % error.  % Error = ((actual mass – measured mass) / actual mass) * 100%
Step 5:    Decide what precision your scale should have (precision is the ability to give the same measurement for a mass, time after time. If you place a 5 gram mass on the scale three separate times, the readings should be the same each time to within +/- so many grams. As with accuracy, smaller masses should have smaller variances between readings.  Precision is usually reported as the range / 2.  Range is easily calculated by “highest measurement – lowest measurement.”  The final answer is given as “average measurement +/- precision.”
Step 6:    Calibrate your scale. Use a series of known masses to calibrate your scale. Use masses near the low end, the middle, and the high end of your working range. You cannot assume that your scale will respond equally to the addition of each mass. Record your work in the data tables provided. You may add space or pages to the Project Write-up section if you need them.
Step 7:    Test your scale. Use a series of known masses to check the function of your scale. This will feel similar to the calibration but this actually a separate step. The way your elastic material behaves may change over time and so you may not get the same readings that you did at the beginning of your calibration. Think of how a sweater “stretches out” as you use it—similar things happen to rubber bands.
Use at least 3 masses, one near the low end, one at the middle and one near the high end of your working range. For each mass, test it at least three times to measure accuracy and precision.
Step 8:    Make at least one significant change to your scale to improve its performance. If the performance of your scale did not meet your expectations, make a change to the way it is constructed or the materials you are using.  If it already met your design parameters, make a change to increase its performance capabilities (allowing it to measure a wider range of masses, for example.
Step 9:    Repeat your test measurements record your data in a table then analyze your results. Did you improve the performance of your scale? If so, by how much?
Step 10:    Suggest additional changes that could improve the performance of your scale even more.
Project Write-up
(20 points) Fill in the following tables and answer the following questions. Don’t forget to use the Grading Rubric as a guide to gauge how effectively you complete this portion of your project.
Prediction (what your scale is designed to do)    Performance (what you scale actually does)
1.    Elastic Material Chosen (be specific, what kind, what size, how many, etc.)
2.    Working Range of Scale
3.    Accuracy of Scale (calculate % error)
4.    Precision of scale (Precision is generally indicated as +/- half the range)
5. (5 pts) Construction: (Describe how your scale is constructed and include a diagram or picture.)
6. (5 pts) Calibration Procedure: Describe the process you used to calibrate your scale.  Create a table that shows what masses were used, and how you measured the change in your scale with each mass.
7. (20 points) Test Data: you have designed your scale to perform in a certain way, but you need to test it against known masses to see if it is performing as desired. Remember that you are testing each of three masses three times.
Mass    Actual Mass    Measured Mass    % Error    Precision
+/- (range / 2)
1 rep1
1 rep2
1 rep3
2 rep1
2 rep2
2 rep3
3 rep1
3 rep2
3 rep3
% Error =( (measured mass – actual mass) / actual mass) * 100%
8. (5 pts) Based on your test data, write a statement describing the accuracy of your scale at each level.
9. (5 pts) Based on your test data, write a statement describing the precision of your scale at each level.
10. (5 pts) What change did you make to improve the accuracy / precision of your scale?
11. (20 points) Test Data (Round 2)
Mass    Actual Mass    Measured Mass    % Error    Precision
1 rep1
1 rep2
1 rep3
2 rep1
2 rep2
2 rep3
3 rep1
3 rep2
3 rep3
12. (5 pts) Based on your test data, what is the accuracy of your scale? How close are you the real value for each mass?
13. (5 pts) Based on your test data, what is the precision of your scale? How close are each of your measurements to each other?
14. (5 pts) Suggestions for additional improvements. What changes do you suggest and why do you think they would be helpful?

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