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Material Type: Lab; Class: Introduction to Engineering; Subject: Engineering Science; University: Boise State University; Term: Unknown 1989;

Typology: Lab Reports

Pre 2010

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Download Measurement Unit - Introduction to Engineering - Lecture Notes | ENGR 120 and more Lab Reports Engineering in PDF only on Docsity! Introduction to Engineering Unit Timeline: Activity 1: Measurement Lab College of Engineering Note: This engineering instructional material was created by Boise State University with support from the William and Flora Hewlett Foundation's Engineering Schools of the West Initiative. Educators may use or copy this document free of charge on the condition that they credit Boise State University and the William and Flora Hewlett Foundation as creators. Measurement Unit Unit Overview As engineers, we often need to make measurements. Understanding the accuracy and precision of the measurement and the capability of the instrument making the measurement is an important part of reporting the data. Knowing how to report the data in a clear, concise, and meaningful way is also important. Different instruments have different capabilities. In general, the more expensive the instrument the less uncertainty there is in the measurement. As an engineer, you will have to balance cost, availability, ease of data collection and calculated uncertainty when you decide which measurement technique to use. The technique with the least uncertainty is clearly the most desirable from an experimental-accuracy standpoint, but it may be too expensive. Through this lab exercise, you will be required to determine not only the numerical value of the answer; you will need to ensure the units or dimensions of a variable in the equation are correct. Dimensional analysis is a technique that will help to ensure your answers are expressed in the appropriate units. If you are unfamiliar with this technique, please see Dimensional Analysis (link to document) for more information. Unit Objectives • Recognize the difference between precise and accurate measurements. • Describe the precision in a set of data using standard deviation. • Apply dimensional analysis in solving equations to demonstrate the correct numerical and unit or dimensional values. • Apply your engineering judgment in justifying your assumptions and answers. Introduction to Engineering – Measurement Unit Page 2 College of Engineering Precision and Accuracy “Are your measurements accurate?” “How precise is that measurement?” In everyday usage, these terms have identical meanings. In scientific fields, however, these terms have very different meanings. Accuracy refers to how close a measurement is to the actual value, and is dependent upon the quality of the measuring device. Precision is the ability of a measurement to be consistently reproduced or how repeatable your measurement is. The figures below contain a series of data samples which illustrate the concepts of precision and accuracy using a target analogy. The desired result is to have both precise and accurate data; however this is not always possible. Note that most of your data can be precise and accurate, with one measurement exhibiting neither characteristic, as is shown in the target on the right. Precise Data Accurate Data Precise and Accurate Data Precise and accurate with outlier data point Precision and accuracy are an important part of industry. At Chrysler, accuracy, precision, and statistics are used throughout the manufacturing process of engine transmissions. Parts are checked and rechecked throughout the design process. If a part does not meet set specifications, it is pulled from the manufacturing line and sent to a troubleshooting area for further investigation. Almost all machine parts must reach 95% or greater accuracy to stay on the line. At Coca-Cola, precision, accuracy, and statistics play a part in Coke’s ability to produce a product that tastes the same every time a new bottle is opened. Coke runs an elaborate laboratory for testing product color, sugar content, carbon dioxide content, pH, and bottle pressurization. The product quality is maintained by statistically running a series of tests on the first and last bottle produced of a specific Coca-Cola product. Example1: You perform an experiment to measure the temperature at which water boils. You set up three containers of water and heat each one. At the instant the water boils you measure the temperature and get the following results: 67°C 68°C 68°C 65°C 66°C Notice these values are precise (they are almost the same, they agree with each other), but they are not accurate. They would have to be at about 100°C, the accepted value, to be accurate. In science, it is important to be as precise and accurate as possible. But, there is such a thing as trying to be too precise. Introduction to Engineering – Measurement Unit Page 5 College of Engineering In summary, accuracy, measured in % error, is the comparison of the measured value to the “true” value or reference value as shown below. Precision, measured by standard deviation, describes the spread of data points from the mean value, as shown below. References: http://nasaexplores.com/show_912_teacher_st.php?id=030103100423 http://phoenix.phys.clemson.edu/tutorials/ap/index.html http://www.studyphysics.ca/newnotes/20/unit01_kinematicsdynamics/chp02_intro/lesson02.htm http://en.wikipedia.org/wiki/Accuracy_and_precision Introduction to Engineering – Measurement Unit Page 6 College of Engineering Activity 1 - Measurement Lab GROUP ASSIGNMENT Names ________________________________ (25 points) ________________________________ ________________________________ The objective of this exercise is not just to get the “right” answer. The objective is to think critically about the way things are measured and the errors that arise from the measurement. In this lab you will determine the weight of a paperclip and determine the density of three materials based on what three objects are on your lab table. You will work in groups of two or three. There are different instruments for measuring dimensions and mass on different lab tables. The instrument closest to you may not be the most appropriate for all of the exercises you need to do. If you need to use another instrument and someone else is using it, move on to another part of the lab that you can do with the instruments that are available. In an effort to develop and reinforce good work practices, remember the following: 1. Show your work! 2. Use dimensional analysis (link to document) course materials 3. Do your work in pencil – you will likely need to erase. 4. Neatness counts – make sure everyone can read your writing. Please work in groups of two or three. Turn in ONE worksheet for the whole group. Introduction to Engineering – Measurement Unit Page 7 College of Engineering Exercise 1: What is the weight of a paperclip? Please make all measurements in grams, using an appropriate number of significant figures. Weigh 10 paperclips together, then 20 paperclips together, then weigh 30 paperclips together, then 40, then 50, then 60, then 70, then 80, the 90, then 100. Measurement # Total number of Paperclips Mass Mass of single paperclip 1 10 2 20 3 30 4 40 5 50 6 60 7 70 8 80 9 90 10 100 What is the mean value of the mass of a single paperclip? What is the standard deviation in your measurements? Introduction to Engineering – Measurement Unit Page 10 College of Engineering You have arrived at the mass of a paperclip in several different ways. Which method is the most accurate? Why? Which method is the most precise? What is the difference between accuracy and precision? You can put 200 lbs of paperclips in a barrel for shipping. How many paperclips will be in that barrel? What is the uncertainty in your answer? (How confident are you of exactly how many paperclips are in the barrel?) As a manufacturer of paperclips, how much uncertainty would you be willing to tolerate? Introduction to Engineering – Measurement Unit Page 11 College of Engineering DENSITY CALCULATIONS Select three objects on your table. Sketch the object and describe its material composition. Calculate the density of each of these objects; express your answer in units of g/mL ( a typical unit for density calculations). Show all your work and check to ensure your units using dimensional analysis (link to document). How confident are you in your answers on the density of these three materials? Describe errors that could have occurred in making these measurements. Describe at least one way to get a more accurate measure of the density.