Earth Science Making Measurements and Reading Scales 1

How to measure – What is important? 2  Instructions:  Mass approximately 5.00 g of sucrose.  Recognize the meaning:  Getting exactly 5.00 g of sucrose is NOT important.  You just need to be near 5.00 g of sucrose (i.e. +/ g – between 4.90 g and 5.10 g).  Key  Once you get near to the desired measurement you are ready to measure your EXACT mass of sucrose (i.e g) IT IS A WASTE OF TIME TO GET 5.00 g!!!!

Reading Scales – What is important? 3  The main idea:  You are communicating scientific information to fellow scientists.  When you give a measurement you communicate the following:  The accuracy of your equipment.  The sensitivity of your equipment.  The uncertainty of your measurement.

Reading Scales – What is important? 4  Example:  Take your 4.93 g measurement we discussed earlier.  The person who reads your report and sees this data knows:  Your equipment can measure to 0.01 g.  The uncertainty of your equipment is likely +/ g  This information will help the scientific reader to evaluate the significance of your data and your findings/conclusions.

Liquid Volumes  When measuring the volume of a liquid, always read the scale from the bottom of the meniscus. The meniscus is the curved surface of a liquid in a narrow cylindrical container. 5

When reading a scale, always strive to avoid parallax errors. Parallax errors arise when a meniscus or needle is viewed from an angle rather than from straight-on at eye level. IncorrectCorrect 6

Reading a Scale  A scale is made up of a series of graduations. Usually, some of the graduations are labeled at regular intervals, with smaller, unlabeled graduations between them.  The rule for reading a scale:  Read the scale using the markings and then read to one more decimal place (which is estimated) 7

Large Scale Increment 8  To read the scale, you must first determine the large scale increment. The large scale increment is the quantity between any two adjacent marked graduations. To find the scale increment, subtract the values of any two adjacent labeled lines.

Small Scale Increment 9  Next, to determine the small scale increment, count that there are ten lines between the labeled lines. Therefore, the scale increment is 10 mL/10 lines = 1 mL/line.  Remember – read this scale to one more decimal place than marked.

Practice – Large Scale Increment  What is the large scale increment on this scale? 10

Practice – Small Scale Increment  What is the small scale increment on this scale?  How many decimal places will be in your final reading?  Is the last place in your answer certain or estimated?  Are the other numbers in your answer certain or estimated? 11

Measuring - Finding the Certain Digits  Use the large scale increment to find the first digit of your measurement.  The first digit of the reading is 5, since the last labeled graduation below the meniscus is 50 (i.e. the reading is between 50 mL and 60 mL). 12

Finding the Certain Digits 13  Next, use the small scale increments to determine the second digit of your reading.  The second digit of the reading is 2, since the last labeled graduation below the meniscus is 52 (i.e. the reading is between 52 mL and 53 mL).  We have now read as far as we can using the markings on the scale. The certain digits of the reading are 52 mL.

Estimate the uncertain digit and take a reading  Finally, estimate the distance that the meniscus lies between the two lines as a decimal.  On this graduated cylinder, the meniscus is about eight tenths (0.8) of the way to the next line.  So, the final digit in the reading is 0.8 mL. (The uncertain digit in the reading is always the last digit)  The correct volume in this graduated cylinder is 52.8 mL. 14

25 mL Graduated Cylinder 15  What is the large scale increment for this cylinder?

25 mL Graduated Cylinder 16  What is the small scale increment between the lines that are unmarked?

25 mL Graduated Cylinder  What is the volume of liquid in this graduated cylinder? Answer 17

Practice Answer 18

Practice 19  Measure this volume. (Be careful this is a little tricky) Answer

Cent-o-Gram 310 Balance  Measure to the nearest 0.1 g Ohaus Web Site 20

Answer – 25 mL Graduated Cylinder 21 Correct Reading mL Return to Previous Slide

Answer – 100 mL Graduated Cylinder 22 Correct Reading 76.0 mL Return to Previous Slide

Answer – 10 mL Graduated Cylinder 23 Correct Reading 6.75 mL Return to Previous Slide

Answer – 25 mL Graduated Cylinder 24 Correct Reading 21.5 mL Return to Previous Slide