1. Making Measurements David A. Krupp, Ph.D.
BIOL 171L General Biology Lab I

2. Making Measurements
Why do we measure?
What do we measure?

3. Variable
A feature or entity that can assume a value (observation) from a set of possible values (observations)
Some examples:
length of a rat tail
number of seeds in a seed pod
phosphate concentration of a water sample
color of a fish
ranking of how well you feel

4. Types of Variables Quantitative variables
Continuous (e.g., length, weight, time, temperature)
Discontinuous (e.g., number of fish in an area, number of seeds in a seed pod; )
Rank (e.g., one-to-five ranking for the quality of instruction)
Derived variables (e.g., density, velocity)
Character variables (e.g., color, gender)
A continuous variable is one that can take any real numerical value. The length of a strip can be anything. A person's height and age can take any real values, within reasonable limits.
Whereas, discrete variables will only have values that are whole numbers, like the number of people on a football team, or the number of major planets in the solar system. No star could ever have 5.62 major planets, for example.
Rank or order the items in your category. Some examples of items that can be ordered are: number of computers sold in a month, students’ GPAs or bank account balances. Anything with numbers or amounts can be ranked or ordered. If you find it impossible to rank or order your items, you have a qualitative item. Examples of qualitative items are “car models,” “types of potato,” “Shakespeare quotes.”
Derived variable- Scales of measurement that are the result of a calculation
Character variable- no numerical meaning

5. Systems of Measure Two systems in use predominantly: English (America)
Metric or SI (European)
Developed by the French in the late 1700’s
Based on powers of ten, so it is very easy to use
Used by almost every country in the world, with the notable exception of the USA
Especially used by scientists
Abbreviated SI, which is French for Systeme International

6. Systems of Measure: English (America)
Disadvantages
No standard base unit for each kind of measurement
Subunits within units not based upon a consistent multiplication factor
Difficult to make conversions between units
Advantages
We already know it

7. Systems of Measure: Metric or SI (European)
Disadvantages
We have to learn it
Advantages
Use a base unit for each type of measure
Subunits/superunits of base unit based upon multiples of ten
Conversions are much easier

8. Metric Prefixes
Regardless of the unit, the entire metric system uses the same prefixes
Common prefixes are:
kilo = 1000
centi = 1/100th
milli = 1/1,000th
micro = 1/1,000,000th

9. Metric Prefixes Example for length:
1 meter (m) = 100 centimeters (cm) = 1,000 millimeters (mm) = 1,000,000 (m)
1 kilometer (km) = 1000 meters

10. Length Length is the distance between two points
The SI base unit for length is the meter
We use rulers or meter sticks to find the length of objects

11. Mass Mass is the amount of matter that makes up an object
A golf ball and a ping pong ball are the same size, but the golf ball has a lot more matter in it. So the golf ball will have more mass
The SI unit for mass is the gram

12. Mass A paper clip has a mass of about one gram
The mass of an object will not change unless we add or subtract matter from it

13. Measuring Mass
We could use a triple beam balance scale to measure mass
It is unaffected by gravity

14. Weight Weight is a measure of the force of gravity on an object
Your weight can change depending on the force of gravity
The gravity will change depending on the planet you are on
The SI unit for weight is the Newton (N)
The English unit for weight is the pound

15. Gravity
Gravity is the force of attraction between any two objects with mass
The force depends on two things:
Distance between the two objects
The mass of the two objects

16. Weight and Mass Jill Earth 1 gravity Moon 1/6th gravity Jupiter
2.5 gravities
On orbit
0 gravity
mass
30 kg
weight
300 N
50 N
750 N
0 N
Notice that Jill’s mass never changes. Her mother will not allow us to take parts off her, or add parts to her, so her mass stays the same. Jill is 30 kg of little girl no matter where she goes!

17. Volume Volume is the amount of space contained in an object
We can find the volume of box shapes by the formula Volume = length x width x height
In this case the units would be cubic centimeters (cm3).
3 cm
5 cm
2 cm
So a box 5 cm x 3 cm x 2 cm would have a volume of 30 cm3

18. Base Units The base SI unit for volume is the Liter (L)
We normally measure volume with a graduated cylinder or a graduated pipette

19. Measuring Volumes
Liquids form curved, upper surfaces when poured into graduated cylinders
To correctly read the volume, read the bottom of the curve called the meniscus

20. Liquid Volume
When the metric system was created, they decided that 1 cm3 of water would equal 1 milliliter (mL) of water and the 1 mL of water will have a mass of one gram (g)
1 cm3 water = 1 mL of water = 1 g

21. Pipette

22. Micropipette Dial in volume 1st click- fill (slowly)
Push to eliminate, 2nd click gets ride of last drop
Use eject button to remove tip

23. Water Mass and Volume 1 cm3 water = 1 mL of water = 1 gram
So what would be the mass of 50 mL of water be?
50 grams
So what would be the mass of 1 liter of water be?
1 L = 1000 mL, so its mass would be 1000 grams or a kilogram

24. Taking Measurements
All measurements include some degree of uncertainty
Sources of uncertainty
Instrument error
Calibration error
User error
A properly taken measurement includes one estimated digit (not always possible with digital readouts)

25. Taking Measurements Measuring devices have units marked on them
When taking a measurement you record:
All known digits: those marked on the measuring device
One estimated digit: a multiple of 1/10 the smallest marked unit on the measuring device

26. Taking Measurements
Value lies between 7.1 & 7.2 cm)

27. Taking Measurements
7.16 cm
estimated digit

28. Accuracy Versus Precision
How close a measured value agrees with the true value
Precision
How closely repeated measurements agree with each other
Good measuring devices are both accurate and precise

29. Precise
Accurate
Precise & Accurate

30. Rounding Off Values
Generally should present values with the number of significant digits measured (including estimated digit)
Thus the value of 7.16 is presented to three significant digits
What would we present if we wished to round off our value to two significant digits?

31. Rounding Off Numbers To three significant digits: 7.237 7.24 7.232
7.23
7.235
7.24

32. Rounding Off Numbers 2.65 x 3.1 = 8.215  8.2
General rule of thumb for presenting the number of significant digits for calculated values:
Use the number of significant digits of the value with the least significant digits
2.65 x 3.1 = 8.215
 8.2

33. Scientific Notation Why is scientific notation useful?
Goal: to express numbers in scientific notation and as ordinary decimal numbers
Scientific notation
A number between 1 and less than 10 multiplied by 10 raised to an exponent.
Examples:
1.63 x 105
2.1 x 103
5.341 x 10-4
Why is scientific notation useful?

34. Scientific Notation Express in scientific notation: 7237 7.24 x 103
7000
7.0 x 103
345
3.45 x 102
0.351
3.51 x 10-1
0.0351
3.51 x 10-2

35. Practice using a pipette and eliminating waste properly.
Task for Today’s Lab Activity
Practice using a pipette and eliminating waste properly.
Practice using a micropipette.

36. Task for Today’s Lab Activity
Work in pairs
Measure 20 koa haole, Leucaena leucocephala, seed pods
length (nearest 0.1 cm)
weight (nearest (0.01 g)
number of seeds per pod
Enter data into Excel spreadsheet

37. Task for Today’s Lab Activity
Prepare single table in Excel that includes all measurement data and for length, weight and number of seeds per pod.
Upload this table into your Dropbox on Laulima.