CE 170:  Water Quality and Supply Engineering

Lab 2:  Use of Laboratory Equipment -- Accuracy and Precision

Purpose of Lab

The purpose of this lab is two-fold:

1.	To become familiar with the use of selected laboratory equipment.

2.	To examine the concepts of precision and accuracy as they relate to laboratory procedures used in environmental engineering.  Statistical tools will be used to evaluate precision and accuracy.

Definitions

Precision:	"Precision refers to the reproducibility of a method when it is repeated on a homogeneous sample under controlled conditions, regardless of whether or not the observed values are widely displaced from the true value as a result of systematic or constant errors present throughout the measurements.  Precision can be expressed by the standard deviation." (Standard Methods)

Accuracy:	"Accuracy refers to the agreement between the amount of a component measured by the test method and the amount actually present." (Standard Methods)

Mean, X:	
where:
x - measurement value
n - number of times measurement was made

Sample Standard Deviation, :	

Coefficient of Variation (percentage), COV:	

95% Confidence Interval:	
where:
t = 1.96 when n is greater than 10

Relative Error, % error:	

Procedures - Mass and Volume Measurements

A.	Measurement of Mass:

Equipment:	Analytical balance

1.	Measure the mass of an index card provided by the instructor using the analytical balance.  Each group should do this independently of other groups.  Be careful to not touch the index card with your hands because this may transfer oils to the card and change its mass.

2.	When all groups have weighed the index card, exchange data with other groups.  Have one member of each group sign the index card.

3.	Repeat the first step.  Exchange that data with the other groups.

4.	Determine the mean, sample standard deviation, and coefficient of variation for the two sets of mass measurements (both before and after the card was signed).  Record your results and comment on the precision of the analytical balance on attached sheets.  Show a sample calculation on separate sheet(s).  In this sample calculation, write out all the equations for the mean, standard deviation, and coefficient of variation.  

B.	Measurement of Volume

Equipment:	
one 10 mL graduated (Mohr) pipette, marked "TD" - to deliver
one 50 mL graduated cylinder, marked "TD" - to deliver
one 100 mL volumetric flasks, marked "TC" - to contain
six 50 mL beakers
Platform balance

1.	Note and record the water temperature.  This temperature will be used to calculate the true volume from the true weight.

2.	Measure and record the mass of the 50 mL beakers.  Be sure to keep them straight.

3.	Pipette 8.4 ml of water from the supply provided by the instructor into the one of the 50 mL beakers. Follow your instructor's directions on how to pipette 8.4 mL from a graduated pipette.  Using the analytical balance, measure and record the mass of the 50 mL beaker and water.  Repeat the process twice more using a new 50 mL beaker each time.  Each person in the lab group should do the measurement at least once.  

4.	Place 44 mL of water into the 50 mL graduated cylinder.  Pour that water into a fresh 50 mL beaker. Using the analytical balance, measure and record the mass of the 50 mL beaker and water.  Repeat the process twice more using a new 50 mL beaker each time. Each person in the lab group should do the measurement at least once.  

5.	Weigh and record the mass of the 100 mL volumetric flask using the platform balance.  Weigh and record the mass of the 100 mL volumetric flask and water.  Repeat the process twice more using the original dry tare weight.

6.	Exchange your volume measurement data with the other groups in the lab.

7.	Determine the mean, standard deviation, coefficient of variation, and 95% confidence interval for each volume measurement.  Provide sample calculations.  In this case, you may use you calculator's statistical functions for mean and standard deviation (i.e., you don't have to write out the equations).

Note:  When using a pipette and volumetric flask, the liquid will have a surface which is curved slightly downward.  This curved surface is called the meniscus, and the bottom of the meniscus should be exactly on the line indicating the desired beginning or end of the dispensation of liquid. This is true for any graduated cylinder, burette, etc.

Discussion Questions  

1.	Comment on the precision of each volume measurement. 

2.	Compare the accuracy of each type of volume measurement.  

3.	How large a role does the lab technicians technique play for these type of volume measurements?