Activity 1.1.3 – Instrumentation
Calibration
Introduction
This activity is a study in accuracy and precision. Both of these concepts are
important tools used in biotechnology engineering. It is very important to know which
is more appropriate for the task at hand.
In terms of arrows shot toward a bull’s eye of a target, it is possible to see a clear
example of accuracy compared to precision.
The three arrows with solid lines represent:
precision
The three arrows with broken lines represent:
accuracy
When using instruments in the lab, it is critical that they provide reliable results
(precise) and are representative (accurate) of what they are supposed to measure.
Lab instruments must be calibrated frequently to ensure accuracy. Alternatively,
certain lab techniques (e.g., pipetting with serological pipettes) have inherent
variability that must be taken into account.
Equipment
•
•
•
•
•
•
•
•
Computer
Excel
Electronic top loading microbalance
Electronic Mettler microbalance
Micropipettes (p20, p100, p1000)
Pipetteman
Serological pipettes (5 and 10 ml)
Large and small plastic weigh boats
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Procedure
Student teams have been contacted by a local biotech company that needs its
pipettes calibrated. You and your team will run the pipettes through a series of tests
to determine if they are calibrated. The local company also requests that your teams
compare the accuracy and precision of micropipettes compared to serological
pipettes.
Step 1:
Create an Excel spreadsheet that looks identical to the one below.
Step 2:
Using the table below, transfer the amount of water shown using the correct pipette.
Place the water onto a weigh boat, tare the microbalance, and record the weight.
Remember that 1 ml of water should weigh 1 mg. Repeat the process 10 times for
each volume of water.
Actual Measured Amounts (g)
Pipette
**Micropipette
P1000**
Serological
pipette 5
Serological
pipette 5
Serological
pipette 10
Volume
40 (μl)
1 (ml)
4 (ml)
4 (ml)
Trial 1
.040
Trial 2
.040
Trial 3
.040
Trial 4
.040
Trial 5
.040
.9
.8
.9
.8
.9
3.7
3.8
3.7
3.8
3.8
3.6
3.8
3.9
3.8
3.9
Table 1 * NOTE: The Mettler microbalance must be used.
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Step 3:
Using an Excel spreadsheet, find the following for each data set (YOU WILL NEED
TO CREATE A NEW TABLE FOR THESE CALCULATIONS):
• Average
• Standard deviation
• Percentage error (difference from expected)
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Actual Measured Amounts (g)
Pipette Volume Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Average % Error STDEV
0.04
0.04
0.04
0.04
0.04
0.04
0%
**Micro
pipette
P1000** 40 (μl)
0
Serologi
cal
pipette
5
1 (ml)
0.9
Serologi
cal
pipette
5
4 (ml)
3.7
Serologi
cal
pipette
10
4 (ml)
3.6
0.8
0.9
0.8
0.9
0.86
14%
0.054772
2557505
166
3.8
3.7
3.8
3.8
3.76
6%
0.054772
2557505
164
3.8
3.9
3.8
3.9
3.8
5%
0.122474
4871391
59
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Step 4:
Graph the standard deviation for each data set and the percentage error.
Step 5:
Using the graphs you created, answer the following questions:
1. Explain the best way to pipette 1 ml?
The best way to pipette 1 mL is using the serological pipette 5 because it can
hold at least 1 mL, but it cannot measure a lot more than 1 mL. It also has a
relatively low percent error, which means that the serological pipette 5 is an
accurate way to measure 1 mL of water (the experimental measurements are
close to the goal measurement. It also has a low standard deviation, which
means it is a precise way to measure 1 mL of water (the measurements are
consistent).
2. Explain how the results match or do not match what you expected.
The results are close to the expected results. It was expected that the
Micropipette would be the most accurate and precise because it is mechanized
and it measures a very small amount of water, meaning that it has less room for
error. It is very accurate (it had a percent error of 0% meaning that there was
no measurable error in the measurements) and it was very precise (it had a
standard deviation of 0, meaning that the results were consistent). It was
surprising that the serological pipette 10 was more accurate than the serological
pipette 5 in measuring 4 mL of water. The serological pipette 10 had a percent
error for measuring 4 mL of water of 5%, while the serological pipette 5 had a
percent error of 6%. This means that the serological pipette 10 was more
accurate (the results were closer to the goal results). Typically, the instrument
with the capacity closest to (but not under). Possible explanations for this
include-- different people were measuring the serological pipette 10 and the
serological pipette 5 for the 4 mL trials, and instruments that must be read
manually (the serological pipettes) can be difficult to read. This results in human
error. Predictions were correct that the micropipette was the most accurate and
precise. It had a percent error of 0% and a standard deviation of 0. This can be
explained because it is mechanized, meaning it measures water mechanically
and decreases the opportunity for human error. It was accurate because the
percent error was 0% and it was also precise because the results were
consistent (there was a standard deviation of 0).
3. How can you determine if all of the micropipettes are calibrated?
The micropipettes are calibrated when the results are consistent. Consistency in
results is also known as precision. The results must have a very low standard
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deviation- which measures how much the results vary from each other. The
results must also be accurate- the measured values must be close to the actual
goal values. Accuracy can be measured by the precent error.
4. What is an acceptable error for pipetting?
An acceptable percent error for pipetting is about 10%.
Conclusion
5.Using this activity as a guide, describe in your own words the difference between
precision and accuracy.
Accuracy is how close the results are to the goal measurement. Accurate results
are close to the goal. Precision, however, is how close the results are to each
other, or the consistency. If results are far away for the goal value, but they are
incorrect by the same amount each time- they are precise, but not accurate.
6. Which is more important for an industry: instrumentation, precision, or accuracy?
Why?
Instrumentation is the most important because it allows the user to know the
measurements of accuracy and precision and to “pick the right tool for the job.”
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