E1C01 09/14/2010
15:40:34 Page 8
Consider a thermodynamics experiment to establish the boiling point of water. The apparatus
for measuring the boiling point might yield the results shown in Figure 1.6 for three test runs
conducted on separate days. Notice the different outcome for each test.
Why should the data from three seemingly identical tests show such different results?
Suppose we determine that the measurement system accuracy accounts for only 0.1
F of the test
data scatter. So another plausible contributing factor is the effect of an extraneous variable. Indeed,
a close examination of the test data shows a measured variation in the barometric pressure, which
would affect the boiling temperature. The pressure variation is consistent with the trend seen in
the boiling point data. Because the local barometric pressure was not controlled (i.e., it was not
held fixed between the tests), the pressure acted as an extraneous variable adding to the differences in
outcomes between the test runs. Control important variables or be prepared to solve a puzzle!
Parameters
In this text, we define a parameter as a functional grouping of variables. For example, a moment of
inertia or a Reynolds number has its value determined from the values of a grouping of variables. A
parameter that has an effect on the behavior of the measured variable is called a control parameter.
Available methods for establishing control parameters based on known process variables include
similarity and dimensional analysis techniques and physical laws (2–4). A parameter is controlled
if its value can be maintained during a set of measurements.
As an example, the flow rate, Q, developed by a fan depends on rotational speed, n, and the
diameter, d, of the fan. A control parameter for this group of three variables, found by similarity
methods, is the fan flow coefficient, C 1 ¼ Q=nd
3 . For a given fan, d is fixed (and therefore
controlled), and if speed is somehow controlled, the fan flow rate associated with that speed can
be measured and the flow coefficient can be determined.
Temperature ºF
215
214
213
212
211
210
209
208
207
206
205
7
6
5
4
3
2
1
Time (min)
Boiling point results
212.1
212.5
211.6
Test 1 (30.2 in. Hg)
Test 2 (29.7 in. Hg)
Test 3 (30.0 in. Hg)
Boiling region
Figure 1.6 Results of a boiling point test for water.
8 Chapter 1 Basic Concepts of Measurement Methods
15:40:34 Page 8
Consider a thermodynamics experiment to establish the boiling point of water. The apparatus
for measuring the boiling point might yield the results shown in Figure 1.6 for three test runs
conducted on separate days. Notice the different outcome for each test.
Why should the data from three seemingly identical tests show such different results?
Suppose we determine that the measurement system accuracy accounts for only 0.1
F of the test
data scatter. So another plausible contributing factor is the effect of an extraneous variable. Indeed,
a close examination of the test data shows a measured variation in the barometric pressure, which
would affect the boiling temperature. The pressure variation is consistent with the trend seen in
the boiling point data. Because the local barometric pressure was not controlled (i.e., it was not
held fixed between the tests), the pressure acted as an extraneous variable adding to the differences in
outcomes between the test runs. Control important variables or be prepared to solve a puzzle!
Parameters
In this text, we define a parameter as a functional grouping of variables. For example, a moment of
inertia or a Reynolds number has its value determined from the values of a grouping of variables. A
parameter that has an effect on the behavior of the measured variable is called a control parameter.
Available methods for establishing control parameters based on known process variables include
similarity and dimensional analysis techniques and physical laws (2–4). A parameter is controlled
if its value can be maintained during a set of measurements.
As an example, the flow rate, Q, developed by a fan depends on rotational speed, n, and the
diameter, d, of the fan. A control parameter for this group of three variables, found by similarity
methods, is the fan flow coefficient, C 1 ¼ Q=nd
3 . For a given fan, d is fixed (and therefore
controlled), and if speed is somehow controlled, the fan flow rate associated with that speed can
be measured and the flow coefficient can be determined.
Temperature ºF
215
214
213
212
211
210
209
208
207
206
205
7
6
5
4
3
2
1
Time (min)
Boiling point results
212.1
212.5
211.6
Test 1 (30.2 in. Hg)
Test 2 (29.7 in. Hg)
Test 3 (30.0 in. Hg)
Boiling region
Figure 1.6 Results of a boiling point test for water.
8 Chapter 1 Basic Concepts of Measurement Methods