E1C08 09/14/2010
14:54:1 Page 348
Because most DAS boards use analog-to-digital (A/D) converters having a Æ5 V full scale, the
signal must be conditioned using an amplifier. Usually a gain of 100 to 500 is sufficient. High gain,
very low noise amplifiers are important for accurate measurements. Consider a J-type thermocouple
using a 12-bit A/D converter with a signal conditioning gain of 100. This allows for a full-scale input
range of 100 mV, which is a suitable range for most measurements. Then, the A/D conversion
resolution is
E FSR
G
ð Þ 2
M
À Á ¼
10 V
100
ð
Þ 1
12
À Á ¼ 24:4 mV=bit
A J-type thermocouple has a sensitivity of $ 55 mV=
C. Thus, the measurement resolution
becomes
24:4 mV=bit
ð
Þ = 55 mV=
C
ð
Þ¼0:44
C=bit:
Thermocouples tend to have long time constants relative to the typical sample rate capabilities of
general-purpose DAS boards. If temperature measurements show greater than expected fluctuations,
high-frequency sampling noise is a likely cause. Reducing the sample rate or using a smoothing
filter are simple solutions. The period of averaging should be on the scale of the time constant of the
thermocouple.
Example 8.11
It is desired to create an off-the-shelf temperature measuring system for a personal computer (PC)based control application. The proposed temperature measuring system is illustrated schematically
in Figure 8.25. The temperature measurement system consists of the following:
A PC-based DAS composed of a data-acquisition board, a computer, and appropriate software
to allow measurement of analog input voltage signals.
A J-type thermocouple and reference junction compensator. The reference junction compensator serves to provide an output emf from the thermocouple equivalent to the value that
would exist between the measuring junction and a reference temperature of 0
C.
A J-type thermocouple, which is uncalibrated but meets NIST standard limits of error.
Reference
junction
compensator
Thermocouple
Figure 8.25 Personal computer (PC)-based
temperature measurement system.
348 Chapter 8 Temperature Measurements
14:54:1 Page 348
Because most DAS boards use analog-to-digital (A/D) converters having a Æ5 V full scale, the
signal must be conditioned using an amplifier. Usually a gain of 100 to 500 is sufficient. High gain,
very low noise amplifiers are important for accurate measurements. Consider a J-type thermocouple
using a 12-bit A/D converter with a signal conditioning gain of 100. This allows for a full-scale input
range of 100 mV, which is a suitable range for most measurements. Then, the A/D conversion
resolution is
E FSR
G
ð Þ 2
M
À Á ¼
10 V
100
ð
Þ 1
12
À Á ¼ 24:4 mV=bit
A J-type thermocouple has a sensitivity of $ 55 mV=
C. Thus, the measurement resolution
becomes
24:4 mV=bit
ð
Þ = 55 mV=
C
ð
Þ¼0:44
C=bit:
Thermocouples tend to have long time constants relative to the typical sample rate capabilities of
general-purpose DAS boards. If temperature measurements show greater than expected fluctuations,
high-frequency sampling noise is a likely cause. Reducing the sample rate or using a smoothing
filter are simple solutions. The period of averaging should be on the scale of the time constant of the
thermocouple.
Example 8.11
It is desired to create an off-the-shelf temperature measuring system for a personal computer (PC)based control application. The proposed temperature measuring system is illustrated schematically
in Figure 8.25. The temperature measurement system consists of the following:
A PC-based DAS composed of a data-acquisition board, a computer, and appropriate software
to allow measurement of analog input voltage signals.
A J-type thermocouple and reference junction compensator. The reference junction compensator serves to provide an output emf from the thermocouple equivalent to the value that
would exist between the measuring junction and a reference temperature of 0
C.
A J-type thermocouple, which is uncalibrated but meets NIST standard limits of error.
Reference
junction
compensator
Thermocouple
Figure 8.25 Personal computer (PC)-based
temperature measurement system.
348 Chapter 8 Temperature Measurements