04-Jan-23
INDUSTRIAL
INSTRUMENTATION
Lecturer: Nguyen Duc Hoang
Department of Control & Automation
Faculty of Electrical & Electronics Engineering
Ho Chi Minh City University of Technology
Email: ndhoang@hcmut.edu.vn
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Chapter 5: Methods to measure
fundamental quantities
• Measurement
of resistance
• Measurement
of capacitance
• Measurement
of inductance
• Measurement
of frequency
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Measurement of resistance
• Sensors based on the variation of the electric resistance of a
device are very common:
•
POTENTIOMETERS
•
STRAIN GAGES
•
THERMISTORS, RTD
•
MAGNETORESISTORS
•
LIGHT-DEPENDENT RESISTORS
•
RESISTIVE HYGROMETERS
•
RESISTIVE GAS SENSORS
•
LIQUID CONDUCTIVITY SENSORS
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Measurement of resistance
• The general equation for a sensor whose resistance changes by a
fraction x in response to a measurand is :
𝑅=𝑅 𝑓 𝑥
Assuming : 𝑓 0 = 1
• For linear sensors we have:
𝑅 =𝑅 1+𝑥
• The range of values for x depends strongly on the type of sensor and
on the measurand span
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Measurement of resistance
• There are two requirements for all conditioners for
resistive sensors:
• they must drive the sensor with an electric voltage or current in
order to obtain an output signal,
• this supply, whose magnitude affects that of the output signal,
is limited by sensor self-heating,
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Measurement of resistance
• Some sensors require particular circuits.
Thermistors require linearization.
• Strain gages require interference cancellation.
• Sensors that yield small outputs require large gains in order for
the dynamic range of the output signal to match the input
range of the ADC .
• Conditioners for remote sensors must be insensitive to
connecting lead resistance or compensate for it.
•
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Measurement of resistance
remote sensors
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Measurement of resistance
• Methods for resistance measurement can be
classified into:
• Deflection methods sense the drop in voltage across the
resistance to be measured or the current through it or
both.
• Null methods are based on measurement bridges.
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Measurement of resistance
•
Deflection method (1)
𝑉 =
𝑉
𝑅 1+𝑥
𝑅
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Measurement of resistance
•
Deflection method (2)
𝑅=𝑅
𝑉
𝑉
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Measurement of resistance
• Voltage dividers :
•
commonly used to measure high-value resistances.
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Measurement of resistance
Example: The MGS1100 CO gas sensor (Motorola) has 1000 kΩ in air,
from 30 k Ω to 300 k Ω (150 k Ω typical) for CO concentration of 60ppm
(R60) , and a ratio R60/R400 = 2,5 (typical). If the allowable voltage across
the sensing resistor and power dissipation in it are 5 V and 1mW, design a
voltage divider for such a sensor if the expected CO concentration range
is from 0 to 400ppm.
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Measurement of resistance
• Wheatstone
bridge: balance measurements
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Measurement of resistance
• Wheatstone
bridge: deflection measurements
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Problems
•
A given Pt100 has R = 100 and  = 6 mW/K when immersed in air and  = 100 mW/K when
immersed in still water. Calculate the maximal current through the sensor to keep the self-heating
error below 0.10C.
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A bridge circuit has R1 = R2 = R3 = R4 = 120Ohm resistances and a 10.0-V supply. Suppose a 3-1/2
digit DVM on a 200-mV scale will be used for the null detector. Find the resistance resolution for
measurements of R4.
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BT5
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