Transcript thermistor
TEMPERATURE MEASUREMENT
CONTENTS
•INTRODUCTION •RESISTANCE TEMPERATURE DETECTOR (RTD) •THERMISTOR •THERMOCOUPLE •CALIBRATION METHOD
INTRODUCTION
TEMPERATURE UNLIKE OTHER QUANTITIES (LENGTH, TIME, MASS) IS AN ABSTRACT QUANTITY THAT MUST BE DEFINED IN TERMS OF THE BEHAVIOUR OF MATERIALS AS THE TEMPERATURE CHANGES
TEMPERATURE CHANGE
CHANGE IN VOLUME OF LIQUID CHANGE IN LENGTH OF BAR CHANGE IN ELECTRICAL RESISTANCE OF A WIRE CHANGE IN PRESSURE OF GAS AT CONSTANT VOLUME ETC MATERIAL BEHAVIOUR
CONTENTS
•INTRODUCTION •RESISTANCE TEMPERATURE DETECTOR (RTD) •THERMISTOR •THERMOCOUPLE •CALIBRATION METHOD
RESISTANCE THERMOMETERS
CONSISTS OF A SENSOR ELEMENT THAT EXHIBITS A CHANGE IN RESISTANCE WITH A CHANGE IN TEMPERATURE , A SIGNAL CONDITIONING THAT CONVERTS THE RESISTANCE CHANGE TO AN OUTPUT VOLTAGE AND INSTRUMENTATION TO RECORD AND DISPLAY THE OUTPUT VOLTAGE TYPES OF SENSOR USED: •RESISTANCE TEMPERATURE DETECTOR (RTD) SIMPLE RESISTIVE ELEMENTS FORMED OF SUCH MATERIALS AS PLATINUM, NICKEL, OR NICKEL-COPPER ALLOY BECAUSE THEY EXHIBIT A POSITIVE COEFFICIENT OF RESISTIVITY, STABLE GOOD REPRODUCIBILITY •THERMISTOR FABRICATED FROM SEMICONDUCTION MATERIALS, SUCH AS OXIDES OF MANGANESE, NICKEL OR COBALT EXHIBIT A HIGH NEGATIVE COEFFICIENT OF RESISTIVITY
RTD
Resistance-Temperature relationship R T =R o {1+ a [T d (0.01T-1)(0.01T) b (0.01T-1)(0.01T) 3 ]} (Calendar-Van Dusen equation) where a , b and d are constants determined by calibration T is temperature in degrees Celcius R o is a resistance at reference temperature (usually 0 0 C) a =0.003921/ 0 C , d =1.49 and b =0 for T>0, b =0.11 for T<0 (US calibration) Advantages using RTD •Platinum RTDs are more accurate than thermocouples •Output response is more linear
US Calibration of Platinum RTD: R versus T
RTD instrumentation Wheatstone bridge for RTD Two wire Assume resistance of two wires low, R 1 =R 4
RTD
R
2
V
supply
V
supply 2
V
2
V o o
Three wire Four wire R 1 =R 4 , if lead wire resistance is not neglected, the change in lead resistance cancelled each other
RTD RTD
R
2
V
supply
V
supply 2
V o
2
V o
R lead V
4 supply
V o
2
V o
R
3
V
supply
V
supply 2
V o
2
V o
R lead V
8 supply
V o
2
V o
Example: An RTD probe has a resistance of 100 W at 0 0 C. The Calendar-Van Dusen constants are a =0.00392, d =1.49 and b =0 for T>0 0 C. What will be the resistance at 350 0 C?
Solution: R T =R o {1+ a [T d (0.01T-1)(0.01T) b (0.01T-1)(0.01T) 3 ]} if all variables are put into this equation R T =232.08 W or from calibration table we have 231.89 W
CONTENTS
•INTRODUCTION •RESISTANCE TEMPERATURE DETECTOR (RTD) •THERMISTOR •THERMOCOUPLE •CALIBRATION METHOD
THERMISTOR
Resistance Temperature relationship
R
R
0 exp b 1
T
1
T
0 where: R=resistance at any temperature T, in K R 0 = resistance at reference temperature T 0 , in K b =a constant, in K Generally respond to an increase in temperature with a decrease in resistance Using semiconductor b depends on thermistor material, typically between 1000-5000 K for metal oxide thermistor
Thermistor Instrumentation
Since the change in resistance for thermistor is so large ( D R/R=80 W /K) a common multimeter (4 digits) can be employed to measure R within ± 1 W , no bridge required
R T
V I
If using DAQ with computing microprocessor, temperature can be approximated very closely using Steinhart-Hart relation 1
A
B
ln
R T
C
(ln
R T
) 3
T
Where A, B and C are coefficients determined from calibration curve
CONTENTS
•INTRODUCTION •RESISTANCE TEMPERATURE DETECTOR (RTD) •THERMISTOR •THERMOCOUPLE •CALIBRATION METHOD
THERMOCOUPLE Seebeck Effect
Temperature gradient between two conductors induces a voltage potential where (sometimes a 0 is used) is average Seebeck coefficient at T1 ≤T≤T2
Thermocouple is a simple temperature sensor consists of two dissimilar materials in thermal contact Thermocouple are only capable of measuring temperature differences.
To measure the temperature of an object, we need a known reference temperature. The thermocouple is used to measure the temperature difference between the object and the known temperature
Law of intermediate metals
EXAMPLE A type R thermocouple system with an ice reference has an output of 9.1 mV. What is the temperature of the sensing junction?
Solution: In table, for R type thermocouple, 7.949 mV corresponds to 800 0 C and 9.203 corresponds to 900 0 C. Linear interpolation gives a temperature of 891.80C for 9.1 mV.
Example: An iron constantan thermocouple is connected to a potentiometer whose terminals are 25 0 C. The potentiometer reading is 3.59 mV. What is the temperature of the thermocouple junction?
Solution: The thermoelectric potential corresponding to 25 0 C from table is E 25 o C=1.277 mV The emf of the thermocouple based on a 0 0 C reference temperature is E T =1.277+3.59=4.867 mV Which corresponds to 92.5
0 C temperature
For laboratory work reference temperature usually forced to 0 0 C using ice bath