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■ Temperature is the measurement of the average energy per molecule of a material
■ Temperature is measured using several temperature units such as Kelvin and Celsius.
1) Absolute Temperature Scales
This scale assigns a zero temperature to a material that has no thermal energy (no molecular vibration).
The two absolute scales are Kelvin(K) and Rankine(oR).
2) Relative temperature scales:
This scale differs from the absolute scales only in a shift of the zero axis.
These two scales are Celsius(C)-related to the Kelvin; and Fahrenheit(°F)-related to the Rankine.
7.3.1 RTD
7.3.1 RTD
Resistance Temperature Detector commonly uses platinum, nickel or copper wire whose resistance varies with temperature.
SOME COMMON MATERIALS USED IN RTD AND TEMPERATURE RANGE
Platinum RTDs provide high accuracy and stability. Most RTD instruments use a Wheatstone's Bridge or its modified version.
Advantages:
Linearity over a wide operating range
Higher temperature operation
Better stability at high temperature
7.3.2 THERMISTOR
7.3.2 THERMISTOR
Thermistor is one of the temperature transducers.
Type of thermistors - negative temperature coefficient (NTC) & positive temperature coefficient (PTC) of resistance.
Negative Temperature Coefficient (NTC)
Exhibits decreasing electrical resistance with increases in environmental temperature
Positive temperature coefficient (PTC)
Exhibits increasing electrical resistance with increases in environmental temperature
A graph of resistance vs temperature for an NTC thermistor
The resistance at room temperature (250C ) for typical commercial units ranges shown above
The various types of thermistors used
7.3.3 THERMOCOUPLE
7.3.3 THERMOCOUPLE
Thermocouples are voltage-generating sensor in which an electromotive force produced is proportional to the temperature. It is constructed by two different metals or wires which are connected on their both ends.
The wires loop produces an electromotive force (emf) which is called the Seebeck effect.
The emf produced is proportional to the difference in temperature between the two junctions.
The Seebeck emf produced is given by : E = a ( T1 – T2 )
A three-wire thermocouple system is used to measure a temperature. To measure the temperature, the reference temperature must be known and the reference junctions must be held at the same temperature.
The reference junctions can be moved to a remote location using extension wires
TM (measured temperature), TR (reference junction) metal A and B are thermocouple wires and metal C connects thermocouple to the measurement apparatus.
An additional video testing of a fully functioning thermocouple with a real life model picture
There are several standard types of thermocouples based on their wire material, each having their own range, linearity and sensitivity.
The graph shows the materials used for several types of thermocouples and some of the thermocouples curves.
It can be seen from the graph that type J, K and N thermocouples are more sensitive than type R and S.
7.3.4 THERMOCOUPLE TABLES
7.3.4 THERMOCOUPLE TABLES
Thermocouple tables give the voltage that results for a particular type of thermocouple when the reference junctions are at 0°C where each type has their own range, linearity and sensitivity.
<< TUTORIALS ON HOW TO READ FROM THE TABLE
(WHILE)
TUTORIALS ON HOW TO USE A THERMOCOUPLE >>
When the temperature or output voltage value is not given in the table, it is necessary to use interpolation equation
VM and TM is the measured voltage and temperature.
VH and VL is the higher and lower voltage in the table.
TH and TL is the corresponding higher and lower temperature
THE DIFFERENCE BETWEEN THERMOCOUPLE VS RTD VS THERMISTOR
THE DIFFERENCE BETWEEN THERMOCOUPLE VS RTD VS THERMISTOR
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