Transcript aakash patel

INSTRUMATION AND PROCESS CONTROL
(2150504)
TEMPERATURE MEASUREMENT: ELECTRICAL
TEMPERATURE SENSORS
Guided By – Prof. D.V. Kapatel
DEPARTMENT OF CHEMICAL ENGINEERING
G.H. PATEL COLLEGE OF ENGINEERING & TECHNOLOGY
VALLABH VIDYANAGAR – 388120
SUBMITTED BY •
•
•
•
•
•
NAME
AAKASH PATEL
MEHUL AMRUTIYA
URVESH CHAUDHARI
DARSH SHAH
TEJJAN DATANI
HARSH MANSURIYA
ENROLLMENT NO.
130110105001
130110105002
130110105003
130110105004
130110105005
130110105006
CONTENTS
• INTRODUCTION
• TEMPERATURE MEASUREMENTS
• ELECTRICAL TEMPERATURE SENSORS
• THERMISTORS
• RESISTANCE-TEMPRATURE DETECTORS
• TERMOCOUPLES
• BIBLIOGRAPHY
INTRODUCTION
• Industrial temperature measurement regulation has gained importance
along with the advancement in technology.
• The objective is to obtain values for the current conditions within the
process.
• The components of a typical continuous measurement device are as
follows:
• Sensors
• Signal Processing
• Transmitters
TEMPERATURE MEASUREMENT
• We need some sort of units to classify the measurement.
• It would be better to express the average thermal energy per molecule
of material in terms of Joule.
• Measurement of the hotness or coldness of a body or fluid is
commonplace in the process industries.
• The three dominant measurement devices used in automatic control
are thermocouples, resistance thermometers, and pyrometers and are
applicable over different temperature regimes.
ELECTRICAL TEMPERATURE SENSORS
THERMISTORS
• A thermistor is a type of resistor whose resistance is dependent
on temperature.
• Thermistors are widely used as inrush current limiter, temperature sensors,
self-resetting overcurrent protectors and self-regulating heating elements.
• Thermistors are nonlinear temperature-dependent resistors.
• Normally only the materials with negative temperature coefficient of
resistance (NTC type) are used. The resistance is related to temperature as:
RT = RTr * [exp ((1/T)-(1/Tr))]
where Tr is a reference temperature, which is generally 298 K.
CHARACTERISTICS
• Sensitivity
• Construction
• Range
• Response Time
• Signal Conditioning
APPLICATIONS
• PTC(Positive temperature coefficient) thermistors can be used as
current-limiting devices for circuit protection etc.
• PTC thermistors were used as heater in automotive industry to
provide additional heat. PTC thermistors are used in lithium
battery protection circuits.
• NTC(Negative temperature coefficient) thermistors are regularly used
in automotive applications. NTC thermistors can be also used to
monitor the temperature of an incubator.
• Thermistors are also commonly used in modern digital thermostats,
monitor the temperature of battery packs etc.
RESISTANCE-TEMPRATURE DETECTORS
• RTDs are more linear than are thermocouples.
• They have positive temperature coefficients (PTC),
• Their
output
is
extremely
linear
producing
very
accurate
measurements of temperature.
• RTD’s are precision temperature sensors made from high-purity
conducting metals such as platinum, copper or nickel.
• Its electrical resistance changes as a function of temperature, similar
to that of the thermistor.
Contd…
• The more common types of RTD’s are made from platinum and are
called Platinum Resistance Thermometer.
• The downside is that Platinum is expensive and one of the main
disadvantages of this type of device is its cost.
• Because the RTD is a resistive device, we need to pass a current
through them and monitor the resulting voltage. To avoid this, the
RTD is usually connected into a Whetstone Bridge network.
• It
has additional connecting wires for lead-compensation and/or
connection to a constant current source
Contd…
• The advantages of platinum resistance thermometers include:
•
High accuracy
•
Low drift
•
Wide operating range
•
Suitability for precision applications
• Limitations Includes –
•
At temperatures above 660 °C it becomes increasingly difficult to
prevent the platinum from becoming contaminated.
•
At low temperatures, the sensitivity of the RTD is essentially zero and
therefore not useful.
There are three main categories of RTD sensors: thin film, wirewound, and coiled elements. While these types are the ones most
widely used in industry there are some places where other more
exotic shapes are used, for example carbon resistors are used at
ultra-low temperatures (-173 °C to -273 °C).
THERMOCOUPLES
• Thermocouples are voltage devices that indicate temperature by
measuring a change in voltage.
• the thermocouple is located inside a metal or ceramic shield that
protects it from exposure to a variety of environments.
• As temperature goes up, the output voltage of the thermocouple is
rises – not necessarily linearly.
• Metal-sheathed thermocouples also are available with many types of
outer coatings, such as Teflon, for trouble-free use in acids and strong
caustic solutions.
Contd…
Consist of two junctions of dissimilar metals, such as copper and
constantan that are welded or crimped together.
One junction is kept at a constant temperature called the reference
(Cold) junction, while the other the measuring (Hot) junction.
When the two junctions are at different temperatures, a voltage is
developed across the junction which is used to measure the
temperature.
Thermocouple Colour Codes:Thermocouple Sensor Colour Codes Extension and Compensating LeadsCode
Type
Conductors (+/-)
Sensitivity
E
Nickel Chromium /
Constantan
-200 to 900oC
J
Iron / Constantan
0 to 750oC
K
Nickel Chromium /
Nickel Aluminium
-200 to 1250oC
N
Nicrosil / Nisil
0 to 1250oC
T
Copper / Constantan
-200 to 350oC
U
Copper / Copper Nickel
Compensating for
"S" and "R"
0 to 1450oC
ADVANTAGES
DISADVANTAGES
• Self-Powered
• Non Linear
• Simple
• Reference Required
• Rugged
• Low Voltage
• Inexpensive
• Reference Required
• Wide Variety
• Least Stable
• Least Sensitive
BIBLIOGRAPHY
• Process Control Instrumentation Technology, Curtis D. Johnson, PHI
•
•
•
•
•
Learning Private Limited, Eight Edition
Perry’s Chemical Engineers’ Handbook, McGraw Hill Publication, Robert H.
Perry Seventh Edition
“Process Instrumentation And Control”, A. P. Kulkarni, Nirali Prakashan
Thermal
Sensors,
https://controls.engin.umich.edu/wiki/index.php/TemperatureSensors, 13-092015
Basic Types Of Thermal Sensors, http://www.wwdmag.com/water/sevenbasic-types-temperature-sensors, 13-09-2015
Thermocouples, http://www.electronics-tutorials.ws/io/io_3.html, 13-092015
QUESTIONS… ??
THANK YOU