Thermocouple Instrumentation Group Project Temperature Measurement in a Nuclear Reactor

Instrumentation Group Project
Temperature Measurement in a Nuclear
Reactor
1.
2.
3.
4.
5.
Cathal
Aidan
Shuang
Libby
Matthew
Introduction to each transducer
Explanation of each transducer
Characteristics
Sensitivity
Project Cost
What Is a Thermocouple?
• Thermocouples are pairs of
dissimilar metal wires joined at
one end
Thermocouple
•
A thermocouple is a sensor used for measuring
temperature.
• Thermocouples are among the easiest temperature
sensors to use and obtain and are widely used in science
and industry.
• They are "simple", rugged, need no batteries and
measure over very wide temperature ranges
•A Thermocouple is made up of a pair of dissimilar metal wires joined
together at least at one end,
•When the junction of the two metals is heated or cooled a voltage is
produced that can be correlated back to a temperature
Thermocouple Types
• Thermocouple are available in different
combinations of metals or calibrations
• When the junction of the two
metals is heated or cooled a
voltage is produced that can be
correlated back to the temperature.
This is the Seebeck effect.
What Is a Thermocouple?
• Core chamber temperature of
reactor is less than 1250°C
• Use Type K for this project
• Type K (Chromel (Ni/Cr alloy) /
Alumel (Ni-Al alloy))
• Needs a Thermocouple
Transmitter with Automatic
thermocouple cold junction
compensation
Cold Junction Compensation
• Traditionally done using an ice bath
• In practice a precision thermistor in contact
with the input connectors is used
• Both temperatures used to calculate the true
temperature at the junction
• Any error in cold junction temperature will
lead to an error in the final temperature
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How the Thermocouple Works
•
When one end of a conductive material is heated the electrons
at the hot end are more thermally energized than the electrons
at the cooler end. These Hot electrons begin to diffuse toward
the cooler end. This redistribution of electrons creates a
negative charge at the cool end and an equal positive charge at
the hot end.
Thermocouple Standard Tables
The thermocouple circuit shown represents the basis upon which
thermocouple standard tables have been established. A new junction
is added and held in an ice bath at T ice (0°C). This allows tables of
thermocouple voltage versus temperature to be developed.
•
Thermocouple Advantages and
Disadvantages
Advantages:
• Low cost.
• No moving parts, less
likely to be broken.
• Wide temperature range.
• Reasonably short response
time.
• Reasonable repeatability
and accuracy.
Disadvantages:
• Low sensitivity
• Requires a known
temperature reference,
usually 0°C ice water.
Platinum RTDs
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Resistance Temperature Detectors
The hotter they become, the larger or higher the value of their
electrical resistance.
Usually they are provided encapsulated in probes for temperature
sensing and measurement with an external indicator, controller or
transmitter
made of pure platinum wire coil encapsulated in ceramic or glass (wire
wound) or a thin film deposited on a ceramic substrate (thin film).
What is a Platinum RTD?
• Resistance Thermometer
• Resistance changes 0.385
Ohms/°C (European
Fundamental Interval)
• Require a small current to
be passed through in order
to determine the
resistance.
2
What is a Platinum RTD?
• Wire-wound in a ceramic insulator - works with
temperatures to -200°C to 850 °C
• Wires encapsulated in glass - resists vibration, offers the
most protection to the detecting wire
• Used to measure reflector inlet temperature and core inlet
temperature in the reactor
How to use Platinum RTD
•Use a Wheatstone bridge circuit to calculate the change in resistance
of the RTD
•As the temperature changes so does the resistance of the platinum
the bridge is balanced and the resulting voltage change can be used to
find the temperature
Advantages and Disadvantages
RTD advantages:
• RTDs have very repeatable
resistance vs. temperature
characteristics over time.
• RTDs can be used with
standard instrumentation cable
for connection to display or
control equipment,
• RTDs are popular because of
their excellent stability, and
exhibit the most linear signal
with respect to temperature of
any electronic temperature
sensor.
RTD disadvantages:
• RTDs are characterized by
a slow response time,
• RTDs are characterized by
a low sensitivity,
• Prone to self-heating.
• RTDs are more expensive
than thermocouples,
Thermistor
•
•
A thermistor is a type of resistor used to measure temperature changes.
As the temperature changes the resistance in the thermistor changes.
•
Thermistors typically work over a relatively small temperature range,
compared to other temperature sensors, and can be very accurate and
precise within that range, although not all are
PTC
•
Resistance goes up as
temperature goes up.
• typically made from
polycrystalline materials that
are normally highly resistive,
but are made semi
conductive by the use of
dopants such as lead and
barium
NTC
•
•
Resistance drops as
temperature goes up.
most commonly composed
of oxides of manganese,
nickel, cobalt, iron, copper
and titanium
What is a Thermistor?
• A thermistor is a type of resistor used to measure temperature
changes, relying on the change in its resistance with changing
temperature
•
Thermistors can be classified into two types. If the resistance
increases with increasing temperature the device is called a
positive temperature coefficient (PTC) thermistor
• If the resistance decreases with increasing temperature, the
device is called a negative temperature coefficient (NTC)
thermistor.
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What is a Thermistor?
• Thermistors are
manufactured from
semiconductor materials
• Oxides such as cobalt and
magnesium are used
• A Wheatstone Bridge is
used to measure the
resistance change and give
a voltage output
corresponding to the
temperature
What is a Thermistor?
• Typical resistance values are 10 KΩ at 0°C and 200 Ω at
100°C
• Not as accurate as Platinum RTD
• Small temperature range -100 °C to +300 °C
• This transducer is not suitable for the nuclear reactor
project
Graph of Thermistor , Thermocouple
and Platinum RTD
Comparison of Common Types of
Temperature Transducers
Thermocouple Characteristic
Thermistor Characteristic
• How does it change
with temperature?
• Linear/Curve or
Exponential?
• How does it
• change with
temperature?
• Linear/Curve or
Exponential?
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Platinum RTD Characteristic
Resistance And Accuracy Table
• How does it
• change with
temperature?
• Linear/Curve or
Exponential?
Sensitivity
Thermocouple Sensitivity
• Sensitivity of measurement is a measure of the change in instrument
output which occurs when the quantity being measured changes by a
given amount:
• Average temperature ranges
-200°C to 1200°C
Change in instrument scale reading
Change in quantity being measured
• Sensitivity approx 41 µV/°C
• Sensitivity of a thermocouple is given by:
mV
Temperature.
• Type K is most sensitive.
• Type K ranges between
approx -250°C to +1400°C
• Sensitivity drift defines the amount by which an instruments
sensitivity of measurement varies as ambient conditions change.
Thermocouples at High
Temperatures
Thermocouples at Low
Temperatures
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Thermistor Sensitivity
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Thermally sensitive resistors.
Two types of Thermistors, negative
(NTC) or positive (PTC).
NTC range -200°C to +1000 °C
Should be chosen when a
continuous change of resistance is
required over a wide temperature
range.
PTC range +60 °C to +180 °C.
Should be chosen when a drastic
change in resistance is required at a
specific temperature or current
level.
Not suitable for this application
Platinum RTD Sensitivity
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Average Temperature range 200 °C to 850 °C.
•
Principle operation measures
the resistance of the platinum
element.
•
Not likely to find an RTD to
cover the range of high
temperatures required for this
application.
Thermocouple Cost, Type K
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