CHAPTER 4
TEMPERATURE TEMPERATURE TEMPERATURE TEMPERATURE MEASUREMENTMEASUREMENTMEASUREMENTMEASUREMENTMEASUREMENTMEASUREMENTMEASUREMENTMEASUREMENT
ObjectivesAfter finish this lesson, you will be able to:
Define the temperature
Convert temp unit
Realize the categories of temp device
Understand the important of thermowells Understand the important of thermowells in temp device
Apply the principle and operation of temp device (RTD. TC, filled-system, bimetallic)
Introduction TemperatureTemperatureTemperatureTemperature is a physical property of
matter that quantitatively expresses
the common notions of hot and cold.
most widely used since 1959 by most widely used since 1959 by
Galileo
technology is continuously improved
typical assembly consists of a
thermowell, temperature element,
extension, temperature transmitter
Measurement Unit
common: Fahrenheit & Celsius
F, C & K recognized internationally
F & C developing from 2 fixed point: ice
& steam, at atmospheric pressure& steam, at atmospheric pressure
conversion
=
59CF 15.273= KC
used to protect the element protect the element protect the element protect the element
If well not required, clear label attached to
element to indicate no well present
T/Ws create time delay. Without well has
Thermowells
T/Ws create time delay. Without well has
1-10s time delay, a well 20-50s delay
Used in most cases, where temperature elements are installed.
There are exceptions to this rule, such as in
Internals of some equipment (compressors, turbines)
Bearings, where space is very limited
Surface temperature measurement
Fast response applications
Air-space temperature measurements
T/W construction & material must carefully matched with process requirement
Material vary with the application & required speed of responsespeed of response
- metal: max varies from 800F (iron) - 2300F (inconel)
- ceramic: max 1900F (fused silica) - 3000F (silicon carbide)
TEMPERATURE MEASUREMENT DEVICES
Principles
every metal unique composition & has a
different resistance to flow electrical
Resistance Temperature Detector (RTD)
different resistance to flow electrical
current
most metal - changes in electrical resistance
directly proportional to change in
temperature; linear
it is called temperature coefficient of
electrical resistance (TCR)
RTD is regarded high precision wire
wound resistor; resistance varies with
temperaturetemperature
by measuring resistance, temperature can
be measured
RTD
ConstructionsConstructionsConstructionsConstructions
pure metals pure metals pure metals pure metals (platinum, nickel, copper)
typical probe contains a coil coil coil coil of very fine metal wire; allowing large resistance change without great space requirementchange without great space requirement
common: platinum RTD - accuracy & linearity
Accurate sensor - can measure
temperature change of 0.00001 C
usually protected from the environment
by a sheathsheathsheathsheath made of stainless steel or
another temperature & corrosion
resistance materialresistance material
element fits snugly inside sheath to
produce high rate of heat transfer
fine powder fine powder fine powder fine powder used to eliminate air pockets
ceramics insulators ceramics insulators ceramics insulators ceramics insulators used to isolate
internal lead wires.
At the end of tube a hermetic seal, a hermetic seal, a hermetic seal, a hermetic seal, to
protects the element
assembly may be terminated with lead
wires/ may supplies with an appropriate
terminal block similar to TC assembly
Operations
To detect small variations of resistance, temperature transmitter in form of Wheatstone bridge Wheatstone bridge Wheatstone bridge Wheatstone bridge is used
the circuit compared RTD value with three the circuit compared RTD value with three known & highly accurate resistors
Wheatstone; available 2 wire, 3 wire, 4 wire elements
Wheatstone bridge consist: 3 resistors, voltmeter, voltage source
when current flow in the meter is zero (voltage point A = voltage point B); null balance
this is set point on RTD temperature this is set point on RTD temperature output.
As RTD temp increase, voltage increase
voltage transducer replaces voltmeter, 4-20 mA signal can be monitored
when RTD is install some distance away some distance away some distance away some distance away
from transmitter, it caused problem
long connecting wires; resistance of wires
changes as ambient temperature fluctuates
variations in wire resistance would variations in wire resistance would
introduce errorerrorerrorerror in the transmitter
eliminate problem; 3-wire RTD is used
(impedance in wires will cancel because
they are in opposite legs of the bridge)
RTD
RTD Advantages
most stable, more accurate at moderate
temperature
less susceptible to electric noise less susceptible to electric noise
operate higher level of electrical signal
response time very fast compared
thermocouple (faction in sec)
more sensitive & more linear than TC
not experience drift problems because not
self-powered
not required special extension cable not required special extension cable
radioactive radiation has minimal effect
RTD Disadvantages
more expensive (purest metal)
not capable measuring as wide temp range as TC
power supply failure, cause erroneous power supply failure, cause erroneous reading
small changes in resistance (vibration, not tight, corrosion) create error
resistance curve vary from manufacturer
accuracy & service life are limited at high
temperature
RTD can found in reactor area temperature
measurement & fuel channel coolant measurement & fuel channel coolant
temperature
Principles
consist 2 pieces of dissimilar metals with their ends joined together (twisting, soldering, welding)
Thermocouple
soldering, welding)
when heat applied, voltage (mV) is generated
the joined produce a thermal electromotive force (emf) when junctions at different temperature
TC is self-powered
Typical response time of bare TC
(0.2 -12s)
Constructions
TC wires are manufactured to close
tolerances & tend to be expensive
their limited is to probe itself
TC extension wires, used as a link between TC extension wires, used as a link between
TC & measuring device/transducer
Thermocouple
3 basic types of TC construction
Ceramic beaded
Insulated (plastic, glass, ceramic fiber)
Metal-sheathed mineral-insulated
(MSMI); extruded (sheath stainless (MSMI); extruded (sheath stainless
steel/inconel & mineral Al2O3/MgO)
TC can be constructed to be protected/
exposed
protected; can be grounded/ungrounded
grounded: give faster response, but
susceptible to electrical noise
ungrounded: slower response, electrically
isolated
TC may be spring-loaded, so the tip & well
surface remain in contact to ensure good
heat transfer
if exposed, the faster response is provided,
but the wires are totally unprotected
when TC get thinner:
the recommended upper temp limit is
reducedreduced
the error decreases & the response is
faster to temp changes
the element becomes more fragile
at high temp, accuracy is more sensitive to
material (wire impurities)
OperationsTC: 2 junctions for measuring.
hot junction: the end inserted in the medium to measured temperature
Hot junction
cold junction: connected to measurement device (e.g. milimeter, potientiometer, galvanometer)
~ voltage generated depends on temperature
Cold junction
in a circuit, loop current
depends on relative
magnitude of voltage.
Detect by galvanometer
to measure temp, one
end contact with end contact with
process, other end kept
at const temp
emf thermocouple
increases when the
difference in junction
temp increases
relationship between total circuit voltage
(emf) & emf at the junction is:
if circuit emf & reference emf known,
Circuit emf = Measurement emf Reference emf
if circuit emf & reference emf known,
measurement can be calculated
convert to 4-20 mA signal, transmitted is
needed: temperature transmitter
the temp measurement circuit consists a TC connected directly to temperature transmitter.
hot & cold junction can located wherever required to measure temp diff
Need monitor the temp rise to ensure the safe operation
T rise of a device is the operating T using ambient or room temperature as the reference.
Types of TC
Copperconstantan (Type T)
pure copper (+ve) element &
constantan (-ve) elementconstantan (-ve) element
Constantan: group of alloys, contain
approximately 55% copper & 45%
nickel.
Iron constantan (Type J)
- iron (+ve) element & constantan (ve)
element
- can be applied in oxidizing / reducing - can be applied in oxidizing / reducing
atmospheres.
Chromel Alumel (Type K)
- Chromel (+ve) element & alumel (-ve)
element
- Chromel is an alloy with a nominal
composition of 90 % nickel & 10 % composition of 90 % nickel & 10 %
chromium
- Alumel contains 95 % nickel plus
aluminum and silicon with manganese
- Chromel alumel TC must be used in
oxidizing / neutral atmospheres
Chromel constantan (Type E)
- Chromel (+ve) element & Constantan (-ve) element
- highest emf per degree change of temp of any commonly used TC
- suitable for oxidizing atmospheres & - suitable for oxidizing atmospheres & not corrode at sub-freezing temp
- operating limits of -180oC and +870oC when protected and is available in wire
Advantageso resistance to corrosion in moist atmosphere,
limits error below 0oC, suitable for
subfreezing temperature measurement.
o can be furnished higher degree of accuracy o can be furnished higher degree of accuracy
for temp between -270 & +400oC than any
other commonly used thermocouple
o can be applied in either oxidizing /reducing
atmospheres between stated temp
o used on most transformers
(hot junction inside the transformer oil & cold junction at the meter mounted on the outside).
o used exclusively around the turbine hall because of their rugged construction and because of their rugged construction and low cost.
o capable of measuring a wider temperature range than an RTD
o self-powered
o simple & rugged
o inexpensive (half-price of RTD)
o wide choice of physical forms
can be calibrated to generate a specific o can be calibrated to generate a specific
curve & easy to interchange
o fast response & measurement at one
specific point
Disadvantages
TC located some distance away from the
measuring device, expensive extension
wires / compensating cables have to be
used.used.
TC are not used in areas where high
radiation fields are present.
generate non-linear output & low voltage
required a reference junction
have a low sensitivity
limited in accuracy
need type-matching extension wiresneed type-matching extension wires
slower response than RTDs
susceptible to stray electrical signal
Principles
metallic assembly consists a bulb, small-diameter tubing (capillary) & Bourdon spring
Filled-Systems
Bourdon spring
indicator linked to Bourdon tube indicates temp
sometimes bellows & diaphragm are used
system filled with a liquid / gas that expands & contract as the temp sensed at the bulb increased & decreased
expansion / contraction translated to mechanical translated to mechanical motion
liquid causes volume changes & gas causes press changes
this device generally used for local indication
Filled-systems
Operations
an improvement
liquidin-glass
thermometer
need no power to need no power to
function, simple,
rugged, self-
contained, accurate
over narrow temp
span
bulb may be too large for existing
application & on system failure, the whole
system must be replaces
capillary tubing is limited to a distance of
250 ft
slow to respond & relative expensive slow to respond & relative expensive
it is susceptible to ambient temp changes
around the capillary & ambient temp
compensation is often required
occasional checking & testing required to maintain accuracy
capillary tubing should be continuously supported & protected against damage
capillarys construction material capillarys construction material should be compatible with the surrounding environment
bulb must be immersed sufficiently to ensure that the actual temp is being measured
Vapor Liquid
Principles
a spiral made of two different metals, having different coefficient of expansion;
Bimetallic
having different coefficient of expansion; expands as the temp increase
movement by expansion drives an indicator on a scale
industrial bimetallic use a helical; coil to fit inside a stem
most temp
switches operate
on this principle;
but guides
provide min of
friction for the friction for the
moving
component
Operations
generally used in local temp gages & switches
To facilitate reading all-angle types usually are selected with a 5 in. diameter usually are selected with a 5 in. diameter dial
capillary type is sometimes used for operating visibility
if vibration exist; thermometer may be
filled with a dampening fluid (compatible
with the process fluid, in case of leakage)
simple construction, has few moving parts
& required little maintenance& required little maintenance
lowest cost among temp device, low
accuracy & provides no remote indication
To calibrate; must be immersed in a bath of
known temp
They are simple,
robust and inexpensive.
Their accuracy is
between +or- 2% to 5%
of the scale.
They are not
recommended for
temperature above
4000C.
When regularly used,
DISADVANTAGES ADVANTAGES
of the scale.
They can with stand
50% over range in
temperatures.
When regularly used,
the bimetallic may
permanently deform,
which in turn will
introduce errors.
Coefficient of expansion.
The following are the important properties a
material should have to be selected for bimetallic
thermometers.
Coefficient of expansion.
Modulus of elasticity.
Elastic limit after cold rolling.
Electrical conductivity.
Ductility.
Metallurgical ability.
Thermometer, resistance bulb, TC & bulb systems are placed in thermowell.
Thermowell protects the measuring device from corrosive and erosive effect of measured
Temperature Measurement Safety
corrosive and erosive effect of measured medium.
Bare TC is install to increase measurement speed and sensitivity.
Primary temp measuring devices can be connected to indicator, recorder & controller.