Measuring Temperature

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Measuring Temperature
ACADs (08-006) Covered Keywords Filled system
thermometer, thermocouple, 3 wire resistance
temperature detector, volatile fluid sensor.
Description Supporting Material
1.1.2.2.2 1.1.2.2.3
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Measuring Temperature
Terminal Objective Given the appropriate
equipment and procedures, the IC Technician will
calibrate and maintain temperature instruments.
Mastery will be demonstrated by successful
completion of a Lab Performance Exercises and
written Exam.
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• Describe the theory of operation of Filled System
  Thermometers
• Describe the theory of operation of a
  thermocouple
• Draw a diagram of a three wire RTD bridge circuit
  and explain it's operation
• Check a Volatile Fluid sensor for proper
  operation per lab instructions
• Given thermocouple tables or graphs, a millivolt
  meter, and a thermometer, determine the
  temperature of the measuring junction of a
  thermocouple within two degrees
• Given a known Resistance Temperature Detector
  (RTD), it's type and it's temperature coefficient
  of resistance, calculate the RTD resistance for a
  given temperature, then verify the results in the
  lab setting

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Class I - Liquid filled (excluding mercury) Class
II - Vapor filled Class III - Gas filled Class V
- Mercury filled
Temperature is
a measure of the average
kinetic energy of the atoms of a substance
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• Operating Principles
• Fill fluid expands as temperature increases,
  increasing in volume
• Liquid in glass thermometers are also limited by
  the ability of glass to handle temperature
  extremes

Mercury becomes solid at minus 39 degrees
C Alcohol doesnt freeze until -150 C
Class I - Liquid filled (excluding mercury) Class
II - Vapor filled Class III - Gas filled Class V
- Mercury filled
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Mercury thermometers can range from -38F to 1110
F Alcohol thermometers range from -328 F to 1110
F
Other thermometer fill fluids include benzene
ether
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Sensing element is a capillary tube filled with a
liquid or gas which expands with an increase in
temperature. This sensing element delivers a
motion of physical change that is applied to the
control element which either indicates, records,
or by comparing the signal to a setpoint can be
used to control the temperature of a process.
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Class II (vapor filled)

• Sensing bulb partially filled with volatile fluid
  
• Common fluids include methylchloride, ether,
  butane, hexane, propane, toluene, sulfur dioxide
• Based upon the principle that in a system
  containing only a liquid and its vapor, at a
  given temperature, a given pressure will exist in
  the system, regardless of system volume
• Actual temperature measurement occurs at
  interface between liquid and vapor
• May exhibit erratic operation when temperature
  being measured swings above and below ambient
• Offers good reliability, inherently accurate,
  non-uniform scales (non-linear)
• Has mounting requirements

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Class III (gas fill)

• Utilizes perfect gas law
• Absolute temperature constant x pressure x
  volume
• (Of course, in real life folume does not remain
  constant, and perfect gasses do not exist)
• Helium approximates perfect gas, but tends to
  leak and is not often used
• Nitrogen usually is used
• Compensation generally not necessary if a large
  bulb is used

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• Two dissimilar metals bonded together
• Metal A has a lower coefficient of thermal
  expansion than metal B
• As temperature increases, metal B expands more
  than metal A
• Frequently used in home thermostats, oven
  thermometers, mercury switches, indicators

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Seebeck Effect
A circuit formed from two dissimilar metals
joined at both ends, develops an EMF (voltage)
proportional to the difference in the two
junction temperatures. So, if the temperature of
one junction is kept at a known value, the
temperature of the other junction can be
determined by the amount of voltage produced.
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Peltier Effect

• Reverse of the Seebeck Effect

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Law of Homogeneous Circuits (also known as the
law of intermediate temperatures)
T2
T1

• If thermocouple wire is homogeneous (all
  thermocouple wire between T1 and T2 and
• If temperature at T1 is known, and temperature at
  T2 is known,
• then the EMF will be known and will not be
  affected by temperature along the wire

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Law of Intermediate Metals
T2
T1
Thermocouple wire
Thermocouple wire
Non-thermocouple wire

• The algebraic sum of the thermo electromotive
  forces (EMF) in a circuit composed of any number
  of dissimilar metals is zero if the circuit is at
  a uniform temperature. -or-
• You can use non-thermocouple wire as long as both
  intermediate junctions are at the same
  temperature without affecting the total EMF

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How to take a thermocouple reading with a
DVM Wrong way! (unless you are going to
mathematically compensate for ref. junction
temperature using thermocouple tables, or the DVM
is set up to do self-compensation)
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How to take a thermocouple reading with a
DVM Right way!
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Reading a thermocouple

• Read the millivoltage for the unknown measuring
  junction temperature
• Obtain the millivoltage for the reference
  junction temperature from the applicable table.
  (reference junction is where the TC wire goes to
  copper)
• Algebraically ADD the two millivoltages
• The sum may then be converted to temperature
  directly from the same table. This is the
  unknown measuring junction temperature
• The calculations are performed automatically
  whenever a thermocouple reading device is used.
  Usually done with a resistive temperature device

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At Palo Verde we use type K thermocouples
Chromel/Alumel
Polarity of thermocouple wire all thermocouple
leads have a red lead which is the negative lead
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Resistance Temperature Detector

• Electrical resistance of certain metals increase
  / decrease in a repeatable manner as temperature
  increases / decreases
• No compensation or reference junction needed
• Slower, but more accurate and more linear than
  thermocouples
• The most commonly used metals for RTDs are
  Platinum, Copper, Tungsten and Nickel. At PV we
  use Platinum

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Resistance Temperature Detectors
Most RTDs at Palo Verde are 100O at 32F We have
a few 200 O RTDs
Whats this called?
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Calculate Temperature using an RTD
Where Rt2 Resistance _at_ temp T2 in O Rt1
Resistance _at_ temp T1 in O a temperature/resistan
ce coefficient (F or C) T2 measurement
temperature (F or C) T1 reference temperature
(F or C) usually 0C or 32F
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Two Wire RTD

• The RTD is one leg of a wheatstone bridge

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Three Wire RTD
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Four wire RTD
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Thermistors

• Solid state device
• Cheap
• Similar to RTD except resistance goes down as
  temperature goes up.
• Less linear than RTD
• Often used in heat detection and compensation
  circuits
• Higher sensitivity to small changes in temperature

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How can you tell if a thermocouple or RTD is in a
thermowell?
Thermowells
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Other Methods of Temperature Calibration
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Discuss Plant Mod 2807626 Read about mod at end
of temperature section in handout Discuss plant
impact of mod This is required by a TCS action
item
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Instrument Loops

• Identify common instrumentation signals
• Explain the operation of a basic measurement loop
• Explain the operation of a basic control loop

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Common Instrument Signals

• Current 4-20 milliamps
• Voltage 0-10 Volts DC
• Pneumatic 3-15 psig

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Basic Pressure Loop
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Basic Flow Loop
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Basic Temperature Loop
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Odds Ends
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There are many ways to destroy test equipment.
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Check voltage before you check contact status on
Ohms
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DVMs are especially sensitive

• Excessive voltage
• Excessive current
• Leads on wrong test point

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Control your test leads
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Check your mini-grabbers
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Dont trust your holding screwdriver, either to
hold the lead or to keep you from getting shocked
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When replacing a transmitter, beware! You are
typically using a 3-valve manifold as your
pressure boundary.
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Know the pressure rating of your test tubing.
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The End