Download Measuring Temperature

Measuring Temperature
ACADs (08-006) Covered
1.1.2.2.2
1.1.2.2.3
Keywords
Filled system thermometer, thermocouple, 3 wire resistance temperature detector,
volatile fluid sensor.
Description
Measuring Temperature
Terminal Objective:
Given the appropriate equipment
and procedures, the I&C Technician
will calibrate and maintain
temperature instruments. Mastery
will be demonstrated by successful
completion of a Lab Performance
Exercises and written Exam.
• 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
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
Operating Principles
•Fill fluid expands
as temperature
increases, increasing
in volume
Mercury becomes
solid at minus 39
degrees C
•Liquid in glass
thermometers are
also limited by the
ability of glass to
handle temperature
extremes
Alcohol doesn’t
freeze until -150 C
Class I - Liquid filled
(excluding mercury)
Class II - Vapor filled
Class III - Gas filled
Class V - Mercury filled
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
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.
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 (nonlinear)
•
Has mounting requirements
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
•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
Celsius  5 / 9( Fahrenheit  32)
Fahrenheit  9 / 5  Celsius  32
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.
Peltier Effect
• Reverse of the Seebeck Effect
Law of Homogeneous Circuits (also
known as the law of intermediate temperatures)
T1
T2
• 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
Law of Intermediate Metals
T1
Thermocouple wire
Non-thermocouple wire
Thermocouple wire
T2
• 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
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)
How to take a thermocouple reading with
a DVM
Right way!
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
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
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
Resistance Temperature Detectors
Most RTDs at
Palo Verde
are 100Ω at
32F
We have a
few 200 Ω
RTDs
What’s this called?
Calculate Temperature using an RTD
Rt 2  Rt1
T2  T1 
Rt1
Where
Rt2 = Resistance @ temp T2 in Ω
Rt1 = Resistance @ temp T1 in Ω
α = temperature/resistance coefficient (F or C)
T2 = measurement temperature (F or C)
T1 = reference temperature (F or C) usually 0C or 32F
Two Wire RTD
• The RTD is one leg of a wheatstone bridge
Three Wire RTD
Four wire RTD
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
How can you tell if a
thermocouple or RTD is in a
thermowell?
Thermowells
Other Methods of Temperature Calibration
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
Instrument Loops
– Identify common instrumentation signals
– Explain the operation of a basic measurement
loop
– Explain the operation of a basic control loop
Common Instrument Signals
• Current
• Voltage
• Pneumatic
4-20 milliamps
0-10 Volts DC
3-15 psig
Basic Pressure Loop
Basic Flow Loop
Basic Temperature Loop
Odds & Ends
There are many ways to destroy test
equipment.
Check voltage before you check
contact status on Ohms
DVMs are especially sensitive
• Excessive voltage
• Excessive current
• Leads on wrong test point
Control your test leads
Check your mini-grabbers
Don’t trust your holding screwdriver,
either to hold the lead or to keep
you from getting shocked
When replacing a transmitter,
beware! You are typically using a 3valve manifold as your pressure
boundary.
Know the pressure rating of your
test tubing.
The End