measuring temperature

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ACADs (08-006) Covered Keywords Filled system thermometer, thermocouple, 3 wire resistance temperature detector, volatile fluid sensor. Description 1.1.2.2.2 1.1.2.2.3

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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. Measuring Temperature. Terminal Objective: - PowerPoint PPT Presentation

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Page 1: 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

Page 2: Measuring Temperature

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.

Page 3: Measuring Temperature

• 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

Page 4: Measuring Temperature

Class I - Liquid filled (excluding mercury)Class II - Vapor filledClass III - Gas filledClass V - Mercury filled

Temperature is a measure of the average kinetic energy of the atoms of a substance

Page 5: Measuring Temperature
Page 6: Measuring Temperature

Class I - Liquid filled (excluding mercury)Class II - Vapor filledClass III - Gas filledClass V - Mercury filled

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 doesn’t freeze until -150 C

Page 7: Measuring Temperature

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

Page 8: Measuring Temperature

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.

Page 9: Measuring Temperature

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

Page 10: Measuring Temperature

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

Page 11: Measuring Temperature

•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

Page 12: Measuring Temperature
Page 13: Measuring Temperature

)32(9/5 FahrenheitCelsius

325/9 CelsiusFahrenheit

Page 14: Measuring Temperature

Seebeck EffectA 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.

Page 15: Measuring Temperature

Peltier Effect

• Reverse of the Seebeck Effect

Page 16: Measuring Temperature
Page 17: Measuring Temperature

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

• 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

T2T1

Page 18: Measuring Temperature

Law of Intermediate Metals

• 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

T2T1 Thermocouple wire Thermocouple wireNon-thermocouple wire

Page 19: Measuring Temperature

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)

Page 20: Measuring Temperature

How to take a thermocouple reading with a DVM

Right way!

Page 21: Measuring Temperature

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

Page 22: Measuring Temperature

Polarity of thermocouple

wire :

all thermocouple leads have a

red lead which is the negative

lead

At Palo Verde we use type K thermocouples – Chromel/Alumel

Page 23: Measuring Temperature
Page 24: Measuring Temperature

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

Page 25: Measuring Temperature

What’s this called?

Resistance Temperature Detectors

Most RTDs at Palo Verde are 100Ω at

32F

We have a few 200 Ω

RTDs

Page 26: Measuring Temperature

Calculate Temperature using an RTD

1

1212

t

tt

R

RRTT

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

Page 27: Measuring Temperature

Two Wire RTD

• The RTD is one leg of a wheatstone bridge

Page 28: Measuring Temperature

Three Wire RTD

Page 29: Measuring Temperature

Four wire RTD

Page 30: Measuring Temperature

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

Page 31: Measuring Temperature

Thermowells

How can you tell if a thermocouple or RTD is in a thermowell?

Page 32: Measuring Temperature

Other Methods of Temperature Calibration

Page 33: Measuring Temperature

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

Page 34: Measuring Temperature

Instrument Loops

– Identify common instrumentation signals– Explain the operation of a basic measurement

loop– Explain the operation of a basic control loop

Page 35: Measuring Temperature

Common Instrument Signals

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

Page 36: Measuring Temperature

A/ S

3- 15 psig

I/P

4- 20 ma

1150- 2050psig

P/ I

I/ E

0- 10 vdc

E/ I C

4- 20 ma

Page 37: Measuring Temperature

Basic Pressure Loop

Page 38: Measuring Temperature

Basic Flow Loop

Page 39: Measuring Temperature

Basic Temperature Loop

Page 40: Measuring Temperature

MCC

Page 41: Measuring Temperature

HTRDRV

S/PCT/V

Page 42: Measuring Temperature

I/ P

A/ S

(100"- 200" WC)(100"- 200" WC)4- 20 MA

LT

100

0

200

I/ E

0- 10 VDC

300

400

500

E/ I

S/ P

C R.A.

(150" WC)5 VDC

Page 43: Measuring Temperature

Odds & Ends

Page 44: Measuring Temperature

There are many ways to destroy test equipment.

Page 45: Measuring Temperature

Check voltage before you check contact status on Ohms

Page 46: Measuring Temperature

DVMs are especially sensitive

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

Page 47: Measuring Temperature

Control your test leads

Page 48: Measuring Temperature

Check your mini-grabbers

Page 49: Measuring Temperature

Don’t trust your holding screwdriver, either to hold the lead or to keep you from getting shocked

Page 50: Measuring Temperature

When replacing a transmitter, beware! You are typically using a 3-

valve manifold as your pressure boundary.

Page 51: Measuring Temperature

Know the pressure rating of your test tubing.

Page 52: Measuring Temperature

The End