Temperature Measuring and Control

Temperature Introduction .................................................................................................. 1 Practical Learning Processes / Task TC and RTD .............................................................. 2 Types of Thermocouples ................................................................................................... 3 Thermocouple Colour coding and Identification ................................................................. 4 Temperature Measuring and Control Panel ........................................................................ 5 Temperature Measuring and Control Loop Diagram ......................................................... 6 Temperature Measuring and Control Wiring ...................................................................... 7 Temperature Measurement and Control PD&I .................................................................... 8 Temperature Panel Start-Up Sequence ............................................................................. 9 Formative Practical Assessment ...................................................................................... 10

Thermocouples Seebeck effect

The Seebeck effect is the conversion of temperature differences directly into electricity and is named after the Baltic German physicist Thomas Johann

Seebeck. Seebeck, in 1821, discovered that a compass needle would be deflected by a closed loop formed by two different metals joined in two places, with a

temperature difference between the junctions. This was because the metals responded to the temperature difference in differen t ways, creating a current loop

and a magnetic field. Seebeck did not recognize there was an electric current involved, so he called the phenomenon the thermomagnetic effect. Da nish

physicist Hans Christian rectified the mistake and coined the term "thermoelectricity".

The Seebeck effect is a classic example of an electromotive force (emf) and leads to measurable currents or voltages in the same way as any other emf.

Electromotive forces modify Ohm's law by generating currents even in the absence of voltage differences (or vice versa); the local current density is given by

Peltier effect

The Peltier effect is the presence of heating or cooling at an electrified junction of two different conductors and is named after French physicist Jean Charles

Athanase Peltier, who discovered it in 1834. When a current is made to flow through a junction between two conductors A and B, heat may be generated (or

removed) at the junction. The Peltier heat generated at the junction per unit time, is equal to

where ( ) is the Peltier coefficient of conductor A (B), and is the electric current (from A to B). Note that the total heat generated at the junction is not determined by the Peltier effect alone, as it may also be influenced by Joule heating and th ermal gradient effects

Thomson effect In many materials, the Seebeck coefficient is not constant in temperature, and so a spatial gradient in temperature can result in a gradient in the Seebeck coefficient. If a current is driven through this gradient then a continuous version of the Peltier effect will occur. This Thomson effect was

predicted and subsequently observed by Lord Kelvin in 1851. It describes the heating or cooling of a current-carrying conductor with a temperature gradient. If a current density is passed through a homogeneous conductor, the Thomson effect predicts a heat production rate per unit volume of:

where is the temperature gradient and is the Thomson coefficient. The Thomson coefficient is related to the Seebeck coefficient as . This

equation however neglects Joule heating and ordinary thermal conductivity

Temperature Introduction

RTD

Wiring configurations

Two-wire configuration

The simplest resistance thermometer configuration uses two wires. It is only used when high accuracy is not required, as the resistance of the connecting

wires is added to that of the sensor, leading to errors of measurement. This configuration allows use of 100 meters of cable. This applies equally to balanced

bridge and fixed bridge system.

Three-wire configuration

In order to minimize the effects of the lead resistances, a three-wire configuration can be used. Using this method the two leads to the sensor are on adjoining

arms. There is a lead resistance in each arm of the bridge so that the resistance is cancelled out, so long as the two lead resistances are accurately the same.

This configuration allows up to 600 meters of cable

Four-wire configuration

The four-wire resistance configuration increases the accuracy of measurement of resistance. Four-terminal sensing eliminates voltage drop in the measuring

leads as a contribution to error. To increase accuracy further, any residual thermoelectric voltages generated by different wire types or screwed connections

are eliminated by reversal of the direction of the 1 mA current and the leads to the DVM (Digital Voltmeter). The thermoelect ric voltages will be produced in

one direction only. By averaging the reversed measurements, the thermoelectric error voltages are cancelled out .

Classifications of RTDs

The highest accuracy of all PRTs is the Standard platinum Resistance Thermometers (SPRTs). This accuracy is achieved at the expense of durability and

cost. The SPRTs elements are wound from reference grade platinum wire. Internal lead wires are usually made from platinum whi le internal supports are

made from quartz or fuse silica. The sheaths are usually made from quartz or sometimes Inconel depending on temperature range. Larger diameter platinum

wire is used, which drives up the cost and results in a lower resistance for the probe (typically 25.5 ohms). SPRTs have a wide temperature range (-200 °C to

1000 °C) and approximately accurate to ±0.001 °C over the temperature range. SPRTs are only appropriate for laboratory use.

Another classification of laboratory PRTs is Secondary Standard platinum Resistance Thermometers (Secondary SPRTs). They are constructed like the

SPRT, but the materials are more cost-effective. SPRTs commonly use reference grade, high purity smaller diameter platinum wire, metal sheaths and

ceramic type insulators. Internal lead wires are usually a nickel-based alloy. Secondary SPRTs are limited in temperature range (-200 °C to 500 °C) and are

approximately accurate to ±0.03 °C over the temperature range.

Practical Learning Processes / Task TC and RTD

OBJECTIVES (WHAT YOU MUST DO / EXPECTATIONS)

1. A learner must be able to select and test a correct working TC or RTD and connect it to the corresponding TC/RTD compensating leads Field Work. Calibrate PUK accordingly, Draw a loop diagram for Temperature Measuring and control,

Connect up the loop completely, do a sequential safety start-up system, Pre-Heat the Ove MANUALLY and slowly to the

required SP, Switch to AUTOMATIC and do the Final Loop Tune to the (PID) system for correct optimization of the Temperature loop.

2. Work safe using all the Safety working procedures and manufacture’s manuals according ly.

3. Calculate and give all the required values accordingly. (Tolerance ± 3°C)

4. Compare the three READINGS and Recorded trends from the front Oven and state the three factors that will affect the measuring in the Oven using thermocouples.

WHAT YOU WILL BE GIVEN

All the necessary equipment e.g. Oven, Temp. Controller, PUK, TC or RTD, Calibration system tool, Information book and testing tools.

HOW WELL YOU MUST DO IT

On completion: If there is any disturbance/upset to the running process, a three step correcting quarter wave dumper should be obtained after any process upset from the Oven.

ADDITIONAL RESOURCES

Your Training and Development Officer

Process instrumentation and Controls Handbook by the TDO

Types of Thermocouples

Type J

Type J (iron – constantan) has a more restricted range than type K (−40 °C to +850 °C), but higher sensitivity of about 50 µV/°C. The Curie point of the iron

(770 °C) causes a smooth change in the characteristic, which determines the upper temperature limit.

Type K

Type K (chromel – alumel) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C (chromel positive relative to alumel

when the junction temperature is higher than the reference temperature). It is inexpensive, and a wide variety of probes are available in its −200 °C to

+1350 °C / -330 °F to +2460 °F range. Type K was specified at a time when metallurgy was less advanced than it is today, and consequently characteristics

may vary considerably between samples. One of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is

that they undergo a deviation in output when the material reaches its Curie point; this occurs for type K thermocouples at around 185 °C.

Type T

Type T (copper – constantan) thermocouples are suited for measurements in the −200 to 350 °C range. Often used as a differential measurement since only

copper wire touches the probes. Since both conductors are non-magnetic, there is no Curie point and thus no abrupt change in characteristics. Type T

thermocouples have a sensitivity of about 43 µV/°C. Note that copper has a much higher thermal conductivity than the alloys generally used in thermocouple

constructions, and so it is necessary to exercise extra care with thermally anchoring type T thermocouples.

Platinum/rhodium alloy thermocouples

Types B, R, and S thermocouples use platinum or a platinum/rhodium alloy for each conductor. These are among the most stable thermocouples, but have

lower sensitivity than other types, approximately 10 µV/°C. Type B, R, and S thermocouples are usually used only for high temperature measurements due to

their high cost and low sensitivity.

Type B, S, R

Type B thermocouples (Pt/Rh 70%/30% – Pt/Rh 94%/6%, by weight) are suited for use at up to 1800 °C. Type B thermocouples produce the same output at

0 °C and 42 °C, limiting their use below about 50 °C. The emf function has a minimum around 21 °C, meaning that cold junction compensation is easily

performed since the compensation voltage is essentially a constant for a reference at typical room temperatures.

Thermocouple Colour coding and Identification

Type Positive metal

physical

properties

Positive (+) Negative (-) Negative metal

physical

properties

Braiding

colour

Colour of compensati

ng cable (+)

Colour of compensati

ng cable (-)

Temperature

range (ºC)

K Non magnetic Chromel Alumel Slightly magnetic Yellow Yellow Red -180 to +1350

J Magnetic Iron Constantan Non magnetic Black White Red -180 to +850

T Copper colour Copper Constantan Non magnetic Blue Blue Red -250 to +400

S Hard Platinum +

10% Rhodium

Platinum Soft Green Black Red -50 to +1750

R Hard Platinum +

05%

Rhodium

Platinum Soft Green Black Red -50 to +1950

THERMOCOUPLE / RTD

CONTROLLER

K RTD R/S

T J

Temperature Measuring and Control Panel

Local Sp

L N

TIC

K

Temp Transmitter

TC

Heat Element Oven Temp

220v Sup

Solid State Relay

24v AC Sup

4 - 20mA

4 - 20mA

mV

Temperature Measuring and Control Loop Diagram

Input

Output

Main

Sup

220V Ac

P/S 24v DC 220V Ac

220v AC

24v AC

SSR

OVEN

TIC

T.Tx 4 - 20mA

4 - 20mA

Temperature Measuring and Control Wiring

24v dc Supply

220V AC Switching for Temp Control

0 - 200°C L Sp

6 5 4 3 2 1

D

FIELD

FRONT CONNECTOR (BANANA PLUGS)

I/O LANDING STRIP

HONEYWELL CONTROLLER

(AI,AO,DO)

O N O L O L

O 4 AL (1) O 5 COMM O 6

O 7 O 8 O 9 OUTPUT 1

O 10 O 11 O 12

O 13 O 14 O 15

O 16 O 17 O 18 INPUT 1

O 19 O 20 O 21

O 22 O 23 O 24

O 25 O 26 O 27

Designed JMK 10/02/2015

Area : AGA Engineering Centre

Revised Project:

Checked JMK 10/02/2015 Project: PAGE No 1

Approved ZS 10/02/2015 Title: LOCAL TEMP CONTROLLER DRAWING

No OF PGS 1

D

C

C

B

B

A

A

6 5 4 3 2 1

TEMPERATURE MEASURING AND CONTROL PD&I

LS

DO

AI

AO

+ O + O - O + O - O

INPUT

4 – 20mA

OUTPUT

4 - 20mA

- O + O - O + O - O

Tx

SSR

24V SUP

220V SUP

Controlled

mV

Sol Pne

Temperature Panel Start-Up Sequence

Q1

Q1-A

Start Stop

Q1-A

Red Light Power available on the Panel

Green Light Panel ready

Start-up Relay

24v DC

220v AC Main Sup

220v AC Power Supply to the whole

Panel

Q1-B

Formative Practical Assessment

Measuring and Control

Select a type …………………… thermocouple/RTD from the rest and connect it correctly to the corresponding TC compensating leads/RTD leads.

Calibrate PUK accordingly for the ………0°C …………To………200°C ………. range temperature,

Draw a loop diagram for Temperature Measuring and control,

Connect up the loop completely according to the loop drawing,

Perform the sequential safety start-up system for your panel,

Pre-Heat the Oven temperature MANUALLY and SLOWLY to the required SP……………….°C,

Switch the controller to AUTOMATIC and do the Final Loop Tune to the (PID) system for correct optimization of the Temperature loop.

Create a step change to the process and note the 1

4 Wave form from the recorder,

Answer the following question: *

What is the real current temperature in the Oven? For this temperature, what is the mA current from the Transmitter?

For this PUK output, what is the input value and units?

State the three factors that will affect the measuring in the Oven using thermocouples.

RTD Process

PRACTICAL ASSIGNMENT MEASURING AND CALCULATIONS

1. Check a R.T.D. temperature Indicator.

a. Disconnect the R.T.D leads on the indicator provided, make note of the connections.

b. Connect a Decade resistance Box, or a R.T.D. simulator to the indicator.

c. Using the Resistance to Temperature Tables supplied at the end of the module, apply the resistance value equal to 0ºC by means of

the Decade box of the R.T.D. simulator.

d. The indicator should indicate 0ºC.

e. Apply the resistance value equal to the upper range value by using the Decade box or the R.T.D. simulator.

f. The indicator should indicate the upper range temperature.

2. Calibrate a R.T.D. transmitter.

a. Connect a 24 v D/C supply to the output side of a R.T.D. transmitter. Make sure that the polarities of the connections are correct.

b. Connect a mA meter in series with the 24v D/C supply and the transmitter.

c. Connect a Decade resistance box or a R.T.D. simulator to the input of the transmitter.

d. Using the Resistance to Temperature Tables supplied at the end of the module, apply the resistance value equal to 0ºC by means of

the Decade box or the R.T.D. simulator.

e. Adjust the Zero setting to obtain a reading of 4mA.

f. Apply the resistance value equal to the upper range value by using the Decade box or the R.T.D. simulator.

g. Adjust the Span setting to obtain a reading of 20mA.

h. Repeat steps d to g until the unit is calibrated accurately.

Thermocouple Process

PRACTICAL ASSIGNMENT MEASURING AND AVARAGE TEMPERATURE CALCULATIONS

Using the equipment assigned to you, you must do the following tasks.

1. Select types “J”, “K” and “T” thermocouples.

2. Connect the correct extension leads to the thermocouples.

3. Place the thermocouples in the oven and allow 10 minutes for the temperature to stabilize before continuing with step 4.

4. Using a Multi-meter, measure the millivolts from each thermocouple.

5. Using the Temperature / Millivolt tables, convert the millivolts readings to ºC.

6. Give the actual temperature in the oven, as measured by the three thermocouples.

7. Ask your Training Officer to check your work.

When you feel satisfied with your knowledge of this module, ask the Training Officer for the Criterion test.