Termosensors - booklet.pdf - Elektryka-2 - tomi78_291

ITS.90

Booklet

EUROTHERM SERIES 2000

The tables in this book should be used with Eurotherm Series 2000 instruments only.

For 800 Series, 90 Series, 900 Series and all older instruments use:-

‘EUROTHERM SENSOR TABLES’ HA016942 which use IPTS-68 Scale.

Contents page

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

THERMOCOUPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Basic theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Additional types of thermocouples . . . . . . . . . . . . . . . . . . . . . . . . . . .2

High stability thermocouple measurement . . . . . . . . . . . . . . . . . . . . .2

Reference junction techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

Sheathed thermocouples - measuring junctions . . . . . . . . . . . . . . . . .4

Compensating cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

Extension cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

Thermocouple application troubles . . . . . . . . . . . . . . . . . . . . . . . . . .6

EXTENSION AND COMPENSATING CABLES, TOLERANCES AND

IDENTIFICATION SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

Tolerance values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

Colour coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Positive conductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Outer sheath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Additional identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

RESISTANCE THERMOMETERS . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Basic theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Lead configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Resistance values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

THE EUROTHERM 239 MILLIVOLT SOURCE . . . . . . . . . . . . . . . .10

CALIBRATING WITH NON-STANDARD THERMOCOUPLES . . .12

Test equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

HA026535 Issue 1

INTRODUCTION

Since 1/1/90 a new international temperature scale has been introduced known as

ITS-90. This replaces the previous scale IPTS-68.

Differences between the old and new temperature scales are quite small being less than

0.4°C below 1000°C and rising to 2.4°C at 3090°C. Below is a graph of the differences

between the temperature scales.

0.02

-0.4

-0.02

-0.04

-0.2

- 200

200

400

100

-0.2

-0.01

-0.2

-0.02

-200

200 400

600 800 1000

Figure 1 The temperature difference (t90 - t68)/°C in the range between the triple

point of equilibrium hydrogen (-259.3467°C) and the freezing point of gold

(1064.18°C) [83.85]

THERMOCOUPLES

Basic theory

An electrical current flows in a closed circuit of two dissimilar metals if one of the

junctions is heated in respect to the other. Reversing the heating polarity results in the

electric current flowing the opposite way, and the magnitude of the EMF generated is

related to the temperature difference of the two junctions. The heated junction is known

as the measuring junction, and the unheated junction as the reference junction.

In practice only a small number of metals and alloys meet the multitude of requirements

needed, the chief of which is the ability to produce a constant voltage of workable

proportions, as linear as possible. For practical purposes they are divided into two

categories, rare or noble metal thermocouples and base metal thermocouples.

-200

Additional types of thermocouples

The thermocouples listed in this booklet are the most commonly used and known as the

standard tables. There are other tables that are not listed in this book which can usually

be found in high temperature applications.

There is a wide range of non-standard proprietary thermocouples, usually aimed at

specialist measurement conditions, i.e. atmospheric, cryogenic. Thermocouple

manufacturers produce batch tables for non-standard thermocouples, as the

characterises can differ batch to batch.

High stability thermocouple measurement

With thermocouple tolerances quoted as say ±3°C plus other variation it would appear a

poor case could be made out for high stability thermocouple measurement, for

example in research and high industrial technology. The key to stability in this field lies

in the careful selection of methods and materials, and the heat treatment and calibration

of the thermocouples. While application conditions do alter techniques, the following

factors are suggested for consideration.

1) Obtain thermocouples with insulated measuring junctions.

2) Specify ‘same melts’ for large installations. This could give problems in

replacement units later.

3) Thermocouple reference junctions should be monitored in a reference unit with at

accuracy of +0.1°C or better.

4) Great care should be taken in running thermocouple circuitry against ‘pick-up’ etc,

with the minimum number of joins in the wiring.

5) Heat treat thermocouples to their most stable condition.

In 5 and 6 assistance can be obtained from the British Calibration Service, National

Physical Laboratory, Queens Road, Teddington, Middlesex.

Reference junction techniques

It must be appreciated that a thermocouple does not measure the temperature at the

measuring junction, but it is a differential measurement between the measuring junction

and the reference junction. It follows that for any form of accurate temperature

measurement, the reference junction must either be held at a fixed known temperature

or accurate compensation be used for any temperature variations of the junction.

Reference junction technology is usually considered as one of the main problems of any

thermocouple installation. Individual instruments actuated by the thermocouples are

generally provided with a temperature compensating coil (often called a ‘cold junction

compensating coil’) or alternatively a solid state assembly. These devices sense the

temperature at the point where the thermocouple is joined to the copper wiring of the

instrument; and apply a corrective signal.

Where accuracy is needed and to accommodate multi-thermocouple installations, larger

reference units are used. These are claimed to have an accuracy of ±0.1°C or better, and

Termosensors - booklet.pdf

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