X hits on this document

21 views

0 shares

0 downloads

0 comments

11 / 11

Technical Information

Technical Information

Differential Temperature ( T) Measurement with Thermocouples

Introduction

Thermocouples are often used for differential temperature ( T) measurement in industrial processes where the differential is of a span sufficient to generate the required signal for the associated instrumentation. Thermocouples provide a possible advantage over other sensor types where small size and ease of interchangeability are important.

Linearity

The output of a given span is not linear for the common thermocouple calibrations. See Table

1. When the process requires temperatures, the non-linearity

T measurement over only a restricted range of working may not be significant. The combined limits of error in

degrees for two thermocouples in

T measurement is equal to the square root of the sum

of the squares of the individual thermocouple limits of error. For J and K calibrations, and standard limits of error for thermocouples below 530F, the combined thermocouple error is _+%(4)2+(4)2 = 5.640F. See Table 2. Intermediate connections should be minimized in the AT thermocouple circuit to lessen the introduction of parasitic EMF’s and the reduction of accuracy that can result. One, or both, of the thermocouples should be ungrounded.

Wiring

Schematic Wiring for T with Thermocouples

Instrument

B

  • -

    +

A

T1 (Hot)

A

(a)

B +

-

A

T2 (Cold)

When two thermocouples are connected for

T measurement either the positive or negative

legs of the thermocouples are connected together, with the remaining legs connected to the instrument. There is a reversal in polarity of the differential temperature signal when the thermocouple junctions change relationship regarding the “hot’ and “cold” designations. If a zero-centered span is not available on the associated instrument, then provisions must be made to reverse polarity at the instrument. Reference junction compensation is not utilized

for the individual thermocouples when they are connected for

T measurement.

A AT measurement is not absolute and, as such, will not give an indication of undesirable over-temperature condition in a process. For safety considerations, an absolute temperature measurement and/or indication may be necessary when measuring a AT condition. In any temperature measurement, attention should be provided to selection of sensors sufficiently rugged for the environment of the process, location of the sensors, and installation details.

Instrument

+

A

-

B

T1 (Hot)

B

(b)

A

  • -

    +

B

T2 (Cold)

One theromocouple grounded

Thermocouple Wire: A (Positive) B (Negative)

-110 to

1100 to

300 to

500 to

700 to

1000 to

-100F

110F

310F

510F

710F

1010F

0.245

0.291

0.306

0.308

0.306

0.316

0.184

0.228

0.224

0.227

0.234

0.237

0.172

0.234

0.279

0.313

0.339

-––

0.272

0.346

0.395

0.426

0.442

0.449

Table 1. Output (mV) for a 10F Span at Various Temperatures for Common Calibrations

Calibration J K T E

Temperature Spa

n

-130 to

100 to

300 to

500 to

700 to

1000 to

-100F

130F

330F

5300F

730F

-1030F

0.723

0.878

0.920

0.924

0.919

0.949

0.543

0.686

0.669

0.683

0.702

0.710

0.508

0.708

0.843

1.044

1.021

––

0.801

1.048

1.293

1.281

1.329

1.348

Table 2. Output (mV) for a 30F Span at Various Temperatures for Common Calibrations

Temperature Span

Calibration J K T E

REFERENCE R.B. Crawford, “Industrial Applications of Temperature Difference Measurements~ Temperature, Its Measurement and Control in Science and Industry” (Reinhold Publishing Corporation, New York, 1962. Volume 3, Part 2, PP. 91 3-925.

Page 8-12

Barber-Colman Company, Loves Park, IL.

1110-PD-001-0-00

Document info
Document views21
Page views21
Page last viewedFri Dec 02 21:08:29 UTC 2016
Pages11
Paragraphs471
Words4783

Comments