Omega TX91 User Manual

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APPROVED

TX91

Thermocouple Two-Wire
Temperature Transmitter

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TX91
Miniature Two-Wire Thermocouple Transmitter

Page

Section 1 Introduction 1

1.1 General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3 Models Available . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Section 2 Unpacking Instructions 9
Section 3 Installation 11

3.1 Mounting the TX91 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.2 Wiring the TX91 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

TABLE OF

CONTENTS

i

TX91
Miniature Two-Wire Thermocouple Transmitter

Page

Section 4 Calibration Instructions 18

4.1 Equipment Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.2 Set-up of Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.3 Calibration Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Section 5 Troubleshooting Guide 25
Section 6 Accessories 26
Section 7 Specifications 27
Appendix A

Intrinsically Safe Interconnection Diagram . . . . . . . . . . . . . . . . . . . . . 30

TABLE OF

CONTENTS

ii

1

1

Introduction

1.1 General Description

The OMEGA®TX90 Series Temperature Transmitters consist of the
TX91 Miniature Two-Wire Thermocouple Transmitter and the
TX92 Miniature Two-Wire RTD Transmitter. This manual is written
for the OMEGA TX91 Thermocouple Transmitter.

The TX91 Transmitter accepts thermocouple sensor types J, K, T,
or E and will produce a standard 4-20 mA output signal propor­tional to that produced by its attached input temperature sensor.
Transmission of the proportional current output may be
accomplished by using copper wires.

Figure 1-1. Photo of TX90 Series Transmitter
Figure 1-2. General Dimensions (in inches)

12

Introduction 1

(For Mounting)

3

1 Introduction

The TX91 transmitter is normally powered by an unregulated DC
power supply as shown in Figure 1-3. The proportionally-transmit­ted signal begins at 4 mA, at the low end of its temperature range,
and increases to 20 mA, at the high end of its temperature range.
(There are various temperature ranges and thermocouple types
available for the TX91. To order, refer to Section 1.3 for correct
Model Numbers and Range Codes.)

Figure 1-3. TX91 Thermocouple Transmitter

Input

THERMOCOUPLE

COPPER

WIRES

TX91

THERMOCOUPLE

TRANSMITTER

DC SUPPLY
11-44 VDC

04

Introduction 1

The TX91 two-wire transmitter receives and measures signals from
thermocouples and sends an output current of 4-20 mA which is directly
proportional to the thermocouple millivolt input. It is designed to
connect with only two copper wire leads that will supply the voltage to
operate the transmitter from a DC power supply, and also carry the
output current. The output current is then used for recording, computing
or controlling.

If the TX91 is mounted inside a protection head, such as the OMEGA
NB1 Protection Head (see Figure 3-1), the thermocouple extension wires
are replaced by two copper wires that carry the 4-20 mA signal and DC
voltage to operate the transmitter. (Refer to the OMEGATemperature
Handbook for information on NB1 Thermocouple Assembly.)

05

Introduction 1

The TX91 has reverse supply polarity protection and will operate with a
wide range of supply voltages (11 to 44VDC). It has an input sensor
break-protection circuit that forces the output current to go upscale when
the thermocouple wire opens. It also is provided with a screw terminal,
where the output current can be measured without interrupting the
power loop. The TX91 does NOT provide isolation between its input and
the 4-20 mA output; therefore, an undergrounded thermocouple junction
is suggested to prevent possible ground loops.

Note that most thermocouple transmitters with 4-20 mA outputs,
including the TX91, are proportional with respect to the thermocouple
input voltage. However, the relationship between temperature and
millivolt for all the thermocouple types is somewhat non-linear. This
leads to maximum error at approximately the midpoint of the range as
shown in Figure 1-4.

Figure 1-4. Straight line Approximation of Curve

06

1 Introduction