Temperature Variation; Considerations; Polarity Of Thermocouple Wires; Shielding And The Guard Terminals - Ectron 1140A Instruction Manual

Thermocouple simulator-calibrator

Table of Contents

Temperature Variation

There are two primary sources of these errors: handling the thermocouple wires or the mini-

connector and air flowing past the terminals. Typically these errors are well under 0.1°C

although some conditions can increase these errors, and precautions must be taken to maintain

the high accuracy of the instrument.

Air that flows past the front-panel terminals from fans or air ducts is a typical source of the

problem. The amount of error depends on the temperature of the air and its velocity as well as the

thermocouple type and the gage of the wires being used.

Shielding the terminals from air currents will minimize this problem and is usually sufficient.

When wires are heated by handling, the usual remedy is to wait for stabilization. When this is not

possible consider using smaller gage wire to reduce the effect.

Among the most common mistakes made when working with thermocouple wires is to reverse

the wire connections. Use the manufacturer's designation and refer to Table

http://www.omega.com/techref/thermcolorcodes.html

wires. The negative wire is usually colored red for the following thermocouple wires: E, J, K, T,

R, S, and N.

When the output mode is set to TEMPERATURE, the positive wire is always connected to the

upper terminal of the binding posts. However, following standard convention, the mini-

thermocouple connector is reversed. For this connector, the top pin is negative, and the middle

pin is positive.

When the output mode is set to VOLTAGE in either meter or source mode, the upper binding post

terminal is positive. Note that the thermocouple connector is also active with reversed terminals;

that is, positive is the middle terminal.

Historically, thermocouple cabling did not include shielding. This was acceptable for accuracies

of a few percent but as the requirement for higher accuracy and more resolution has increased,

the need for shielding increases. In addition, the preferred wiring is twisted-pair shielded cables.

2 Courtesy of Omega Engineering, Inc.

When in either source or measure modes using ther-

mocouples, it is necessary to set the instrument to

the connector in use since the sensing of tempera-

ture of each of the terminals in use is required to

precisely cancel temperature effects at these junc-

tions. Setup for the connection in use is accom-

plished through the Main Menu and its sub-menus.

NOTE

to determine the positive and negative

5-3

Applications

4-1

and to

Table of Contents