Keithley 2001-TCSCAN User Manual

A
Thermocouple Basics
A-1
A.1 Definitions
The following terms are defined as they relate to thermocou-
ple circuits and thermocouple switching cards:
Cold junction — The junction that is held at a stable known
temperature. Also known as the reference junction.
Hot junction — The junction of two dissimilar metals that
is used to measure an unknown temperature. Also known as
the measurement junction.
Isothermal block or cover — The metal block or cover that
equalizes the temperature of thermocouple connections on a
switching card.
Reference accuracy — The maximum error between sensor
and channel inside the isothermal environment. Also known
as temperature offset.
Reference channel — The channel that measures the tem-
perature of the isothermal environment.
Reference output — The output signal that represents the
temperature of the reference channel. Commonly specified
by a temperature coefficient of µV/°C and an offset voltage
in millivolts at 0°C.
A.2 Theory
A thermocouple is a junction formed between two dissimilar
metals. If the temperature of the thermocouple junction con-
nected to the Model 2001-TCSCAN is T, a voltage E is de-
veloped between leads A and B as shown in Figure A-1.
When connected to a voltmeter, two more junctions (C and
D) are formed with the meter terminals. The measured volt-
age is proportional to the difference between temperatures T
and T
1
.
To determine the difference, the thermoelectric properties of
the thermocouple are needed. Data is available to determine
the voltage versus temperature relationship based on a refer-
ence temperature (T
1
) of 0°C. Thus, if the thermocouple-to-
copper junctions were maintained at 0°C, it would be possi-
ble to determine T by referring to the Thermocouple Refer-
ence Tables. (See NIST Nomograph 125.) The tables list
temperature as a function of the meter reading E
t
. Since these
junctions are not 0°C, a voltage E
r
is introduced, where:
E
t
= E - E
r
Terminal
C
D
E
t
= E - E
r
Isothermal
E
t
E
T
Copper
V
Copper
E
r
T
1
Terminal
A
B
Figure A-1
Thermocouple measurement

Keithley 2001-TCSCAN Specifications

General

Brand	Keithley
Model	2001-TCSCAN
Category	Test Equipment
Language	English