Delta AS Series Manual page 198

A S S er i es Mo d u le Ma n ua l
7.1 Overview
This chapter describes the specifications for the ASTC-A module, its operation, and its programming. The
AS04TC-A is a temperature measurement module that converts temperatures received from thermocouples
(type J, K, R, S, T, E, N, or B, with ±100 mV voltage inputs) into digital signals. You can select either Celsius
(resolution: 0.1° C) or Fahrenheit (resolution: 0.1° F) as the unit of measurement.
An introduction to thermocouples
A thermocouple uses the Seebeck effect to measure differences in temperature. Generally speaking, a
thermocouple consists of two conductors of different materials that produce a voltage at the point where the
two conductors contact. The voltage produced depends on the difference of temperature between the
junctions with other parts of those conductors, and it ranges from several dozen microvolts to several
thousand microvolts. Because the voltage is so low, it needs to be amplified.
Differential operations are used to eliminate external noise. Thermocouples are more stable than thermistors,
resistance thermometers, and thermal resistors, so thermocouples are widely used in industrial applications.
A thermocouple consists of a circuit having two wires of different metals or metal alloys welded together or
joined at both ends. One of the junctions—normally the cold junction—is maintained at a known reference
temperature, and the other junction is at the temperature to be sensed. A temperature gradient across the
junction of the wires gives rise to an electric potential according to the Seebeck effect. The voltage produced
is proportional to the difference of temperature between the junctions with other parts of those conductors.
The voltage can be derived from the following equation.
T (
∫
V = 2 Q
T
1
_7
where Q and Q
A
temperatures of the two junctions.
Because Q and Q
A
equation (B).
V=α(T - T )
2 1
7 - 2
)
− Q dT
A B
(A)
are the thermopowers (Seebeck coefficient) of the metals A and B, and T
B
Principle of operation
are almost unrelated to temperature, formula (A) above can be approximated as in
B
(B)
and T are the
1 2