Operator`s manual

SECTION 13. 21X MEASUREMENTS
13-10
FIGURE 13.3-7. Half Bridge Configuration
for YSI #44032 Thermistor Connected to 21X
Showing: A) Large source resistance,
B) large source resistance at point P, and
C) configuration optimized for input settling
FIGURE 13.3-8. Measuring Input Settling
Error with the 21X
FIGURE 13.3-9. Incorrect Lead Wire
Extension on Model 107 Temperature Sensor
13.4 THERMOCOUPLE
MEASUREMENTS
A thermocouple consists of two wires, each of a
different metal or alloy, which are joined
together at each end. If the two junctions are at
different temperatures, a voltage proportional to
the difference in temperatures is induced in the
wires. When a thermocouple is used for
temperature measurement, the wires are
soldered or welded together at the measuring
junction. The second junction, which becomes
the reference junction, is formed where the
other ends of the wires are connected to the
measuring device. (With the connectors at the
same temperature, the chemical dissimilarity
between the thermocouple wire and the
connector does not induce any voltage.) When
the temperature of the reference junction is
known, the temperature of the measuring
junction can be determined by measuring the
thermocouple voltage and adding the
corresponding temperature difference to the
reference temperature.
The 21X determines thermocouple
temperatures using the following sequence.
First, the temperature of the reference junction
is measured. If the reference junction is the
21X panel, the temperature is measured with
the built-in thermistor (Instruction 17). The
reference junction temperature in
o
C is stored in
an input location which is accessed by the
thermocouple measurement instruction
(Instruction 13 or 14). The 21X calculates the
voltage that a thermocouple of the type
specified would output at the reference junction
temperature if its reference junction were at
0
o
C, and adds this voltage to the measured
thermocouple voltage. The temperature of the
measuring junction is then calculated from a