VTI Instruments EX1000A, User Manual

Page: 51 / 320

www.vtiinstruments.com
EX10xxA/EX10xxA-51C/RX10xx Preparation for Use 51
ANSI Thermocouple Standard IEC Thermocouple Standard
Thermocouple + - Thermocouple + -
Type J White Red Type J Black White
Type K Yellow Red Type K Green White
Type T Blue Red Type T Brown White
Type E Violet Red Type E Violet White
Type S Black Red Type S Orange White
Type R Black Red Type R Orange White
Type B Gray Red Type B Gray White
Type N Orange Red Type N Pink White
T
ABLE
2-2: S
TANDARD
T
HERMOCOUPLE
R
EFERENCE
D
ESIGNATIONS
In most applications, the length and gauge of the thermocouple wire do not affect the accuracy of
the measurement. Due to the high input impedance and lack of dynamic switching in the signal
conditioning circuitry of the EX10xxA, the resistance and capacitance of the thermocouple wire are
normally not important factors. If, however, maximum system accuracy is desired, the resistance of
the thermocouple wire must be considered as a system error source. As an example, 650 ft of
24 gauge type T wire has a resistance of about 500 Ω. Against the EX10xxA’s typical input bias
current of 7.5 nA, this creates a voltage error of:
V 3.75 nA 7.5
Ω
500

 
To convert this error to its representative temperature error, it is then divided by the slope of the
thermocouple characteristic at the temperature of interest. For example, the slope of the type T
characteristic at 0 ºC is 39 µV/ºC. The error at this point is then:
C 0.1 C V/ 39 V 3.75
   
 
This example demonstrates how to evaluate the potential error that a specific wire installation
represents. The user is encouraged to evaluate each individual application to ensure that the error is
within acceptable bounds. In general, best results will be achieved with the shortest and largest wire
that the physical requirements of the application can support.
The EX10xxA offers excellent noise rejection through its high common mode rejection and
selectable bandwidth limiting, which allows for high integrity, noise-free measurements with even
less-than-ideal wiring setups. However, some common instrumentation wiring practices can be used
to achieve or ensure maximum performance.
Shielded thermocouple wire can be used to raise the system’s rejection of electrical interference.
Shielded wire encloses the two thermocouple wires with a low impedance conductor that should be
terminated by the user to a convenient earth ground. The EX10xxA provides an external ground
stud that can be used for this purpose, but any earth ground point is acceptable.
Magnetic interference, which is present wherever high currents are flowing, is conversely decreased
by minimizing the loop area represented by the two thermocouple wires. That is, the wires should
be run closely together from the thermocouple junction to the connections in the thermocouple jack.
Fortunately, most thermocouple wire comes with a sheath that covers the two thermocouple
conductors, inherently creating a small loop area.
Many test applications involve the monitoring of a test article in a chamber, requiring the routing of
numerous types of signals through the chamber’s cable access ports. It is recommended that the
thermocouple wires be run through a separate port and conduit from cables carrying power or high
frequency signals.