VTI Instruments EX1401, Руководство пользователя

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EX1401 Preparation for Use 35
T
IME
C
ONFIGURATION
By default, the EX1401 will be configured to receive its time through PTPv2 (Precision Time
Protocol). The user can also set time using SNTP (Simple Network Time Protocol) or the time can
be set manually. The manual setting is necessary if the network environment is such that the unit
cannot reach the Internet/Intranet time server. For more information, see
Time Configuration in
Section 4.
I
NPUT
C
ONNECTIONS
/W
IRING
Thermocouple Connections
The EX1401 employs an uncompensated (Cu-Cu) mini-thermocouple female jack as its input
connector. This connector provides a solid, reliable connection that is also easily changeable. Since
it is not thermocouple-type specific, different thermocouple types can be mixed throughout the unit
without hardware modification. The input jack is polarized and will only accept its mating connector
in one orientation. The mating connector is a standard mini-thermocouple male jack. A popular
source is the SMPW series from Omega Engineering. For specified accuracy performance, the input
connector must be of the same thermocouple type as the wire being connected.
Thermocouple wire is polarized, and it is critical to consider this polarity when connecting the
thermocouple wire to the thermocouple jack. For reference, the color designations and polarizations
of the most popular thermocouple types are listed in Table 2-2 for both ANSI (American) and IEC
(European) standards.
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 switching in the signal conditioning
circuitry of the EX1401, 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 feet of 24 gauge
type T wire has a resistance of about 500 Ω. Against the EX1401’s typical input bias current of
7 nA, this creates a voltage error of:
V 5 . 2 nA 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 0.064 C V/ 39 V 2.5
 =  
 
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