© National Instruments Corp. 7 FP-TC-120 and cFP-TC-120
Detecting Open Thermocouples
The [c]FP-TC-120 can detect open thermocouples. When you
select a temperature range for a channel, open-thermocouple
detection for that channel is enabled. When the [c]FP-TC-120
detects an open thermocouple, it reports an
Open thermocouple
error for that channel and lights the corresponding red LED.
Open-thermocouple detection works in part by using the resistors
shown in Figure 3. This circuit produces a 35 nA input current at
IN(–), a 35 nA output current at IN(+), and an input impedance of
20 MΩ. The open-thermocouple detection circuitry normally
results in negligible errors when used with thermocouples. Other
voltage sources with higher source impedance can introduce more
significant errors. The following gain and offset errors result from
this circuit:
• Offset error: 0.035 µV per ohm of source resistance
• Gain error: 0.05 ppm (parts per million) per ohm of source
resistance
Thus, a voltage source with a source impedance of 1 kΩ has
an additional offset error of 35 µV and an additional gain error
of 50 ppm.
Cold-Junction Compensation
Connecting thermocouple leads to the terminal base or connector
block creates new thermocouple junctions between the
thermocouple wires and the terminals. These cold junctions affect
the measured readings of the thermocouple. The [c]FP-TC-120
automatically enables cold-junction compensation when you select
a temperature range but not when you select a voltage range.
The [c]FP-TC-120 reads the temperature of the terminals from
temperature-measuring elements built into all FieldPoint terminal
bases and connector blocks. The module uses this temperature data
to compensate for cold junctions and reports it as
CJ Temperature in software. If the FP-TC-120 senses a
temperature of less than –50 °C or more than 85 °C, the
FP-TC-120 reports
Out of range for the cold-junction
temperature and for all of the channel temperatures.
For optimal cold-junction compensation, NI recommends that you
use the FP-TB-3 isothermal terminal base with the FP-TC-120 and
the cFP-CB-3 isothermal connector block with the cFP-TC-120.
To Next Page
Loading...
Need help?
General
