Using Programmable Open-Thermocouple Detection - National Instruments NI 4350 User Manual

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Using Programmable Open-Thermocouple Detection

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Table 3-2. Using Programmable Ground-Referencing
Signal
Source
Floating
Ground-referenced
The default setting for programmable ground referencing is on in volts
measurement mode and off in 4-wire ohms mode.
The NI 4350 instruments have software-programmable,
open-thermocouple detection on every channel, which you can use
to detect an open or broken thermocouple. This feature connects
CH+ to +2.5 V through a 10 MΩ resistor. This resistor acts as a pull-up
resistor and, consequently, the voltage between CH+ and CH– rises
rapidly above 100 mV if your thermocouple breaks open. All
thermocouples functioning under normal conditions generate a voltage
of less than 100 mV, even at very high temperatures, which makes this
conclusion possible. You can detect this voltage level in software and
conclude that your thermocouple is open.
To understand how setting open-thermocouple detection affects
the accuracy of measurements, refer to the programmable
open-thermocouple detection section later in this chapter. You can set
open-thermocouple detection on a channel-by-channel basis. Table 3-3
summarizes the settings you should use for open-thermocouple
detection.
Ground-Referencing
3-5
Chapter 3 NI 4350 Operation
Programmable
On
Off
NI 4350 User Manual