Using Programmable Open-Thermocouple Detection - National Instruments NI 435 Series User Manual

Note The default setting for programmable ground referencing is On in volts
measurement mode, and Off in 4-wire ohms mode.

Using Programmable Open-Thermocouple Detection

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Table 2-2. Using Programmable Ground-Referencing
Signal Source
Floating
Ground-referenced
The NI 435x devices 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 the
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 the thermocouple is open.
To understand how setting open-thermocouple detection affects
the accuracy of measurements, refer to the Using Programmable
Open-Thermocouple Detection section. You can set open-thermocouple
detection on a channel-by-channel basis. Table 2-3 summarizes the settings
you should use for open-thermocouple detection.
Chapter 2
Ground-Referencing
2-5
Operating the NI 435x Device
Programmable
On
Off
NI 435x User Manual