Breaking the hot junction of a thermocouple or disconnecting a thermocouple signal results in an open thermocouple channel. This causes the channel to return invalid data. Using open thermocouple detection, you can determine if a thermocouple channel is disconnected.
Devices have a set of pull-up and bias resistor networks that connect to channels, forming a voltage divider when a thermocouple is connected and working properly. When a thermocouple breaks or becomes open, the open thermocouple detection circuitry uses a small current source to push the input voltage out of range, which is possible to detect in software using limits or conditions. In normal operation, this current causes a voltage error on the input, known as a lead offset, when pushing against a source resistance. In many applications, this error is minimal, but in some applications where high accuracy is required and there are large source resistances caused by long, narrow gauge thermocouple wires, the error can be significant.
To determine if a thermocouple is disconnected or becomes open on the FD-11613, FD-11614, NI 4353, NI 9212, NI 9213, or NI 9214, you can use the OpenThrmcplChansExist and OpenThrmcplChans attributes/properties. OpenThrmcplChansExist returns a Boolean of true if one or more channels were disconnected since the last time the attribute/property was queried, and OpenThrmcplChans returns the virtual channel names of the channels that were disconnected.
To eliminate lead offset, you can disable open thermocouple detection, which eliminates the bias current. If you are using an SCXI module, you can disable open thermocouple detection by removing the bias resistor networks. On the NI 4353 and NI 9214, you can disable OTD current with the OpenThrmcplDetectionEnable attribute/property.
If an application requires high-accuracy and high-resistance thermocouple wires, but you do not want to disable the ability to detect open thermocouples, you can perform lead offset nulling. On the NI 9214, you can calibrate lead offset nulling programmatically. Lead offset nulling calibration compensates for lead offset error by taking the difference between a measurement with the bias current from open thermocouple detection and one without the bias current. This difference is then stored in the AI.Thrmcpl.LeadOffsetVoltage attribute/property and is applied to measurements to compensate for current from open thermocouple detection.