When introducing a current source such as IOTD in figure 4, there are some errors that can creep up due to parasitic properties of the thermocouple wires themselves. Since the thermocouple wires consist of two dissimilar metals (with unique resistivities), each leg of the thermocouple has a unique resistance associated with it. These resistances increase with longer or narrower gauge thermocouple wire. Increased resistance can contribute to offsets in acquired data. The effect of the offset voltage may be negligible or critical depending on the particular measurement system.
Figure 4: Offset voltage caused from OTD current
The total offset voltage created by the detection current can be calculated from Ohms law V=I*R, as,
where RP and RN are the resistances of the positive and negative legs of the thermocouple. Whether the offset produced causes significant error depends on the accuracy requirements of the application as well as the sensitivity of the deployed thermocouple type in the range of operation.
Products with OTD functionality from National Instruments generally specify OTD current so that users can determine the impact of this circuitry on their particular system. To facilitate comprehension of the topics discussed above we will work through a couple of examples.
PXIe-4353 deployed with 15ft, 24AWG K-type Thermocouples, at 500-800°C
First let’s calculate the resistance of the thermocouple. To be complete, one could refer to reference tables of resistivity by each element of the thermocouple. However, many vendors provide reference tables sorted by thermocouple type that give resistance as a function of length and gauge. Omega provides a table  in their catalog as well as online that shows K-type resistance per double foot (double foot means that it includes both the positive and the negative elements) as 1.490Ω at 24AWG. Fifteen feet of this wire would therefore only be 22.35Ω.
Second we need the specification of the OTD current for the PXIe-4353. OTD current is specified as 17nA when enabled.
Third, we calculate the voltage error.
Fourth, we convert this voltage error to a temperature error at the measurement temperature of interest. According to NIST Monograph-175 , the sensitivity of K-type at 500°C is 42.628µV/°C. And the sensitivity at 800°C is 41.000µV/°C. It is slightly less sensitive at 800°C, so a small offset voltage creates a slightly larger temperature error at 800°C. We will use this less sensitive point to calculate the worst case temperature error given our OTD induced offset voltage.
In this case, the OTD current produces less than 1/100th of a degree Celsius of error. This is probably negligible in any thermocouple application.
NI-9211 deployed with 1500ft, 30AWG E-type Thermocouples, at 0-900°C
First let’s calculate the resistance of the thermocouple. The table from the vendor specifies E-type resistance per double foot as 7.169Ω at 30AWG. Fifteen hundred feet of this wire would therefore be 10.75kΩ.
Second we need the specification of the OTD current for the NI-9211. OTD current is specified as 50nA.
Third, we calculate the voltage error.
Fourth, we convert this voltage error to a temperature error at the measurement temperature of interest. According to NIST Monograph-175 , the sensitivity of E-type at 0°C is 58.666µV/°C. And the sensitivity at 900°C is 76.835µV/°C. It is less sensitive at 0°C, so an offset voltage creates a larger temperature error at 0°C. We will use this less sensitive point to calculate the worst case temperature error given our OTD induced offset voltage.
In this case, the OTD current produces more than 9°C of error even at a measurement temperature of 0°C. This magnitude of error is much more likely to cause trouble than the 1/100th of a degree of error from the first example. If an application with 1500ft of 30AWG required better than 9°C of accuracy, then some form of compensation should be used in order to reduce the effect of the offset voltage.