Chapter 3
Signal Connections
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National Instruments Corporation
3-9
These polynomials are accurate only within the temperature ranges
specified. Also, all terms must be included to achieve the specified
accuracy.
To avoid the long computation time required for these high-order
polynomials, the operating range of a thermocouple can be subdivided into
several smaller ranges. Each of the smaller ranges can then be
approximated by a much lower order polynomial (i.e., third or fourth
degree). Further examples of polynomials, including lower order
polynomials for subdivided temperature ranges, can be found in NIST
Monograph 175: Temperature-Electromotive Force Reference Functions
and Tables for the Letter-Designated Thermocouple Types Based on the
ITS-90.
Differential Measurements
Connect the temperature sensor to channel 0 and channel 32 (differential
channel 0) by configuring jumper W1 as shown in Table 2-2,
. Connect the thermocouples to the appropriate pairs of
input channel screw terminals (for example, CH1 and CH33, CH2 and
CH34, and so on). Notice that some thermocouples, such as those from
Omega Engineering, have red insulation on the negative terminal. Check
with the vendor to determine the output polarity of any particular
thermocouple. Since thermocouples are floating signal sources, you must
attach a bias return resistor between the negative channel and ground,
which is connected to the MIO board AIGND (analog input ground) pin.
The signal path of each channel has component locations for such resistors.
Refer to the
Connecting Nonreferenced or Floating Signal Sources
section
, for the resistor component locations.
For more information about signal sources and their connections, see the
configuration chapter of the user manual that came with your MIO board.
An Example of Using Thermocouples (Differential or Single-Ended)
For this example, assume that a J-type thermocouple is connected to
differential channel 1 and is being used to measure a high-pressure boiler
system. The maximum temperature that can be reached is 300 °C. Before
taking readings, it is necessary that you configure the MIO board for the
maximum resolution possible. A thermocouple table shows that the output
voltage will never exceed 16 to 17 mV (recall that the exact voltage
measured is a function of the AMUX-64T temperature as well as the
temperature being measured). Therefore, you could select either a ±5 V
input range with a gain of 100 (±50 mV maximum signal) or a ±10 V input
range with a gain of 500 (±20 mV maximum signal). In this case, the ±10 V
