Chapter 3
Signal Connections
3-12
©
National Instruments Corporation
Using More Than One AMUX-64T
Two cold-junction compensation options are possible when thermocouples
are being used with two or more AMUX-64T boards connected to one
MIO board. If all AMUX-64T boards are at approximately the same
temperature, only one needs to have the temperature sensor connected to
channels 0 and 32. These two channels on the other boards are then free for
more thermocouples. If the AMUX-64T boards are not at approximately
the same temperature, you should use the temperature sensor on each
AMUX-64T. Using the temperature sensors on each board reduces the
chance of cold-junction compensation error, but it does increase software
overhead and reduce the number of channels available for general use.
If you use two AMUX-64T boards, the sensors will appear at channel 0 for
board A and channel 64 for board B. If you use four AMUX-64T boards,
the sensors will appear at channel 0 for board A, channel 64 for board B,
channel 128 for board C, and channel 192 for board D (see the
Single-Board and Multiple-Board Configurations
section in Chapter 2,
Configuration and Installation
).
Sources of Error
There are several major sources of error when making thermocouple
measurements with the AMUX-64T and an MIO board. These sources of
error are compensation, linearization, measurement, and thermocouple
wire errors.
Compensation error can arise from two sources, inaccuracy of the
temperature sensor and temperature differences between the sensor and the
screw terminals. The sensor on the AMUX-64T is specified to be accurate
to ±1 °C. You can minimize temperature differences between the sensor
and the screw terminals by keeping the AMUX-64T away from drafts,
heaters, warm equipment, and so on. Linearization error is a consequence
of the fact that the polynomials are by nature approximations of the true
thermocouple output. The linearization error is dependent on the degree of
polynomial used. Table 3-3 lists the linearization errors for the NIST
polynomials.
Measurement error is the result of inaccuracies in the MIO board. These
include gain and offset errors. If the board is properly calibrated, the offset
error should be zeroed out. The only remaining error is a gain error of
±0.08% of full range (see the MIO board specifications). If the input range
is ±10 V and the gain is 500, gain error will contribute 0.0008 x 20 mV, or
16
µ
V of error. If the Seebeck coefficient of a thermocouple is 32
µ
V/±C,
this measurement error will add 0.5 °C of uncertainty to the measurement.
