National Instruments AMUX-64T, Руководство пользователя

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Chapter 3 Signal Connections
© National Instruments Corporation 3-11 AMUX-64T User Manual
Procedure 2
1. Read the voltage from the temperature sensor (channel 0). If you are
using NI-DAQ, you can use the
AI_Read
and
AI_Scale
functions to
do the reading. This voltage is 10 mV/°C, so the gain should be either
1 or 10 (10 for the best resolution). Multiply the voltage by 100 to get
the AMUX-64T temperature in degrees Celsius. For example, if the
reading is 0.25 V, the AMUX-64T is at 25 °C.
2. Read the voltages on any thermocouple channels. If you are using
NI-DAQ, you can use the
AI_Read
and
AI_Scale
functions to read
each channel, or you can use the
SCAN
functions to scan all the
channels at once. If you use the
SCAN
functions, use a sample interval
of 100 µs between channels. For the example given, the gain is at 500
for channel 1. Other thermocouple types may require other gains.
3. Translate the reading into a temperature using either a look-up table or
a polynomial such as one from Table 3-3. For example, when reading
9.39 mV from a type J thermocouple, the temperature is 175 °C.
4. Add the cold-junction temperature from step 1 to the temperature
obtained in the previous step. This result is the temperature at the
measuring end of the thermocouple. For the example given, the
temperature is 175 °C + 25 °C = 200 °C.
Comments
Procedure 1 is more accurate, but it requires two temperature-voltage
conversions. Procedure 2 is faster, but introduces an error of 2 °C. To
determine if the error from procedure 2 is acceptable or not in your
application, you can work through some examples both ways. Use a
thermocouple reference table and consider voltages and temperatures
close to those in your application.
Single-Ended Measurement
Connect the temperature sensor to channels 0 and 32 by configuring jumper
W1 as shown in Table 2-2,
Temperature Sensor Selection . Connect the
positive leads of the thermocouples to any AMUX-64T input channels
except CH0 and CH32. Connect the negative leads to GND. 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. Configure the MIO board
for ground-referenced, single-ended inputs. For more information about
signal sources and their connections, see the Signal Connections chapter of
the user manual that came with your MIO board.