Chapter 2
SC-2070 Board
© National Instruments Corporation
2-21
SC-207X Series User Manual
Differential Measurements
Connect the temperature sensor to channel 0 and channel 8 (differential channel 0) by configuring
jumper W1 as shown in Figure 2-3. Connect the thermocouples to the appropriate pairs of input
channel screw terminals (for example, CH1 and CH9, CH2 and CH10, and so on). Notice that some
thermocouples, such as those from Omega Engineering (Stamford, CT), have red insulation on the
negative terminal. Check with the vendor to determine the output polarity of any particular
thermocouple. Because thermocouples are floating signal sources, a bias-return resistor in the range of
10 to 100 k
Ω
must be attached between the ( - ) channel and GND, which is connected to the MIO-16
AIGND (analog input ground). For more information about signal sources and their connections, see
Chapter 2 in your MIO-16 board user manual.
An Example of Using Thermocouples in Differential Mode
Assume that you are measuring the temperature of a high-pressure boiler system using a PC AT
computer, an AT-MIO-16L board in differential mode, an SC-2070 board, and a J-type thermocouple.
The maximum temperature that can be reached in this example is 300
°
C. Before taking readings, you
must configure the AT-MIO-16L board for the maximum resolution possible. A thermocouple
table shows that the output voltage never exceeds 16 mV to 17 mV (recall that the exact voltage
measured is a function of the SC-2070 temperature as well as the temperature being measured).
Therefore, 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) can be selected. In this case, the
±
10 V input
range with a gain of 500 gives the best resolution. Set the jumpers on the AT-MIO-16L for differential
input,
±
10 V input range. Jumper W1 on the SC-2070 board is set to select the temperature sensor as
shown in Figure 2-3. To connect the thermocouple to differential channel 1, connect one lead to the
CH1 terminal and the other to the CH9 terminal. Notice that no thermocouples can be read on channel
0. A bias-return resistor in the range of 10 k
Ω
to 100 k
Ω
is soldered to R10 for the bias current return
path. For this example, you do not add any lowpass filters or broken thermocouple detection.
The procedure for this example is as follows:
1. Read the voltage from the temperature sensor (channel 0). If you are using an NI-DAQ software
package, you can use the
AI_Read
and
AI_Scale
functions to do the reading. The temperature
sensor 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 SC-2070 temperature in degrees Celsius. For example, if the
reading is 0.25 V, the SC-2070 board is at 25
°
C.
2. Read the voltage on analog input channel 1, using a gain of 500. If you are using an NI-DAQ
software package, you can use the
AI_Read
and
AI_Scale
functions to read the channel.
3. Translate the reading into an uncompensated temperature using either a look-up table or a
polynomial such as that described in Formula 2-4. For example, assume that the reading from the
thermocouple is 9.39 mV. By applying Formula 2-6 and the coefficients from Table 2-4, you can
calculate that the uncompensated temperature is 175
°
C.
4. Add the cold-junction temperature from step 1 to the uncompensated temperature obtained in step
3. This result is the temperature at the measuring end of the thermocouple. For the example given,
the temperature of the boiler system is 175
°
C + 25
°
C = 200
°
C.
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