SC-2070 Board
Chapter 2
SC-207X Series User Manual
2-22
© National Instruments Corporation
Single-Ended Measurements
Connect the temperature sensor to channel 0 by configuring jumper W1 as shown in Figure 2-3.
Connect the positive leads of the thermocouples to any SC-2070 input channels except CH0. Connect
the negative leads to AIGND. 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. The MIO-16 board should be configured
for ground-referenced, single-ended inputs. 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 Single-Ended Mode
Assume that you are measuring the temperature of a high-pressure boiler system with a PC AT
computer, an AT-MIO-16L in single-ended 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 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 single-ended 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 single-ended channel 1, connect the positive thermocouple
lead to channel 1 and the negative thermocouple lead to AIGND. 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. 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 the voltage on channel 0. 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 SC-2070 temperature in degrees
Celsius. For example, if the reading is 0.25 V, then the SC-2070 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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