Chapter 3
SC-2071 Board
© National Instruments Corporation
3-19
SC-207X Series User Manual
An Example of Using Thermocouples
Assume that you are measuring the temperature of a high-pressure boiler system using a PC/XT
computer, a Lab-PC, an SC-2071 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
Lab-PC 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-2071 board temperature as well as the temperature being measured). Therefore, you
should select a
±
5 V input range with a gain of 100 (
±
50 mV maximum signal). Set the jumpers
on the Lab-PC board for
±
5 V input range. Set Jumper W1 on the SC-2071 board to select the
temperature sensor, as shown in Figure 3-4. To connect the thermocouple to channel 1, connect
one lead to the CH1 terminal and the other to the AIGND terminal. For this example, 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 for this reading (10 for the best resolution). Multiply the
voltage by 100 to get the SC-2071 board temperature in degrees Celsius. For example, if the
reading is 0.25 V, the SC-2071 board is at 25
°
C.
2. Read the voltage on analog input channel 1, using a gain of 100. 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 voltage reading into an uncompensated temperature using either a look-up
Table or a polynomial such as that described in Formula 3-4. For example, assume that the
reading from the thermocouple is 9.39 mV. By applying Formula 3-6 and the coefficients
from Table 3-4, 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 is 175
°
C + 25
°
C = 200
°
C.
The method of temperature calculation used in this example yields only an approximate value.
For a discussion on the preferred method of temperature calculation, which uses voltage
compensation calculations, see National Instruments Application Note 043, Measuring
Temperature with Thermocouples.
Sources of Error
When making thermocouple measurements with the SC-2071 board and a Lab/1200 device, the
possible sources of error are compensation, linearization, measurement, and thermocouple wire
errors.
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