National Instruments Corporation SC-207 Series, Руководство пользователя

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SC-2071 Board Chapter 3
SC-207X Series User Manual 3-14 © National Instruments Corporation
Example:
Using the values in Figure 3-12,
G =
10 k
Ω
10 k
Ω
+ 10 k
Ω
=
1
2
Therefore, V
Lab
= (1/2) V
SC
With the Lab/1200 device configured for
±
5 V inputs, the device can acquire
±
10 V signals with
this attenuator circuit.
Working with Thermocouples
One of the most frequently used temperature transducers is the thermocouple. A thermocouple is
created whenever two dissimilar metals are brought in contact, and the contact produces a small
voltage as a function of temperature. In most measurement situations, a thermocouple is placed
where the temperature is to be measured; then two wires, consisting of the same type of metal as
each side of the thermocouple, are run back to a termination panel such as the SC-2071 board.
Additional dissimilar junctions are formed where these wires contact the screw terminals. Each
of these junctions generates its own temperature-dependent voltage. Other temperature-
dependent voltages are generated at ribbon cable junctions and solder joints. However, the
effects of these junctions are negligible and tend to cancel in most cases. The voltage measured
by the DAQ device is the sum of the voltages of all dissimilar metal junctions in the circuit, not
just the thermocouple junction used to measure temperature.
Only the temperature of the screw terminal junctions must be known to compensate for all
dissimilar metal junctions in a system with constant temperature (Thermocouple Law of
Intermediate Metals). Therefore, if you know the temperature at the screw terminals, you can
easily calculate the temperature at the point of interest from the voltage measured. A National
Semiconductor LM-35CZ is used on the SC-2071 board to determine the temperature of the
screw terminals. The dissimilar metal junctions at the screw terminals are also referred to as the
cold junction, and the process of using the known temperature at that point to find the
temperature at the unknown point is called cold-junction compensation (CJC).
Thermocouples are usually more desirable than direct-reading sensors for several reasons.
Thermocouples are more rugged than most other sensors, and can tolerate several hundred
degree temperatures without degradation. Semiconductor sensors, the easiest to use, usually
cannot be used above 70
°
C. Thermocouples are inexpensive and physically small, a feature that
enables them to track temperature changes rapidly.
Thermocouple output voltages are very low, on the order of 50
µ
V/
°
C. The temperature
coefficient changes with temperature, resulting in a nonlinear curve. The temperature can be
determined from tables available from thermocouple vendors or calculated from polynomial
equations such as those given in the next section.