Interface To Transducers, Thermocouples, Etc - Keithley DAS-8 Series User Manual

Interface to Transducers, Thermocouples, etc.

Low-level transducers such as thermocouples and strain gage bridges
(load cells, pressure/force transducers) require amplification before
applying to the high level DAS-8 series board inputs. The EXP-16
expansion multiplexer incorporates an instrumentation amplifier that can
provide stable amplification. It also includes circuitry that allows
cold-junction compensation of thermocouples. EXP-16 handles most
interfacing requirements to DC output transducers and also includes
spaces for filters, shunts, and attenuators.
For inexpensive temperature measurement in the –50 to +125 degree
Celsius range, semiconductor temperature transducers are a good choice.
The most popular types are the AD590 (Analog Devices), which behave
like a constant-current source with an output of 273µA at 0˚C and a
scaling of 1µA/˚C. The LM335 (National Semiconductor) has an output
of 2.73 volts at 0˚C and a temperature coefficient of 10mV/˚C. Both of
these devices can be powered from the +12V available from the computer
and directly interfaced to DAS-8 Series boards.
For measuring high temperatures (up to 1800˚C or more), thermocouples
are the most satisfactory solution. The base metal thermocouples, Types J,
K, T, and E have outputs around 40µV/˚C, while the platinum and
tungsten types are generally used for the highest temperature
measurement. Types S, B, and R tend to have lower outputs in the 6 to
12µV/˚C range. A further complication encountered in the use of
thermocouples is the cold-junction compensation.
Where the thermocouple wire is terminated to the copper DAS-8 Series
connections, a second thermocouple junction is formed by the two
dissimilar metals. As the temperature of this connection varies, an error is
introduced. The error can be mathematically compensated for by sensing
the connector temperature using a semiconductor sensor on another
channel and correcting the thermocouple readings using software. This is
required only at the highest levels of accuracy, since in most cases
connector temperature (usually room temperature) varies little.
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Setup and Installation