MiniPuck Model ST132-06x0 Two-Wire TC/mV Transmitter
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Acromag, Inc. Tel:248-295-0880 Fax:248-624-9234 Email:
19
Turn Cold Junction Compensation (CJC)
On, or Off…
This model embeds a very accurate temperature sensor in the space
between the input + and
– terminals in order to cold-junction compensate
the thermocouple signal. To explain, the voltage measured from the T/C
reflects the difference in temperature between each end. Thus, in order to
discern the actual temperature being sensed, it is necessary to know the
temperature at the other end, and this is usually referred to as the Cold
Junction.
Further, the connection between the thermocouple and the copper terminals
of the cold junction introduces additional thermocouples into the circuit.
However, because these errant thermocouples that occur at the junctions of
the ± terminals are close together and at identical temperatures, their effect
on the principal measurement cancels out of the derivation. But to keep
error to a minimum, you should still avoid any environmental or installation
effects that could drive a difference in temperature between the ± input
terminals. For example, touching one terminal and not the other.
You normally turn CJC On via this control for making T/C measurements,
but you can elect to turn it off temporarily, if you wish to calibrate the T/C
input using a voltage source that connects to the module via copper wires
(as opposed to the wire materials of the various thermocouple types).
Note: CJC temperature values are only resolved to 0.1°C using the internal
lookup tables for the T/C type. As such, units configured for small input
spans may appear less accurate with CJC ON, as ±0.1°C becomes a
greater percentage of a smaller span. Keep this in mind when resolving
measurements with short spans and high gains.
Set the I/O Scaling (You can set this up to be Reverse Acting, too)
…
This control is used to map your input range, or a portion of your input
range, to the nominal 4mA (0%) and 20mA (100%) output range endpoints.
Refer to the specifications to determine the full input range capability of the
various input types. Then select valid input range endpoints to map to 4mA
and to 20mA. Note that it is possible to exchange the order of these value
assignments in order to define a reverse-acting output signal.
You need to select the input temperature or millivoltage that is to correspond
to 4mA of output signal and type this value into the corresponding field for
4mA output. You also need to do this for the 20mA output endpoint.
Note:
Note that the effective input resolution does not rescale itself for input
spans smaller than the nominal input range. That is, input resolution
diminishes proportionally as you reduce the input span with smaller input
ranges than nominal (see Specifications section for nominal ranges and
resolution).
Note that some under-range and over-range is built-into the unit, as the
output can swing as low as 3.5mA, and as high as 24mA. Actual endpoint
limits will vary slightly between units.
CONFIGURATION
STEP-BY-STEP
Reconfiguration