FACTS Engineering Direct Logic 405, Manual

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INSTALLATION AND WIRING 2.7
MAXIMIZING THERMOCOUPLE MEASUREMENT PRECISION
Ambient Temperature Variations
The FACTS Engineering Temperature Controller module has been designed and tested to operate within
specifications over a 0 to 60
E
C ambient temperature range.
Thermocouple cold junction compensation is calibrated for operation in a still air environment. If used in
an application with forced convection cooling (such as an enclosure with air circulated by a fan) then 2-3
E
C of error is typically introduced into the cold junction compensation circuit. In this case, ladder logic
can be used to correct SetPoint and indicated temperature values.
To maintain specifications, external devices must be prevented from creating a temperature gradient
across the terminal block. If an external device releases heat into one end of the terminal block, small
channel to channel reading variations may occur. Heat generating devices should generally be mounted
above and away from the Temperature Controller.
If the module is exposed to abrupt temperature changes (such as opening the control system enclosure
door) it may take several minutes for the automatic cold junction compensation circuit and terminal block
to stabilize. Errors introduced due to abrupt ambient temperature changes normally will be less than 4
E
C.
Thermocouple Wiring Distance Limitations
The maximum resistance of the thermocouple should be kept below 1000
S
. This will assure that errors,
induced by the small amount of burn-out detection current which flows through the thermocouple, are
less than .3
E
C.
The following chart shows the maximum allowable distance for the Type 'K' thermocouple. The Type 'K'
can be considered the worst case since it has the highest ohms per foot among the supported
thermocouple types. For more information refer to the wire specifications of the thermocouple
manufacturer.
WIRE GAUGE OHMS PER FOOT MAXIMUM DISTANCE
20 .586 1706 feet
24 1.490 671 feet
26 2.381 420 feet