FEC920: USER GUIDE
Page 74
September 18
For mA inputs this offset is removed automatically, without user intervention.
For mV inputs the offset depends on the value of the voltage source impedance and is
equal to 199.9
ȝ
V/
ȍ
. This offset can be compensated for either by using this Offset2
parameter, or by carrying out the ‘Input Adjust’ procedure (Section 6.1.9).
For Dual T/C inputs, it is recommended that the ‘Input Adjust’ procedure be used instead
of Offset2 as the use of Offset2 results in an offset which is non-linear over the
thermocouple range.
Input filter
Damping can be used to filter out noise from slowly
changing signals so that the underlying trend can be
seen more clearly. Valid input values are between 0
and 60 seconds.
CJC Type
For thermocouple input types only, this allows the user to select ‘None’, ‘Internal’,
‘External’ or ‘Remote 1’ to ‘Remote 4’. For Dual T/C inputs, both primary and secondary
inputs use the same cold junction.
None: No Cold junction compensation applied.
‘Internal’ uses the instrument’s internal cold junction temperature measurement.
‘External’ means that the cold junction is to be maintained by the user, at a fixed, known
temperature. This temperature is entered in the ‘External CJ Temp’ field which appears
when ‘External’ is selected.
Remote 1 (2) (3) (4) means that the cold junction temperature is being measured by input
channel 1 (2) (3) (4) respectively. (This must be a different channel from that currently
being configured).
Ext. CJ Temp
Appears only if CJC type is set to ‘External’, and allows the user to enter the temperature
at which the external cold junction is being maintained.
Sensor Break Type Defines whether the sensor break becomes active for circuit impedances greater than
expected.
‘Off’ disables Sensor Break detection.
Break Low: Sensor break active if measured impedance is greater than the ‘Break Low
impedance’ value given in Figure 68.
Break High: Sensor break active if measured impedance is greater than the ‘Break High
Impedance’ value given in Figure 68.
For mA inputs, limits are applied, such that if the process value lies outside these limits, a
sensor break is assumed to have occurred. These limits are (Input lo - 4% Span) and
(Input high + 6% Span). For example, for a 4 to 20mA signal, an input below 3.36mA or
above 20.96mA will trigger a sensor break event.
Figure 68 Minimum impedances for sensor break detection
Note:
Applying a filter to an input channel can affect the operation of any Rate-of-change alarms
configured to act on that channel.
Input
3x
Input filter
Instrument
Response
Range
Break Low
impedance
Break High
Impedance
40mV
80mV
2V
10V
~5k
ȍ
~5k
ȍ
~12.5k
ȍ
~12.5k
ȍ
~20k
ȍ
~20k
ȍ
~70k
ȍ
~120k
ȍ
A5E45696052A Rev-AA
