Model TT231-0600
Two-Wire RTD Transmitter w/USB
Acromag, Inc. Tel: 248-295-0880
- 23 - http://www.acromag.com
- 23 -
http://www.acromag.com
Over-Scale & Under-Scale Thresholds
TIP – Namur Limits:
For Namur
compliant output limits, you
generally need to produce a linear
output range from 3.8mA to
20.5mA, and have a failure high
limit greater than or equal to
22.5mA, and a failure low limit less
than or equal to 3.6mA.
TIP – Error Detection:
Note that a
checksum error can be
distinguished at the output signal
from a lead break error by
selecting an under-scale limit that
is greater than the minimum
threshold setting. This is because
a checksum error always sends the
output signal to a level that is
0.4mA below the lowest threshold
setting until reset (~1.8mA). If you
select an under-scale threshold
value greater than the minimum,
then you ensure that the
downscale alarm level limit (0.4mA
below the threshold) does not
overlap with the checksum error
level indication.
Select The Over/Under-Scale Thresholds & Alarm Levels…
This unit allows you to select over-scale and under-scale output range thresholds
which determine the linear operating range of your output. They also indirectly
define the upscale & downscale alarm/error limits, as the downscale detent will be
set to a current level ~0.4mA below the under-scale threshold, and the upscale
detent will be set approximately 1.0mA above the over-scale threshold. In this way,
a lead break or open sensor fault can be easily discerned from simply an over-range
or under-range input signal.
The range of adjustment for the under & over-scale thresholds is calibrated at the
factory and indicated via the “Min” and “Max” value fields adjacent to the slide
controls. Note that the threshold levels can vary as much as 10% of span between
units for the same digital setting, and this will be reflected by differing values for
Min and Max between units. The Min/Max limits of adjustment are calibrated at
the factory.
CAUTION:
For a low resistance or shorted load, and a high loop supply voltage,
excessive over-range current does drive excessive power dissipation in the output
pass transistor of the transmitter and will cause the unit to get warm. This could be
troublesome at elevated ambient temperatures and in hazardous environments,
particularly for output currents near 30mA.
Use the
Under-Range Limit
slide control to select an approximate under-scale
threshold. You have 8 levels of under-scale threshold adjustment between Min
& Max, typically between 2.1mA and 3.6mA. Your selection will be indicated in
the field just above the control.
Use the
Over-Range Limit
slide control to select an approximate over-scale
threshold. You have 16 levels of over-scale threshold adjustment between Min
& Max, typically between 21mA and 30mA. Your selection will be indicated in
the field just above the control.
After making your adjustments, click the
“Submit O/U Configuration”
button to
write your adjustments to non-volatile EEPROM memory.
The linear operating range of your output is now defined between the limits you
specified. Your under-scale and over-scale thresholds indirectly correspond to a
linear operating reqion that usually extends outside of the input zero and full-scale
limits you specified. Additionally, the sensor fault/break detent output levels are
set outside the linear operating region so that you can discern them from simply an
over-range or under-range input signal.
You should check your under-scale and over-scale threshold levels. For example,
you could disconnect an RTD lead to check your O/U alarm limits, which should be
~0.4mA below the under-scale threshold for a downscale break, or 1mA above your
over-scale threshold for an upscale break.