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12.1 OVERVIEW
This section is a general description of how the Model
5081-T Transmitter operates. This section is for those
users who desire a greater understanding of the trans-
mitter’s operation.
12.2 CONDUCTIVITY
The conductivity sensor produces a “conductance signal”
that is proportional to the conductivity of the process
solution. The transmitter subtracts a baseline zero con-
ductivity signal from the sensor signal and multiplies the
result by the cell constant and the cell factor. This
absolute conductivity is then corrected to the reference
temperature (usually 25°C) using the process tempera-
ture measured by a RTD located in the conductivity sen-
sor. In the “n SALt”, “CAtion” and “rStvty” modes, the
Model 5081-T automatically calculates the amount of
correction needed.
In conductivity mode “LInEAr”, the microprocessor also
adjusts the amount of correction required for temperature
compensation by means of a temperature slope adjust-
ment. This slope may be adjusted between 0 to 5%/°C
either manually via the Infrared Remote Control Keypad
or automatically during bench or process calibration. This
slope controls the amount of correction required in the
temperature compensation circuit, and is specific to the
process, giving you the most accurate conductivity read-
ing possible.
12.3 HART COMMUNICATION
A MODAC (An application specific Integrated Circuit) is
connected across the current loop to read and transmit
the superimposed HART communications. The transmit-
ter communicates via the HART protocol which uses an
industry standard BELL 202 frequency shift keying (FSK)
technique. This FSK signal is an AC signal, whose fre-
quency is shifted higher or lower, depending upon the
condition of the digital signal (High or Low). This commu-
nication conforms to the Rosemount HART
®
specifica-
tion and is used to configure and interrogate the trans-
mitter.
12.4 OUTPUT LOGIC
Normal transmitter operation specifies that the output
tracks the process. However, the transmitter can be put
into other modes of operation.
These modes are:
Fault Mode (in the event of a fault). Sets the transmit-
ter output to the value set during configuration. (Between
3.80 and 22.00mA). This mode is over-ridden by the
HOLD or TEST modes.
Hold Mode (manually placed in hold). Holds the output
current to the value set during configuration. This value
may be between 3.80 and 22.00 mA.
Hold mode supersedes the fault mode value. The current
output measurement is “Frozen” while the transmitter is
in the Hold Mode.
Test Mode (manually placed to test output). Can only
be accessed through the Program menu, and is only
active during the time the prompt is visible.
Output is set to the entered value and supersedes the
Hold and Fault modes, if such exist.
Test mode also disables the normal timeout feature (2
minutes after the last keystroke is made) for 20 minutes.
Timeout. The display will normally timeout and default to
the Main Display two (2) minutes after the last keystroke
is made.
While the output is being tested, or if a 2-point calibration
is being performed, the timeout is adjusted to 20 minutes.
If a custom curve is being programmed, no timeout will
be applied.
SECTION 12.0
THEORY OF OPERATION
MODEL 5081-T SECTION 12.0
THEORY OF OPERATION