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The TX787 and T788 is capable of performing many functions in addition to the simple example outlined in the Quick Start exercise.
Filtering is provided to optimize the stability and response time of the transmitter output signal. The filter is set by specifying a Filter Band and a Damping
Time Constant. To simplify the definition of the filter values, click on the "Set to Optimal" button on the configuration screen.
T
he Filter Band allows the transmitter to be configured to react quickly to significant changes in input while smoothing out small changes. This band
defines the range above and below the current input reading within which the Damping Time Constant will be applied. It is defined as a percent of
the configured span of the transmitter. For example, a 0.5% Filter Band on a transmitter configured for an input range of 200 to 700 deg F would mean
that the Damping Time Constant will be applied to input changes of less than 2.5 deg but that changes of more than 2.5 deg would be immediately
reflected in the output. In general, the larger the full scale range, the smaller the Filter Band should be. Conversely, a larger Filter Band is generally
appropriate for smaller input ranges. Clicking the "Set to Optimal" button triggers the calculation of an appropriate setting based on the currently defined
input range.
Damping defines the length of time, in seconds, that the input will be averaged over to determine the output value when the input change is less than
the Filter Band. Changes in the input that are greater than the Filter Band bypass the Damping filter and are immediately reflected in the output. The
default value of 15 seconds, when used with the optimal Filter Band yields a very stable output while maintaining a fast response to significant changes in
the input.
Because of slight variations between sensing elements or other loop characteristics, it is sometimes beneficial to "trim" the transmitter so that its output
corrects for these effects. If the measurement error is known, the transmitter may be trimmed "off-line." "ON-line" trimming allows the characteristics of an
individual sensor to measure and match to the transmitter to which it will be connected in the field. Trimming allows the basic calibration of the transmitter to
be left intact.
This option allows a loop to be tested dynamically verifying control logic as well as wiring. The transmitter's current loop output can be controlled via four
parameters: simulate an input value in raw units (ohm, mV, etc.); simulate an input value in engineering units (e.g. 450 DEG); set an ouput in percent; or set
an ouput in mA. After an initial simulation is selected, the ouput may be incremented or decremented in 1% or 10% steps.
The transfer function is the "formula" that the transmitter uses to determine the appropriate output signal for a given input signal. A "Linear" function means
that output is changed in a straight line fashion as the input changes within the configured input zero and full scale range: 0% of range input yields 4 mA
output, 25% of range input yields 8 mA, etc. A "Standard" function means that the transmitter will apply a standard linearization curve for temperature inputs.
The accuracy of RTD temperature sensing can be increased significantly when the temperature transmitter is "matched" to the individual sensor using a
method called Callender Van Dusen linearization. Doing so requires entering the sensor's Alpha, Beta (measured for temperatures above zero and for
temperatures below zero), Sigma, and Ro values into the transmitters' configuration. These values can be requested from the manufacturer when the
sensor is purchased or may be measured by a testing lab.
Select this function if the transmitter's output is to vary with the square root of the input.
The transmitter can use up to a 10th order polynomial equation (y = A
into the transmitter's configuration.
The transmitter can use a User Defined table by specifying a .tbl file. The data in the file should be two columns, tab delimited. Up to 140 data pairs may
be listed. The input values in column one must be equally spaced.
E
ach unit may be assigned an I.D. Tag name of up to eight characters, Job Number up to eight characters, and a "Message" of up to sixteen characters;
stored in the transmitter's memory. Click on the desired field, enter the text, and click the download icon.
+ A
x + A
x
2
+ ... + A
x
10
) as the transfer function by inputting the coefficients
0
1
2
10
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