Emerson Rosemount 644, Reference Manual

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Reference Manual
00809-0200-4728, Rev RA
Section 2: Configuration
April 2015
Configuration
The value chosen for damping affects the response time of the transmitter. When set to zero
(disabled), the damping function is off and the transmitter output reacts to changes in input as
quickly as the intermittent sensor algorithm allows. Increasing the damping value increases
transmitter response time.
With damping enabled, if the temperature change is within 0.2% of the sensor limits, the
transmitter measures the change in input every 500 milliseconds (for a single sensor device) and
outputs values according to the following relationship:
At the value to which the damping time constant is set, the transmitter output is at 63% of the
input change and it continues to approach the input according to the damping equation above.
For example, as illustrated in Figure 2-14 , if the temperature undergoes a step change—within
0.2% of the sensor limits—from 100 degrees to 110 degrees, and the damping is set to 5.0
seconds, the transmitter calculates and reports a new reading every 500 milliseconds using the
damping equation. At 5.0 seconds, the transmitter outputs 106.3 degrees, or 63% of the input
change, and the output continues to approach the input curve according to the equation above.
For information regarding the damping function when the input change is greater than 0.2% of
the sensor limits, refer to
“Intermittent Sensor Detection” on page 35 .
Figure 2-14. Change in Input vs. Change in Output with Damping Set to 5 Seconds
Damped Value
N P
–
( )
2
T U –
2
T U
+
------------------
 
 
× P +
=
P = previous damped value
N = new sensor value
T = damping time constant
U = update rate