Rosemount 248, Reference Manual

Page: 30 / 80

Reference Manual
00809-0100-4825, Rev BB
August 2005
Rosemount 248
3-6
Set Output Units
The Set Output Unit command sets the desired primary variable units. Set the
transmitter output to one of the following engineering units:
• Degrees Celsius
• Degrees Fahrenheit
• Degrees Rankine
• Kelvin
• Ohms
• Millivolts
50/60 Hz Filter
The 50/60 Hz Filter command sets the transmitter electronic filter to reject the
frequency of the AC power supply in the plant.
Terminal Temperature
The Terminal Temp command sets the terminal temperature units to indicate
the temperature at the transmitter terminals.
Process Variable (PV) Damping
The PV Damp command changes the response time of the transmitter to
smooth variations in output readings caused by rapid changes in input.
Determine the appropriate damping setting based on the necessary response
time, signal stability, and other requirements of the loop dynamics of the
system. The default damping value is 5.0 seconds and can be reset to any
value between 0 and 32 seconds.
The value chosen for damping affects the response time of the transmitter.
When set to zero (or disabled), the damping function is off and the transmitter
output reacts to changes in input as quickly as the intermittent sensor
algorithm allows (refer to “Intermittent Threshold” on page 3-12) for a
description of the intermittent sensor algorithm). Increasing the damping value
increases the transmitter response time.
With damping enabled, the transmitter outputs values according to the
following relationship. At time t
Damping Value =
At the time 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.
After one damping time constant following a sensor input step change, the
transmitter output will be at 63.2% of that change. The output will continue to
approach the input according to the damping equation above.
Fast Key Sequence 1, 3, 2, 1, 2, 2
Fast Key Sequence 1, 3, 5, 1
Fast Key Sequence 1, 3, 2, 2
Fast Key Sequence 1, 3, 3, 1, 3
P N P –
( )
1 e
t
–
T
----
–
⎝ ⎠
⎜ ⎟
⎛ ⎞
×
+
P =previous damped value
N =new sensor value
T = damping time constant
U =update rate