Rosemount 3491, Reference Manual, Page: 49 / 152

Reference Manual
00809-0100-4841, Rev. AA
May 2007
4-17
Rosemount 3490 Series
P127 is a low cut-off parameter that allows the user to force the channel 2
output (D852) to be zero when the calculated value falls below a user defined
value:
Table 4-4. P127 Options
This feature is particularly useful in an Open Channel Flow application (OCF)
where a small standing or remaining liquid level in the channel would cause
continued totalising of flow when no actual flow exists. For example, set P127
to a +ve (positive) value of flow units, usually around 2% of maximum flow, to
overcome this problem.
P210 allows the user to apply damping to the calculated value of D852.
The final output of Channel 2 is readable on D800 and is the 3490’s second
PV, which is not shown on the factory default display.
Figure 4-16. 4-20mA Txr Input
Blocks and Parameters (Ch. 2)
Notes:
Optional damping of mA readings:
mA = [mA ((mAnow - mAbefore)/(1+10 * P321))], ten times a second
P121 Channel 2 Input Source
Select "mA In 1" from list of options. (All other options are for HART.)
P122 Channel 2 Input Offset
Use for optional adjustment to the D842 value.
P123 Channel 2 Profile Selection
Keep the "Scaled" option unless requiring a profile calculation (see Section
“Profile Calculations for Contents and Flow Applications“ on page 4-24.)
P127 Channel 2 Output Action
+ve value Drop immediately to zero.
None Continue measuring.
0.0 Hold output at zero.
-ve value Rise immediately to zero.
Conditioning
and damping
mA Input
P321
Normalise
mA to %
Input
Source
Select
P121
D842
mA mA
% %
Apply
Input
Offset
P122
Channel
Profile
Select
D802
(TV)
D840
Apply
Input
Scaling
P124
% % %
mA
D801
(SV)
P123
Low
Cut-off
Check
Scaled
Pr
o
fil
e
s
Non-Linear Profile Post Scale NLP
(PV)
D800
4-20mA Limit
Alarm Check
<3.7mA
>20.75mA
P127
P125 P126
D852
(CH2 O/P)
P121 = “mA in”
PV
Damping
P210