Reference Manual
748384-C
September 2003
Rosemount Analytical Inc. A Division of Emerson Process Management
Introduction 1-23
NGA2000 Reference
1-10 RESPONSE TIME
The analyzer has a natural response time based
on its gas passage design, and the response
time of its sensor. However it is possible to add
additional filtering to the signal, which will have
the effect of increasing the response time while
reducing the noise level.
The natural response time may be modified by
changing the flow rate through the analyzer.
This affects both the rise time and the delay
time - the rise time is the time it takes from the
beginning change in the output to the time that
90% of the final change has occurred, while the
delay time is the time from the introduction of
gas to the back of the analyzer to when that ini-
tial reading change occurs. The delay time will
be reduced by increasing the flow rate, and the
rise time may also be reduced in the same way,
but only for flow sensitive analyzers such as the
NDIR types. Note that increasing the flow may
also change the response level, due to pres-
surization of the sample cell. The term “re-
sponse time" is loose - sometimes it can refer to
the rise time and sometimes to the overall re-
sponse, rise time plus delay time. In the NGA it
is used to refer to the rise time only.
In general, increasing the amount of filtering by
editing the response time setting will first in-
crease the order of the "Median" filter (this will
also slightly increase the delay time), and then
to increase the time constant of the third order
infinite-impulse-response (IIR) software filter.
The IIR filter is disabled for response times be-
low about 0.5 seconds. Setting the response
time to zero will completely disable all filtering.
You should normally set the response time for
the longest possible value for your application.
The delay time should be set so as to delay the
fastest responding analyzer in the system so
that it responds at the same time as the slowest.
The delay you need to be concerned with is the
time from a sample gas change at the entry port
to the time the reading just begins to move, not
until it reaches its full change. You are then
compensating for differences in sample path, as
well as differences in analyzer performance.
The delay time adjustment was disabled in V2.2
of the PMD and NDIR analyzers, but enabled
for V2.2.1 and later. Also on this screen is the
LON update rate control. This sets the speed at
which this particular analyzer updates its PVA
variable -the primary or process variable, the
main reading - over the LON. In almost all
cases, setting this to 10 per second will be satis-
factory. You can set it to ASAP, in which case it
will update the PVA at up to 33 Hz, but doing so
may make the system less responsive, particu-
larly if you have several analyzers all on the
LON together. The symptoms of excessive
network traffic include difficulty getting PCs to
update variables, and occasional lost menus -
the system will seem to lock up, and only the left
arrow key will work, then it will apparently come
back to life again. They also include random
failures generally in the network, such as I/O
modules that do not respond to their analyzers.
If this latter occurs, set the LON update time as
above and then if the symptom does not go
away immediately, rebind the I/O modules.
Network variables: AFT90_, AMDELAYTIME,
LONPVUPDATE
a. Basic
Instructions
Set the analyzer flow rates as desired.
Enter the
Main menu
.
Select
Expert setup and controls...
Select
Analyzer module setup.
Select
Gas measurement parameters...
Select
delay/response time parameters...
Edit the response time for each range as
desired.
Edit the delay time as desired for the fastest
analyzers in the system.
Edit the LON update rate for the desired
rate, normally 10 per second.
