Reference Manual
748384-C
September 2003
Rosemount Analytical Inc. A Division of Emerson Process Management
Introduction 1-9
NGA2000 Reference
1-4 ZERO AND SPAN GAS SETTINGS
It is essential that the zero and span gases are
correctly described to the analyzer. It zeros and
spans by making the reading it is getting from its
measurement signal equal to what it has been
told is the zero or span gas value. If it is zeroing
or spanning ranges separately it may well pro-
duce bogus answers if the zero or span gases
are entered incorrectly.
Zero and span gas values are entered into the
Calibration gases menu under Expert controls
and setup.
a. Basic
Instructions
Select the analyzer you want to zero.
Press "MENUS" or "HOME" to enter the
Main menu
.
Select Expert controls and setup
Select Analyzer module setup…
Select Calibration gas list…
Edit the zero and span gases until they refer
to the values on the bottles.
Press "HOME" to return to the
Main menu
.
b. Advanced
Instructions
Zero gases are normally simply of zero con-
centration, but in some cases it is desirable
to "zero" off a non-zero gas. In this case, it
may be necessary to iterate between zero
and spanning a few times until the results
are accurate enough.
Gas cylinders are notoriously inaccurate.
Even when they are specified at
±
2% certi-
fied accuracy, you can still have consider-
able errors. Some users prefer to zero and
span off gases of mediocre quality, but to
"name" the gases by comparison with a
known high precision reference gas. In this
case, carefully zero and span the analyzer
on the reference gas, and then measure the
"span" gas with the analyzer. Enter the re-
sult as the span gas "name" both on the
bottle and in the
Calibration gas list
. When
it is time to replace this bottle, "name" the
replacement in the same way.
The span value has to be between 20% and
110% of the fullscale range. Ideally it
should be about 80% of the range.
The FID analyzer can be set to be cali-
brated on various kinds of hydrocarbons,
such as methane or propane. Its response
factor must be adjusted accordingly. On the
same
Calibration gas list
screen, the FID in-
cludes a line marked
Calibration gas HC re-
sponse factor:.
Edit this to the desired
response factor, 1 for methane, 3.14 or so
for propane. The analyzer will then regard
the span concentration as referring to that
kind of gas.
c. Details
The analyzer compares its reading on the
span or zero gas with the value entered into
the
Calibration gas list
value for the range it
happens to be on. It adjusts its internal
zero and span factors so as to make its
reading correspond to the value entered as
the span or zero gas concentration. It is
therefore vital that the value entered be cor-
rect.
The
Calibration gas list
is located under the
Expert controls
menu so as to keep it away
from the lower skilled operators. Entry of an
incorrect value will completely invalidate the
analyzer's readings thereafter.
Network variables: SPANGAS, ZERO-
GAS, NOSPANGAS, NOXSPANGAS,
ZERO, SPAN, CARBON_ATOMS,
NO_NOX
