General Zero/Span Setup; Calibrate Ranges Separately Or Together; Calibrate Ranges Using An Autocal Module - Emerson NGA2000 Reference Manual

Reference Manual
748384-C
September 2003
1-5

GENERAL ZERO/SPAN SETUP

During a zero or span the analyzer reads the
signal from the gas detector, calculates the ap-
parent gas level, compares it with the desired
level as entered in the Calibration gas list, and
modifies the internal calibration factors until the
reading is within a small error value of the de-
sired value. The error is 0.02% of the upper
range value.
During the calibration sequence the analyzer
monitors the reading over a period of time set
by the CALTIME variable, located in the Calibra-
tion parameters. screen under Analyzer Module
setup .... If it detects a drift of the reading, it will
wait for a longer time until the reading is stable,
or until the time out period set by CALTIME-
OUT, also located in the Calibration parameters
menu as Calibration time out: This value is nor-
mally set at about 2 minutes. The analyzer will
perform its zero or span anyway at the end of
the timeout time.
If the gas value is wrong, or the analyzer has
drifted excessively, the analyzer will check the
reading on the calibration gas against the
named value in the Calibration gas list, and if it
is set to check its limits, it will not calibrate.
Conversely, if the analyzer is set to ignore limit
checking, it will calibrate anyway, unless the
span gas value is outside the allowed range of
20% - 110% of the range value.
You can select whether to zero or span all the
ranges together or separately. If you make
them separate, unless this is the first time that
this analyzer has ever been calibrated, it will
only zero or span the range that you are on.
Other ranges will not be affected. If you elect to
zero or span them together, the results will de-
pend on the gas values and the range values al-
ready set in the analyzer.
If the ranges are all zero based, and the zero
gas is of zero concentration, the analyzer will
have no difficulty zeroing all the ranges at once.
It does this by changing the range to range 1,
zeroing it, changing to range 2, waiting for sta-
bility and zeroing that, and so on.
When it tries to span the ranges, if the span gas
is above 10% and below I/O% of the range
1-10 Introduction
NGA2000 Reference
value on any range, it will span that range, but if
not it will not perform a span. Thus if the span
gas is 900 ppm, and the ranges are 10, 100,
1000 and 10,000 ppm, and you are on range 3
(0 - 1000 ppm), it will only span range 3, even if
you tell it to span all the ranges together. In
general, with widely spaced ranges such as this
example, it is best or indeed essential to span
the ranges separately.
Bear in mind that the NGA analyzers have very
good linearity and dynamic range, and it is nor-
mally quite practical to operate them over a dy-
namic range of 30:1 or more on a single range,
if you are using a digital output.
a.

Calibrate Ranges Separately Or Together

From the Main menu:
Select Expert controls and setup
Select Analyzer module setup
Select Calibration parameters
Edit the zero and span calibration lines as
desired.
b. Calibrate Ranges Using An Autocal
Module
There are two Autocal modules available for
the NGA systems (other than the on-stack
sample control module). One of them is
designed to work with a single analyzer,
and to calibrate individually its separate
ranges, the other is designed to work with
up to four analyzers, providing up to four
span gases to span any number of analyzer
ranges. You can set these modules up to
zero and span the analyzers on a timed ba-
sis, and you can make them perform a zero
or span or both at will. You achieve the lat-
ter by entering their control screen, under
Expert controls and setup then Auxiliary
module controls.. ., then select the Autocal
module. If you have already setup the
module to zero and span as you desire, you
can simply press the "START" button and it
will put the gases on the analyzer(s), wait
the appropriate time, zero them and span
them. It is a common error to assume that
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