Emerson NGA2000 Reference Manual page 28

Reference Manual
748384-C
September 2003
shown on the bottom of the screen. This
will produce a zero output on the gas you
have flowing.
Autocal Module Present
Select the analyzer you want to zero.
Enter the Main menu.
Select Expert controls and setup
If you have not configured the Autocal mod-
ule, do so.
Select Auxiliary module controls...
Press "START".
Alternatively, select Manual calibration...
Select the gas desired, the range desired
and the operation desired, then press
"START".
c. Details
Certain analyzers can take a long time to
get a good zero reading. If the analyzer has
been exposed to a high level gas, it may
take many time constants to get down to
zero again. This means that you should be
careful about doing a "Zero - span - zero"
check: the analyzer may not read the same
thing on the second zero, simply because it
has not had a chance to get back to it. The
NGA analyzers are capable of reading
gases with a great deal of precision, so you
can easily see if the zero reading is not cor-
rect, and this can be confusing.
Two kinds of analyzer are particularly prone
to zero problems. FID analyzer measure
hydrocarbons, and they are more respon-
sive to high molecular weight hydrocarbons
than anything else. Oils, in particular, pro-
duce high readings due to the large size of
the molecules. If there are oils or other high
molecular weight hydrocarbons in the sam-
ple system, they will stick to the sample tub-
ing walls and take a very long time indeed
to evaporate away. If you expose the ana-
lyzer to a really high concentration of such
materials, such as by spilling gasoline near
1-14 Introduction
NGA2000 Reference
the sample intake, you may have to disas-
semble it and clean all the plumbing before
it will read a good zero again.
The symptom of hydrocarbon contamination
is that the analyzer reads high on zero gas,
and this reading very slowly decays over a
matter of weeks. You can speed up the
process by cleaning out the tubing with al-
cohol and distilled water, or by flowing ar-
gon through it while heating the tubing to
red heat with a flame. Needless to say, this
should not be done to Teflon tubing! You
should use stainless steel components and
tubing, and the tubing should be cleaned
before use. Otherwise, if you do use Teflon
or copper tubing, and you do not take pre-
cautions, you will suffer from this bleeding
down effect for a long time.
Trace oxygen and water vapor analyzers
have similar zero problems. Both water va-
por and oxygen are present in the air at
very high vapor pressures, and thus will
leak or diffuse into sample systems with
great ease. Water vapor will stick to walls
even worse than hydrocarbons do, and may
take many weeks to be removed. Both wa-
ter and oxygen will readily diffuse through
any plastic components, and you will not be
able to reach levels around 1 ppm unless
you make the entire sample system out of
stainless steel, including valve and regulator
diaphragms. Even when you have made
the sample system perfect, you will find that
getting down to sub 1 ppm levels may take
several days. After exposure to air for a
short time, for example while replacing a
sensor, the system may take a day to come
back to zero.
For these reasons, zeroing trace analyzers
is not an easy thing to do. If you go ahead
and zero them anyway, you may find that
you get negative readings as the sample
system cleans out, and your readings
around the zero point may be quite unreli-
able.
Trace levels around the 10 ppm value are
normally comparatively unaffected by the
above considerations, unless you have
made a gross error in the sampling system.
Rosemount Analytical Inc. A Division of Emerson Process Management