Sub ppm and ppb detection are not just business as usual Presented by ETA Associates and Interscan Corporation KEY POINT Once you enter the world of low concentration measurements issues that previously were hidden are now magnifiedand are thus all too apparent ID: 598988
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Slide1
Issues with low concentration gas detection in ambient air:
Sub-
ppm
and ppb detection are not just business as usual. Slide2
Presented by:
ETA Associates
and
Interscan CorporationSlide3
KEY
POINT
Once you enter the world of low concentration measurements, issues that previously were hidden are now magnified—and are thus all too apparent.Slide4
Compliance levels are always being lowered
So this is…
Far from just an “academic” concernSlide5
Calibration issuesSlide6
At best, it will be difficult!Permeation tubes, usually not cylinder gasSlide7
A permeation tube is a sealed permeable membrane containing solid, liquid or liquefiable gases that permeate through the walls of the membrane at a constant rate.Slide8
The device is maintained at a constant temperature to establish constant vapor pressure inside the device.
The vapor escapes through the walls of the permeable membrane at a constant rate as long as the set point temperature is maintained. Slide9
A measured flow of an inert gas is passed through the permeation chamber resulting in known volumetric concentration in ppm/ppb.
By varying the dilution flow rate one is able to generate a wide range of concentrations using a single device.
Slide10Slide11
There is an alternativeElectronic Calibration Service (ECS)Slide12
A certified spare sensor is kept on hand, to be put into the instrument, while the presently used sensor is sent back to the factory for re-certification.
The
ECS
certification details zero and span adjustments that are to be made on the instrument, to set it up with the specified newly certified sensor. Slide13
For some gases…Even permeation tubes are not available
e.g. H2O
2Slide14
In such cases….
Specialized lab techniques will be required
With ECS or a surrogate to the customerSlide15
Zero gas issuesSlide16
Zero gas must
Contain ≤ 1% of expected value of target gas
Exclude potential interferents, as necessarySlide17
BUT
This may not always be possibleSlide18
Consider best availablezero gas
Total hydrocarbons < 0.01 ppm
CO < 0.01 ppm
NO
x
< 0.001 ppm
SO
2
< 0.001 ppm
N
2
O < 0.001 ppmSlide19
Thermo Model 48i offers 0-1 ppm measuring range for CO
But, a reading of 0.2 ppm would be subject to a 5% error--just from the zero gas
And what about the calibration error?
mnSlide20
With gases such as SO2
The situation is even worse, for very low range measurementsSlide21
Check out our Knowledge Base article
“In search of zero”Slide22
Interference issuesSlide23
Interferences that pose negligible problems at higher concentration measurements…
Can become significant at lower range measurementsSlide24
Consider this example…
The effect of NO
2
on SO
2
measurements
OSHA
PEL
for SO
2
is 5 ppm
Slide25
In our sensor…
24 ppm NO
2
will produce a 1 ppm negative effect on SO
2
And, that’s not so bad
Slide26
But, if you wanted to read
0.1 ppm
The effect has been magnified by a factor of ten
And, can now become a problem
Slide27Slide28
Joel MyersonSpecial thanks to…
Caroline
Ronten