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Issues with low concentration gas detection in ambient air:
Sub-ppm and ppb detection are not just
business as usual.
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Presented by:
ETA Associatesand
Interscan Corporation
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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.
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Compliance levels are always being lowered
So this is… Far from just an “academic” concern
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Calibration issues
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At best, it will be difficult!
Permeation tubes, usually not cylinder gas
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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.
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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.
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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.
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There is an alternative
Electronic Calibration Service (ECS)
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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.
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For some gases…
Even permeation tubes are not available
e.g. H2O2
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In such cases….
Specialized lab techniques will be required
With ECS or a surrogate to the customer
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Zero gas issues
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Zero gas must
Contain ≤ 1% of expected value of target gas
Exclude potential interferents, as necessary
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BUT
This may not always be possible
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Consider best availablezero gas Total hydrocarbons < 0.01 ppmCO < 0.01 ppmNOx < 0.001 ppm
SO2 < 0.001 ppm
N2O < 0.001 ppm
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Thermo Model 48i offers 0-1 ppm measuring range
for COBut, a reading of 0.2 ppm would be subject to a 5% error--just from the zero gas
And what about the calibration error?mn
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With gases such as SO2
The situation is even worse, for very low range measurements
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Check out our Knowledge Base
article
“In search of zero”
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Interference issues
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Interferences that pose negligible problems at higher
concentration measurements…
Can become significant at lower range measurements
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Consider this example…
The effect of NO2 on SO2 measurements
OSHA PEL for SO2 is 5 ppm
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In our sensor…
24 ppm NO2 will produce a 1 ppm negative effect on SO2
And, that’s not so bad
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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
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•Calibration issues1•Zero gas issues2•Interference issues3
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Joel Myerson
Special thanks to…
Caroline Ronten