Page 316 - gas transport in porous media

Chapter 18: Measurement of Vapor Concentrations
Type 1a -
Flourescence or phosphorescence for direct
spectroscopic measurement of contaiminant 315
Light
source/
Detector
Type 1b - Refractive index changes Vapor
Light
source/ Water
Detector
NAPL
Type 2 - Reversible binding with indicator molecules changes color,
refractive index, or flouresces
Light
source/
Detector
Indicator coating
Reaction product
Type 3 - Chemical/biochemical reactions with contaminants
Light
source/
Detector

Chemical reagent
added to system
Figure 18.3. Three types of fiber-optic sensors

a particular chemical reagent. Hach sells pre-measured, unit-dose reagent that react
with water samples. To test water samples, the pocket colorimeter compares a reacted
sample with a sample blank and yields results in concentration units.
Pros: Portable, simple to use. Visual evidence of gas detection event. Not prone to
interferences.
Cons: Limited chemical sensitivity to individual VOCs; needs actual water samples
(cannot be used in situ); most kits do not meet U.S. EPA method requirements and
may not be used for compliance monitoring. Requires visual inspection and is not
amenable to long-term in-situ applications.

18.5.3 Infrared Sensors
Infrared sensors can be used to detect gases, which, in general, have unique infrared
absorption signatures in the 2–14 µm range. The uniqueness of the gas absorption
spectra enables identification and quantification of chemicals in liquid and gas
mixtures with little interference from other gases. These devices are typically com-
prised of a source of infrared radiation, a detector capable of seeing the infrared
radiation, and a path between the detector and source that is exposed to the gas