5.4. Direct Reading Instrumental Analysis: Portable Analyzers
A range of different types of portable instruments suitable for odor measurement
are currently available.
5.4.1. Portable Gold Leaf Analyzers
Portable gold leaf analyzers are frequently used to monitor hydrogen sulfi de in
the gas phase. This type of detector utilizes the change in resistance of a gold
fi lm sensor caused by adsorption of H2S molecules, within an output proportional
to the H2S concentration. Eventually the gold leaf becomes saturated and
has to be regenerated. A common gold fi lm monitor is the Jerome 631-X H2S
analyzer (Able Instruments and Controls Ltd.), which has a reported detection
limit of the order of 0.003 ppm and can measure up to 0.005 ppm H2S. Sampling
and measurement time depend on the level of sulfi des present, but typically it is
less than 1 min.
5.4.2. Paper Tape Monitors
Paper tape monitors contain a chemically impregnated tape, which when exposed
to a gas sample changes color in direct proportion to the amount of gas present.
A tape is selected that will react with the gas of interest (IPPC H4-Part 2, 2002).
Today a wide range of compounds can be quantifi ed depending on the instrument
selected. A particular advantage is that the monitor can be set to sample
at regular intervals, exposing an unreacted section of the tape each time, and
so leaves a permanent record of the concentration of each sample. However,
given that the sampling times can be of the order of minutes rather than seconds,
this type of monitor is not recommended when a large number of samples
are required in a short period of time. Furthermore, the equipment is generally
expensive, and although relatively simple to operate, suffi cient information relating
to process parameters/activities at the time of measurement must be collected,
thus allowing correct interpretation of the concentration data obtained.
Some instruments are affected by the presence of moisture, which limits their use
for stack monitoring.