Electricity: measuring and testing – Using ionization effects – For analysis of gas – vapor – or particles of matter
Reexamination Certificate
1999-06-11
2001-11-20
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Using ionization effects
For analysis of gas, vapor, or particles of matter
C324S459000, C250S382000, C250S281000, C250S289000, C250S384000
Reexamination Certificate
active
06320388
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a volatile gas detector and particularly to a portable photo-ionization detector (PID).
2. Description of Related Art
Photo-ionization detectors (PIDS) can detect volatile gases.
FIG. 1
shows a conventional portable PID
10
that includes an ultraviolet (UV) lamp
12
and an ionization chamber
14
. UV lamp
12
produces UV light including UV photons having energy up to 8.4 electron volts (eV) or more. The UV photons pass through an optical window
16
into ionization chamber
14
. In ionization chamber
14
, the UV photons collide with and ionize volatile gas molecules having ionization potentials below the energy of the photons, creating ions and electrons.
PID
10
further includes an ion detector
18
having a pair of electrodes
20
and
22
, which are typically made of a metal. Ion detector
18
has a high voltage (150 V or more) applied across electrodes
20
and
22
to generate an electrical field. In particular, electrode
22
is biased to a high voltage to attract negatively charged particles (electrons) and repel positively charged particles (ions), and electrode
20
is grounded to collect the positively charged particles (ions). The movement of the ions to electrode
22
produces a current, from which the concentration of the volatile gas can be determined. More specifically, the magnitude of the current depends on the number of ions produced from the collisions between volatile gas molecules and UV photons. Accordingly, the magnitude of the current depends on the concentration of ionizable volatile gas molecules and the intensity of UV light in ionization chamber
14
. If the UV light intensity is constant, a measurement of the current directly related to the concentration of ionizable gases.
During use of PID
10
, a gas sample in ionization chamber
14
can contain air mixed with one or more volatile gases that have ionization potentials lower than the maximum energy of the UV photons from UV lamp
12
. PID
10
, which has a single ion detector
18
, measures ion current and the total concentration for the ionizable gases of all types in the sample. PID
10
cannot determine the concentrations of individual gases in the gas sample.
U.S. Pat. No. 5,393,979, which is herein incorporated by reference in its entirety, discloses a PID that includes multiple single channel PIDs that measure the concentrations of different types of gases in a gas sample. For instance, the PID may include three UV lamps having different maximum photon energies of 9.8, 10.2, and 11.7 eV and an ionization chamber including three ion detectors respectively in front of respective UV lamps. When a gas sample flows through the ionization chamber, each of the UV lamps, which are arranged in tandem, ionizes only the gases having ionization potentials below the maximum photon energy of the lamp, and the associated ion detector measures a current that the lamp generates from the gas sample. The three measured currents from the ion detectors differ from one another if the gas sample contains gases that can only be ionized by UV light from some of the lamps. The concentrations of gases having ionization potentials below each photon energy can be determined from the respective measured currents.
SUMMARY OF THE INVENTION
In accordance with an embodiment of the present invention, a PID measures the concentrations of volatile gases in a gas sample that flows through an ionization chamber of the PID. The PID includes a UV lamp having an optical window that is divided into multiple window zones. Each window zone produces UV photons having a distinctive energy distribution.
The ionization chamber includes multiple ion detectors that are in front of respective window zones of the optical window. Each ion detector measures the current generated when the UV photons from the corresponding window zone ionize the gas sample. Since the energies of UV photons passing through the optical window from the UV light vary according to the window zones through which the UV photons pass, the UV photons from different zones ionize different components of the gas sample. Accordingly, the currents measured at the ion detectors can differ from one another, and the concentrations of the various component gases can be determined from the separate current measurements.
The differentiated zones of the optical window can be formed by modifying the material characteristics of the optical window from zone to zone, changing the dimensions (e.g., thickness) of the optical window, or using different optical materials in each zone. For example, different coatings or thicknesses of the optical window transmit different wave lengths of UV light and permit selection of the photon energies to identify specific gases.
Each ion detector has a pair of electrodes. One is a bias electrode, and the other is a measurement electrode. In one embodiment of the invention, the measurement electrodes of the ion detectors are separate from one another, but the bias electrodes can be either separate or common.
Another embodiment of the invention provides a method of determining the concentrations of specific gases or classes of gases in a gas sample. The method comprises: producing a plurality of UV light beams having different spectrums; passing the UV light beams through the gas; measuring a plurality of current signals caused by the beams ionizing gas molecules; converting the current signals to concentrations of gas molecules ionizable by each beam; and determining the concentration of the selected gas compounds by finding a difference between a first concentration of gas molecules ionizable by a first UV light beam and a second concentration of gas molecules ionizable by a second UV light beam. The method can further identify the specific gases by comparing ratios of the current signals to a table of ratios associated with the gases.
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Hsi Peter C.
Sun Hong T.
Brown Glenn W.
Hamdan Wasseem H.
Heid David W.
RAE Systems, Inc.
Skjerven Morrill & MacPherson LLP
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