Radiant energy – Invisible radiant energy responsive electric signalling – Including ionization means
Reexamination Certificate
1998-10-22
2001-05-01
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Including ionization means
C250S374000, C250S281000
Reexamination Certificate
active
06225633
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to volatile gas detectors and particularly to a portable photoionization detector (PID).
2. Description of the Related Art
Photo-ionization detectors (PID) can detect volatile organic gases or compounds. A conventional portable PID
10
is illustrated in FIG.
1
. PID
10
includes an ultraviolet (UV) lamp
12
, which produces high energy photons having an energy above 9.2 electron volts (eV). The high energy photons from UV lamp
12
are directed into an ionization chamber
14
through an optical window
16
. The some of the photons collide with molecules of volatile gases having ionization potentials below the energy of the photons. Such collision ionizes the molecules, creating detectable ions and electrons.
PID
10
additionally includes an ion detector
18
having a pair of electrodes
20
and
22
. Ion detector
18
, typically made of a metal, has a high voltage (e.g., greater than 150 V) applied across electrodes
20
and
22
to generate an electrical field. Accordingly, first electrode
20
is electrically biased to attract positively charged particles and second electrode
22
is biased to attract negatively charged particles. Second electrode
22
repels the ions towards first electrode
20
which is simultaneously collecting the volatile gas ions. As a result, a current is produced with which the concentration of the volatile gas can be measured. The magnitude of this measurement current depends on the number of ions produced and therefore on the concentration of ionizable molecules and the intensity of the UV light in ionization chamber
14
. If the UV light intensity is constant, the measurement current can be converted to the concentration, in part per million (ppm), of the volatile organic compounds.
In PID
10
, there is a space
24
between optical window
16
and second electrode
22
. Space
24
is a “dead zone,” in which positive ions are trapped. The positive polarity of second electrode
22
prevents positive ions in space
24
from reaching first electrode
20
. Accordingly, the configuration of electrodes
20
and
22
with dead space
24
inhibits the production and collection of ions and can reduce the sensitivity or accuracy of PID
10
. For example, current PID devices typically can measure concentrations up to about 2,000 parts per million (ppm) of ionizable gases.
As mentioned above, the measurement current can be converted to yield the concentration of the volatile gases if the UV intensity from lamp
12
remains constant. However, UV intensity typically diminishes during the normal operation of PID
10
due to a variety of factors, including degradation of lamp
12
, contamination of optical window
16
, and introduction of interfering substances such as methane, carbon monoxide, or water which block or absorb UV photons in ionization chamber
14
. A UV monitor
26
, which is a biased electrode, is disposed in ionization chamber
14
to measure the intensity of the UV light. The UV light by striking UV monitor
26
releases electrons to produce a monitor current indicative of the intensity of the UV light. The monitor current can be used to correct for UV intensity variations when calculating the volatile gas concentration from the measurement current. The monitor current can also be used to adjust the intensity of UV lamp
12
, for example, by increasing the supply voltage to lamp
12
when the monitor current indicates a low UV intensity. The monitor current, however, inaccurately measures the intensity of UV lamp
12
in the presence of ionized volatile gases. Biased monitor electrode
26
collects positive ions. As a result, the monitor current increases in the presence of ionizable gases, resulting in a less than accurate measurement of the UV intensity. Accordingly, a more accurate UV monitor is needed.
As discussed-above, contamination of PID
10
, including optical window
16
, reduces the UV intensity. The contamination is often a polymer-like coating caused by the deposition of metal atoms, oil film, or dust particles, during the normal use of PID
10
. A user must often disassemble PID
10
to clean optical window
16
. This cleaning is time consuming and burdensome. Accordingly, it is advantageous to provide a self-cleaning PID system.
SUMMARY OF THE INVENTION
The present invention provides a photo-ionization detector (PID) comprising a detector housing having an ionization chamber configured to receive volatile gases. An ultraviolet (UV) lamp transmits UV light through an optical window to ionize the volatile gases in the ionization chamber. An ion detector is disposed in the ionization chamber. The ion detector comprises a pair of differentially biased electrode structures which produce an electrical field that is perpendicular to the direction of the UV light propagation. The ion detector captures ions produced by the ionization of the volatile gases and produces a current which is used to measure the concentration of the volatile gases. A pump is also incorporated into the detector housing to circulate the gases into and out of the ionization chamber. The direction of the flow of gases is perpendicular to the direction of the electrical field and the direction of the UV light propagation. Because the aforementioned directions are perpendicular to each other, formation and collection of ions are more efficient, and the ion detector of the present invention can accurately measure concentrations up to about 10,000 ppm of ionizable gases.
In accordance with another aspect of the invention, a UV monitor, that measures the intensity of the UV light, includes a pair of monitor electrodes which release electrons when struck by the UV light. As a result, a current is induced by which the intensity of the UV light can be measured. Since capture of volatile gas ions by the monitor electrodes would interfere with measurement of the UV intensity, the UV monitor is disposed in a UV monitor chamber that prevents the monitor electrodes from being significantly exposed to the ionized gases. The UV monitor chamber has an optical window so as to allow the UV light to strike the monitor electrodes. The UV light is blocked from propagating through an electrical field between the monitor electrodes.
In accordance with another aspect of the invention, the PID additionally includes electronic circuitry for the operation of the PID. A bias circuit biases the electrodes of the ion detector and the UV monitor to induce an electrical field between each pair of electrodes. A measurement circuit senses the currents for measuring the concentration of the ionized gases as well as the intensity of the UV light. The measurement circuit provides a signal indicative of the currents to a microprocessor. A pump and lamp driver circuit, connected to the lamp and the pump, respectively, also communicate with the microprocessor.
In accordance with still another aspect of the invention, a method for producing electrodes for the ion detector and the UV monitor includes forming an electrode layer on a substrate, selectively patterning the electrode layer to produce an interdigital electrode structure, and removing the substrate from the interdigital electrode structure. Alternatively, the substrate can be patterned to match the shape of the interdigital electrodes. Alternatively, the substrate may be transparent to UV light.
In accordance with still another aspect of the invention, a method for self-cleaning the PID and the optical windows of the ionization and UV monitor chambers includes introducing a gas containing oxygen into the ionization chamber, transmitting UV light into the ionization chamber to create ozone, and allowing the ozone to accumulate in the ionization chamber. Ozone is a strong oxidant which etches and removes the contamination from surfaces, including the optical windows, of the ionization chamber.
REFERENCES:
patent: 4013913 (1977-03-01), Driscoll et al.
patent: 4376893 (1983-03-01), Whetten
patent: 4398152 (1983-08-01), Leveson
patent: 4429228 (1984-01-01), Anderson
pa
Hsi Peter C.
Sun Hong T.
Gabor Otilia
Hannaher Constantine
Millers David
RAE Systems, Inc.
Skjerven Morrill & MacPherson LLP
LandOfFree
Photo-ionization detector for volatile gas measurement and a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Photo-ionization detector for volatile gas measurement and a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photo-ionization detector for volatile gas measurement and a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2561716