Electricity: measuring and testing – Using ionization effects – For analysis of gas – vapor – or particles of matter
Reexamination Certificate
1999-03-17
2001-11-06
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Using ionization effects
For analysis of gas, vapor, or particles of matter
Reexamination Certificate
active
06313638
ABSTRACT:
BACKGROUND
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
FIG. 1
illustrates a known photo-ionization detector (PID)
10
for detecting volatile gases. PID
10
includes an ultraviolet (UV) lamp
12
, an ion detector
18
and a UV monitor
26
. In operation , UV lamp
12
produces high-energy photons having energy above 9.2 electron volt (eV) which emanate through an optical window
16
into an ionization chamber
14
. In ionization chamber
14
, the UV photons collide with gas molecules including volatile gas having ionization potentials below the energy of the UV photons. This ionizes the volatile gas molecules, creating detectable ions and electrons.
Ion detector
18
includes a negative electrode
20
and a positive electrode
22
which have a high voltage difference (e.g., greater than 150 V). Accordingly, negative electrode
20
attracts positively charged particles such as ions, and positive electrode
22
attracts negatively charged particles such as electrons. As a result, the production of volatile gas ions causes a current from electrode
22
to electrode
20
that depends on the number of ions produced. The concentration of the volatile gases in ionization chamber
14
can be determined by measuring the current and the intensity of UV light. At a constant UV light intensity, the measured current is nearly proportional to the volatile gas concentration, and the measured current can be simply converted to the concentration, in parts per million (ppm), of the volatile gases.
PID
10
has a space
24
between optical window
16
and positive electrode
22
. Space
24
is a “dead zone” in which positive ions can be trapped. The positive polarity of electrode
22
prevents positive ions in space
24
from reaching electrode
20
. Accordingly, the configuration of electrodes
20
and
22
with dead space
24
inhibits the collection of ions and can reduce the range and sensitivity of PID
10
. For example, PID
10
typically has a detection range of about 2,000 ppm of ionizable gases.
As mentioned above, the measured current can be simply converted to a concentration of volatile gases if the UV intensity from lamp
12
remains constant. However, the UV intensity typically diminishes during a normal operation of PID
10
due to a variety of factors, including degradation of UV lamp
12
, contamination of optical window
16
and the presence of interfering substances such as methane, carbon monoxide or water which block or absorb the UV photons in ionization chamber
14
. UV monitor
26
, which includes a negatively biased electrode, measures the intensity of the UV light by measuring a current caused by the photoelectric effect of the UV light. In particular, when struck by the UV photons, UV monitor
26
releases electrons which cause a monitor current indicative of the intensity of the UV light. The monitor current can be measured to determine UV intensity variations when calculating the volatile gas concentration. The monitor current can also be used when adjusting the intensity of UV lamp
12
, for example, by increasing a supply voltage to lamp
12
in response to the monitor current indicating a low UV intensity. However, the presence of ionizable gases around UV monitor
26
increases the monitor current because a positive electrode of UV monitor
26
also collects positive ions. Accordingly, the monitor current inaccurately measures the UV intensity. Absorption of the UV light along the path from UV lamp
12
to UV monitor
26
further reduces the accuracy of the monitor current as an indicator of the UV intensity. Therefore, a PID that can accurately measure the UV intensity, is needed.
SUMMARY
In accordance with an embodiment of the present invention, a dual-channel photo-ionization detector (PID) includes a UV light source, a first ion detector that measures a first current primarily resulting from the ionized gases, a second ion detector that measures a second current resulting from the ionized gases and photoelectric emission of electrons. A UV shield blocks the UV light so that the first ion detector is exposed to less UV light than is the second ion detector. The ion detectors are otherwise structurally identical and symmetric in relation to the UV source. The differential shielding of the ion detectors enables the PID to separate the UV light intensity dependency from the measurement of the concentration of the ionizable gas irrespective of variations in the UV light intensity. Embodiments of the PID can determine the ionizable gas concentration accurately to a ppb (parts per billion) level without frequent calibrations of the UV light intensity.
In accordance with another aspect of the invention, a PID includes a heater that maintains the temperature inside the ionization chamber to prevent condensation and stabilize parameters affecting concentration measurements. With the heater, operation of the PID in a humid environment does not cause condensation inside the ionization chamber. Further, the heater reduces thermal variations which might affect measurements. Accordingly, the PID with a heater can provide better accuracy of volatile gas concentration measurements in a wider variety of environments.
The determination of the gas concentration being independent of the UV light intensity and a self-cleaning capability of the PID eliminate the need for manual cleaning of the UV light source or the ionization chamber and allow integration of a PID sensor module including a UV lamp and an ionization chamber with detector electrodes enclosed. The module can be sealed to prevent disassembly for cleaning. Accordingly, delicate components in the sealed sensor module are less subject to damage. Further, the module positions and configures its components for optimal performance and reduces the need for calibrations since the configuration is fixed and not changed by disassembly, cleaning, or reassembly. The module as a unit can be plugged to the PID having other parts for operating the sensor module.
In one embodiment of the sensor module, a single housing of the sensor module includes the UV light source, the electrodes for the ion detectors, and the UV shield. The sensor module may further include a heater.
Another aspect of the invention provides a method for calculating the gas concentration in the above-described PID. The method includes: shielding the first ion detector from the UV light; exposing the second ion detector to the UV light; measuring a first and second currents with a first reference gas having a known concentration in the PID; and measuring a first and second currents with a second reference gas having a known concentration in the PID. The currents measured at the first and second ion detectors can be modeled as functions of a concentration of the ionizable gases and an intensity of the UV light. Since the two ion detectors are identical except for their exposures to the UV light, the difference between the first and second currents is due solely to the UV light intensity. Parameters for the models of the currents can be determined from the current measurements with the reference gases, and the concentration of ionizable gases can be calculated irrespective of the UV light intensity.
REFERENCES:
patent: 5528150 (1996-06-01), Sterns et al.
patent: 5561344 (1996-10-01), Hsi
patent: 6023169 (2000-02-01), Budovich et al.
Hsi Peter C.
Sun Hong T.
Kerveros James C
Metjahic Safet
RAE Systems, Inc.
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