Interference Free Gas Measurement
This application claims the benefit of U.S. Provisional Application No.
61/970,564 filed March 26, 2014, which is hereby incorporated by reference in its
entirety as if fully set forth herein.
BACKGROUND OF THE INVENTION
The cost of traditional monitoring instrumentation for air quality is high, and
there is an increasing requirement to lower the cost. One approach is to use less
expensive sensors such as electrochemical gas sensors, however such sensors suffer
from a lack of selectivity - they respond to gases other than the target gas. It would be
advantageous to improve their selectivity.
The measurement of NO2, SO2, H2S and Cl2 gases in ambient air by electrochemical
gas sensors is very difficult, due to the interference by ambient ozone levels. Ozone gas
will cause a positive response in NO2 and Cl2 electrochemical sensors and a negative
response in SO2 and H2S electrochemical sensors.
Needs exist for improved air quality sensors.
SUMMARY OF THE INVENTION
The present invention provides improved air quality sensors at low cost.
It would be advantageous to compensate for the interference by ozone by using
a sensor which is selective to ozone, but which is of a similar cost to the
electrochemical sensors.
It was discovered that a heated metal oxide sensor operated at high temperature
so as to generate a selective response to ozone could be used to compensate for the
ozone interference.
If the ozone sensor is co-located with an electrochemical sensor or better still
incorporated within the same gas sampling apparatus and data from the sensors is
collected at the same time, then the actual N0 2, Cl2, S0 2 or H2S concentrations could
be calculated using the equation below:
Gas concentration = a*(Electrochemical sensor +/- b* 0 3 sensor) + c
(Eql)
where a, b, c are constants which can be calculated through calibration at
known humidity, temperature and gas concentrations. The constants may exhibit a
dependence on humidity and temperature and therefore it is advantageous to calculate
their dependence through calibration and to incorporate temperature and humidity
sensors into the gas measurement apparatus to adjust the constants in response to
changing gas conditions.
0 3 increases a NO sensor response and a Cl2 sensor response, and the O3 sensor
response must be subtracted in Equation 1. O3 decreases sensor responses for S0 2 and
H2S, and the O3 sensor response must be subtracted in Equation 1.
The invention provides an instrument containing a selective ozone sensor and
one or more electrochemical gas sensors which exhibit an interfering response to
ozone. A microprocessor is connected to the one or more electrochemical gas sensors
and to the ozone sensor. An ozone sensor signal from the selective ozone sensor is
used to adjust an electrochemical gas sensor output from the one or more
electrochemical gas sensors to produce an accurate measurement from the
electrochemical gas sensors.
The one or more electrochemical gas sensors are N0 2, S0 2, H2S and Cl2
electrochemical gas sensors.
The selective ozone sensor is a heated metal oxide gas sensor.
The electrochemical sensors and the selective ozone sensor are located within
10 meters of each other so that the sensors are sampling substantively the same air
parcel at the same time.
The heated metal oxide gas sensor is substantively composed of one or more of
W0 3, Sn0 2, In2 0 3, Mo0 3 or ZnO.
A method of measuring concentrations of one or more of N0 2, S0 2, H2 S and
Cl2 gases in ambient air uses one or more electrochemical gas sensors. Co-located
with the one or more electrochemical gas sensors is a selective ozone sensor.
Producing an ozone signal with the selective ozone sensor and using the ozone signal to
adjust the one or more signals from the electrochemical gas sensors produces an
accurate measurement of the one or more gases.
These and further and other objects and features of the invention are apparent in
the disclosure, which includes the above and ongoing written specification, with the
claims and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1is a schematic representation of the new sensor apparatus and method.
Figure 2 is a graph produced from the new sensor apparatus and method.
DETAILED DESCRIPTION
As shown in Figure 1, a N0 2, S0 2, Cl2 or H2S electrochemical sensor 1 has
means of contacting gas samples. A microprocessor 2 receives and records sensor
outputs, calculates gas concentrations and communicates results to an external logger.
Heated metal oxide ozone sensor 3 has means of contacting the gas sample.
A housing 4 contains the components.
A temperature and relative humidity RH sensor 5 is in contact with a gas
sample.
A line power source may be connected to the housing with a step-down
transformer, an inverter and resistors for operation the electrochemical gas sensors and
the microprocessor and for heating and operating the metal oxide ozone sensor.
Operating power may be provided by a battery in the housing or by a low voltage input.
Results of an example are shown in Figure 2.
The graph shows ambient data 20 from one example using an NO sensor. In
this case electrochemical sensor 1 is an NO2 sensor. The electrochemical NO2 sensor
1produces an output signal 22 of parts per billion NO2. The metal oxide ozone sensor
3 produces an output signal 24 related to parts per billion ozone. Outputs of the No2
sensor and the ozone sensor are provided to the microprocessor. A reference analyzer
using microprocessor 2 subtracts from the NO2 sensor response 22, the (ref NO2)
response 24. The microprocessor 2 subtracts from the output signal response 22. A
part of the ppb is the result of the sensing in NO2 sensor 1 that O3 adds to the NO2
sensor response, and NO2 true 26 is calculated from the electrochemical NO2 sensor 1
and a heated metal oxide ozone (O3) sensor 3 using Eq 1 with a=l, b=l and c = 32 and
the +/- sign being a plus. Application of equation 1 has dramatically improved the
correlation between the N02 measured and the reference analyzer. The
microprocessor provides an output signal 26 that is the true N0 2 ppb.
N0 2, S0 2, H2S and Cl2 sensors 1 are used. The output of the 0 3 sensor 3 may
be used by subtracting the 0 3 sensor output from the N0 2 and Cl2 sensor outputs and
adding the 0 3 sensor output to the S0 2 and H2S sensor outputs. Each electrochemical
sensor may have its own associated 0 3 sensor, or the output from one 0 3 sensor may be
stored and used to compensate output from the different electrochemical sensors.
Known temperature and relative humidity effects upon the sensor outputs are
used to calculate the true ppb of the sensed gas or gases at standard temperature and
relative humidity. For that reason the housing 4 has a temperature and relative
humidity sensor 5 attached or close by. An output signal of the temperature and
relative humidity sensor 5 may be passed to the microprocessor for compensating the
input signals 22 and 24 or their comparison when producing the output signal 26.
The true sensed gas output signal from the housing 4 may be sent to an onboard
or remote recorder along with the temperature and relative humidity signal.
While the invention has been described with reference to specific
embodiments, modifications and variations of the invention may be constructed
without departing from the scope of the invention, which is defined in the following
claims.

I claim:
1. Apparatus comprising an instrument containing one or more
electrochemical gas sensors which exhibit an interfering response to ozone, a selective
ozone sensor, a microprocessor connected to the one or more electrochemical gas
sensors and to the selective ozone sensor, and wherein an ozone sensor output signal
from the selective ozone sensor is used by the microprocessor to adjust one or more
electrochemical gas sensor output signals from the one or more electrochemical gas
sensors to produce accurate gas concentration measurement signal from the one or
more electrochemical gas sensors.
2. The apparatus of claim 1, wherein the one or more electrochemical gas
sensors comprise N0 2, S0 2, H2S and Cl2 electrochemical gas sensors.
3. The apparatus of claim 1, wherein the selective ozone sensor comprises
a heated metal oxide gas sensor.
4. The apparatus of claim 3, wherein the heated metal oxide gas sensor is
substantively composed of one or more ofW0 3, Sn0 2, In2 O3, M0O 3 or ZnO.
5. The apparatus of claim 1, wherein the electrochemical sensors and the
selective ozone sensor are located within 10 meters of each other, wherein the sensors
are sampling substantively the same air parcel at the same time.
6. The apparatus of claim 1, wherein the electrochemical sensors and the
selective ozone sensor are located within adjacent housings, wherein the sensors are
sampling substantively the same air parcel at the same time.
7. The apparatus of claim 1, wherein the electrochemical sensors and the
selective ozone sensor are located within one housing, wherein the sensors are
sampling substantively the same air parcel at the same time.
8. A method comprising providing a gas sensing instrument, providing
one or more electrochemical gas sensors, measuring concentrations of one or more
gases in ambient air using the one or more electrochemical gas sensors, providing a
selective ozone sensor, and co-locating the selective ozone sensor with the one or more
electrochemical gas sensors, producing an ozone concentration signal with the
selective ozone sensor, producing one or more gas concentration signals with the
electrochemical gas sensors and using the ozone concentration signal for adjusting the
one or more gas concentration signals from the electrochemical gas sensors to produce
an accurate concentration measurement of the one or more gases.
9. The method of claim 8, wherein the providing of one or more
electrochemical gas sensors comprises providing one or more of N0 2, S0 2, H2S and
Cl2 sensors.
10. The method of claim 9, wherein the measuring concentrations of N0 2,
S0 2, H2 S and Cl2 in ambient air use the instrument of claim 1 wherein each true gas
concentration equals a* (electrochemical sensor reading - (b*0 3 sensor reading) ) +
c, wherein a, b, c are determined by calibration of the temperature and humidity of the
sensed gases.