Interference Free Gas Measurement

Abstract

One or more inexpensive electrochemical gas sensors are paired with a selective ozone sensor. Ozone in ambient air influences the output signals of the electrochemical gas sensors. The unwanted ozone effects are removed from the output signals of the electrochemical gas sensors by comparing them with the selective ozone sensor output signals. The selective ozone sensor signals are removed from and/or added to output signals from the electrochemical gas sensors. True indications of concentrations of the sensed gases in the ambient air result from the compensation for ozone interference.

Claims

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 (1) comprise NO.sub.2, SO.sub.2, H.sub.2S, NH.sub.3 NO and Cl.sub.2 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 of WO.sub.3, SnO.sub.2, In.sub.2O.sub.3, MoO.sub.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 (1) comprises providing one or more of NO.sub.2, SO.sub.2, H.sub.2S, NH.sub.3 NO and Cl.sub.2 sensors.

10. The method of claim 9, wherein the measuring concentrations of NO.sub.2, SO.sub.2, H.sub.2S and Cl.sub.2 in ambient air use the instrument of claim 1 wherein each accurate gas concentration equals a*electrochemical sensor reading+/(b*O.sub.3 sensor reading)+c, wherein a, b, c are determined by calibration of the sensors to O.sub.3 and the sensed gases.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a schematic representation of the new sensor apparatus and method.

[0019] FIG. 2 is a graph produced from the new sensor apparatus and method.

DETAILED DESCRIPTION

[0020] As shown in FIG. 1, a NO.sub.2, SO.sub.2, Cl.sub.2 or H.sub.2S 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.

[0021] Heated metal oxide ozone sensor 3 has means of contacting the gas sample.

[0022] A housing 4 contains the components.

[0023] A temperature and relative humidity RH sensor 5 is in contact with a gas sample.

[0024] 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.

[0025] Results of an example are shown in FIG. 2.

[0026] The graph shows ambient data 20 from one example using an NO sensor. In this case electrochemical sensor 1 is an NO.sub.2 sensor. The electrochemical NO.sub.2 sensor 1 produces an output signal 22 of parts per billion NO.sub.2. The metal oxide ozone sensor 3 produces an output signal 24 related to parts per billion ozone. Outputs of the No.sub.2 sensor and the ozone sensor are provided to the microprocessor. A reference analyzer using microprocessor 2 subtracts from the NO.sub.2 sensor response 22, the (ref NO.sub.2) response 24. The microprocessor 2 subtracts from the output signal response 22. A part of the ppb is the result of the sensing in NO.sub.2 sensor 1 that O.sub.3 adds to the NO.sub.2 sensor response, and NO.sub.2 true 26 is calculated from the electrochemical NO.sub.2 sensor 1 and a heated metal oxide ozone (O.sub.3) sensor 3 using Eq 1 with a=1, b=1 and c=32 and the +/ sign being a plus. Application of equation 1 has dramatically improved the correlation between the NO2 measured and the reference analyzer. The microprocessor provides an output signal 26 that is the true NO.sub.2 ppb.

[0027] NO.sub.2, SO.sub.2, H.sub.2S and Cl.sub.2 sensors 1 are used. The output of the O.sub.3 sensor 3 may be used by subtracting the O.sub.3 sensor output from the NO.sub.2 and Cl.sub.2 sensor outputs and adding the O.sub.3 sensor output to the SO.sub.2 and H.sub.2S sensor outputs. Each electrochemical sensor may have its own associated O.sub.3 sensor, or the output from one O.sub.3 sensor may be stored and used to compensate output from the different electrochemical sensors.

[0028] 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.

[0029] 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.

[0030] 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.