Measurement of gases

This application claims the benefit of U.S. Provisional Application No. 62/093,455 filed Dec. 18, 2014, which is hereby incorporated by reference in its entirety as if fully set forth herein.

The measurement of low levels of gases in ambient air by electrochemical gas sensors is made difficult due to interferences such as humidity, temperature and other gases to which electrochemical sensors respond.

The invention solves the problem and compensates for those interferences by using a comparative measurement between two different states. The first state is one in which the sensor responds to a sample of air which includes the gas of interest plus the combination of interferences. The second state is one in which the gas of interest is removed from the sample using a selective scrubber, and the sensor responds to the combination of interferences. The concentration of the gas of interest in the incoming air sample is proportional to the difference between the electrochemical sensor signals in first and second states. This can be achieved with a new measurement system as described herein.

FIG. 1 is a schematic representation of the apparatus, system and method of the invention.

FIG. 1 is a schematic representation of the apparatus, system and method of the invention. In FIG. 1 , the numeral 1 represents an optional selective scrubber. Element 2 is a selective scrubber for the gas of interest to be determined. Element 3 represents a valve or a combination of valves that can be controlled to switch at a specific interval between the gas streams to the electrochemical sensor 5 in a housing. Element 4 represents a nafion tubing to buffer humidity. Pump 6 brings the air flow to the electrochemical sensor 5 . T 7 allows the incoming air sample 8 to be routed through scrubber 2 or directly to the sensor. Valve 3 either directs the incoming air sample 8 or the sample which has been scrubbed by the target gas scrubber 2 through the humidity buffer 4 to the electrochemical sensor or directs the incoming air sample 8 through the humidity buffer 4 to electrochemical sensor 5 , bypassing the scrubber. The concentration of the gas of interest in the incoming air sample is proportional to the difference between the electrochemical sensor signals in first and second states.

The electrochemical sensors 5 that may be deployed in the improved instrument include sensors for the measurement of SO 2 , H 2 S, CO, NH 3 , O 3 and NO 2 . Suitable selective scrubbers 2 depend on the gas of interest.

Measurement of SO 2 was improved by using an electrochemical sensor 5 such as Alphasense SO 2 -AF, a three-way solenoid valve 3 to switch sample flow between lines 10 and 11 . Line 10 bypasses the selective target gas scrubber 2 . Line 11 draws the incoming gas through scrubber 2 . Scrubber 1 is a piece of PTFE tubing containing 0.2 g of silver wool. Scrubber 2 contains 10 g of marble (calcium and magnesium carbonate). The difference in response from the electrochemical sensor 5 when presented the air sample directly or via the scrubber 2 was found to be proportional to SO 2 concentration, and the measurement was substantively free of interference from temperature and humidity changes and carbon monoxide.

In other examples, a different scrubber 2 would be used to remove a different target gas and to eliminate its inclusion in gases sensed in sensor 5 .

In other examples, plural scrubbers 2 are used with plural parallel lines 11 . Switch 3 may have plural input connections for plural lines 11 to select individual lines 11 , and the different individual lines 11 have different scrubbers 2 for connection to sensor 5 .

In other examples, plural switches are connected between plural scrubbers and switch 3 to select specific scrubbers.

In one example, a device for determining the concentration of a target gas in ambient air transfers the sample air through an optional selective scrubber 1 to an electrochemical sensor 5 . A solenoid valve 3 or valves cycles the air to the sensor 5 via a direct path 10 or a path 11 containing a selective scrubber 2 . Electronics control the position of the valve 3 or valves and measure the sensor response at a sample interval between five and one thousand seconds after the valve 3 switches the sample flow.

The new method determines the concentration of a target gas in ambient air. The gas concentration is proportional to the difference in sensor signals between when the gas sample flows directly to the sensor 5 and when the gas sample flows via the selective scrubber 2 to the sensor.

The sensor signal after a predetermined time of sample flow through the scrubber 2 to the sensor 5 represents the signal of the sensor in the absence of the target gas. The sensor signal after a predetermined time of sample flow directly to the sensor represents the signal of the sensor in the presence of the target gas.

The sensor signal after a predetermined time of sample flow through the scrubber 2 to the sensor 5 represents the signal of the sensor in the absence of the target gas. The sensor signal after a predetermined time of sample flow directly to the sensor represents the signal of the sensor in the presence of the target gas.

In other examples, the electrochemical sensor is for the measurement of SO 2 , H 2 S, CO, NH 3 , O 3 or NO 2 .

For the measurement of SO 2 , the first optional scrubber 1 is a catalyst containing a silver catalyst, MnO 2 catalyst, potassium iodide or ferrous sulphate for the scrubbing of ozone and NO 2 , and selective scrubber 2 contains activated carbon, marble chips, calcium carbonate or an adsorbent for acidic gases.

For the measurement of H 2 S, the first optional scrubber 1 is a catalyst containing a silver catalyst, MnO 2 catalyst or potassium iodide for the scrubbing of ozone and NO 2 , and selective scrubber 2 contains activated carbon or iron oxides or a purafil chemisorbent for H 2 S.

The invention provides a device for determining the concentration of a target gas in ambient air 8 which transfers the sample air through an optional selective scrubber 1 to an electrochemical sensor 5 , via a solenoid valve 3 or valves which cycles the air to the sensor via a direct path or a path containing a selective scrubber 2 . Electronics control the position of the valve 3 or valves and measure the sensor 5 response at a sample interval between 5 and 1000 seconds after the valve 3 switches the sample flow.

The invention provides a method of determining the concentration of a target gas in ambient air and where the gas concentration is proportional to the difference in sensor signals between when the gas sample flows directly to the sensor or flows via the selective scrubber 2 .

In the new method the sensor signal after a predetermined time of sample flow through the scrubber 2 to the sensor 5 represents the signal of the sensor in the absence of the target gas. The sensor signal after a predetermined time of sample flow directly to the sensor 5 represents the signal of the sensor in the presence of the target gas.

In the new method the sensor signal after a predetermined time of sample flow through the scrubber 2 to the sensor 5 represents the signal of the sensor in the presence of the target gas, and the sensor signal after a predetermined time of sample flow directly to the sensor 5 represents the signal of the sensor in the absence of the target gas.

The invention provides a new device with the electrochemical sensor 5 for the measurement of SO 2 , H 2 S, CO, NH 3 , O 3 or NO 2 .

In a new device as described herein for the measurement of SO 2 , the selective scrubber 1 includes a catalyst containing a silver catalyst, a MnO 2 catalyst, potassium iodide or ferrous sulphate for the scrubbing of ozone and NO 2 , and selective scrubber 2 contains activated carbon, marble chips, calcium carbonate or an adsorbent for acidic gases.

In a new device described herein for the measurement of H 2 S, the selective scrubber 1 includes a catalyst containing a silver catalyst, a MnO 2 catalyst, potassium iodide or ferrous sulphate for the scrubbing of ozone and NO 2 , and selective scrubber 2 contains activated carbon, iron oxides or a purafil chemisorbent for H 2 S.

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.