GAS DETECTION APPARATUS

Abstract

An apparatus detects a target gas in ambient air. The apparatus has a GC column, a sensor downstream of the GC column, a pump, a gas storage chamber and a pneumatic circuit. The pneumatic circuit has two states. In a first state, the pump draws in ambient air and supplies it to the gas storage chamber to store ambient air under pressure within the chamber, while trapping a sample of ambient air within the pneumatic circuit. In the second state, the gas storage chamber is connected to the GC column to cause pressurised air drawn from the storage chamber to act as a carrier gas to advance the trapped sample through the GC column and sensor. A filter is filters out any target gas present in the air entering into, or the air drawn from, the storage chamber, to avoid the presence of any target gas in the carrier gas.

Claims

1. Apparatus for detecting a target gas in ambient air, the apparatus comprising a GC column, a sensor located downstream of the GC column, a pump, a gas storage chamber and a pneumatic circuit incorporating a valve that is operative in a first state to connect the pump to the gas storage chamber in order to store ambient air under pressure within the chamber, while trapping a sample of ambient air within an internal conduit of the valve, and in a second state to connect the gas storage chamber to the GC column to cause pressurized air drawn from the storage chamber to act as a carrier gas to advance the trapped sample through the GC column and the sensor, wherein a filter is provided to filter out any target gas present in the air entering into, or the air drawn from, the storage chamber, so as to avoid the presence of any target gas in the carrier gas.

2. Apparatus as claimed in claim 1, wherein the filter is positioned in the path of the air drawn from the storage chamber.

3. Apparatus as claimed in claim 1, wherein the gas storage chamber is a variable volume working chamber.

4. Apparatus as claimed in claim 3, wherein the variable volume working chamber has a movable wall defined by a rolling diaphragm.

5. Apparatus as claimed in claim 1, wherein the valve is a rotary 4-port two-position changeover valve, the internal conduit being formed within the rotor to connect two of the four ports in one position of the valve and the other two ports in the other position of the valve.

6. Apparatus as claimed in claim 1, wherein the sensor is a PID sensor.

7. Apparatus as claimed in claim 1, wherein the filter, or an additional filter, serves to remove water vapour from the air serving as the carrier gas.

8. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention will now be described further, by way of example, with reference to accompanying drawings, in which:

[0032] FIGS. 1 and 2 are a schematic diagrams showing the manner in which 6-port valves are used in conventional GC-PID apparatus,

[0033] FIGS. 3 and 4 are schematic diagrams of an embodiment of the invention that uses a 4-port changeover valve,

[0034] FIG. 5 shows a schematic section through a 4-port rotary valve that may be used in the embodiment of FIG. 3,

[0035] FIG. 6 is a section through the valve of FIG. 5 in the plane A-A, and

[0036] FIG. 7 is a section through the valve of FIG. 5 in the plane B-B.

DETAILED DESCRIPTION OF THE DRAWINGS

[0037] A conventional GC-PID apparatus 10 is shown in FIGS. 1 and 2. The apparatus comprises a GC column 12 followed by a PID sensor 14. The apparatus also comprises a gas pump 16, a source of a carrier gas 18 and a two position 6-port valve 20.

[0038] In a first position of the valve 20, shown in FIG. 1, a carrier gas, which is devoid of the target gas, fed under pressure from the supply 18 to the GC column 12 and flows out to ambient atmosphere through the PID sensor 14. The gas supply 18 may either be a pressure cylinder containing the carrier gas, or it may comprise a pump that pumps ambient air through a filter, such as an active carbon filter, into the GC column 12. At the same time, a separate pump 16 sucks ambient air into a loop that contains a reservoir 22 for the sample to be analysed.

[0039] To introduce the sample into the GC column, the valve 20 is rotated to the position shown in FIG. 2. In this position, carrier gas is supplied to the loop containing the sample reservoir 22, to transport the sample into the GC column 12 for analysis. During this time, the pump 16 merely draws in ambient air and discharges it as exhaust.

[0040] Such an apparatus is difficult to miniaturise for several reasons explained above. If the gas supply 18 is a pressure cylinder it would be cumbersome and heavy to permit the apparatus to operate continuously for an acceptable length of time. If it comprises a pump, then the need for both this pump and the pump 16 to operate continuously would place a heavy burden on the electrical power supply. The size of the sample reservoir and of the GC column result in long elution times, while in a portable apparatus it is desired to minimise the detection time.

[0041] A further disadvantage is that ambient air is sucked into the reservoir 22 and the sample resides in the reservoir 22 at sub-atmospheric pressure. If the sample remains under sub-ambient pressure on reaching the PID sensor 14, it creates a risk of ambient air being drawn into the sensor, if the sealing of the sensor is not perfect.

[0042] Existing valves of the type used in the pneumatic circuit as shown in FIGS. 1 and 2 have other inherent disadvantages, in that they have un-swept and contorted dead volumes that degrade the sample. Furthermore, they are costly, require a high current and may include component parts that interact with the sample.

[0043] FIGS. 3 and 4 show an apparatus 100 embodying the present invention and using a 4-port changeover valve 120. The valve 120 has an internal conduit of fixed volume formed in its rotor and represented in the drawings by an arrow 125. The conduit in FIG. 3 connects the ports designated 122 and 124 and in FIG. 4 it connects the other two ports, designated 121 and 123.

[0044] Port 122 is connected to receive the ambient atmosphere 128 that is to be analysed. In the position of the valve 120 shown in FIG. 3A, a pump 110 connected to the port 124 draws the ambient atmosphere from the port 122 through the internal conduit 125 of the valve 120 and feeds the air under pressure into a variable volume storage chamber represented in the drawing by a bellows 114. A pressure sensor 112 sensing the output pressure of the pump 110 is used control the pump 110. The output of the pump 110 and the mouth of the bellows 114 are also connected by way of a carbon filter 116 to the port 121.

[0045] The port 123 of the valve 120 is connected to a GC column 118. Gas discharged from the GC column flows through a PID sensor 126 before being discharged to exhaust. In the position of the valve 120 shown in FIG. 3, the ports 121 and 123 are isolated so that no gas can reach the GC column 118 nor flow through the carbon filter 116.

[0046] To commence sample analysis, the rotor of the valve 120 is turned to the position shown in FIG. 4, in which ports 122 and 124 are isolated, while the internal conduit 125 of the valve 120 connects port 121 to port 123. In this position of the valve 120, pressurised ambient air stored in the bellows 114 flows through the carbon 116 filter to produce a carrier gas devoid of the target gas. The carrier gas flows through the internal conduit 125 to the GC column 118, sweeping ahead of it the fixed volume of ambient air trapped in the internal conduit 125, this volume being the sample to be analysed.

[0047] The gas sample now flows through the GC column 118 and its constituents leave the column 118 after different elution times. The target gas, if present, will reach the PID sensor 126 at a known time following the changeover of the position of the valve 120 and the strength of the output signal of the PID sensor 126 at this time will be indicative of the concentration of the target gas.

[0048] It will be appreciated that the carbon filter may be positioned between the output of the pump 110 and the input of the storage chamber 114, to remove target gas from the ambient air before it enters the storage chamber 114 instead cleaning the air after it has left the storage chamber, to allow it to serve as the carrier gas.

[0049] As well as filtering out the target gas, or VOC's generally, the filter 116, or a separate filter containing a desiccant, may be used to reduce the moisture content of the carrier gas to avoid condensation.

[0050] While it would be possible to use a fixed volume storage chamber 114, one having a variable volume is desirable as it helps keep to a minimum the volume of air that has to be pumped and filtered. If using a variable volume working chamber, a rolling diaphragm has been found to be the most efficient manner of achieving a movable wall.

[0051] FIGS. 5 to 7 show a suitable construction of a 4-port valve 200. The valve has a rotor 210 into the top surface of which there is machined a spiral groove 212, best seen in FIG. 6, this being the internal conduit in which the sample is stored. The inner and outer ends of the spiral groove 212 are connected to bores 214, 216 that lie at equal distances from the axis of rotation of the rotor 210. The stator 220, as shown in FIG. 7, has four ports 222, each fitted with an O-ring 230, that can be selectively aligned, two at a time, with the bores 214 and 216 in the stator that connect to the ends of the spiral groove 212.

[0052] In operation, the apparatus starts in the position shown in FIG. 3 in which the bellows is charged until a desired pressure is sensed by the sensor 112. As an alternative, or in addition, to a pressure sensor 112, a mechanical sensor may be used to indicate when the variable volume working chamber 114 has been expanded to a desired size. Once the storage chamber 114 has been sufficiently charged, the pump 110 is switched off and the valve 120 moved to the position shown in FIG. 4. After completion of analysis of the sample stored in the internal conduit 125 of the valve 120, the valve 120 is returned to the position shown in FIG. 3 and the pump 110 is again operated for a brief interval, sufficient to recharge the storage chamber 114.

[0053] The valve shown in FIGS. 5 to 7 offers several important advantages. In particular, it will be noted that the ports 222 are arrange in two pairs with the ports of each pair located close to one another. Aside from allowing a quick changeover and requiring little movement of the rotor (thus minimising power consumption), the mouths of the bores 214 and 216 that communicate with the spiral groove containing the sample volume, are sealed by the O-rings 230 substantially the entire time during their transition between ports, thus avoiding any contamination of the trapped sample. Furthermore, those ports that are not in communication at any time with the internal conduit in the rotor of the valve are blocked off by the O-rings 230 sealing against the lower surface of the rotor 210 (as viewed in FIG. 5). Thus, in the position of the valve shown in FIG. 3, air cannot flow into, nor out of, the carbon filter through the port 121 and no gas can reach the GC column 118 from the port 123. During times that the bellows 114 is being recharged, the GC column remains filled with the carrier gas from the preceding sensing cycle and is therefore ready to receive the next sample to commence a new sensing cycle. In the position shown in FIG. 4, the blocking of the port 124 prevents the bellows from being discharged on account of reverse flow of air through the pump 110.

[0054] Despite the many advantages of the described and illustrated 4-port valve, it should be stressed that it does not form an essential part of the invention and may be replaced, for example, by electrostatic valves. Indeed, the entire pneumatic circuit using a 4-port valve is only given as an exemplary implementation of the invention.

[0055] There are several advantages presented by the disclosed embodiment of invention as compared with GC-PID apparatus provided by prior art. In particular: [0056] Congruent with the requirements of a portable system, the apparatus employs only one pump which operates during only a fraction of the cycle time. The use of one pump reduces the size of the portable apparatus, and provides for relatively easy manufacture and service. [0057] The variable volume storage chamber can be designed, such as by the use of a rolling diaphragm, to ensure that during the charging cycle the pump delivers a flow and pressure commensurate with its standard operation. [0058] None of the flow is wasted in bi-passes, thereby conserving energy. [0059] The pneumatics are considerably simplified by the provision of a simple two stage process in which the gas sample is entrained. [0060] The disposition of the gas sensor near the exhaust, avoids picking up detectable gas from leak sites. [0061] A single absolute pressure sensor can be provided to maintain system fault diagnostics. [0062] A smaller injection assembly is achieved.