GB2273779A

GB2273779A - Ionisation gas detector

Info

Publication number: GB2273779A
Application number: GB9226663A
Authority: GB
Inventors: Christopher David Jones; Richard Frederick Griffiths
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1992-12-22
Filing date: 1992-12-22
Publication date: 1994-06-29
Anticipated expiration: 2012-12-22
Also published as: GB2273779B; GB9226663D0

Abstract

A gas detector includes a conduit 10 defining a gas flow passage 11 having an inlet 12 and an outlet 13, an ultraviolet light source 14 mounted to direct a beam of ultraviolet light 15 through the gas flow at right angles to the flow direction to ionise a teat gas in the gas flow, the conduit 10 having a substantially right angled bend 16 immediately after the light source 14 and having a pair of electrodes 17, 18 positioned immediately after the bend 16 for measuring ionisation current. The conduit may be of rectangular, preferably square, section with flat plate electrodes 17, 18 or of tubular, preferably circular, section with electrodes 19, 20 of concentric annular form. The detector may be used to detect tracer gases in an air flow. <IMAGE>

Description

Gas Detection Devices.

The present invention relates to gas detection devices, particularly those of the type which use the ionisation of gases by ultra- violet light. One use of such devices is in the detection and investigation of gas flows, when a tracer gas is introduced into a main gas flow. Another is in the detection of the presence of an extraneous gas in a flow of a particular gas.

The main gas flow with which these devices is used is of air, and this specification will therefore, for convenience, refer to thn main gas flow as airflow. Similarly, for convenience, any other gas in the airflow, whether a tracer gas or an extraneous gas, will be referred to as a test gas.

Devices of this type operate by passing the airflow through an ultra-violet light from a lamp which is preferably tuned for maximum ionisation effect on the test gas, and measuring the current flow between two electrodes caused by ionisation of the gas as the airflow is passed between the electrodes. Such devices are described in, for example, GB 1576474 and PCT/GB92/01313. In the latter of these an ionising ultra-violet light is passed through a gas-carrying airstream which then passes between two co-axial potentially biassed annular electrodes, ionisation being substantially completed before the airflow enters the gap between the electrodes. The device is calibrated so that the current between the electrodes provides a measure of the concentration of the test gas in the airstream.

It has been discovered, however, that in devices of this type the response in terms of current flowing as the presence of the test gas is increased is linear up to a certain concentration, then rises less rapidly, and then actually declines. For example, when propylene is used as a tracer gas in an airflow the response is linear up to a concentration of 100 parts per million (ppm), then increases non-linearly up to a maximum concentration of 1000 ppm, after which it decreases. The concentrations for which these devices can be used is therefore limited.

One method of increasing the concentration up to which the scale remains linear is to increase the potential across the electrodes between which the flow passes. However this can only be achieved at the expense of greater complexity, especially in the cases of instruments intended for use in environments having a wide range of concentrations and hence requiring changes in electrode potential during use.

According to the present invention a gas detection device includes a conduit defining a gas flow passage having an inlet and an outlet, and an ultraviolet light source mounted to direct a beam of ultraviolet light at right angles to a direction of gas flow within the passage to irradiate a gas flow, the conduit having a substantially right angled bend immediately after the light source and having a pair of electrodes positioned immediately after the bend.

The bend will usually be such that the conduit thereafter extends parallel to the ultraviolet light beam in a direction opposite to the direction of projection of the beam.

The Applicant has found that with this arrangement any ions formed within the irradiated gas flow are more quickly subjected to the influence of a potential applied across the electrodes and that as a result the response remains linear up to a higher concentration of ionisable gas within the gas flow.

Some embodiments of the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, of which: Figure 1 is a graph showing the relationship between the concentrations of ionisable gas in a gas flow and the current across two electrodes between which the gas flow passes, at three levels of potential difference between the electrodes, and Figure 2 shows a side elevation, in section along line III-III in Figure 3, of two embodiments of a gas detection device according to the invention, and Figure 3 shows an end elevation, in section along line II-II in Figure 2.

When an airstream carrying a tracer gas is passed through a gas detector such as, for example, that described in PCT/GB92/01313, the response is as illustrated in Graphs A, B, C in Figure 1, where the tracer gas was propylene, and A, B and C have progressively increasing potentials applied across the electrodes. It will be seen that the current across the electrodes increases substantially linearly with increasing tracer concentration up to a level A', B', C' where it becomes non- linear and eventually starts to actually decrease. It can be seen how the response can be maintained in the linear form up to higher levels of concentration by raising the potential across the electrodes.The recovery of linearity and the shifting of the maximum signal to higher values at higher concentrations on altering the advection speed and the distance to the electrodes from the lamp is also shown on the graph. For example the lines labelled BL and As show the recovery of linearity achieved as compared with the line AL by virtue of changing these two variables.

In a gas detector according to the present invention (Figures 2, 3) a conduit 10 defines a gas flow passage 11 having an inlet 12 and an outlet 13, and an ultraviolet light source 14 mounted to direct a beam of ultraviolet light 15 at right angles to a direction of gas flow within the passage 11. Immediately after the light source 14 the conduit 10 is bent at right angles as shown at bend 16 so that it lies substantially parallel to but extending in a direction opposite to the beam of ultraviolet light 15.

Immediately after the bend 16 are positioned a pair of electrodes 17, 18 (full lines) or 19, 20 (dotted lines). The electrodes are connected by electrical circuitry 21 to a current recording device 22.

In one embodiment of the invention (full lines) the conduit 10 is of rectangular, preferably square, section with the electrodes 17, 18 in the form of plates, whilst in another (dotted lines) the conduit 10 is of tubular, preferably circular, section with electrodes 19, 20 in the form of concentric cylinders.

In use air containing a tracer gas is drawn through the gas flow passage 11 by means (not shown) such as a suction fan and is irradiated by the beam of ultraviolet light 15 from the lamp 14. A potential is applied across the electrodes 17, 18 or 19, 20. The tracer gas is ionised by the light beam 15 and on passage between the electrodes causes a current to flow. The magnitude of the current is measured by the recording device 22 and can be related by prior calibration to the concentration of tracer gas in the airstream.

It has been found that, with this arrangement, the response curve corresponding to the lines A, B, C in Figure 1 remains linear up to high concentration levels with lower potentials across the electrodes than is required in conventional detectors.

It will be realised that alternative arrangements of the invention are possible- for example the conduit 10 may be bent to run in a direction other than opposite to the direction of the beam 15.

However it will also be realised that with the arrangement as described above with reference to Figures 2 and 3 there is the added advantage that there is absolutely no danger of the beam of ultraviolet light 15 impinging on any of the electrodes 17, 18 or 19, 20.

Claims

What is claimed is: 1. A gas detection device including a conduit defining a gas flow passage having an inlet and an outlet, and an ultraviolet light source mounted to direct a beam of ultraviolet light at right angles to a direction of gas flow within the passage to irradiate a gas flow, the conduit having a substantially right angled bend immediately after the light source and having a pair of electrodes positioned immediately after the bend.
2. A gas detection device as claimed in Claim 1 wherein the bend is such that the conduit thereafter extends parallel to the ultraviolet light beam in a direction opposite to the direction of projection of the beam.
3. A gas detection device as claimed in Claim 1 or in Claim 2 wherein the conduit is of rectangular section and the electrodes are flat plates.
4. A gas detection device as claimed in Claim 3 wherein the section is square.
5. A gas detection device as claimed in Claim 1 or in Claim 2 wherein the conduit is of tubular section and the electrodes are of concentric annular form.
6. A gas detection device as claimed in Claim 5 wherein the conduit is of circular section.
7. A gas detection device substantially as herein described with reference to Figures 2 and 3 of the accompanying drawings.