Gas detection devices

Details Gas detection devices Gas detection devices

1995-09-18

1997-12-23

Berman, Jack I.

Radiant energy

Invisible radiant energy responsive electric signalling

Flow metering

7386109, G01F 164

Patent

active

057010099

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
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.
2. Discussion of Prior Art
The main gas flow with which these devices is used is of air, and this specification will therefore, for convenience, refer to the 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 biased 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.
In the device of PCT/GB92/01313 the airflow therethrough is driven by a fan. However, the device is intended to be used in the open air, where the airspeed may be affected by local wind effects. This can affect the results. It is therefore desirable that the airspeed through the device be known. Conventional airspeed measuring devices (i.e. pivot tubes, anemometers) tend to the disproportionately expensive or complicated, and also in some cases may disrupt the measurement process.


SUMMARY OF THE INVENTION

According to the present invention a gas detector includes structure defining a gas flow passage, means for passing an ultraviolet light beam across the passage, and first and second electrodes downstream of the ultra-violet light means, at least one of the first and second electrodes being in the form of a series of electrically separate segments extending in a downstream direction, the electrodes being connectable to potential applying means and to means whereby the current passing through each segment of a segmented electrode can be measured.
In use air containing a concentration of ionisable test gas is passed through the air passage and subjected to the beam of ultra-violet light which causes a degree of ionisation in the test gas. Potential is applied across the electrodes, which causes ions to be collected by the electrodes, the resultant currents being measured and the measurements passed to analysing means which compare the readings from consecutive segments to provide a measurement of the air speed through the passage and which also sum the segment readings to provide the concentration of the test gas. The measurement of air speed may be displayed, and will usually be used directly by the analysing means.
There may be only one segmented electrode, in which case the potential applied between the unsegmented electrode and the segments will be the same in each case, or both electrodes may be segmented, in which case different potentials can be applied between associated pairs of electrodes.
In one form of the invention the gas passage is rectangular and the segments are in the form of flat rectangular strips. In another form the gas passage is circular, the electrodes are in co-axial form as in PCT/GB92/01313, and the segments are in annular form, in the form of sections of a rod, or both.
The detector may be provided with means, such as a fan, for inducing airflow through the passage, or may rely on flow speed (for example wind speed, or air speed in a duct) to induce flow, in which cas

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patent: 3718043 (1973-02-01), Fishman et al.
patent: 4304124 (1981-12-01), Biblarz
patent: 4393719 (1983-07-01), Wiegand et al.
patent: 5047723 (1991-09-01), Puumalainen
patent: 5455417 (1995-10-01), Sacristan

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