ULTRA VIOLET LAMPS

Abstract: An ultra-violet lamp (19) has a pair of electrodes (31, 32) within the lamp, the electrodes being connected to a DC power supply (34). There is a coil (37) wound round the lamp, the coil being connected to an RF power supply (38). The DC power supply (34) and the RF power supply (38) are controlled by control means (39) adapted to activate an RF voltage in the coil (37) to initiate the supply of ultra-violet light (20) from the lamp, to activate a DC voltage between the electrodes (31, 32), and to switch off the RF voltage when the DC voltage is sufficient to maintain the supply of ultra-violet light.

Description: ULTRA VIOLET LAMPS The present invention relates to Ultra Violet lamps, and in particular to such lamps for use in environmental testing apparatus of the type described in Patent Application PCT/GB92/01313. PCT/GB92/01313 describes an apparatus in which an ionising ultra-violet light is passed through an airstream, carrying a test gas, which then passes between two co-axial potentially biassed annular electrodes, ionisation of the test gas 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. The lamp providing the ultra-violet light may be powered either by a DC source or by an RF source, but it has been found that there are certain disadvantages with using an RF source. However, whilst a DC ultra-violet lamp can be operated using a voltage of about 300 volts this voltage is insufficient to initiate operation. The usual procedure is therefore to apply a voltage of about 1.5kV, which initiates operation, and then reduce the voltage to 300 volts. Unfortunately the use of 1.5kV is dangerously high for some operational purposes, for example when the apparatus is being used in testing for the presence of a flammable gas. According to the present invention an ultra-violet lamp has: a pair of electrodes within the lamp, the electrodes being connected to a DC power supply: and a coil wound round the lamp, the coil being connected to a RF power supply: the DC power supply and the RF power supply being controlled by control means adapted to activate an RF voltage in the coil to initiate the supply of ultra-violet light from the lamp, to activate a DC voltage between the electrodes, and to switch off the RF voltage when the DC voltage is sufficient to maintain the supply of ultra-violet light. The coil will usually form part of a resonant circuit, and an RF drive might conveniently include a solid state circuit powered by a low voltage DC source such as a battery. The RF voltage is preferably supplied in the range of 30 to 1000MHz. According to another aspect of the invention a method of activating an ultra-violet lamp consists of the steps of applying a RF voltage to a coil surrounding the lamp to initiate supply of light and of applying a DC voltage between two electrodes within the lamp to maintain the supply of light, the RF voltage being discontinued upon initiation of the supply. An example of an instrument in which the invention can advantageously be employed, and an example of the invention will now be described with reference to the accompanying diagrammatic drawings, of which: Figure 1 is an elevation, in section, of an environmental testing apparatus, and Figure 2 is an elevation of an ultra violet lamp according to the invention and suitable for use with the apparatus of Figure 1. An environmental testing apparatus 10, as described in PCT/GB92/01313. has structure 11 defining an air passage 12 containing a cylindrical electrode 13 mounted on the structure 11 by mountings 1-4. Co-axial with the electrode 13 and with the axis of the structure 11 is a circular electrode 15- The electrodes 13. 15 are connected to terminals 16, 17 which form part of an electrical circuit (not shown) which includes means for establishing a potential differenced across the electrodes and for measuring any current passing between the electrodes. A fan 18 is positioned in the air passage 12 so as to cause airflow through the passage in the direction of the arrow A, and an ultra-violet lamp 19 is positioned such that a beam of ultra-violet light 20 from the lamp can be directed across the passage 12 upstream of the electrodes 13. 15 and at right angles to flow through the passage. The apparatus is calibrated such that current passing between the electrodes 13. 1 can be related to the concentration of a test gas in airflow through the passage 12. In use, air is passed through the passage 12, the beam of light 20 is shone across the airstream, and a potential difference is established between the electrodes 13. 15- If there is any test gas in the airstream there is a degree of ionisation, and as the ionised airstream passes between the electrodes 13. 15 an electrical flow is established which is measured and related to the the concentration of the test gas. An ultra-violet lamp 19 suitable for use with the apparatus 10 and according to the present invention is shown in more detail in Figure 2. The lamp 19 has a casing 30 within which are lamp electrodes 31. 32 connected to an electrical circuit 33 having a DC supply means 3^ and a switch 35- At one end of the casing 30 is an ultra-violet translucent window 36. Surrounding the casing 30 is coil 37 which forms part of a resonant circuit with a RF voltage source 38- The switch 35 and RF voltage source 38 are connected to control means 39• In use, when it is required that the lamp 19 supply ultra-violet light the control means 39 actuate the RF voltage source to supply an RF voltage to the coil 37. This initiates the supply of ultra-violet light. The control means 39 also actuates the switch 35 to establish a DC potential between the lamp electrodes 31. 32 at a level sufficient to maintain the supply of ultra-violet light. Once the supply of light is established the RF supply is discontinued by the control means 39• The DC supply will normally be about 300 volts. The RF supply, which may include, for example, a solid state circuit powered by a low voltage source such as a battery, will be no greater than this. The voltages are therefore within levels for which apparatus, such as the apparatus 10, can easily be designed for safe operation in the presence of flammable gases. The RF voltage will usually be supplied at a frequency in the range 30 to 1000MHz. It has been found that with this system the supply of ultra-violet light will usually be established within a few seconds. The control means 39 might, therefore, switch both DC and RF supplies on at the same time. Alternatively, there might be a delay after switching on the RF supply so that the DC supply only starts after the k . supply of light has been established. Many alternative versions of the present invention will be readily apparent to those skilled in the art.

Claims: Claims What is claimed is: 1. An ultra-violet lamp having a pair of electrodes within the lamp, the electrodes being connected to a DC power supply: and a coil wound round the lamp, the coil being connected to a RF power supply: the DC power supply and the RF power supply being controlled by control means adapted to activate an RF voltage in the coil to initiate the supply of ultra-violet light from the lamp, to activate a DC voltage between the electrodes, and to switch off the RF voltage when the DC voltage is sufficient to maintain the supply of ultra-violet light. 2. A lamp as claimed in Claim 1 wherein the coil forms part of a resonant circuit. 3. A lamp as claimed in Claim 1 or in Claim 2 wherein the RF power supply includes a solid state circuit powered by a low voltage DC source. 4. A lamp as claimed in Claim 3 wherein the DC source is a battery. 5. A lamp as claimed in any one of Claims 1 to 4 wherein the RF voltage is supplied in the range of 30 to 1000MHz. 6. A lamp as claimed in any one of Claims 1 to 5 wherein the DC power supply operates at 300 volts. 7. A lamp substantiall as described herein with reference to Figure 2 of the accompanying drawings. 8. An environmental testing apparatus including means for drawing an airstream containing a test gas through an ultra-violet light and then between a pair of potentially biassed electrodes, there being means for measuring electrical flow between the electrodes and relating this flow to the concentration of the test gas, wherein the ultra-violet light is provided by a lamp as claimed in any one of Claims 1 to 7• 9- A method of activating an ultra-violet lamp consisting of the steps of applying a RF voltage to a coil surrounding the lamp to initiate supply of light and of applying a DC voltage between two electrodes within the lamp to maintain the supply of light, the RF voltage being discontinued upon initiation of the supply.

Inventors: GRIFFITHS RICHARD FREDERICK, JONES CHRISTOPHER DAVID, GRIFFITHS, RICHARD, FREDERICK, JONES, CHRISTOPHER, DAVID
Applicants: GRIFFITHS RICHARD FREDERICK, JONES CHRISTOPHER DAVID, SECR DEFENCE BRIT, GRIFFITHS, RICHARD, FREDERICK, JONES, CHRISTOPHER, DAVID, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1995
Main classification: H05B41
IPC codes: G01N27, H05B41