APPARATUS FOR DETERMINING THE DEWPOINT

Abstract: In a humidity or dewpoint-measuring instrument of the chilled mirror type, a light source (16), such as an LED and a photodetector (18) are mounted at locations remote from the cooled mirror (10) and fibre-optics (28, 30, 32) carry light from the light source (16) to the mirror (10) and, after reflection by the mirror (10), to the photodetector (18). This arrangement allows the mirror (10), with the fibre-optics (28, 30, 32), to be mounted within an elongate probe (114) of small diameter, and the light source and photodetector to be located, for example, in a box to which the probe is connected. Thus, the apparatus may be used to assess the humidity of hot gases, for example, without the light source (16) or photodetector (18) being affected by the temperature of such gases.

Description: DESCRIPTION OF INVENTION Title: "Apparatus for determining the dewpoint" THIS INVENTION relates to apparatus for determining the dew- point or for measuring humidity by determination of the dew- point. Instruments for measurement of humidity and dewpoint employing the chilled mirror principle are well known. Generally, in such an instrument, a mirror is mounted on a cooling device and a light source is arranged to direct light on the mirror whilst a photodetector is arranged to receive such light after reflection from the mirror. In operation, the mirror is cooled progressively by the cooling device and the formation of dew on the mirror is detected by sensing the reduction in intensity of the reflected light, as sensed by said photodetector, occurring when dew forms on the mirror, the temperature of the mirror at this point being sensed by a temperature sensor. A problem with this arrangement is that the minimum diameter of casing or the like within which the cor.ponents can be mounted depends on the size of the light source and photodetector and the housing in which, typically, the lamp and photodetector are mounted to give the correct angle of reflection. Usually, for compactness, robustness and reliability and from considerations of performance and cost, the photo¬ detector is a semi-conductor device such as a photo-diode or photo-transistor, whilst the light source may also be a semi-conductor device such as a light-emitting diode, (LED). In such an arrangement, as the lamp and photodetector (the opto electronics devices) are in close proximity to the mirror, the temperature at which measurements can be made is limited to the range of safe operating temperatures for such devices, (the upper limit of such range generally being relatively low, for example 70°C). It is an object of the present invention to provide a dewpoint or humidity measuring apparatus in which the above- noted disadvantages may be avoided or reduced. According to the invention, there is provided a dew- point or humidity measuring instrument including a mirror, means for cooling the mirror, means for sensing the temper¬ ature of the mirror, a light source and a photodetector and wherein a fibre-optic or light pipe arrangement is provided extending from the light source to the mirror and from the mirror to the photo-detector, whereby the light source and photodetector can be spaced substantially from said mirror with the interposition of said fibre optic or light-pipe arrangement and thus can readily be maintained at a temper¬ ature substantially different from said mirror. An embodiment of the invention is described below by way of example with reference to the accompanying drawings, in which:- FIGURE 1 is a fragmentary view in longitudinal section of a known dew-point instrument, FIGURE 2 is a fragmentary view in longitudinal section of a first form of dew point instrument embodying the invention, and FIGURE 3 is a view, corresponding to Figure 2, of another embodiment. Referring to Figure 1, in a known dew-point measuring instrument, a mirror 10 is mounted on a cooling device 12 (for example a Peltier cooling device). Disposed at some distance above the mirror 10 are a light source 16 and a photodetector 18 mounted in a holder or housing 20 which supports the light source 16 and photodetector 18 in predet¬ ermined positions relative to one another such that light from the light source 16, after striking the mirror 10, will be reflected onto the photodetector 18. The device 12, mirror 10 and holder 20 with the light source 16 and photo¬ detector 18 are supported within a cylindrical casing 14. In such a conventional arrangement, the minimum value of the diameter D of the casing 14 is determined by the sizes of the light source 16 and photodetector 18 and of the housing 20, and especially by the relative positions required for the light source 16, photodetector 18 and mirror 10 in order to give the correct angle of reflection (from the light source 16 to the detector 18). Consequently the casing 14 is inconveniently wide for some applications. Referring to Figure 2, (in which parts corresponding with parts in Figure 1 have the same reference numerals), a mirror 10 and a cooling device 12 on which the mirror is mounted are disposed within an elongate cylindrical casing 114 * of a diameter indicated at D1. Spaced longi¬ tudinally from the mirror 10 along the axis of the casing 114 are shown a light source 16 and a photo¬ detector 18 (e.g. photo-diode or photo-transistor). Extending longitudinally within the casing 114 from the mirror 10 to the light source 16 and the photo¬ detector 18 are fibre optics 24, for example optical glass fibres, such as used to form fibre optic cable. The optical fibres are arranged in a single, (e.g. generally cylindrical) bundle 28 in the vicinity of the mirror, such single bundle terminating in an end face 26 spaced somewhat from the mirror 10. Some of these optical fibres extend to the light source 16 and some to the photo¬ detector 18, the fibres leading to the light source 16 being preferably separated from the fibres leading to the photo¬ detector 18 at a position relatively remote from the mirror and being formed into respective separate bundles 30,32, led respectively to the light source and the photodetector, so that the fibre bundle extending from the mirror branches into two smaller bundles 30 and 32 extending to the light source 16 and the photodetector 18 respectively. Within the larger bundle 28, in the region close to and extending to the bundle end face 26 directed towards the mirror, the optical fibres leading to the light source 16 are preferably interspersed evenly with the fibres leading to the photodetector 18. In operation, light from the light source 16 is conducted along the optical fibres extending from the light source 16, to emerge from the fibres and strike the mirror 10 substantially normally and is reflected by the mirror 10 back towards the bundle end face 26 so that at least some of the reflected light enters the optical fibres leading to the photodetector 18 and thus is directed to the photodetector 18. It will be appreciated that there is no necessity for the light source 16 and photodetector 18 to be at the same axial position along the casing 114. In Figure 3, parts corresponding with parts in Figures 1 and 2 again have the same reference numerals. Figure 3 shows an arrangement similar to that of Figure 2 but with two separate fibre optic bundles, one (30) extending from the light source 16 to a position just in front of the mirror 10 and the other (32) extending from just in front of the mirror 10 to the photodetector 18. The arrangement of Figure 3 operates in the same way as that of Figure 2 except that the optical fibres of the two sets 30,32, leading res¬ pectively to the light source 16 and the photodetector 18, are not interspersed. The last-noted feature also makes it possible for the end faces 26a, 6b of the two bundles of optical fibres to be angled slightly with respect to each other and the mirror surface, so that the light is reflected at an angle from the mirror, without the need significantly to increase the diameter D1 of the casing 114. It will be appreciated that in the embodiments of Figures 2 and 3 the bundles of optical fibres referred to may, in practice, be bundles of pre-formed strands, each strand comprising a large number of very fine individual fibres, and that the reference, in relation to Figure 2, to the even distribution of fibres from the light source and fibres from the photodetector should be taken, in such a case, as being a reference to even distribution of such strands from the light source with such strands from the photodetector. It will be understood that, in the arrangements of Figures 2 and 3» as in the known arrangement of Figure 1, ports (not shown) are provided in the wall of the outer casing 114 for the passage of air or other gas of which the moisture content (or other vapour content) is to be assessed, to and from the region adjacent the mirror 10. Such ports may, for example, be located in the wall of the casing 114.adjacent the mirror 10. The light source 16 may be a light emitting diode, or a filament lamp or any other light source of controllable intensity. (The remote location of the light source per¬ mitted by the fibre optics of course allows the use, if desired, of light sources which cannot conveniently be made small enough to fit within a practicable probe). One advantage of the arrangements shown in Figures 2 and 3 is that a much smaller diameter, (shown in Figures 2 and 3 by D1) is required to house such an arrangement, (as compared with the diameter D in Figure 1 required to house the conventional arrangement). Also the light source and photodetector can be mounted at a considerable distance from the mirror. Accordingly, the casing 114 together with the compon¬ ents 10, 12, 28, 30, 32, 16 and 18 mounted therein may take the form of an elongate thin probe. Furthermore, there is, of course, no need for the lamp and photodetector to be disposed in line with the mirror and it is possible, for example, for the mirror and cooling device to be mounted within a thin tubular probe, (which may be rigid or flexible), with the optical fibre bundle or bundles extending along the probe to emerge into a box, for example, on which the probe is mounted and which houses the lamp, photodetector and the bulk of the electronic cir¬ cuitry, etc. of the instrument. The fibre optics can withstand much higher temperatures than the opto electronics (16,18), which need not be in the same hot environment, and accordingly the instrument can be used to determine high- temperature dew-points, for example in exhaust gases or of gases incorporating vapours of non-aqueous liquids of high boiling point. In each of Figures 2 and 3 > (as in Figure 1) there is indicated at 40 a heat conductor or heat pipe arranged co- axially within the casing 114 for conducting heat away from the device 12. Power supply conductors (not shown) for the Peltier cooling device 12 and conductors (not shown) leading to electrical or electronic temperature transducers incor- porated in the cooling device 12, for carrying electrical signals indicative of the temperature of the mirror 10, extend from the device 12 along the casing 114 alongside the heat conductor 40. A support member, (not shown), for example in the form of a spacer or spider around the conductor 40, serves to centralize the conductor 40 within the casing 114. Whilst, for clarity of illustration, the heat conductor 40 is illustrated as extending from the region of the mirror 10, along the interior of the casing, in the opposite direction from the fibre optic bundles 24, 30, 32, it will be understood that, in practice, it will generally be desired to place the mirror 10 close to a free end of the probe, with the fibre optic bundles 28, 30, 32 being led past the mirror 10 and along the interior of the probe casing 114 adjacent the heat conductor 40 and in the same direction as the heat conductor 40.

Claims: CLAIMS : 1. A dewpoint or humidity measuring instrument including a mirror, means for cooling the mirror, means for sensing the temperature of the mirror, a light source and a photo¬ detector and wherein a fibre-optic or light pipe arrangement is provided extending from the light source to the mirror and from the mirror to the photodetector, whereby the light source and photodetector can be spaced substantially from said mirror with the interposition of said fibre-optic or light pipe arrangement and thus can readily be maintained at a temperature substantially different from said mirror. 2. A dewpoint or humidity measuring instrument according to Claim 1, wherein said fibre-optics or light pipe arrange¬ ment comprises a plurality of optical fibres which, adjacent the mirror form a unitary bundle having an end surface, opposing the mirror, in which the optical fibres terminate, said optical fibres, remote from the mirror, being formed into two subsidiary bundles, a first one of which terminates adjacent the light source, in an end surface in which the optical fibres of that subsidiary bundle terminate such that light emitted by the light source can enter the fibres of said first subsidiary bundle via the last-mentioned end surface, and the second of which subsidiary bundles termin¬ ates adjacent the photodetector, in an end surface in which the optical fibres of said second subsidiary bundle terminate such that light transmitted along the fibres of said secondary bundle can pass through the last-mentioned end surface, to the photodetector. 3- A dewpoint or humidity measuring instrument according to Claim 2, wherein the fibres which are included in said first subsidiary bundle are interspersed, in said unitary bundle, with the fibres which are included in said second subsidiary bundle. 4. A dewpoint or humidity measuring instrument according to Claim 1, wherein said fibre-optic or light pipe arrange¬ ment comprises a first and a second bundle of optical fibres, the first said bundle having an end surface, adjacent the light source, in which the fibres of that bundle terminate and through which light from the light source can enter the fibres of that bundle, the second such bundle having an end surface adjacent the photodetector in which the fibres of the second bundle terminate and through which light transmitted along the fibres of said second bundle can pass to the photodetector, each said bundle terminating, adjacent the mirror, in a respective end face, opposing the mirror, said end faces of said bundles adjacent the mirror being angled with respect to each other and to the mirror so as to ensure that light emerging from the first bundle, to strike the mirror, will, after reflection from the mirror, be directed along such a path as to strike said end face of the second bundle to enter the fibres of said second bundle.

Inventors: DADACHANJI FALI MINOCHER, DADACHANJI, FALI, MINOCHER
Applicants: PROTIMETER PLC
Application year: 1991
Main classification: G01N25
IPC codes: G01N25