GB2043242A

GB2043242A - Dew point meter

Info

Publication number: GB2043242A
Application number: GB8004374A
Authority: GB
Original assignee: Protimeter PLC
Current assignee: GE Protimeter Ltd
Priority date: 1979-02-14
Filing date: 1980-02-08
Publication date: 1980-10-01

Abstract

In a dew-point meter of the kind in which dew formation on a cooled mirror 10 is detected by the resultant scattering thereby of light directed thereon by a light source 12, as detected by a light sensitive detector 14, the mirror 10, light source 12 and detector 14 are housed in a casing partly comprised by an air permeable element 18 providing no line of sight passages and formed of opaque material, preferably of sintered metal particles, which allows air and water vapour to enter the casing, while excluding light and also dust. <IMAGE>

Description

SPECIFICATION Dew point meter Apparatus is known, for measuring the dew point, which is electrically operated and which includes a body (for example a mirror) having an optically smooth surface, cooling means, for example thermo-electric cooling means, for cooling said body, a light source arranged to direct a light beam onto said optically smooth surface, and a light sensitive element, such as a photocell, arranged to detect the scattering from said smooth surface of light from the light beam directed onto said surface, said scattering resulting from the formation of dew on said surface.

Apparatus of the above noted general character is herein referred to as being "of the kind specified".

There are various modes of operation which may be employed, in apparatus of the kind specified, in order to achieve a determination of the dew-point of the ambient air, but whatever the mode of operation it is clear that it is necessary that the space in the immediate vicinity of said optically smooth surface should be in communication with the atmosphere around the apparatus and at the same time that said surface and the light sensitive element should be shielded from light from outside the apparatus. To this end apparatus of the kind specified is normally provided with an outer casing which is light-tight but in free communication with the surrounding air, this being achieved, in the known constructions, by the use of labyrinthine light traps or similarly elaborate constructions, which contribute to increased bulk and add to manufacturing expense.

It is an object of the present invention to provide an improved dew-point measuring apparatus of the kind specified in which the need for elaborate light traps and the like is avoided.

According to the present invention there is provided a dew-point measuring apparatus of the kind specified in which the body having the optically smooth surface, the light source and the photosensitive element are enclosed in a casing partly constituted by a porous air-permeable element providing no line-of-sight passages therethrough, the material of said element being opaque whereby no light is transmitted therethrough.

Preferably said porous air-permeable element comprises a mass of granules sintered together.

In a preferred embodiment of the invention the air-permeable element comprises fine granules or balls of metal, (e.g. of a few microns in diameter) sintered together to form a porous mass. Such elements are known perse as filter elements. The thickness of the element may be of the order of several millimetres.

The casing enclosing the light source, photosensitive element, said body and the cooling means therefor may, with the elements therein and said porous element, form a compact probe connected with the part of the apparatus including processing circuitry, power supplies and the like by a flexible cable, the porous sintered element forming for example a cap which may be removed for access to the interior of the casing.

Embodiments of the invention are described below with reference to the accompanying drawings wherein: Figure lisa schematic sectional view through part of one form of apparatus embodying the invention, Figure 2 is a schematic sectional view through part of another form of apparatus embodying the invention, and Figure 3 is a schematic sectional view through part of a yet further form of apparatus embodying the invention.

In Figure 1, a part of a dew-point measuring apparatus, in the form of a sensing body or probe electrically connected with remote circuitry (not shown), comprises a casing within which is disposed a mirror 10, means (not shown) associated with the mirror, for cooling the mirror, means (not shown) for measuring the temperature of the mirror, an electric lamp 12, and a photo-sensitive electronic device 14 of known kind. In the arrangement shown the lamp 12 and device 14 are mounted in a base portion 15 of the casing which also supports a structure 16 which in turn supports the mirror 10 and the associated cooling and temperature measuring means.

The elements referred to are, of course, so arranged that light from the lamp 12 strikes the mirror 10 and is reflected thereby onto the device 14.

The structure 16 is encircled by a porous air permeable annular element 18 in the form of a hollow cylinder affording a peripheral wall of the probe, the cylindrical element 18 engaging the base portion 15 of the casing at one end, whereby said one end of the element 18 is closed, the element 18 being engaged at its other end by a circular opaque cap 20, for example of metal, closing the other end of the cylindrical element. The element 18 is preferably releasably clamped between the base porion 15 and the cap 20, the latter being, for example, screwed onto a projection extending from the structure 16.

The element 18 is preferably formed from fine granules or balls of metal, for example having diameters in the range 5 to 50 microns, sintered together to form a porous mass. The element 18, being permeable to air and water vapour, does not interfere appreciably with the access of ambient air and water vapour to the mirror 10, but as the element 18 affords no line-of-sight passages therethrough, and is thus entirely opaque, allows no light to enter the casing which might produce spurious responses in the device 14. Access to the interior of the casing, for maintenance or inspection of mirror 10, etc., may be had very readily by removal of cap 20 followed by removal of the sintered element 18.

Electrical connections, for example for the supply of current to lamp 12 and to the electrical cooling means associated with mirror 10, and for carrying electrical signals from the device 14 and the temperature sensing means associated with mirror lotto remote circuitry, extend in the form of a flexible cable from the sensing body or probe to a remotely disposed part of the apparatus comprising processing circuitry, display means, power supplies and the like.

Figure 2 shows a variant of the sensing body or probe of Figure 1, in which the base part 25 of the casing is in the form of a cup or box closed by a lid in the form of a flat, porous, light impermeable disc 28, corresponding to the element 18 of Figure 1, and again preferably formed from fine granules or balls of metal e.g. having diameters in the range 5 to 50 microns, sintered together to form a porous mass.

As shown, the mirror 10 and the mounting therefor, with the associated electrical cooling and temperature sensing means may be disposed within the cavity defined within the cup or box, on a side wall of the latter, with the lamp and light sensitive device, (not shown separately in Figure 2 but together indicated at 29 in Figure 2) disposed opposite the mirror on the opposite side of this cavity. The disc 28 may be secured to the base part 25 by a ring or frame 30 engaging the periphery of the disc 28 and detachably secured to the upper ends of the wall or walls of the base part 25.

The variant of Figure 3 corresponds substantially to that of Figure 1 except that the porous, light impermeable element, instead of being in the form of a cylinder clamped between a cap and the base portion of the casing, is in the form of an inverted cup 38, with a frusto-conical downwardly diverging side wall 39 terminating in a peripheral flange 40 engaged by a clamping ring 41 by means of which the inverted cup 38 is secured to the base part 35 of the casing, the mirror and associated structure being housed within the inverted cup, in much the same way as in the embodiment of Figure 1.

In each of the embodiments described, the sizes of the interstices between particles in the sintered elements 18, 28 or 38 is such that the element also acts as a dust filter.

Claims

1. A dew-point measuring apparatus of the kind specified in which the body having the optically smooth surface, the light source and the photosensitive element are enclosed in a casing partly constituted by a porous air-permeable element providing no line-of-sight passagestherethrough, the material of said element being opaque whereby no light is transmitted therethrough.

2. Apparatus according to claim 1 wherein said porous air-permeable element comprises a mass of granules sintered together.

3. Apparatus according to claim 1 wherein the air-permeable element comprises fine granules or balls of metal, sintered together to form a porous mass.

4. Apparatus according to claim 2, or claim 3 wherein the diameters of the granules sintered together lie in the range from 5 to 50 microns.

5. Apparatus according to any preceding claim wherein the thickness of the element is of the order of3 mm.

6. Apparatus according to any preceding claim wherein said porous, air-permeable element is in the form of a flat disc.

7. Apparatus according to any of claims 1 to 5 wherein said porous air permeable element is in the form of a ring defining a peripheral wall of at least part of the casing, the ends of the ring being closed off by opaque members constituting further parts of the casing.

8. Apparatus according to any of claims 1 to 5 wherein said porous, air-permeable element is in the form of a hollow cap or cup the open side of which is engaged by an opaque member constituting a further part of the casing and at least some of the elements of the apparatus being enclosed within said cap or cup.

9. Apparatus according to any preceding claim wherein the casing enclosing the light source, photo-sensitive element, said body and the cooling means therefor, with the elements therein and said porous element, form a compact probe connected with a part of the apparatus including processing circuitry, by a flexible cable.

10. Apparatus according to claim 1 and substantially as hereinbefore described with reference to, and as shown in, Figure 1 of the accompanying drawings.

11. Apparatus according to claim 1 and substantially as hereinbefore described with reference to, and as shown in, Figure 2 of the accompanying drawings.

12. Apparatus according to claim 1 and substantially as hereinbefore described with reference to, and as shown in, Figure 3 of the accompanying drawings.

13. Any novel feature or combination of features described herein.