GB1572084A - Means for measuring the water content of a gas - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO MEANS FOR MEASURING THE WATER CONTENT OF A GAS (71) I, SECRETARY OF STATE FOR DEFENCE, LONDON, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to apparatus for measuring the amount of water in the atmosphere. It is particularly concerned with airborne instrumentation to measure the amount of water in the local environment of flying aircraft.

According to the present invention apparatus for measuring the amount of water in an atmosphere includes a capacitor comprising a plurality of electrical conductors each separated by a gap accessible in use to the atmosphere to be measured, at least one of the conductors being coated, at least within the gap, with a layer of solid insulating material the surface whereof has a non-zero angle of contact with water, intake means for ducting moisture from the atmosphere to the gap, and moisture removal means for removing moisture from the capacitor. Preferably the whole of the opposing face of each conductor is coated entirely with the said solid insulating material. In a simple form of the invention the conductors are plates.

While the conductors may be made of metal the use of semi-conducting material of good conductivity is also possible. In this latter case the insulating material may be a semi-conductor of low dielectric constant and low conductivity compared with water.

According to a first aspect of the invention one type of capacitor, arranged for direct impingement by incident airflow, may be so housed that the airflow will not remove water from the gap during a measurement stage of operation. The gap is then preferably of capillary dimensions, that is, smaller than the diameter of the drops or droplets of water collected therein from the atmosphere.

In apparatus of this type, i.e. capillary, the preferred solid insulating material is a coating on the or each opposing face having an angle of contact with water neither so low as to give rise to excessive wetting of the insulant surface nor so high as to inhibit capillary action between the plates (in fact leaving a fairly broad band of acceptable angles and materials). In order to minimise peripheral region problems with this type of capacitor the solid insulant coating in those regions is preferably one having a high angle of contact with water, whereby the mobility of water thereon is enhanced.This difference in requirement of the insulation surfaces within the gap and on the periphery thereof may be accomplished by first coating the conductors with a plastics insulant having a medium angle of contact with water and then adding to those faces thereof which will in use oppose within the gap a layer of varnish having a higher angle of contact with water.

According to a second aspect of the invention the apparatus may include collection means arranged to collect water droplets and to supply them to the capacitor. The collection means may comprise a rotatable member having a cylindrical surface, or it may include a centrifuge device of the type adapted to filter a gas flow and comprising a tube containing vanes in spiral array leading to an outlet for cleaned gas surrounded by another for moisture rich gas, the latter being arranged to deliver moisture to the capacitor. In apparatus according to the second aspect the gap may be larger than capillary.

In one form of this aspect of the invention a rotatable cylinder may be arranged for presentation of part of its surface at a time to the impinging atmosphere, and for conveyance of moisture collected thereon to removal means, and a plate capacitor of the type hereinbefore described may be arranged to collect the moisture from the removal means. In place of a rotatable cylinder a flexible tape may be used.

In another form of this aspect of the invention, the cylinder or tape may form one plate of the capacitor, the other plate having a surface corresponding thereto. Thus the apparatus may include two members having co-operating cylindrical surfaces, one member being arranged for rotation with respect to the other, the outer member having at least one aperture permitting impingement of water droplets on the inner. The outer member may have a plurality of apertures, of different sizes and facing different directions. Scraper means may be provided preferably on the inner surface of the outer member and bearing on the inner member, to enable more water to be collected than that which impinges on the inner member during an arc of revolution from an aperture to corresponding scraper means.

In non-capillary embodiments of the invention an insulative layer may be preferred which has an angle of contact with water relatively higher than that of opposing faces in devices according to the first aspect of the invention.

For the purposes of this specification a cylinder is a body of generation including a sphere.

Particularly in apparatus according to the first aspect of the invention but also possibly in that of the second the plates need not be parallel, but may be further apart at the opening where water is collected than remote therefrom to assist the collecting forces on aggregating the water into a single homogenous mass and thus minimise fringe problems. In this case the consistency of the capacitor output signal may be preserved by a suitable variation in insulation thickness whereby changes in drop contact area are compensated. There may indeed be advantages in apparatus according to the invention wherein the air gap is partly closed by insulant so that it is in the form of a channel. Conductors of circular cross-section may therefore be used to form the plates of the capacitor.

According to another feature of the invention the moisture removal means comprises a purge facility arranged to blast gas through the capacitor after a measurement cycle.

Alternatively the moisture removal means may comprise a heater which may also prevent formation of ice thereon. In many cases both a purge facility and a heater may be preferred.

The heater may be part of or associated with one or more of the plates. Alternatively, it may be located downstream of the capacitor. This can be particularly suitable when a flat plate capacitor is employed. If the heater is of radiant type it can enable inspection of the capacitor plates for dirt and damage.

Although it is probable that vibration of the carrier and the effect of the airflow will assist in the amalgamation of droplets in the capacitor, the apparatus may include a vibrator for this purpose.

It will be appreciated that apparatus in accordance with the invention generally operates on a cycle basis, there being collection, measurement and discharge stages in the cycle. The measurement stage may involve the generation of an atmospheric water content signal, a function either of the time taken to reach a given capacitance value or of the capicitance value after a given sampling time. A capacitance signal may be derived conventionally, for example in an alternating current bridge circuit or in a resonant circuit arrangement.

The capacitance signal may be utilised to operate a visual or audible indicator (digital, analogue or threshold) or to provide a signal suitable for recording and subsequent analysis together with associated environmental parameters such as temperature and airspeed. If the output is directly displayed the reading relates to the total collection of water droplets and ice crystals whose trajectories impinge on the collection area at that speed. This quantity is of direct interest when considering the amount of water deposited on related parts of an aircraft structure. If, however, one is more concerned with the actual atmospheric water content the signal should be divided by a velocity signal, an operation which can of course be performed electronically.

The capacitance of capacitors employed in apparatus according to the invention is proportional to (A-aw) Ei Ea aw EI Ew + 2di Ea + d, Ei 2di Ew + cia Ei where A is the plate area and aw is the area covered by water, di and da are the thicknesses of the insulating layer and the gap respectively, and Ei, Ea, Ew are the dielectric constants for the insulating layer, air and water respectivelv. The value of Yew is 80.3 at 200C and the temperature coefficient is approximatelv -().230C-'. The dc conductivity of pure water is negligible but additions of even small amounts of impurity rapidly increase this and add also to the value of Ew. In order to render the apparatus insensitive both to airborne impurities and temperature variations within the atmosphere range, the insulating layer may be made sufficiently thick for the capacitance when filled with water to be determined largely by the characteristics of the insulators while the impure water provides a low impedance path between them. Variations in this impedance will then have a negligible effect on the overall capacitance. An insulating layer dielectric constant (Ei) of the order of 3.0 is suitable and various electrical varnishes and adhesive plastics films meet the requirements for the solid insulating layer.

Various examples of apparatus in accordance with the invention will now be described with reference to the drawings accompanying the provisional specification, of which: Figure 1 is a diagrammatic cross section of capacitance plates, Figure 2 is a diagrammatic cross section of a first sensor, Figure 3 is a diagrammatic cross section of a second sensor, Figure 4 is a diagrammatic cross section of a third sensor, Figure 5 is a diagrammatic cross section of a fourth sensor, Figure 6 is a side view of the fourth sensor, Figure 7 is a diagram of a typical electrical circuit, Figure 8 is a longitudinal section of a fifth sensor, Figure 9 is a longitudinal section of a sixth sensor, Figure 10 is a view on section X-X in Figure 8, Figure 11 is a view on section XI-XI in Figure 9, and Figure 12 is a graph of results with an experimental apparatus.

The capacitor plates illustrated in Figure 1 comprise flat metal plates 10 coated entirely with a solid insulating layer 11 of plastics material the outer surface of which, when cured, has a high angle of contact with water. The opposing faces of the coated plates are further coated with a layer 12 of electrical varnish having a low angle of contact with water. The plastics coating is rounded at a forward side of the plates to further encourage water into the air gap between them.

The sensor illustrated in Figure 2 comprises a resin housing 20, sheathed in a metal skin 21 and aerofoil in shape. The housing 20 has an opening at its leading edge within which is mounted a plurality of capacitor plates 22 of the type shown in Figure 1. The plates 22 are each spaced apart by a gap of the order of 0.2 mm. To the rear of the plates 22 is a plenum chamber which also communicates with the opening via pressure equalisation ducts 23. A purge blast orifice is sited in the plenum chamber at 24. To the rear of this the chamber is partitioned by a translucent screen 25, behind which is situated a radiant heater 26. There are also heaters 27 embedded in the housing 20 and operable to maintain a temperature above freezing in the openign and capacitor cavity.

The sensor illustrated in Figure 3 has a resin housing 30 sheathed in a metal skin 31 and aerofoil in shape. Within an opening at the leading edge of the housing 30 is a right cylinder 32 attached to a motor (not shown). Within the housing to the rear of the cylinder is a capacitor unit 33 comprising capacitor plates of the type described with respect to Figure 1, spaced apart by air gaps of the order of 0.2 mm wide. A scraper 34 is formed on the housing 30 to act on the surface of the cylinder 32 and to feed the water collected on the cylinder to the capacitor unit.

A heater 35 is embedded in the housing. Purging is accomplished spanwise by ducts not shown.

In the sensor illustrated in Figure 4, a capacitor is constructed at the mating faces of a housing 40 and a right cylinder 41. The housing and cylinder are formed of a plastics material, the housing being aerofoil in shape and having a leading edge cavity receiving the cylinder.

One capacitor plate 42 is formed on the circumference of the cylinder and the other 43 on the cavity wall. An insulating layer 44 of plastics material covers each plate. There is left a gap of the order of 1 mm between the insulating faces. A scraper 45 is formed on a lip of the housing 40, to bear on the cylinder 41. A heater element 46 is embedded in the cylinder 41. The cylinder is rotatable by a motor (not shown) and the capacitor clearable by purge means (not shown) acting spanwise.

The sensor illustrated in Figures 5 and 6 comprises a right cylinder 50 made of plastics material and a right cylindrical housing 51 coaxially surrounding it. A capacitor plate 52 is formed on the whole cylindrical surface of the cylinder 50 and coated with a layer 53 of insulant. The housing 51 is formed of plastics insulant having a capacitor plate endoskeleton 54. Apertures 55 are formed in the outer cylinder at right angular positions and various locations therealong. Each is surrounded by an external lip 56. Adjacent appropriate aperture edges the housing forms scrapers 57 acting on the inner cylinder 50. A heater 58 is embedded in the inner cylinder 50. The motor 59 is connected to the cylinder 51 and purge means 60 are located at one end of the sensor.

Figure 7 illustrates a measurement circuit. In it a 1 k resistor 70 and an opposing pair of Zener diodes 71, 72 are connected between a 10 volt 400 Hz supply and earth. A sensor capacitance unit 73, a diode 74 and an analogue moisture content indicator 75 are in parallel with the diodes 71, 72 and a switch 76, diode 77and a 1 k load 78 are in parallel with the diode 74 and the indicator 75. The switch is controlled by the time circuit (not shown) which opens and closes in at fixed time intervals and which also controls the operation of the purge means and where appropriate the motor rotating the cylinder 32, 41 or the housing 51. When the switch is closed the circuit is in the measuring phase of operation, when open in the leakage current detection mode. The circuit is thus applicable to any of the sensors hereinbefore described.

The sensors illustrated in Figures 8 to 11 comprise a capacitor assembly situated within a perforated metal screen 80, 90 which is shown as being of a cylindrical construction. The elements of the capacitor are in each case mounted on a central insulating core 81,91 which in the case of Figure 8 includes connections to the individual capacitor plates. A plurality of insulating separators 82, 92 serve to position the capacitor assembly and prevent the free passage of air and water between circumferentially separated perforations in the screen 80,90.

eaters, not shown, within the separators 82, 92 serve to maintain the temperature of the screen above the freezing point of water. The core 81,91 also contains heaters, not shown, by which the temperature of the capacitor can be varied as desired.

The sensor illustrated in Figures 8 and 10 includes a capacitor comprising a series of parallel connected plates 83 which are positioned alternately with parallel connected plates 84. In the related construction depicted in Figures 9 and 11 the pair of conductors 93 and 94 comprising the capacitor are of spiral form. In both cases the elements of the capacitor are coated with an insulating layer of enamel 0.001"-0.002" thick.

The sensor described with respect to Figure 2 is particularly suitable for use in situations where the airflow to be sensed is substantially unidirectional, for example on a fixed wing aircraft or associated with a vane whereby it is alignable with airflow. It is deployed with its leading edge opening facing the airflow. The heater is arranged to maintain a temperature above freezing at the opening and within the capacitor unit cavity. As moisture laden air enters the opening moisture is deposited on the plate 22 and moves, because of the plate edge insulant surface and the capillary action into the air gaps. In the gaps the drops coalesce, aided by the nature of the local insulant surface and the pressure equalisation ducts 23. This changes the capacitance and a corresponding reading of the indicator 75 is generated.After a predetermined time interval the indicator reading is frozen and the purge system operated to clean out the capacitor unit. The cycle is then recommenced. Examination of the cleanliness and state of this sensor on the ground can readily be carried out with the heater switched on.

The sensor described with respect to Figure 3 is suitable for use on any aircraft, or on fixed wing aircraft where a comparatively low moisture content determination is required, and is deployed with the leading edge opening facing an airflow to be analysed. The heater 35 is arranged to ensure a temperature above zero at the opening end in the region of the capacity unit. The cylinder rotates constantly to convey moisture incident thereon to the scraper 34 where it is removed and absorbed by the capacitor unit 33. After a predetermined time interval the reading of the indicator is frozen and the purge means operated.

The sensors described with respect to Figures 4, 5 and 6 are operated in a similar manner to that described with respect to Figure 3. The sensor described with respect to Figures 5 and 6 is particularly suitable however, for use with helicopters and aircraft where the airflow may vary considerably in direction.

The sensors described in association with Figures 8 to 11 are particularly suitable for operation where variations in the direction of airflow may occur. An improved degree of symmetry of the environment with respect to the device may be obtained in an allied embodiment by means of a continuous rotation or oscillation of the sensor about its cylindrical axis.

The sensors described with respect to Figures 8 to 11 may be operated in a similar manner to that described with respect to Figure 2. In addition the sensors are suitable for continuous operation. This latter mode of operation is achieved by offsetting the rate of arrival of moisture through perforations in the screen by a process of evaporation and convection which serves to release water from the device in a controlled manner.

The temperature of the core 81, 91 which governs the rate of evaporation from the capacitor, may be adjusted manually to maintain the amount of collected water below a maximum, or alternatively, the temperature may be varied automatically according to the amount of water present and the rate of increase of the amount. Saturation of the capacitor, which does not have as large a storage capacity as other embodiments, is effectively prevented by means of the servo control of the capacitor temperature. A time history of incident water may be obtained from a record of heater power since the energy input depends on the amount of water evaporated for a given environmental and screen temperatures.

The results illustrated in Figure 12 were obtained from an experimental device employing a capacitor unit comprising twenty-one anodised aluminium plates 3 in x 1/2 in x 1 mm, each sprayed with electrical varnish of specification DTD 900/4165 AFS 569, the separation between the varnished plates being 0.2 mm. Two 18 watt lamps provided heating from behind frosted glass screen to the rear of the capacitor unit. The curves indicate that there is a gaugeable co-relation between the actual moisture content of the air and the output of the capacitor unit. The anomalous rise in curve 2 was due to an insulation failure and the fall in curve 4 to icing.

Claims (29)

WHAT I CLAIM IS:

1. Apparatus for measuring the amount of water in an atmosphere and including a capacitor comprising a plurality of electrical conductors each separated by a gap accesible in use to the atmosphere to be measured, at least one of the conductors being coated, at least within the gap, with a layer of solid insulating material the surface whereof has a non-zero angle of contact with water, intake means for ducting moisture from the atmosphere to the gap, and moisture removal means for removing moisture from the gap.

2. Apparatus as claimed in claim 1 and wherein the whole of the opposing face of each conductor is coated entirely with the said solid insulating material.

3. Apparatus as claimed in claim 1 or claim 2 and wherein the conductors are plates.

4. Apparatus as claimed in any one of claims 1 to 3 and wherein the conductors are made of metal.

5. Apparatus as claimed in any one of claims 1 to 3 and wherein the conductors are made of semi-conducting material of good conductivity and the insulating material is a semiconductor of low dielectric constant and low conductivity compared with water.

6. Apparatus as claimed in any one of claims 1 to 5 and wherein capacitor is arranged for direct impingement by incident airflow, and so housed that the airflow will not remove water from the gap during a measurement stage of operation.

7. Apparatus as claimed in claim 6 and wherein the gap is of capillary dimensions.

8. Apparatus as claimed in claim 7 and wherein the solid insulating material is a coating on the or each opposing face having an angle of contact with water such that the material is substantially wetted by the water and yet permits capillary action between the plates.

9. Apparatus as claimed in any one of claims 6 to 8 and wherein the solid insulant coating in capacitor peripheral regions is one having a high angle of contact with water, whereby the mobility of water thereon is enhanced.

10. Apparatus as claimed in any one of claims 1 to 5 and having collection means associated with the intake and arranged to collect water droplets and to supply them to the capacitor.

11. Apparatus as claimed in claim 10 and wherein the collection means comprises a rotatable member having a cylindrical surface.

12. Apparatus as claimed in claim 11 and wherein the rotatable cylinder is arranged for presentation of part of its surface at a time to the impinging atmosphere, and for conveyance of moisture collected thereon to removal means, and a plate capacitor of the type hereinbefore described is arranged to collect the moisture from the removal means.

13. Apparatus as claimed in claim 10 and wherein the collection means includes a flexible tape arranged for presentation of part of its surface at a time to the impinging atmosphere, and for conveyance of moisture collected thereon to removal means, and a plate capacitor of the type hereinbefore described is arranged to collect the moisture from the removal means.

14. Apparatus as claimed in claim 10 or 11 and wherein the collection means provides one plate of the capacitor.

15. Apparatus as claimed in claim 14 and comprising an outer and an inner member having co-operating cylindrical surfaces providing the capacitor plates, one member being arranged for rotation with respect to the other, and the outer member having at least one intake and associated collection means whereby in use water droplets enter the intake and impinge on the inner member and the collection means assists in agglomerating them.

16. Apparatus as claimed in claim 15 and wherein the collection means comprises a scraper.

17. Apparatus as claimed in claim 16 and wherein the scraper is located on the inner surface of the outer member and bears on the inner member.

18. Apparatus as claimed in any one of the preceding claims and wherein the gap is wider approximate to intake than remote therefrom.

19. Apparatus as claimed in claim 18 and wherein the insulation thickness is varied to compensate for changes in drop contact area.

20. Apparatus as claimed in any one of the preceding claims and wherein the moisture removal means comprises a purge facility arranged to blast air through the capacitor after a measurement cycle.

21. Apparatus as claimed in any one of claims 1 to 19 and wherein the moisture removal means comprises a heater.

22. Apparatus as claimed in claim 21 and wherein the heater is also arranged to prevent formation of or remove ice on the apparatus.

23. Apparatus as claimed in claim 20 and having a heater for preventing formation of ice thereon.

24. Apparatus as claimed in claim 21 or 22 and wherein the heater is part of at least one of the conductors.

25. Apparatus as claimed in claim 21 or 22 and wherein the heater is of radiant type.

26. Apparatus as claimed in any one of the preceding claims and having a vibrator for assisting in the amalgamation of droplets in the capacitor.

27. Apparatus as claimed in any one of the preceding claims, and including an indicator.

28. Apparatus as claimed in any one of the preceding claims, and wherein the insulating layer has a dielectric constant of the order of 3.0.

29. Apparatus as claimed in claim 1 and substantially as hereinbefore described with reference to the drawings accompanying ffie provisional specification.