GB2181257A - A probe for use in a device for measuring the liquid content of a gas - Google Patents

A probe for use in a device for measuring the liquid content of a gas Download PDF

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GB2181257A
GB2181257A GB08624694A GB8624694A GB2181257A GB 2181257 A GB2181257 A GB 2181257A GB 08624694 A GB08624694 A GB 08624694A GB 8624694 A GB8624694 A GB 8624694A GB 2181257 A GB2181257 A GB 2181257A
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probe
material
gas
ofthe
ceramic
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GB2181257B (en
GB8624694D0 (en
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Paul Christopher Osbond
Roger William Whatmore
John Paul Auton
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Plessey Co Ltd
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Plessey Co Ltd
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N25/00Investigating or analyzing materials by the use of thermal means
    • G01N25/56Investigating or analyzing materials by the use of thermal means by investigating moisture content
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00Investigating or analysing materials by the use of electric, electro-chemical, or magnetic means
    • G01N27/02Investigating or analysing materials by the use of electric, electro-chemical, or magnetic means by investigating the impedance of the material
    • G01N27/04Investigating or analysing materials by the use of electric, electro-chemical, or magnetic means by investigating the impedance of the material by investigating resistance
    • G01N27/14Investigating or analysing materials by the use of electric, electro-chemical, or magnetic means by investigating the impedance of the material by investigating resistance of an electrically-heated body in dependence upon change of temperature
    • G01N27/18Investigating or analysing materials by the use of electric, electro-chemical, or magnetic means by investigating the impedance of the material by investigating resistance of an electrically-heated body in dependence upon change of temperature caused by changes in the thermal conductivity of a surrounding material to be tested

Abstract

A probe for use in a device for measuring the liquid content of a gas, is fabricated in part from a material 1 which is thermally self-stabilising and which preferably has a positive temperature coefficient of resistance. In use the probe is electrically heated and disposed in a flowing gas or is adapted for movement through a static gas so that droplets of liquid are deposited on the surface of the probe. A particular application for the probe is that of detecting the liquid water content in the air flowing around helicopters and other aircraft, so that the crew can be given warning when they are flying into icing conditions. A further application is to monitor the air/fuel mixture in a carburettor of an internal combustion engine. <IMAGE>

Description

SPECIFICATION A probe The present invention relates to a probe for use in a device for measuring the liquid content of a gas, and in one aspectto a probe for use in a device for measuring the liquid water content (LWC) in the atmospherearound aircraft, such as helicopters.

Helicopters are particularly susceptible to the build-up of ice on their rotor blades and engine air intakes when operating under cold, damp conditions both because of the increase in the weight of the aircraft and because ofthe rapid loss of lifting efficiency due to the ice layers that build up on the aerofoils. There is, therefore, a needfora device employing a probe to measure the LWC in the atmosphere around an operational helicopter so thatthe crew can be given warning when theyareflying into icing conditions.

Various techniques, including both optical and platinum-wire measurements, are capable of determining the atmospheric LWC but most of these are complex and require a considerable amount of electronicsfor control.

According to the present invention there is provided a probe for use in a device for measuring the liquid content of a gas, wherein the probe comprises a material which is thermallyself-stabilising.

Essentially the probe utilises the relatively large cooling effect due to the evaporation of droplets of liquid deposited on the surface of the thermally self-stabilising material from an impinging gas stream, compared with the cooling due to convection, conduction and radiation. The gas may be a flowing gas or a static gas but there must be relative movement between the probe and the gas for the measurement of liquid content to take place.

In one embodiment the probe comprises a material having a positive temperature coefficient of resistance (PTCR).

Advantageously the probe is substantially cylindrical in shape and is fabricated from at least one ceramic toroid manufactured from the thermally self-stabilising material. Convenientlythe probe isfabricated from a number of ceramic toroids wired electrically in parallel and stacked on top ofone anothertoform a cylinder. An embodiment of the present invention has the ceramictoroids stacked with an interleaving electrically conductive shim between adjacent pairs of toroids. The shims are shaped to allowtwo electrically conductive elements to extend through the central portion ofthe stack, each element making electrical contact with a respective major face of each toroid,the major faces ofthetoroids being coated with an electricallyconductive material.Advantageously when the probe is assembled it is coated with a polymer two protect the probe from the environment in which it is to be used.

In one embodiment of the present invention the probe comprises a material having a positivetemperature coefficient of resistance, the material being in the shape of a toroid having a wall thickness of at least0.2mm.

Preferably the probe is assembled by a method comprising stacking a plurality oftoroids, each ofwhich is manufactured from a thermally self-stabilising material, an interleaving electrically conductive shim between adjacent pairs oftoroids, providing electrically conductive elements to extend through the central portion of the formed stack, each element making electrical contact with an electrically conductive material on a respective majorface of each toroid, and coating the surface of the assembled probe with a polymer to protect the probe from the environment.

The present invention will be described further by way of example with reference to the accompanying drawings wherein: Figure lisa graph showing the resistance of a doped barium titanate ceramic as a function oftemperature; Figure2 is a graph illustrating the variation in electrical resistivity ofthe doped barium titanate ceramic with temperature on which a thermal performance model is based; Figure3 illustrates a doped barium titanate ceramic slab used in the thermal model; Figure 4 illustrates the circumferential variation ofthe Nusselt number Nu surrounding a cylindrical probe for a range of Reynolds number Re; Figure 5illustrates current voltage characteristics for different values ofsurface heat transfer rates (a) for the probe; Figure 6is a schematic diagram of a probe according to one embodiment of the present invention;; Figure 7 illustrates a construction jig forassembling a probe likethat shown in Figure 6; Figure8 illustrates the shape of a ceramictoroid, a numbs - which are assembled together in the probe of Figure 6; Figure 9 is a cross-section through a copper disc positioned between adjacent ceramictoroids in the probe of Figure 6; Figure loins a graph illustrating the power output from a probe according to one embodiment of the present invention,the power output being shown as a function of liquid water flow rate (LWF) at two constant air velocities, 60 knot and 100 knot (30.87ms-' and 51 .44ms-') ) and as a function of the square root ofthe velocity heat constant LWF; and, Figure 11 illustrates a method of measuring the liquid droplet concentration of a static gas using a probe according to the invention.

Description ofa preferred embodiment The embodiment ofthe present invention described below with reference to the figures makes use of a probe the operation of which is dependent on the electrical properties of a suitable positivetemperature coefficient of resistance (PTCR) material. Certain doped barium titanate semiconducting ceramics are particularly suitable in this respect. For example, Figure 1 shows the resistance of a ceramic with the composition (Bao.825Sro.175)o.99s5La0.0035Tj1.07303 as a function oftemperature. These ceramics are well known and are available commercially. Their preparation will not be the subject of further discussion in this specification.

These ceramics can be madeto act asthermally self-stabilising heating elements.

When suitable electrical contacts are made to the ceramic material and a potential difference is applied,the initial current at room temperature is large as the resistivity of the material is low. Any heat lost at the surface of the ceramic material is equal to the electrical power input at constanttemperature. If the potential difference is sufficiently high, the joule heating effect raises the temperature of the ceramic material to above Tc in Figure 1 where a rapid increase in resistance reduces the current and the power dissipated. At a temperature above Tca small change in ceramic temperature will give rise to a large change in resistance, and to a large change in dissipated heat. Therefore, large changes in heat lost at the ceramic surface can be tolerated with only small changes in ceramictemperature.Any increase or decrease in the heat dissipation from the surface ofthe ceramic material causes a corresponding increase or decrease in the current flowing through the material and hence a change in the power dissipated. Underthese conditions, the ceramic material behaves as a thermally self-stabilising heater, tending to stabilise at a temperature just above Tc. The temperature at which stabilisa- tion occurs is dependent on the potential difference and the gradient of the curve above in Figure 1 .The higherthe value of the gradient the more closely controlled the stabilisation temperature will be.At a constant voltage the electrical currentthrough the ceramic material is closely related to and therefore a measure ofthe ambient conditions at the surface ofthe ceramic material As will be described below a probe comprising the ceramic material can be utilised in a device which is designed and fabricated to measure the liquid droplet content of a gas, and in the case of a wet gas, the water in dropletform. Such a probe will hereinafterforthe purpose of this description be referred to as a liquid droplet/gas probe (LDGP).

The way such a ceramic material is configured in the LDGP makes use of the relatively large cooling effect due to the evaporation of droplets of liquid deposited on its surface from a gas stream flowing over itwhen compared with the cooling dueto radiation and convection and conduction duets the gas alone.

A good model ofthe physical characteristics of a PTCR ceramic such as that shown in Figure 1 is given by equations relating the electrical resistivity and temperature of the material: p = pO.exp (-n1 (6- 6N) ) 66N (1) p = PO.exp (n2 (6 - ON) ) 6 > 0N (2) At a characteristictemperature By, the electrical resistivity p is taken to have a minimum value pO,asshown in Figure 2. Below that temperature, the resistivity rises ns % per 0C as temperature falls. Above that temperature, the resistivity rises n2% per 0C as temperature rises.The Curie temperature, which is taken as the temperature at which the rate of change of the resistivity gradient is a maximum, is close to ON The selection of a suitable PTCR ceramic for application as an ice probe involves the specification of attainable values of p,, ON, n1 and n2 (and tolerances in those values) which result in acceptable thermal performance of the probe.

By the inclusion of insolvent impurities in PTCR barium titanate ceramic the Curie temperature can be modified to suitthe particular requirements. For a material to be suitable, the generated power has to vary sufficiently widely across the operating rangeto enable changes in the ambient atmospheric conditions to be sensitively discriminated.

Considering a slab of PTCR ceramic with electrodes on the faces as shown in Figure 3 the heat is removed by convection from the two faces X eat + d/2. Assuming the electrodes are subject to 200V/cm, ON = 60"C, p0 = 20 cm, n1 = 1 of2 OC-ll n2 = 0.2 oC-ll ambient temperature Oc = 20"C and the thermal resistivity ofthe ceramic k =0.03Wcm -1 "C-l, it is found thatthe slab temperature and power generated in the slab rise to a maximum as the thickness is increased, and are constant above a saturation thickness of d/2 = #0.2mm. The saturation thickness is based on a practical maximum surface heat transfer coefficient of az 1Wcm-2"C-l. The effectof using lowervalues ofa is to flatten the internal temperature in the slab and to raise the wall temperature.

For a typical constant value of a, the current reaches a maximum at a certain voltage. In the lowervoltage range, the siab has a temperature less than 0N Above the critical voltage however,the temperature exceeds ON For a constant voltage applied to the slab, the current drawn varies with the surrounding conditions.

Provided the voltage exceeds the turning voltage for the maximum value of a, the variation of the currentwith a is substantial, and depends on the value of n2which is the resistivity gradient of the selected PTCR ceramic.

By making the dimension d/2verysmall,the slab can be considered as an elemental strip of a hollow cylindrical probe being cooled from only one side. It can then be used to model the behaviour of the probe undervarying conditions. For applications with helicopters, the probe is required to operate at relative air velocities between 20ms- and 90me71, and these velocities correspond to Reynolds numbers between 4,000 and 20,000. At these Reynolds numbers, the heat transfer coefficient a is not constant around the periphery of the probe as can be seen in Figure 4. Corresponding curves can therefore be obtainedforthecurrent drawn by the probe (running dry) by integrating the family of current-voltage curves in Figure 5.

In dry air at 90m/s and the Reynolds Number Re = 21,000 and the mean Nusselt Number Nu = 105 corresponding to the mean surface heat transfer co-efficient = 0.06Wcm-2 C-1.Assuming a 0.2mm wall thickness, the local rates of heat loss and heat transfer coefficient at certain angular positions (for the PTCR material discussed above) are approximately:: Angular Position PTC Temperature "C Heat Loss W/cm2 "C ss Surface Core w/cm2 a 0 68 70 9.0 1.06x10- 30 69 71 8.0 9.7 x10-2 90 72 73 3.5 4.0 x10-2 The surface temperature only falls to the characteristictemperature 6N x 60 C if &alpha; > 0.5Wcm-2 C-1.

However, sincethe maximum power loss atthe 0" position on the probe even atthe highest airdroplet concentration (90ms-,1.6g/m3) is 13.0Wcm-2 in addition to the 9.0Wcm-2 heat transfer rate, the surface temperature of the probe does notfall to the characteristictemperature in practice.

When particles or droplets of liquid land on the surface of the probe, the device is cooled partly by heat convection and partly by mass convection as the drops evaporate.

Fora single drop of mean size20 m the region ofthe probe where the drop lands gives up heat rapidlyso that the drop is heated through. A drop of this size responds at such a rate thatthe outside is in equlibrium with the probewall in about3 milliseconds. Assuming 1.6g/m3 moisture content and 90ms-' airvelocity,the average period before the next drop will land on the same point can be calculated statistically as being 0.3 seconds. The drop surfacetherefore has to be hot enough to cause it to evaporate into the ambient airstream within that period, ifthe moisture layer is not to build up and coverthe probe with a layerofliquid.

If the probe is made with a thicker wall (i.e. more than 0.2mm),there is a response ofthe inside of the probe which alters conditions on the outer surface, and this will follow in a period which is about 1 second aftera change in probe environment. The thermal mass of the probe is, of course, large compared with that of impinging water drops, so thatthe temporary localised temperature depression in the region where the drop lands can be ignored.

The theoretical study described above indicates that the PTCR material required requires a value of ON above 50 Candthetemprature coefficient n2 should be greaterthan 0.10 C-1. The dimensions ofthe probe are not critical provided that the surface area exposed to the gas flow is known. Probes which have been fabricated and tested in wind tunnels and on operational helicopters have been composed of a hollow cylinder of ceramic which is 8.0mm in diameter and 40mm in length. The cylinder is divided into a number of sections (ortoroids) which are connected electrically in parallel so as to reduce the operating voltage.It has been found experimen tally that ceramics with the composition (Ba0.825Sr0.175) 1 yLayTi1+803 (with y = 0.0035 and 8 = 0.013) and containing a second phase addition of 0.1 mole % Li203,0.15 mole%Al2O3 and 0.5 mole% SiO2, exhibitthe required electrical properties with ON and n2 equal to 70"C and 0.17 C-1 respectively. Other materials which may also be suitable in this application are Cr-doped V203 ceramics which exhibit a PTCR behaviourwhenthey undergo a metal-insulatortransition at a temperature determined by the concentration of Cr. Fabrication ofthe ceramic probe is described in detail below.

In orderto calculate the liquid contentofa gas, the power Q (watts) required to maintain the cylinder at its operational temperature, the true gas velocity V(ms-01), the static gas temperature Tst(K) and the static gas pressure Pst (N.m-2) are required. The air velocity can be measured with a piton tube and the airtemperature determined by a thermistor. The liquid content of a gas can be obtained by subtracting the convective power from the measured power; the convective power is a once-only measurement and the constants derived are then fixedfora particular installation. Measurements are required of probe powerovera range ofairspeeds and temperatures in dry air.

If the surface temperatu re Tsf (K) ofthe probe is known theTf(K),thedropletfilm temperature can be taken as the arithmetic mean Tf = (Tsf + Tst)/2 (3) and the dynamicviscosity of air f atTf is given bytheformula: <img class="EMIRef" id="027098177-00030001" />

knowing thethermal conductivity kf of air atTf and the density of airatTst, Nu and Re can be calculated to be Nu = hD/Kf (5) <img class="EMIRef" id="027098177-00040001" />

andRePsVD (7) f where h, the convective heat transfer coefficient is calculated by the formula::- <img class="EMIRef" id="027098177-00040002" />

As is the probe surface area, D is the probediameterand Pa(kgm-3) is the density of airatTst. The gradient N and intercept B of a graph of log Nu against log Re can then be used to calculate the LWC (denoted M).

The total power applied to the probe equals the power lost by convection plus that required to evaporate the water, Q = h.A5(T5f - Tet) + EApVM(Le + Cw(Tsf - Tet)) (9) where E = overall droplet catch efficiency for the probe Ap = probe projected area Le = latent heat of evaporation of water = 2.36 x 106Jkg- Cw = specific heat of water = 4.2 x 103Jkg -1 C-1 <img class="EMIRef" id="027098177-00040003" />

It can be shown that <img class="EMIRef" id="027098177-00040004" />

and K (12) E=K + He where He is the efficiency parameter = #/2 + 0.121 Rd0.6 + 0.754x10-4Rd1.38 (13) Rd is the droplet Reynolds Number K = droplet inertia parameter <img class="EMIRef" id="027098177-00040005" />

(d2 = means droplet diameter, Pw is the density ofwater and a is the viscosity of air).

Hence substitution of equations 11 and 12 into equation 10 gives the value of M i.e. the mass ofwater in Kg per cubic meter of gas. The only variables for any one installation are Q, V, Tst and (Pet = pe.TsPst.287.1). In the case of sensing the LWC around an operational helicopter, the Q is derived directly from the current drawn by the probe and the other variables from the other helicopter instruments. Asuitable computation of LWC can then be performed using an on-board processor.

The method for fabricating a probe is described below and involves (i) plating PTCR material with nickel,(ii) design of components used to make electrical connections to the ceramics, (iii) design of an assembly jig (iv) application of solder (v) grinding the probe and (vi) insulating the internal and external surfaces of the probe.

The description which follows applies specifically to semiconducting barium titanate ceramic as the PTCR material, but a similar method could be used with other materials as discussed above.

Figure6showsa practical designforthe probe and Figure7 demonstrates a jig which issuitableforprobe fabrication. Twenty PTCR ceramic toroids as shown in figure 8 with the basic composition of (Ba0.825 Sr0.175)1 yLayTi1+ÔO3 (withy = 0.0035 and 8 = 0.013) are plated on their major faces using an electroless nickel plating technique such as has been previously described. The plated ceramics are annealed at 300 C so as to minimize the contact resistance between the metal and ceramic and the electrodes are then coated with a thin layer of resin-based flux/preservative to prevent further oxidation of the nickel and to allow good solderability.

In orderthatthe operating voltage is keptto minimum,the ceramictoroids haveto be connected electrically in parallel. This is achieved using copper discs 2, one of which is shown in Figure 9, the copper discs are positioned between thetoroids 1 shown in Figure 6. The discs 2 which are also prefluxed before assembly, have a central cutout 10 shown in Figure 9 which permits them to be placed centrally on a stainless steel post 16shown in Figure 7 of the jig. The cut-out 10 includes a narrow slot 12 for a pair of tinned copperwires3 shown in Figure 6 to be soldered to, and alternate discs are rotated by 180" with respect to each other so that the wires only make electrical contact to alternate toroid faces.

The complete device, including a threaded copper base 4 is soldered together by using two 5 micron thick foil washers 5 made of 60/40 lead/tin solder (Figure 6) which are positioned between every plated toroid face and everycopperdisc 2. The copper base can be of any design which is suitableforsecuring the devicewhen operating and could be omitted if, for example, the probe was bonded to its support using a high temperature epoxy. The solder washers 5 allow a consistently uniform layer of solder between surfaces but other methods of applying the solder can be employed if thickness of deposition is controlled (forexample, thedeposition of solder alloys by electrolysis).Electrical contact between the copper discs 2 and the tinned copper wires 3 is ensured by placing a small blob of solder paint (a suspension of solder particles in a fluid) around the slot 12 through which the wires pass.

Before assembly is placed in an oven, pressure on the probe is applied from above using a thick brass plate 18 (Figure 7) which slides overfourguiding rods 20 ofthe assemblyjig. Hence, when the probeandjig are heated to a temperature above the melting point ofthe solder (200 C) the flux is squeezed out from the surfaces and the strength of the solder bonds is optimized.

Afterthesolderhas melted and the probe is cooled and removed from the jig, it can be operated as a liquid content sensor. However, the probe surface is rough and irregular so that the surface area is difficultto calculate. Also, electrical insulation is desirably required on the outside and inside surfaces ofthe device if it is going to be operated in an environment containing an electrolyte, for example water.

The probes can be ground on a latheto a known diameter using a diamond grinding wheel.The inside ofthe probe is desirably filled with a high temperature epoxy 7 (Figure 6); conventional epoxy resins are not suitable since the inside ofthe probe can be above 1000C (depending on the value of 6N chosen).

To prevent electrolysis occurring on the surface of the probe a suitable coating has to be applied which will not only resist electrolytes but also has to be resistant to erosion from particles in the atmosphere. However, since the operation ofthe probe relies on the cooling effectofdroplets impinging on the surface, external coatings will slow the time response of the device. Therefore a durable coating of between 10 and 50 am is required if the time response ofthe probe isto be keptto about 1 second althoughthickercoatings are suitable if a device with a time constant of several seconds is permissible.It has been found that polyimide coatings offerthe greatest protection against electrolysis and erosion, although devices coated with epoxy resins and other polymers have been tested in simulated conditions and were found to operate satisfactorily.

Nolimid 32 (a registered Trade Mark), a high purity modified polyimide, was found to be particularly suitable for coating LDGP's because of its mechanical and chemical properties at raised temperatures. The resin is applied to the surface ofthe probe in thin layers and each layer is dried at 1300for 15 minutes priorto curing at 250Cforafurther 15 minutes. Although the melting pointofthe Pb/Sn solder is around 2000C,the high temperature epoxy7 resin which fillsthe centre ofthe probes enables the lattertowithstandthis high curing temperature without affecting the mechanical strength ofthe device.

Results from tests carried out on a PTCR ceramic LDGP in a wind tunnel are given in figure 10. This showsthe power drawn bythe probe at constanttemperature (-1SOC)for different liquid waterflow rates (Iwf) andfor constant air velocities of 60 and 100 knots (30.87ms-1 and 51 .44ms-1). A constant potential of 28 volts was applied to the probe so that the power drawn is directly proportional to the currentflowing. It can be seen that a linear relationship exists between power dissipation and Iwf and that the power drawn at constant liquid water flow is proportional tothesquare rootofthe airspeed.The slight vertical power displacement in thisfigure between the 60 knot and 100 knot lines is due to the convective term in the heat transfer coefficient. The thermal self-stabilisation of the probe is demonstated by the fact that an increase in the LWC from Ogm-3to 1 .5g m -3 at constant air velocity increased the current drawn by the probe by 104% but only decreased the probe temperature by3.6 C.

The linearityofthe curves shown in figure 10 means that it is a relatively simple matterto compute the LWC in the atmosphere when the other variables (V, Tet and Pet) are measured.

The primary application ofthe LDGP is in the measurement of the liquid water droplet content in the atmosphere around helicopters and fixed wing aircraft. This is of particular importance in sensing when the aircraft is flying in icing conditions.

The LDGP, as described above, could also be used to measure the liquid droplet concentration in a static atmosphere, provided that probe is moved through the gas at a known speed. One method of achieving this is to secure the LDGP on a rotating boom (Figure 11). Such a device for example could then give an accurate measurement ofthe liquid water content of fog and could therfore be used as a warning device on motorways as well asa measurementdeviceforfog irrigation of plants in greenhouses, measurement ofliquid droplet entrainment in gases used in chemical processes and measurements of liquid droplet entrainmentforthe characterising of atomising sprays.

Another possible application ofthe LDGP is the monitoring offuel supplied to internal combustion engine carburettors. The air/fuel mixture from the carburettorwould pass over a probe stabilized at a suitable operating temperature and the power required to maintain the LDGP at a constanttemperature could be used to compute the rate at which fuel is injected into the engine. The device employing such a probe could be used to optimisethefuel consumption ofan engine iftheflowoffuel into the carburettor could be adjusted asa result of data from the LDGP.

In all the applications above including the aircraft LWCthe probe need not be a cylinder but could for example be a small flat plate.

The embodiment ofthe probe described above utilises a material having a positive temperature coefficient of resistance (PTCR) which is thermally self-stabilising. In other embodiments ofthe present invention the probe comprises a material having a negative temperature coefficient of resistance (NTCR) which isthermally self-stabilising, butthese other embodiments are however less preferred embodiments.

It will also be appreciated that the probe configuration does not have to be cylindrical but it is convenientto employ a cylindrical configuration as it considerably simplifies the mathematical model which is used to measure the water content. In one of the preferred embodiments of the present invention a cylindrical probe is employed which operates at between approximately 65"C and 755C, using a ceramic of semiconducting Barium Strontium Titanate, such a probe has been found practical for measuring the liquid water contents in the air flowing around helicopters and other aircraft.

For the purpose ofthe above described embodiments by referring to thermally self-stabilising material is meant one which when subjected to controlled electrical voltages or currents will maintain its temperature closetosome predetermined temperature.

Claims (15)

1. A probe for use in a device for measuring the liquid content of a gas, wherein the probe comprises a material which is thermally self-stabilising.
2. A probe as claimed in claim 1 wherein the material has a positive temperature coefficient of resistance.
3. A probe as claimed in claim 1 orclaim 2whereinthe probe is substantially cylindrical in configuration.
4. A probe as claimed in claim 3 wherein the probe structure comprises at leastoneceramictoroid manufactured from the thermally self-stabilising material.
5. A probe as claimed in claim 4wherein the probe is fabricated from a pluralityoftheceramictoroids wired electrically in parallel and stacked on top of one anothertoform a cylinder.
6. A probe as claimed in claim 5 wherein the ceramic toroids are stacked with interleaving electrically conductive shims between them.
7. A probe as claimed in claim 6 wherein the shims are shaped to allow two electricallyconductive elements to extend through the central portion of the stack, each element making electrical contactwith a respective majorface of each toroid, the major faces of the toroids being coated with an electricallyconductive material.
8. A probe as claimed in any preceeding claim wherein the material comprises a doped barium titanate (BaTi03) semiconductor ceramic.
9. A probe according to claim 1 wherein the material has a negative temperature coefficient of resistance.
10. A probe as claimed in any one of claims 1 to 8 wherein the probe in use operates between approximate ly 65"C and 75"C, the probe material being a ceramic of semiconducting Barium Strontium Titanate.
11. A probe as claimed in claim 10wherein the material is formed in the shape ofatoroid having awall thickness of at least 0.2mm.
12. A probe as claimed in claim 1 wherein the probe is assembled bya method comprising stacking a plurality oftoroids, each of which is manufactured from a thermally self-stabilising material, an interleaving electrically conductive shim between adjacent pairs oftoroids, providing electrically conductive elements to extend through the central portion of the formed stack, each element making electrical contact with an electrically conductive material on a respective major face of each toroid, and coating the surface ofthe assembled probe with a polymerto protectthe probe from the environment.
13. A probe for use in a device for measuring the liquid content of a gas, the probe being substantially as hereinbefore described with reference to Figures 6,8 and 9 ofthe accompanying drawings.
14. A probe as claimed in claim 1 wherein the probe is assembled buy a method substantially as herein be fore described.
15. Adeviceformeasuring the liquid content of a gas, the device comprising a probe as claimed in anyone of claims 1 to 14 and means four detecting a change in the electrical resistance ofthe probe in responseto a change in the heat loss rate due to the evaporation of droplets of liquid from the surface of the material.
GB08624694A 1983-10-08 1986-10-15 A probe Expired GB2181257B (en)

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GB838326982A GB8326982D0 (en) 1983-10-08 1983-10-08 Atmospheric sensor
GB08624694A GB2181257B (en) 1983-10-08 1986-10-15 A probe

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GB08624694A GB2181257B (en) 1983-10-08 1986-10-15 A probe

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GB2181257A true GB2181257A (en) 1987-04-15
GB2181257B GB2181257B (en) 1988-06-29

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2308192A (en) * 1995-12-14 1997-06-18 Jtl Systems Ltd Liquid-sensing apparatus
US5691466A (en) * 1995-06-28 1997-11-25 J.T.L. Systems Ltd. Liquid-sensing apparatus and method

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GB2181257B (en) 1988-06-29
GB8624694D0 (en) 1986-11-19

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