Indicating Device

The present invention relates to an indicating device for indicating the boiling point of fluids, especially hygroscopic fluids such as hydraulic fluids, for example automotive brake and clutch fluid.

Brake fluid as used in the hydraulic braking systems of motor-cars and vans is hygroscopic; i.e. is capable of absorbing moisture, and this can have an adverse effect on braking performance, especially in the long term. In particular, the absorption of moisture in the brake fluid causes a lowering of the boiling point of the fluid and very possibly to such a degree that temperature increase in the braking system due for example to heavy braking or some fault condition can result in the brake fluid boiling and as will be appreciated this has a very deleterious effect on braking. Therefore it is beneficial to be aware of a serious lowering of the boiling point of brake fluid so that steps can be taken to change the faulty fluid.

A method and device for determining the boiling point of hygroscopic liquids, such as automotive brake and clutch fluid, is described and claimed in EP-A-0074415 (corresponding to U.S. Pat. No. 4,484,823). The method of EP-A-0074415 involves the heating of the liquid to be tested using a heater immersed in the liquid until the liquid evaporates and gas bubbles rise along the heater, the temperature of the heater effected by the gas bubbles being measured whence the boiling point of the liquid can be ascertained. A further pressure fluid boiling point testing device is described in GB-A-2139763, the device of GB-A-2139763 including a heater for fluid which also serves as a temperature sensor, the device relying on the formation of gas bubbles on the heater for a boiling point testing operation. These prior art devices have the disadvantage of not being particularly accurate and/or forming noxious vapours due to the evaporation of hydraulic fluid.

It is the principal object of the present invention to provide an indicating device for indicating the boiling point of brake fluid which avoids these disadvantages and which is convenient to operate but which gives accurate results.

According to the present invention there is provided an indicating device for indicating the boiling point of fluid especially a hygroscopic fluid such as hydraulic fluid comprising a meter preferably of the portable hand-held type including a probe portion for insertion into fluid in a fluid reservoir, heating means being provided in the probe portion for heating fluid in which the probe portion is immersed, said meter additionally including monitoring means for monitoring the temperature rise of fluid heated by said heating means so as to indicate the boiling point temperature of the fluid, characterised in that said probe includes a casing defining a housing wherein a portion of fluid to be tested is semi-encapsulated, the temperature monitoring means sensing the temperature of fluid in the housing heated by the heating means, and in that fluid inlet means are provided permitting fluid flows to and from the housing.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings wherein:

FIG. 1 shows a pictorial view of a fluid boiling point meter in accordance with the present invention, and especially intended for sensing the boiling point of automotive brake fluid;

FIG. 2 shows a cross-sectional elevation of the probe portion of the meter of FIG. 1;

FIG. 3 shows the lower part of the probe portion (i.e. below line X-Y) to a larger scale (and greater than actual size);

FIG. 4A shows a view in the direction of arrow A in FIG. 3, while 4B shows a similar view but for an alternative arrangement of the probe bottom;

FIG. 5 shows a view in the direction of arrow A but with the probe bottom wall removed;

FIG. 6 shows in basic block form the electronic circuit arrangement of the fluid temperature monitoring means of the meter; and

FIG. 7 shows a typical graph of temperature against time for the temperature of heated brake fluid measured by the meter.

Referring to the drawings, a temperature indicating device or meter 1 for measuring the boiling point of automotive brake fluid comprises a main casing 2 housing electronic temperature monitoring equipment and a probe 3 projecting downwardly from the casing 2 for insertion into brake fluid contained in a fluid reservoir as schematically illustrated at 4 and located, for example, in the engine compartment of a motor vehicle. The probe 3 houses a heating element 5 for heating brake fluid while a temperature sensing head 6 of a thermocouple (serving as a mechanical electronic transducer) in the probe feeds heated fluid temperature information to the temperature monitoring equipment in the casing 2.

Referring especially to FIGS. 2 to 5, the probe 3 comprises an outer tubular sheath 7 defining a shroud while the heating element/thermocouple pack is carried by a spaced pair of heat-proof insulator members 8, 9 which in the probe assembled condition bear against the inner wall of the outer sheath 7, and the sheath 7 includes a cap 7B at its upper end which also serves to support the pack. The sheath 7 has a bottom closure wall 7A including a series of fluid apertures 10 (see FIGS. 4A/4B), and a separate inner tubular casing or sheath 11 is located on a stepped portion 8A of the insulator member 8, the casing 11 having an open bottom which rests on the bottom wall 7A of the sheath 7. The apertures 10 may take the form of slots (FIG. 4A) or a series of holes (FIG. 4B). An annular fluid space is present between the sheaths 7, 11, the sheaths 7, 11 can be made of suitable material such as stainless steel. A sealing ring 12 (e.g. of Tufnol (TM)) is housed in the casing 11 and the zone in the casing 11 below this ring 12 defines a housing 11A wherein a portion of fluid to be tested is semi-encapsulated or entrapped, the heating element 5 and the temperature sensing probe 6 being located in this housing 11A. In particular, the heating element 5 is located in the lower half of the housing 11A, and the probe 6 extends into the heart of the element. Initial filling of the housing 11A with fluid is achieved via the apertures 10 and 13. In FIG. 4B, the outer holes 10 are preferably aligned with the space between the shrouds 7, 11. Further, the casing 11 includes an annular series of apertures 13 adjacent to the sealing ring 12 whereby fluid can flow between the housing 11A and the annular fluid space between the sheaths 7, 11.

The leads 14A/B and 15A/B for the heating element 5 and thermocouple head 6 respectively extend up through the probe 3. The outer sheath 7 includes a level mark L (FIG. 1) to which the probe 3 should be immersed to ensure that the heating element 5 is satisfactorily immersed. Vent holes 16 are provided on the sheath 7.

A suitable circuit for the electronic temperature monitoring equipment is shown in FIG. 6. The major item in this circuit is a micro-processor 17 which transmits each monitored temperature value in digital form to a digital display 18 (liquid crystal form). More particularly, a temperature sensing thermocouple 19 of the probe 6 is connected to a closely coupled amplifier and micro-powered thermocouple cold junction compensation chip 20, to obtain most satisfactory results. To convert the output of the matched chips into a form which the micro-processor 17 understands the output from item 20 is fed to an analogue to digital converter 21 (say 10 bit). The processor 17 is driven by an erasable programmable memory (EPROM) 22 and constantly measures and monitors the temperature of the fluid within the housing 11A. The eprom 22 holds the temperature measurement information (program) which is used in sensing the precise boiling point of the fluid and also to generate to BCD (binary coded decimal) code fcr the display 18. A suitable latch can be present between the EPROM 22 and the processor 17. The processor 17 serves to produce all the information for the various items of the monitoring circuit equipment. Once the equipment senses the fluid boiling point the display 18 will indicate the appropriate temperature value. Additionally, a temperature control switch 5A for the heater 5 receives signals from the microprocessor 17 to halt power to the heater when the measured fluid temperature rises to a predetermined level.

Power for the meter 1 can be taken from the vehicle battery via flexible leads 24 and terminal clips 25 and/or the meter 1 can carry its own battery (e.g. 12 volts) for example located in housing 26. The casing 2 carries on-off buttons 27, 28 for the power supply to the heating element 5 and to the display equipment respectively, while indicator lamps 29, 30 indicate when the appropriate power via button switches 27, 28 is on, although a single indicator lamp could suffice for both functions.

All the parts of the meter are housed in the casing 2 or in the probe 3 and the meter is a conveniently portable hand-held device.

In operation of the meter 1 to test the quality of brake fluid in a vehicle, the cap of the fluid reservoir housing the brake fluid to be tested is removed and the probe 3 is inserted into the fluid to the level indicated L. Brake fluid flows via the apertures 10, 13 to immerse the heating element 5 and such that a semi-encapsulated body of fluid is located in the housing 11A. Button 27 is now pressed to heat element 5 and consequently cause heating of the encapsulated brake fluid in the housing 11A. With button 28 actuated, the temperature monitoring equipment will monitor the increasing fluid temperature values.

The temperature reading rises to a first maximum or peak (B1 in the graph in FIG. 7) then falls slightly before rising again, and subsequently the temperature/time graph will have an oscillating characteristic. The reason for this feature is believed to be due to the fact that, whereas prior to the boiling point the encapsulated fluid in the housing 11A is in a substantially static state, at the on-set of boiling fluid flow paths of a reflux nature are established within the housing 11A, encouraged by the constricted nature of the fluid in the housing and by the location of the heating element 5. During this fluid movement some fluid is discharged via the apertures 10 and 13, although the apertures restrict direct flow of fluid from the housing and the fluid flow paths at boiling will generally be as indicated by the arrows at the bottom end of the probe 3 in FIG. 3. The reflex fluid flow results in instantaneous cooling at the temperature sensing zone of the probe 6 causing the temperature reading fall subsequent to the first temperature peak (B1). The highest temperature reached prior to the first temperature drop is taken as the fluid boiling point temperature --in essence this represents the equilibrium reflux boiling point temperature of the fluid. Thus the continuous graph line in FIG. 7 is produced from continuous readings from the thermocouple 6 and as will be noted at point B1 there is a temperature drop: point B1 therefore represents the fluid boiling point. The monitoring equipment will have the facility to set the value B1 for reference. Also, it is possible for the monitoring equipment to be arranged so as to equate the measured boiling point as tested to any standard required e.g. as represented by the dashed lines I, II, etc in FIG. 6. Further, ambient temperature can be automatically compensated for and batch tests can be carried out without waiting for cooling. The complete test from start to finish can be carried out very quickly and very conveniently (without the creation of mess or fumes) and, furthermore, other than the brake fluid reservoir cap, nothing has to be removed from the vehicle. The test using the meter 1 should give a tested boiling point reading very close to the correct value i.e. within 2 or 3 degree centigrade.

At a predetermined elevated temperature the switch 5A is actuated by the micro-processor 17 to halt the supply of electrical power to the heating element 5 thereby avoiding or minimising the risk of formation of noxious brake fluid vapours. The heating element 5 can provide intense local heating in the housing 11A so enabling very rapid testing of the fluid, and this is condusive to the achievement of accurate results. Also, the meter can be used satisfactorily to test liquids where normally there would be the risk of decomposition of the liquid.

Modifications are of course possible. For example a downwardly projecting shroud (indicated by dashed line 35 in FIG. 3) could be provided and this would have the advantage of "holding" the external fluid portion adjacent to the slots (apertures) 10 so enabling satisfactory testing even when the probe 3 is moved about in a reservoir of fluid to be tested.

Whereas the meter 1 was described above for testing brake fluid the meter could be used for testing other fluids, especially hydraulic fluids. In particular, transformer fluid could be tested.