GB1581327A - Detection of levels in conductive oils - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE DETECTION OF LEVELS IN CONDUCTIVE OILS (71) We, THOMAS ELECTRONICS LIMITED, a British Company of 69 Strines Road, Marple, Cheshire, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the detection of levels in conductive oils, such as vegetable oils used for example in vehicle hydraulic brake and clutch systems.

There is a need for a level detecting device which will give warning when the level of hydraulic oil in a brake or clutch system falls below a safe level.

An object of the invention is to provide a detector device which is capable of giving a warning when the level of a conductive oil in a system falls below a predetermined level.

According to the invention, there is provided a detector for detecting a level of a conductive oil in a reservoir, comprising a probe for mounting in the reservoir at the level to be detected, and a circuit means for connection to the probe for sensing the current flow from the probe through the oil to the reservoir or a ground electrode for operating a warning device when the level to be detected is reached, characterised in that a test circuit is provided for indicating that the detector is operative when the circuit means is first connected to an energising source, the test circuit comprising a capacitor arranged to be charged at a predetermined rate when the circuit means is first energised and to cause the operation of the warning device until charged.

An embodiment of the invention will now be described with reference by way of example to the accompanying drawing which is a schematic circuit diagram of a level detector in accordance with the invention.

A reservoir 1 has secured therein by an insulating bush 2, a probe 3 which is connected to a terminal 4. The reservoir 1 is connected to earth by its mountings or, for example, to ground terminal 5 so that a small current, say 1 microamp, flows from the supply rail 6 through resistors R5 and R6, the probe 3 and brake fluid 7 to the earthed reservoir 1. The normal level of fluid 7 is shown at 8.

Transistors T1 and T2 together with associated resistors R1, R2, R3 and R4 form a differential amplifier, the base of transistor T2 being connected to the junction of resistors R5 and R6. The small current through probe 3 is sufficient to maintain transistor T2 in conduction. The collector of transistor T2 is connected to the base of a transistor T3 which is reverse biased when transistor T2 is in conduction. Transistor T3 together with transistors T4 and T5 and associated circuits form an amplifier to operate a warning device 9 which may be visual and/or audible. Transistor T3 maintains reverse bias on transistors T4 and T5, when it is itself reverse biased.

Should the level of liquid 7 fall below the position of probe 3, i.e. to the minimum level indicated by broken line 10, the current through the probe will markedly reduce to the leakage current causing transistor T2 to become reverse biased and its collector to become rapidly negative in accordance with the normal action of a differential amplifier. This now allows transistor T3 to become forward biased causing transistor T4 and T5 to conduct enabling warning device 9 to operate. When the level of the liquid is restored warning device 9 again become inoperative.

A diode D1 inserted in the supply rail 6 protects the circuit against damage if the supply source to be connected across terminals 11 and 12 is wrongly polarised.

Capacitor Cl and resistor R12 provide a test circuit when the supply voltage is first applied, i.e. the equipment is first switchedon after a period of non-use. Capacitor Cl charges through resistor R12 and the baseemitter path of transistor T4, causing transistors T4 and T5 to become conductive and alarm device 9 to operate. As soon as capacitor Cl becomes charged, the charging time being dependent on the time constant involved, transistor T4 becomes and is maintained reverse biased as described above. However, there has been a temporary operation of the warning device which checks its satisfactory operation.

In one example of the invention, used in conjunction with the brake fluid reservoir of a motor vehicle, satisfactory operation was obtained between temperatures between 40tC and +90"C and with probe-toreservoir resistance between 0 and 5 megohms.

WHAT WE CLAIM IS: 1. A detector for detecting a level of a conductive oil in a reservoir, comprising a probe for mounting in the reservoir at the level to be detected, and a circuit means for connection to the probe for sensing the current flow from the probe through the oil to the reservoir or a ground electrode for operating a warning device when the level to be detected is reached, characterised in that a test circuit is provided for indicating that the detector is operative when the circuit means is first connected to an energising source, the test circuit comprising a capacitor arranged to be charged at a predetermined rate when the circuit means is first energised and to cause the operation of the warning device until charged.

2. A detector according to claim 1, wherein the probe is mounted in an insulating bush located in a conductive reservoir, the reservoir being connected to earth by the mountings or to a ground terminal.

3. A detector according to claim 1 or 2, wherein the circuit means comprises a differential amplifier.

4. A detector according to any preceding claims, wherein the warning device comprises a visual and/or audible device.

5. A detector for detecting a level of a conductive oil in a reservoir substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. applied, i.e. the equipment is first switchedon after a period of non-use. Capacitor Cl charges through resistor R12 and the baseemitter path of transistor T4, causing transistors T4 and T5 to become conductive and alarm device 9 to operate. As soon as capacitor Cl becomes charged, the charging time being dependent on the time constant involved, transistor T4 becomes and is maintained reverse biased as described above. However, there has been a temporary operation of the warning device which checks its satisfactory operation. In one example of the invention, used in conjunction with the brake fluid reservoir of a motor vehicle, satisfactory operation was obtained between temperatures between 40tC and +90"C and with probe-toreservoir resistance between 0 and 5 megohms. WHAT WE CLAIM IS:

1. A detector for detecting a level of a conductive oil in a reservoir, comprising a probe for mounting in the reservoir at the level to be detected, and a circuit means for connection to the probe for sensing the current flow from the probe through the oil to the reservoir or a ground electrode for operating a warning device when the level to be detected is reached, characterised in that a test circuit is provided for indicating that the detector is operative when the circuit means is first connected to an energising source, the test circuit comprising a capacitor arranged to be charged at a predetermined rate when the circuit means is first energised and to cause the operation of the warning device until charged.

2. A detector according to claim 1, wherein the probe is mounted in an insulating bush located in a conductive reservoir, the reservoir being connected to earth by the mountings or to a ground terminal.

3. A detector according to claim 1 or 2, wherein the circuit means comprises a differential amplifier.

4. A detector according to any preceding claims, wherein the warning device comprises a visual and/or audible device.

5. A detector for detecting a level of a conductive oil in a reservoir substantially as hereinbefore described with reference to the accompanying drawings.