GB2048495A

GB2048495A - Liquid level sensor

Info

Publication number: GB2048495A
Application number: GB8012237A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1979-04-26
Filing date: 1980-04-14
Publication date: 1980-12-10
Also published as: GB2048495B

Abstract

A liquid level sensing system comprises a variable resistor 1 having a substrate 9 comprising insulating material resistant to volatile liquids (e.g. petrol or petrol/alcohol mixtures) and on it printed resistive means 8, which provides the requisite law characteristic according to the shape of the container of said volatile liquid, and wiper contact means 3 arranged to selectively connect part of said printed resistive means in circuit to vary the effective resistance value of said resistor in response to the actuation of a float or equivalent device as the liquid level changes in said container. The float is coupled to extention 7 so that wiper assembly 2 pivots about pin 5. The substrate may be ceramic or porcelainized steel and the resistive means a composition of glass frit and a precious metal or oxide thereof. <IMAGE>

Description

SPECIFICATION Improvements relating to variable resistors This invention relates to variable resistors and relates more specifically to such resistors eminently suitable for use in the sensing of levels of volatile liquids such as for example the level of petrol in a petrol tank.

Hitherto, variable resistors employed in petrol level sensing arrangements have conveniently been of the wire-wound or metal foil type in which the effective resistance of the resistor varies in accordance with the movement of a wipe contact over turns of wire wound on to or a metal foil adhering to an insulating board or substrate. Movement of the wiper contact is produced in response to movement of the usual float device as the petrol level in the tank changes. These known forms of variable resistors, due to movement or even breakage of the wire turns, or due to peeling or deterioration of the metal foil on the substrate, more especially as a result of severe attack by petrol/methanol mixtures currently in use in some countries, are prone to failure or serious measuring or sensing errors.

According to the present invention there is provided, in a volatile liquid level sensing system, a variable resistor having a substrate comprising insulating material resistant to volatile liquids (e.g.

petrol or petrol/alcohol mixtures) and printed resistive means on said substrate which provides the requisite law characteristic according to the shape of the container of said volatile liquid and wiper contact means arranged to selectively connect part of said printed resistive means in circuit to vary the effective resistance value of said resistor in response to the actuation of a float or equivalent device as the liquid level changes in said container.

The wiper contact means may be arranged to sweep over a single printed resistive track or said wiper contact means may be arranged to move across a series of printed conductors connected at respective points along the printed resistive means thereby to select partofthe printed resistive means in dependence upon the position of the associated float or equivalent device. The wiper contact means in the latter construction will be arranged so that during movement thereof it always makes contact with at least one of said printed conductors (i.e.

make-before-break action) in order to avoid discontinuities in the level sensing circuit arrangement.

In carrying out the present invention the substrate of the resistor may comprise ceramic or porcelainised steel for example whereas the printed resistive means may comprise a composition of glass frit and a precious metal including oxides thereof (e.g. oxide of palladium or ruthenium.

Terminalsforthe printed resistive means may conveniently be of precious metal or alloys thereof such as alloys of silver including palladium silver or platinum silver. However, it is also possible to use base metal compositions for such terminals.

The shape of the container for volatile liquid will usually require the law characteristic of the resistive means to be non-linear and to achieve this a single resistive layer may be suitably shaped (e.g. stepped) or a plurality of layers of different shapes and ohmic values may be successively printed and fired on the substrate.

By way of example the present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a variable resistor in a liquid level sensing arrangement for a petrol tank; and Figure 2 shows a substrate for a different form of variable resistor according to the invention.

Referring to Figure 1 of the drawings the petrol level sensing arrangement comprises a variable resistor 1 having an associated wiper assembly 2 which has a bifurcated twin contact arm 3 and which is pivotally mounted on a fixed support structure 4 by means of a pivot pin 5 which bridges a U-shaped part 6 of the wiper assembly 2 as shown.

An upstanding extension 7 of the wiper assembly is mechanically coupled to a movable float device (not shown) within the petrol tank so that the twin contacts of the wiper contact arm 3 move across a printed resistive track 8 provided on a card or substrate 9 comprising electrically insulating material resistant to petrol and petrol vapour. As the wiper contacts move across the resistive track 8 the resistive value of that portion of the track 8 between the twin contacts of the wiper arm which ensure good contact reliability and a terminal land 10 changes in accordance with the position of the float referred to and thus the level of petrol in the petrol tank, the printed resistive track in the present example being shaped as shown to produce a non-linear resistance characteristic which is predetermined by the shape of the petrol tank.

The substrate 9 of the variable resistor preferably comprises a ceramic or porcelainised steel block or card which is resistant to petrol or petrol/alcohoi mixtures (e.g. petrol/methanol mixture).

The printed track is applied to the substrate in a plurality of successive films or layers with the successive layers having different shapes and ohmic values to furnish the non-linear resistance characteristic. After firing of the resistive layers or films fine adjustment of the characteristic to provide accurate measurement of liquid level may be achieved by use of a laser or air abrasive trimming technique.

The composition of the printed resistive track may comprise powdered precious metal or an oxide of palladium or ruthenium in a glass frit. The terminal land 10 at the end of the track 8 may be composed of silver or silver alloy, such as palladium silver or platinum silver.

The wiper contact arm 3 may for instance be composed of nickel silver (e.g. 55% copper, 18% nickel and 27% zinc) phospher bronze or beryllium copper.

In a further embodiment of the present invention the variable resistor comprises a substrate with printed resistive means provided thereon as shown in Figure 2 of the drawings.

Referring to Figure 2 this shows an insulating substrate 11 which may be of the same composition as that described with reference to Figure 1. Applied to the substrate 11 are printed resistive areas 12 and 13 of stepped and curved form and bridging these resistive areas are a multiplicity of parallel printed conductor strips 14. The left-hand ends of the resistive areas 12 and 13 are interconnected by a broader printed conductor 15 and a further resistive area 16 is located between the conductor 15 and a terminal land 17.

A wiper contact arm (not shown) mechanically coupled to a float device will be arranged to sweep across these parallel conductors 14 in response to movement of the float device and by so doing connect selected parts of the resistive areas 12 and 13 in circuit. The wiper arm will have a single contact which during movement of the wiper over the conductor strips 14 will make contact with the next contact before breaking contact with the last. Since the wiper contact moves over the conductors 14 and not over the resistive areas 12 and 13 the metal of wiper contact may be of a softer character (e.g.

silver). In order to modify or trim the characterisitic of the variable resistor according to requirements further printed resistive strips 18, 19, 20, 21 and 22 may be provided between selected conductors 14 and these may be readily severed from the circuit if not required.

The variable resistor described with reference to the drawings will exhibit good stability in the present of organic solvents, such as a petrol or petrol/alcohol mixture which may contact the variable resistor when the latter is located within the petrol tank and which have deleterious effects on the wire-wound and metal foil type variable resistors hitherto employed in petrol level sensing arrangements.

Although in the specific embodiments described the wiper contact arm is actuated by a float device in the petrol tank other devices for producing such movement might alternatively be used.

Claims

1. In a liquid level sensing system, a variable resistor having a substrate comprising insulating material resistant to the liquid concerned and printed resistive means on said substrate which provides the requisite law characteristic according to the shape of the container of the liquid and wiper contact means arranged to selectively connect part of said printed resistive means in circuit to vary the effective resistance value of said resistor in response to the actuation of a float or equivalent device as the liquid level changes in said container.

2. Avariable resistor as claimed in claim 1, in which the wiper contact means is arranged to sweep over a single printed resistive track to vary the effective resistance of said resistor.

3. A variable resistor as claimed in claim 1, in which the wiper means is arranged to move across a series of printed conductors connected at respective points along the printed resistive means thereby to select part of the printed resistive means in dependence upon the position of the associated float or equivalent device.

4. Avariable resistor as claimed in claim 3, in which the printed resistive means comprises twon spaced elongate resistive areas joined by the printed conductors and effectively connected in parallel.

5. A variable resistor as claimed in claim 3 or claim 4, in which the wiper contact means is effective to make bridging contact between adjacent printed conductors.

6. A variable resistor as claimed in any preceding claim, in which the substrate comprises ceramic or porcelainised steel.

7. A variable resistor as claimed in any preceding claim, in which the printed resistive means comprises a composition of glass frit and a precious metal or an oxide thereof.

8. A variable resistor as claimed in any preceding claim, in which terminals for the printed resistive means are composed of silver or silver alloy.

9. A variable resistor as claimed in any preceding claim, in which the law characteristic of the resistive means is non-linear.

10. A variable resistor substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.