WO1998039623A1

WO1998039623A1 - Probe and sensor for liquids

Info

Publication number: WO1998039623A1
Application number: PCT/GB1998/000667
Authority: WO
Inventors: Steven John Daniels; Martin Orrell; Graham Vesey
Original assignee: NULECTROHMS Ltd
Current assignee: NULECTROHMS Ltd
Priority date: 1997-03-06
Filing date: 1998-03-06
Publication date: 1998-09-11
Anticipated expiration: 1999-09-06
Also published as: GB9704681D0; AU6407498A

Abstract

A device for determining physical characteristics of a liquid in a storage container, comprising a probe operably located within the liquid and comprising a level sensor (30) for determining the position of the interface between the liquid and another fluid, and preferably a temperature sensor (70) positioned proximal with the level sensor. Also, a device for determining physical characteristics of a liquid, particularly an explosive liquid, in a storage container, comprising a probe (24) operably located in the liquid, and means for preventing ingress of liquid into the probe whilst allowing electrical connection (28) to a sensor (30, 32) within the probe, wherein the ingress preventing means comprises a substrate (130) having electrically conductive tracks (132) thereon means (134) for sealing the substrate within the probe.

Description

PROBE AND SENSOR FOR LIQUIDS

The invention relates to a device for determining physical characteristics of a liquid

and in particular an explosive liquid held within a storage vessel. More particularly,

the invention relates to a device comprising a sensor for determining the temperature

of the liquid or a sensor for determining the position of a boundary interface of a liquid

for example with water.

It is known to store explosive liquids such as petrochemicals in large storage tanks,

wherein the explosive liquid is kept away from the bottom of the tank using a layer of

neutral liquid such as water. In order to determine the exact quantity of stored

explosive liquid, it is necessary to determine the boundary interface position between

the explosive liquid and water. Additionally, it is preferable to know the temperature

of the explosive liquid and its pressure.

It is known to provide a probe having several temperature sensors disposed along its

length in order to determine the temperature at several different layers within the stored

liquid. It is also known to provide a pressure transducer within the storage tank itself

at a fixed location, e.g. 1 metre above the bottom of the tank.

A problem in manufacturing probes for measuring the physical characteristics of an

explosive liquid is the intrinsic safety which must be provided for all internal electrical

systems in order to prevent any potential of electrical breakdown or overheating which might lead to an explosion.

Objects of the invention include providing a device for determining physical

characteristics of a liquid and particularly an explosive liquid comprising a sensor for

measuring a boundary interface position of the liquid, sensors for measuring two or

more physical characteristics of the liquid, and providing means for preventing ingress

of liquid into the device or probe.

According to one aspect of the invention, there is provided a device for determining

physical characteristics of a liquid in a storage container, comprising a probe housing

a sensor and means for preventing ingress of liquid into the probe whilst allowing

electrical communication to the sensor, the ingress preventing means comprising a

substrate having electrically conductive tracks thereon and means for sealing the

substrate within the probe. Beneficially multiple electrical connections can be made

through the sealing means since the tracks on the substrate can be substantially flat

and configured to meet requisite separation criteria to provide the necessary safety.

Another aspect of the invention provides a device for determining physical

characteristics of a liquid in a storage container, comprising a probe operably located

in the liquid, which probe houses a level sensor for determining the position of the

interface of the liquid with another fluid and preferably a temperature sensor for

determining the temperature of the liquid or other fluids. Preferably, the temperature sensor is located proximal to the level sensor thereby enabling determination of the

temperature at or near to the interface. Preferably the level sensor is a capacitance

device for example comprising an elongate conductive member and outer plate or tube.

A further aspect of the invention provides a level sensor for determining the position

of an interface between a liquid and another fluid, which comprises an elongate

electrically conductive member preferably having a dielectric sheath sealed at one end.

Beneficially, the sensor can be made by heat- shrinking a sheath of plastics material

such as PTFE onto the conductive member, or by electrostatically coating the member.

A yet further aspect of the invention is a device for determining physical characteristics

of a liquid in a storage container, comprising a probe operably located in the liquid

having at least two of a level sensor for determining the position of an interface of the

liquid with another fluid, a temperature sensor for determining the temperature of the

liquid, and a pressure sensor for determining the pressure of the liquid at a known

position.

Embodiments of the invention will now be described, by way of example only, with

reference to the accompanying drawings, in which:

Figure 1 is a schematic side elevation view of a storage tank having a device

according to the invention; Figure 2 is a schematic sectional side elevation view of a level sensor and

temperature sensor according to the invention;

Figure 3 is a schematic view of another embodiment of a level sensor and

temperature sensor according to the invention;

Figure 4 is a schematic sectional side elevation view of a temperature sensor for

the probes shown in Figures 2 and 3;

Figure 5 is schematic block diagram of a control system for a level sensor

according to the invention;

Figure 6 is a schematic sectional side elevation view of part of a probe

according to an aspect of the invention;

Figure 7 is a sectional side elevation view of another probe according to another

aspect of the invention;

Figure 8 is a sectional side elevation view of the upper end of a probe according

to the invention; and

Figure 9 is a sectional front elevation view of the upper end of a probe according to another aspect of the invention.

Referring to Figure 1 there is shown a storage tank 10 for an explosive liquid 12 such

as a petrochemical. Liquid 12 sits above a lower layer of water 14. Liquid is passed

in and out of tank 10 via inlet 16 and outlet 18. The tank 10 comprises a sensor 20

positioned at the top of a well 22 which passes down into liquid 12. Sensor 20 can for

example be an ultrasonic or radar device for detecting the position of the upper level

of liquid 12. Tank 10 further comprises a probe 24 according to the invention which

is positioned in a well 26 which passes through liquid 12 and down to the bottom of

the tank into water, or other neutral fluid, layer 14. Means 28 is provided for

communicating and/or controlling sensors within probe 24 such as level sensor 30,

temperature sensor 32 and pressure sensor 34. Two slightly different version of a level

sensor according to the invention are shown in Figures 2 and 3; they are labelled 30a

and 30b respectively, and like components are given the same two figure designation.

The level sensor 30a comprises a cable exit and entry port 36, a locking nut 38 and a

tubular casing 40 for housing a printed circuit board and electrical components 42.

The circuitry 42 is rigidly held within casing 40 using a potting compound or cement

44. The potting compound preferably and beneficially provides a complete seal across

the internal diameter of the casing 40, thereby preventing movement of gases and

liquid along the probe. Also, the mass of the compound 44 beneficially provides a heat sink for the circuit 42.

A first electrical connection 46 is made between the level sensor circuitry 42 and end

cap 48, and a second electrical connection 50 is made to elongate member 52 which

is preferably tubular. Tube 52 is held in position using a retaining nut 54 for example

of nylon, fastened against internal threading of end cap 48; resilient members 56 such

as 0-rings; sealing bush 58; and sealing cap 60. Sealing cap 60 threadingly engages

an outer threading on end cap 48 and cooperates therewith through pressing sealing

bush 58 and resilient members 56 into a fluid-tight seal against end cap 48 and tube 52.

Additionally, sensor 38 comprises a protective tube 64 having a series of apertures 66

which allow ingress of fluid into contact with tube 52. It is the variation in the material

between tube 52 and tube 64 which enables determination of the position of the

interface between two liquids.

In the embodiment shown in Figures 2 and 3, an optional temperature sensor 70 is

provided within tube 52 which has electrical connectors such as wires 68 which pass

along the length of the probe 24. A screened multi-core cable can be provided for a

number of temperature sensors positioned along the length of probe 24. In this

end-most unit of the probe, the screening is preferably terminated at connection 74

before the circuitry 42 and additionally, a cable junction 72 for wires 68 is also

preferably provided before circuitry 42. Sensor 30a further comprises an end cap 76 to help retain tube 52 in a known position

within tube 64. Additionally, an eyebolt can be provided at the lowermost end of

probe 24, for example, to allow attachment or a weight or a guide means to position

probe 24 within a storage tank.

Sensor 30b shown in Figure 3 is substantially similar to that shown in Figure 2, except

that wiring details are not shown and sealing bush 58 comprises a tapered surface 59

for cooperating with a tapered surface 61 on the inside of sealing cap 60. Accordingly,

by fastening sealing cap 48 with respect to end cap 48, more effective compression of

resilient members 56 in contact with tube 52 is achieved.

Referring to Figure 4, there is shown greater detail of temperature sensor 70. A

temperature sensor element 80 such as a resistance thermometer such as PtlOO or

thermo-couple sensors such as a type K (NCNA) device is provided. It is connected

at joints 82 and 84 to one or more cores from a multi-core cable 68. Each joint 82 and

84 is sealed for example by a heat-shrinkable material 86. Element 80 is positioned

within a casing 88 for example of stainless steel and rigidly held in place by a body of

potting compound or cement 90. Preferably the screening from cable 68 is connected

to casing 88 and finally, sensor 70 comprises an outer protective layer 94 for example

of a heat-shrinkable plastic sleeve. The sensor 70 can be positioned anywhere along

the length of tubular elongate member 52 and held in position using a cement or other

attaching means. Referring to Figure 5, there is shown a schematic block diagram of the control circuitry

42 for the level sensor which comprises tubes 52 and 64. The tubes combine to form

a capacitance rod sensor 96 which operably communicates with a frequency generation

circuit 98 positioned within circuitry 42 shown in Figures 2 and 3 for example. The

variation in capacitance depending on fluid between elongate member 52 and outer

plate or tube 64 is preferably achieved by making both components of a good electrical

conductor such as steel. Preferably, elongate member 52 is coated with a dielectric

material such a plastic sheath of PTFE or FEP. The dielectric material can be

electrostatically applied to member 52. However, in another form, a sleeve of material

is heat shrunk around member 52 and sealed at the end remote from the circuitry 42.

Preferably the thickness of the sheath is relatively thin and for example less than or

about 1 mm thick. In a preferred form the sheath thickness is about 0.25 mm thick.

Beneficially, the relatively thin sheath thickness enables the probe to be media

independent and therefore operable with a range of oils without recalibration. This

effect is believed to be achieved due to the electrical conductivity of water at the lower

end of the probe when measuring the position of an oil-water interface.

The frequency generation circuit 98 operably communicates with a microprocessor

circuit 100 which is programmed to enable calibration of sensor 96 and thereafter,

determination of the level of an interface between two given fluids, e.g. a

petrochemical and water, along the length of tube 52, for example by using a look-up table of predetermined capacitance values. The measurement is fed to digital analogue

converter 104 which is connected to a voltage reference 102, and a power supply

circuit and current converter 106. Communication, including power input and

measurement output is via wires 108 such as through a two-core screened cable or two

individually sheathed wires for example. The output can be linear or stepped

(switched) for example. In a stepped configuration, a number such as four levels of

output can be used indicative of water required, water level okay, take water out, and

an alarm level for the water being too high.

Referring to Figures 6 and 7, there are shown two temperature sensors which might be

used at any position along the length of probe 24 or in the endmost position, hence,

eyebolt 78.

Temperature sensor 32a is a spot sensor comprising an element substantially similar

to temperature sensor 70 described earlier. Hence, a temperature sensing element 80

is held in a potting compound in a casing 88 which is then covered by an insulating

sheath 94. Electrical connection to element 80 can again be via individually sheathed

wires 68 or a screened multi-core cable. In this example, element 80 is connected in

series by an insulated sheathed or single-core wire which is terminated at a connection

110 by which it is also connected to a return wire 112 which passes along the entire

length of probe 24. The wiring 68 is retained in a protective sleeve 114 which is

connected to a termination block 116 which clamps the sleeve 114 onto a spacer or end cap 118. Furthermore, an outer casing 120 is provided which can for example be made

of steel convolute or nylon tube.

Temperature sensor 32b is an averaging temperature sensor which comprises an

elongate temperature sensor element 110 of say between 10 and 20 cm long, again

such as a resistance thermometer, which is housed in an insulation protective sleeve

122. Communication to the sensor element 120 is again via cable or cables 68. A

termination connection 124 is provided within protective sleeve 114 and again

termination 116 and end cap 118 are provided within a protective outer casing 120.

In this embodiment a further spacer 126 such as a rigid cylindrical member, is also

provided in order to locate and maintain the position of sensor element 120 within

probe 24.

Any number of averaging or spot (local) sensors can be provided along the length of

a probe 24. The temperature sensors can be wired independently or have a common

return (or feed) to reduce the number of connection wires. These wires to the

temperature sensor, a pressure sensor, and/or level sensor, exit the top of probe 24 as

shown in Figures 8 and 9. Wires such as wire 68 or 108 passing up through the probe

24 are first passed through a seal 136 into a region W of potting compound 134 which

fills the internal diameter of the probe. The potting compound 134 surrounds a

substrate 130 having tracks 132 thereon. For example, substrate 130 can be a printed

circuit board having electrically conductive tracks 132 positioned thereon in a suitable manner to meet requisite safety standards. Accordingly, individual wires can be bared

and soldered or otherwise connected to one end of a track 132 and then reconnected

to another wire in order to exit through the top of potting compound 134 and into

output bundle 28. In the version shown in Figure 7, a flange 128 is provided for

resting the top of the probe 24 or otherwise fitting the probe 24 to a storage tank.

In the version shown in Figure 8, some of the wires, for example wires 108 to the level

sensor 30 are passed through potting compound 134 separately from other wires.

Preferably, these wires 108 are sheathed in a heat-shrink sheath 138 between seal 136

and the top of the potting compound 134. Preferably also, a heat-shrink 140 is

provided at the top of the probe 24 to collect all the wires as they exit potting

compound 134. Naturally, the wires have their own insulation coating in accordance

with requisite standards when entering and leaving the region W of potting compound

134. Beneficially however, a large number of wires can be passed through a relatively

narrow sheath 120 whilst at the same time providing a seal which prevents the ingress

of liquid into tube 120 at the upper end.

The probes and sensors described are applicable to a variety of liquids such as

non-explosive organic liquids like benzene and non-explosive inorganic liquids,

including slurries or mud for example. Additionally, further sensors can be

incorporated in the probe such as pH and dissolved gas sensors which rely on changes

in capacitance or electrical resistance. For certain uses, it is possible to suspend the level sensor a long way down from an

anchor point, for example when locating a level sensor in a cavern. In such

applications, it is appropriate to use armoured cable to suspend the probe in order to

provide sufficient inherent strength in the suspension means for it not to extend under

its own weight and the weight of the probe.

Claims

1. A device for determining physical characteristics of a liquid in a storage

container, comprising a probe locatable within the liquid and comprising a level sensor

for determining the position of the interface between the liquid and another fluid, said

sensor comprising a first elongate member having a temperature sensor positioned

along the length of the level sensor.

2. A device according to claim 1 wherein the final elongate member is electrically

conductive.

3. A device according to claim 2 wherein the first elongate member is tubular and

the temperature sensor is located within the tube.

4. A device according to claim 3 wherein the first elongate member comprises a

metallic tube such as a stainless steel tube.

5. A device according to any preceding claim wherein the elongate member

comprises a dielectric sheath such as a PTFE or FEP coating.

6. A device according to claim 5 wherein the dielectric sheath is applied by heat-

shrinking a sleeve of dielectric material on to the elongate member, or by electrostatically coating the member with a dielectric material.

7. A device according to Claim 5 or 6 wherein the dielectric sheath is sealed at one

end.

8. A device according to claim 5, 6 or 7 wherein the dielectric sheath is less than

lmm thick and preferably in the order of 0.25mm thick.

9. A device according to any preceding claim comprising a second electrically

conductive elongate member proximal the first.

10. A device according to claim 9 wherein the second member is substantially

coaxially disposed about the first member.

11. A device according to claim 10 wherein the second member comprises apertures

to allow movement of fluid into contact with the first elongate member.

12. A device according to any preceding claim further comprising means for sealing

against the first elongate member thereby to prevent ingress of liquid into the first

elongate member.

13. A device according to claim 12 wherein the sealing means comprises two co- operating components having associated tapered surfaces which effect clamping on the

elongate member.

14. A device according to any preceding claim wherein the temperature sensor

comprises a temperature sensing element sealed within a body such as potting

compound.

15. A device according to claim 14 wherein a rigid sheath is provided around the

body, for example the sheath can be made of metal such as stainless steel.

16. A device according to any preceding claim wherein screened multi-core cable

is provided to communicate electrically within the probe to a sensor.

17. A device according to any preceding claim wherein a control circuit is provided

for a sensor, which circuit comprises a processor to enable calibration and/or digital

analysis of measurements made by the sensor.

18. A device according to claim 17 further comprising storage means such as read¬

only memory enabling the processor to store a calibrated look-up table to interpret the

output from a sensor.

19. A device according to claims 17 or 18 wherein the processor is programmed to provide a linear or switched output from the sensor.

20. A device for determining physical characteristics of a liquid in a storage

container comprising a probe, operably located in the liquid and housing two or more

of a temperature sensor for determining the temperature of the liquid, a level sensor

for determining the position of an interface of the liquid with another fluid, and/or a

pressure sensor for determining the pressure of the liquid at a predetermined

positioned.

21. A device according to claim 20, wherein the probe comprises a level sensor at

one end and a pressure sensor operably above the level sensor.

22. A device according to claim 20 and 21, comprising a plurality of temperature

sensors positioned along the length of the probe, preferably at regular intervals.

23. A device according to any preceding claim comprising an elongate tubular body

and means for preventing ingress of liquid into the body whilst allowing electrical

connection to a sensor within the probe, wherein the ingress preventing means

comprises a substrate having electrically conductive tracks thereon to provide electrical

connection to the sensor which substrate is encapsulated is sealing means.

24. Device for determining physical characteristics of a liquid in a storage container, comprising having an elongate tubular body, a probe operably located in the

liquid, and means for preventing ingress' of liquid into the probe whilst allowing

electrical connection to a sensor within the probe, wherein the ingress preventing means

comprises a substrate having an electrically conductive track to provide electrical

connection to the sensor and which substrate is encapsulated in sealing means.

25. The device according to claim 23 or 24 wherein the substrate is substantially

flat.

26. A device according to claims 23, 24 or 25 wherein the electrically conductive

tracks are substantially flat.

27. A device according to any of claims 23 to 26 wherein the sealing means

comprises a potting compound and/or a sealing plug.

28. A device according to any preceding claim wherein the substrate comprises a

printed circuit board.

29. A device according to any preceding claim comprising a level sensor having a

first electrically conductive elongate member having an outer dielectric sheath, and a

second electrically conductive elongate member substantially co-axially disposed about

the first member.

30. A device for determining physical characteristics of a liquid in a storage

container, comprising a probe operably located in the liquid and comprising a level

sensor for determining the position of the interface between the liquid and another

fluid, said level sensor having a first electrically conducted elongate member having

an outer dialectric sheath, and a second electrically conducted elongate member

substantially co-axially disposed about the first member.

31. A device according to any preceding claim comprising means for suspending

the level sensor at a desired location within a storage container, which suspension

means is preferably flexible.

32. A device according to claim 31 wherein the suspension means comprises

armoured cable.