Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
11304 ~
~he present invention relates tG a conten-ts gauge for
use with a li~uicl s-torage tank, Ic~r e~a~le a petroleu~
storage tark. More s~ecifically the gauge accordi~r to the
present invention measures and gives an indication of the
volume o liquid y:cesent in the tcL~ll{ and can be readily
2,dapted for use ~ th taL~ks of any shape.
Enown designs o-f gauge can only be used ~,lith tanks of
one parti~u~a~ shaL~e and size and re-calibration for ta-nks
of other shapes and sizes is difficuLt.
According to the present invention there is provided
a liquid stor2ge tank contents gauge comprising me2nS
providlng an electrical signal corresponding to the vertical
height o the liquid in a tankS electronic means for deriving
the volume of liquid cor-responding to said vertical height,
and an output means for providing a visual out~ut of said
volume.
Preferred embodiments of the ~resent invention will now
be described with reference to the accomL~anying d~a-.Jings
in which:
~igvrQ~ 1a and 1b form Fi~ure 1 ~!~hich is a partial~y
sectioned slde view o the sensing unit of the gaugre according to
the present inventiorl shown installed in a tanX.
Figure 2 is a cross section of the ~au.ge taken along
the line ~.-A in l~igl,lre lb.
~5 ~igure '~ is a block dia~ram o an electronic circuit
associated ~ith the ~au~e of t~e preseL~t invention.
~ ~e d is a block diagr~m of ar alternative electronic
circuit e.r~olo~r:inG a microprocessor.
r~he sensin~ iO sho~r. in ~ig~e 1 comLr.rises an el.onsate
JO t~ e 11 J at the lc~er end of ~hich is m~o~nted an ~Ltraso~lc
- 2 - ~ ,
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transducer 12. The tube 11 i5 suspended inside the tank for
which the contents are to be gau~ed ana the length of the tube
11 is chosen such that the transducer 12 is positioned at, or
close to, the bottom of the tanX. ~he transducer is orientated
such tb~t its signal is propagated vertically upwards inside
the tube to the surface of the liquid stored in the tank.
Holes 1~ in the tube 11 allow the liquid to enter the tube 11
and ~or the liquid level to correspond to the level in the tank.
The type of mou~ting just described with the transducer
- 10 inside the tube has the advantage that the transducer is
protected ~rom damage during install~tion or by falling
objects. Severe blows could also cause it to produce
high voltages due to the piezoelectric effect. Ho~ever
in some situations it is preferable to adopt some other form
of mounting such as fi~ing the transducer to the upper surface
of a horizontal plate posltioned at the bottom of the tank.
It is also possible to mount the transducer ou-tside but in
contact with the bottom of the tank. 3uch a configuration
reduces the strength of the echo signal received,
requiring e~tra amplification, but can be a much simpler
mechanical arrangement. With large tanks, mounting
the transducer and adjusting its orientation from the
top by a long rod or similar device may be inconvenient
and entry via a hole in the side of the tank near the
bottom may be more appropriate.
As sho~n in Figure 3, the signal to the transducer 12
is supplied from an oscillator 14 via a frequency divider 15,
a pulse sha~er circuit 16 and a modulator 17. A second
oscillator 1~ controls the modulztor 17 such that signal
passed to the transducer 12 is in the form of pulses each
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composed of a burst several cycles long, at regular intervals.
This signal is passed to the transducer via a power
amplifler 19.
An example of a transducer used in practice is a lead
5 zirconote titanate disc having a resonant frequency of 500 KHz.
l~lhen the ultransonic signal reaches the liquid surface
some of its energy, at least, is reflected by the liquid/air
interface and returns to the transducer 12. This reflected
signal is converted back into electrical form by the
transducer and amplified by amplifier 20. The time taken by
the signal to travel to the surface and return to the trans-
ducer 12 ~ equal to twice the depth divided by the velocity
of propagation of the ultransonic wavesO Thls velocity is a
constant for any particular li~uid and hence the depth ^an
be readily calculated if the time is measured. o measure
the time interval between transmission of a burst and
reception of its echo, circuitry is arranged to count the
n~mber of pulses of a high frequency pulse train of known
repetition rate that occur during this interval. This
count is referred to as C1. To avoid ambiguity the rate of
transmission of signal bursts is sufficiently low to ensure
that, even with maximum liquid depth, the echo returns to the
transducer before the next signal is transmitted.
The signal from amplifier 20 is passed through a
detector circuit 21 and a second amplifler 22. The output
of amplifier 22 is fed to a gating waveform generator 23
as a reset signal, the other input to the generator 23
being derived from the output of the pulse shaper circuit 16.
The ga~ing generator 23 acts to control a gate 24 ~Thich gates
30 the pulses from the osclllator 14 which are operating a
4 --
11~0~
counter 25.
In applica~ons where the temperature of the
stored liquid is subject to variation, compensa-tion
for the change of velocity of propagation of ultrasonic
waves with temperature is ircluded. ~he temperature is
measured by a sensor such as a thermistor, a thermocouple
or a resistance thermometer 29 and the electrical signal
produced controls a voltage controlled oscillator.
The number of cycles of the ouput of tnis oscillator
occurring in the time interval between tranmission of a
burst and reception of its echo is counted~ This count c2
is added to the count c1 referred to above, o~ the constant
frequency. The relationship between temperature and frequency
of the voltage controlled oscillator is arranged so that
the total count, c - (c1 ~ c2), for a particular depth, does
not vary if the ~trasonic velocity changes ~"ith temperature.
The total count c occurring in the time interval between
transmission of a burst of ultrasonic energy and the
reception of its echo is a direct measure of the liquid
depth. When the liquid is contained in a tank o~ non
constant cross sectional area that varies with depth,
the relationship between depth and volume is non-
linear. This is a common occurrence in many types of
storage tank. ~or e~ample, petrol is frequently kept in
cylindrical tænks mounted with their axes in a horizontal
plane.
To convert from depth to volume, program~a3le read only
memory (PROrI) stores 26 are used. The number of high frequency
pulses occurring du~ing the transmlsslon to echo reception
interval is obtalned in binary orm as an output from counter 25
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and is used as the input address to a store. ~he output
of a store for ar~ particular input, is the corresponding
volume in binary form. This is converted into bir~ry
coded decimal format by decoder and decade driver circuits
27 and used to drive a decimal display 28 indicating the
stored vol~e. To allow alternative readout in either
gallons or litres, two sets of progr~ ble read only me~ories
are used and either can be switched into operation.
The vertical diameter of the tank is quant~edinto
1~ more than 1000 equal segments. Thus by making the systèm
sense changes of le~el at least as small as one of these
segments an accuracy of better than 0.17? is obtaired.
Correspondirg to each of the possible liquid heights is a
16 bit word representing the appropriate volume of liquid.
Each word has to be 16 bits long because the display has ~our
main decades each requiri~ a 4 bit BCD (binary coded decima])
input.
An example of suitable PROM units are those manufactured
by Signetics (type 92S115) and are 4096 bit Bipolar devices
organised as 512x8 bytes, Since 1000x15 words are required.
~our 82S115 units are r`equired per instrument to read gallons.
As a gallon/litres option is included another four are
incorporated.
A printout facility may also be included that allows
the operator to h~ve the liquid volume recorded on a roll of
paper. The printing ~echanism is supplied with a similar
signal to that which actuates the declmal display but only
operates ~hen required by the operator,
To indicate ~lhen the tar~ is full a ~arning alarm is
also pro-v-ided. This is operated by a c-rcuit that detects
.
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when the n~be~ in t,le binary cc~nter 25, operating over the
time i.n-ter~-al bet~een the trans~.ittea signal and the echo
retl~n, rea.ches a predetermined :Level. The alarm can take
the I Ol'm o F a light or an audiblesound or both and is
principcllly of use durinS the ope~ation of filling tanks,
In so~e applications the signal processing
may be carried out bv using a microprocessor instead of
dedicated digital circuits, as shown in Figure 4. With the
microprocessor system the compensation for the variation of
ultrasonic ~Locity with temperat~re may be achieved b~
multiplying the apparent depth of liquid, as derived from
the time int2rval between the trans~itted and echo pulses, by
a correction factor. The appropriate factor is selected from
the system memor~r by addressing the latter ~ith the digitised
output of the temperature sensor. ~his .nethod of compensation
is used instead of the one described earlier in T.~hich a
voltage depenclent oscillator is controlled by an analog~le
signal correspondin~ -to temperature.
A variation in the method of converting from
depth to vol~e ~,Then a microprocessor is incorpora~ed is
also appropriate in some circumstances, ins-tead of
storin~ all the pairs of values of depth/volume ~-1antities
in a PRO~i memory.
~s shoT;~n in Figure 4 7 the signal to the transducer
12 is derived .-.ro~ a signal genera~or 3~ eauivalent to the
oscillator -i~ eqllenc,r divider 15, pu~1se sh~er15
and mod-~lator 17 controlled b~ oscillator 18, and is
ampliIied ~y ~lr)lifier 19 as be,?ore, In ad.lition each p~se
lron the siglal generator 30 is fed to a microprocessor 31
'jO 2ncl th.e ret~r. p-ulse is also fed to the microprocessor 31
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via amp]if,-i*.r 20 and detector circuit 21. A signal
from the temperatllre sensor 29 :is ~lso fed to the micro-
processor 31 a.nd the variation -in the propagation velocity
~rith tem~erature are automaticall.y compe~sated for.
The microprocessor i9 ~)rogra~med to calcula-te the
volume from the tank dilnensions and the measured depth.
With ~ome t.ank shapes however, a readily computable
rel~tionship between depth and ~olume cannot be obtained
and a combin2tion of the two methods of (a) storing all the
depth/volume pairs, and (b) calculating the volume each
time is used. The corresponding values of depth and volume
for a number o-f depths between the minimum and maximum are
held in store. The values of volwme for intermediate
depths are obtained by interpolation. ~inear interpolation
is used when the reference de~ths areIairly close together or the
volume/depth relationship does not depart greatly from
lin~rand higher order interpolation schemes are used in
other cases as appro~riate~ The output from the
microprocessor ~1 is fed to a visual d~splay 32.
In ~ome si~uations, especially when tanks are used
for petrol stora~e, water can collect in the bottom of a
tank. ~ecause its den~ity is ~greater th.an that of petrol
and also bec~use o~ chemical differencesinhibitin~ -their
ml.~ing, the ]iqui.ds form two layers wJith the water belo~.
~ warnir,~ mechanism is in~.. uded to indica-te when the le~el
o-E the ~wanted water e.~ceeds a cert~in nredetermined
thre~'1old,
The 1iater e~ensing mechanism com~rises a pair o-f concentric
annular C~CitOl' pl.ate~ 42 mo~u~ted i~lside the -tube 11 a
3~ fe~ inches above the -tr.ansd-lcer 12. ~he ~resence of tiater
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is detected by sensin~ a change in the capaci-tance between
the plates 42, -the dielectric constan-ts of fuel, oil or petrol
and water bein~T, different,
When water is presen-t in the bottom of a tank containing~
llquid o~ a lower dielectric constant, it increases the
capacitance of the open plate capacitor i-ormed by the plates
42- This capacit~nce controls the frequency of a squarewave
oscillator to an inverse relationship and so the presence of
water decreases the frequency. Each cycle of the oscillator
output triggers a mono~able circuit producing pulses of
fixed duration, These are integrated and smoothed to produce
a direct voltage. When the value of this voltage drops
below a preset thresilold value, an alarm is activa-ted.
This a~arm ma~ take the form of a audible warning, a visible
light or both.
'~he tu~e 11 is mo~nted on an assembly 33 which allows the
axis of the tube 11 to be aligned truly vertical. The assembly
is screwed into the top of the tank by a cap 34 with screw
thread 3~. To allow adjustment of the assembly 33
permittin~ the axis of the transducer to be positioned
at right angles to the liauid surface the tube 11 is
secured to a plate 36.
Plate 36 is suspen~ed from cap 34 by threaded set screws
37. Springs 38 positively hold cap 34 and late 36 apart,
Screws 37 are in tapped holes in cap 34 and turning any ons
of them ~lters its vertical positlon. Adjusting all three
screws to~,ether rai~es or lowers plate 36 but alte-ring any
one or t~io of -them ~lso varie, the angle the axis of
tran~llcer a~ n~l~r 12 ~akes with the horizontal surface
of the ~ i(l, To ~btail maxi~llm re~3~?cted sigral the
11~0 441
transducer a,cis is ad,justed to be vertical. Locking nuts 37
are then ti;.htened to maintaln this positioning,
The t:r.~nsducer 12 ~ 1 the capaci~or plates 42 are
connec-ted to external electrical control equipment by way
of coa.~i~l cabLes 3~ whi.ch pass up inside the tube 11 to a
cable conduit 39 secured to the ca 34 , The tube 11 is earthed
by ~ay of ear~h tags 40 wh:ich are co.nnec-ted to an earth terminal
stud 4-l from where an earthin~ conductor passes to the cable
conduit 39.
In an alternative construction, the tube 11 is connected
to a trunnion ring at two diameterically opposite points
~rhich allows the tube to swing i.n a vertical plane relative to
thetr~on ring. The trunnion ring i~self is pivoted in a
housing about an a~is at right angles to the pivotal axis
between the tube and the ring. The housing is secured to
the tank by wa,y of an ad~ustable threaded mount. ~he trunnion
ring acts as ~ gimbel mountlng ~hich allows the tube 11 to
hang truly vertical. Such a mounting is especially suitable
for situations ~rhere the tanlc may be +ilted, as for example
in mobile applications~
When the liquid level sensor is to be used in a large
capacity, vertical cyli.ndrical bulk storage tank, the sensor
unit is preferabLy ~ounted on the side of the tank~ adjacent
its lo;ler end to avoid the use of a tube 1t of an impracticably
long len~th. In this situtation some form o~ pivoting connection
is utili~.ed -to allo~ the transducer axis to be aligned truly
ver-tica:l .
For ~nlic~tions ~!here t~le tar,k containing the liquid
may be tilted (e~c~,. in mobile tar.kers) tne be~ from an
ultrasonic tr~nsducer l~ay not arrive ?er~endicular to the surface~
In such sl 't~a'ti ons a cap,~cltl~e sensor :is preferab1..e,
_ 10 -
B
1~ ~V'~41
The eiez-trodesof such a capacitor ~ransducer
are so arra.1ged that the dielectric space between them
is increas~ngly filled ~ith the liauid to be measured
as its level rises. As its dielectric constant is different
to that o-~ air the capacitance changes with level. Some
electrode configurations used are (a) ver~ical coaxial
cylinders, (b) a double helix of metallic stri or wire,
(c) vertical parallel p]ates (d) interleaved 'fingers' o~ metal
deposited on an insulating material.
To measure liquid depth the output of a high stability,
constant ~requency oscilla~or is counted for a time depending
on the capacitance sensed. The count is used to read out
the corresponding vol~ne as described earlier. In some
instruments the depth dependent capacitance is used as the
~requency determining element in an oscillator whose output
is counted for a fi~ed ti~,e but this is merely an alternative cir
cuit arrangement lor producing a count dependent on depth.
