Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Length and te~ J~ n~
apparatus for tank installations
The invention relates to a length and temperature
measuring apparatus for tank installations, having a
measuring tube and a movable element surrounding the
measuring tube for causing movement of a slider located
within the measuring tube.
Such an apparatus is already known and measures the
propagation time required for ultrasonic waves to travel
Erom an upper ultrasonic generator/receiver to the slider,
which is formed as a reflector and then back to the ultra-
sonic generator/receiver. From the propagation time thusmeasured, the propagation distance, i.e. the actual posi-
tion of the slider, can be calculated as a measure of -the
actual fluid level within the tank, provided the tempera~
ture prevailing in the propagation path is approximately
known as this exerts an influence on the speed of
propagation. Alternatively, an average temperature
prevailing within the measuring tube can be determlned
from a propagation path of known length~
The temperature in the measuring tube is however sub-
jected to considerable variations from time to time andalso from place to place depending upon the tank contents
and the particular fluid level, so that the assumption of
constant predetermined temperature at all times leads to
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incorrect results in the length measurement. This known
apparatus permits the determination of an unknown, i.e.
either the fluid level or the average temperature. Since,
however, in general the fluid level cannot be determined
in another way, this known apparatus is as a rule exclu-
sively used for measurement of the fluid level.
In view of this, an object of the invention is to
provide a measuring apparatus of the above mentioned type
that makes possible an accurate distance measurement,
especially of the depths of fluids in tanks or the like,
and in addition enables an accurate measurement of the
average temperature in a simple and dependable way.
According to the invention there is provided a length
and temperature measuring apparatus for tank installa-
tions, comprising: a measuring tube having a measuring
span embracing the lengths to be measured; first and
second resistance elements longitudinally located within
the measuring tube over the entire measuring span thereof,
said elements having terminals at each end thereof; a
slider located within the measuring tube, ~aid slider
having first and second sliding contacts sliding along
said first and second resistance elements, respectively;
a movable element surrounding said measuring tube for
causing movement of said slider located within the measur-
ing tube; a reference voltage source; an electrical
measuring unit; and circuitry enabling, in a first step,
said reference voltage source to be connected to the
terminals of the first resistance element and the
electrical measuring unit to be connected to one of said
terminals and to said first sliding contact for measuring
the partial voltage between said terminal and said contact
and hence the length between said terminal and said
contact; and said circuitry, in a second step, enabling
both said reference voltage source and said electrical
measuring unit to be connected to one of said terminals
of the second resistance element and said second sliding
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contact to measure the resistance value and hence the
average temperature of the second resistance element
therebetween.
The advantage of the invention in particular lies
in that, in a first measuring step, a partial voltage of
the reference voltage applied to a resistance element is
measured at a point where the sliding contact of the
slider, which taps the partial voltage, contacts the
resistance element, said point being at exactly the level
of the fluid. The partial voltage, which is drawn from
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the reEerence voltage, thereby directly reproduces thefluid leve] or measured length oE the total length of
the resistance element. The average temperature within
the measuring tube has no influence on the measurement
of the fluid depth.
Preferably the measuring tube contains a second
longitudinally extending resistance element. The lower
terminal of the second resistance element is likewise
guided out of the measuring tube, while the upper ter-
10 minal terminates within the tube. The slider has asecond sliding contact which engages the second resist~
ance element and has a terminal which extends to the
outside. In a second method step, the reference voltage
is applied between the second sliding contact and the
15 lower terminal of the second resistance element, and
then the resistance of the part of the second resistance
element therebetween is measured. From this partial
resistance, the average temperature of that part of the
resistance element can be determined from known data for
20 the resistance material employed.
Al-ternatively, in the second measuring step, the
reference voltage may be applied between the first slid-
ing contact of the slider and the lower terminal of the
first resistance element, and the thereby-produced par-
25 tial resistance measured by the measuring device and theaverage temperature in the region therebetween determined
Erom the measured value by use of the previously deter-
mined fluid depth. The use of a second resistance element
can then be avoided.
According to the invention, the measurement of the
fluid depth, or some other length determined by the
position of the slider is based on -the measurement of
a partial voltage, and further the determination of
the average temperature in the filled zone is made by
35 measurement of a partial resistance, whereby a great
degree of accuracy and dependability can be obtained
for comparatively low cost. The determined fluid depth
is not dependent upon the average temperature in the mea-
suring tube and is therefore relatively accurate. This
accurate value of the liquid depth is employed in the
followiny partial resistance measurement. From the
measured partial resistance, the average temperature
prevailing in that zone of the measuring tube in which
the partial resistance is located can be concluded
directly, i.e. without further corrections.
Preferably, the resistance elemen-ts are each formed
of resistance wire which is stretched by a tensioning
spring at one end. Particularly preferably, the re-
sistance wire is helically wrapped around a strip of
insulating material, and the strip of insulating material
is stretched by the tensionlng spriny. Thereby, the
useful length of the resistance wire is sukstantially
increased, so that a very precise determination of the
measured potential can be obtained.
Preferably, the lower terminals of the resistance
20 elements are guided to the top of the tube by means of a
common guideway, e.g. between the two resistance elements,
and through the upper cap of the tubeO The utilizable
volume of the tank is in this way left free of the
measuring equipment.
The temperature distribution present in the measuring
tube is as a rule characterized in that the ~luid tank
contents in the lower region of the measuring tube and the
gaseous atmosphere in the upper regions of the measuring
tube each have a definite temperature. The high heat
30 conductivity of the measuring tube guarantees that the
temperature of the tank contents also essentially prevails
in the corresponding submerged zone of the measuring
tube. Thereabove, with a gradual transition, the zone
of the measuring tube above the submerged zone takes on
35 the temperature of the atmosphere material. An homo-
geneous temperature division is thereby located within
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the measuring tube, whereby the accuracy of the first
measuring step, namely the determination of the fluid
depth, is improved, because then the voltage drop over
a single division of the resistance element better ful-
fills the hypothesis on which the effec-t is based that
the partial voltage is proportional to the length of the
part of the resistance elemen-t being measured.
Alternatively, the measuring tube may be made from a
material of low heat conductivity. This is for example
advatangeo~s if the tank employed is relatively small and
is quickly filled or emptied, as then based on the heat
insulating properties oE the measuring tube the same
homogeneous temperature distribution is obtained in the
measuring tube, directly after the Eil:ling or emptying of
lS the tank as was present before the filling or emptying,
and this is advantageous for the determination of the
fluid ~evel of the full tank or the mostly empty tank.
If the accuracy of the system has to be increased,
there may be provided, according to the invenkion, a
temperature sensor with an accompanying temperature
measuring circuit at one end of the measuring tube, in
order to measure the ]ocal temperature at this end of the
tube. Additionally, the resistance of the full length of
a resistance element may be measured e.g. in a measuring
step preceding the main measurements, and thereby the
average temperature in the measuring tube determined.
With these values obtained from the local temperature
sensor and for the average temperature within the tube,
the measurement of the fluid level may then be corrected -
perhaps by reference to a corresponding positionvariational function of the specific resistance of
the resistance element. In order to carry out such a
correction it is also possible to locate a temperature
sensor having a temperature measuring circuit on the
slider.
If the measuring tube is made of metal, the slider
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may be provided with a third sliding contact which slides
along the inner wall oE the tube ancl is electrically
connected to the ~irst and second sliding contac-ts. The
wa]l of the tube then serves as the lead to the first and
second sliding contacts, so that additiona] moving leads
within the tube can be avoided.
Further advantages of the invention are apparent from
the following description.
A preferred embodiment of the invention is explained
in detail below with reference to the drawings, in which:
FigO 1 shows a side view in cross-section of the
measuring tube of the apparatus of the invention; and
Fig. 2 shows a block circuit diagram of the apparatus
of the invention.
lS Fig. 1 shows a side view in cross-section of a mea-
suring tube 2 of a measuring apparatus according to the
invention. The measuring tube 2 is located vertically in
a tank 1~. ~ magnet float 1~, 16 encircles the measuring
tube 2 and is provided with a sleeve 15 which is integral
with the float body 16. A magnet 14 is provided on the
sleeve 15 at the immersion level of the Eloat body, the
magnet projecting into the interior of the body. A
cylindrical slider 20 made of a magnetized or magnetizable
material is slidah:ly mounted within the measuring tube 20
and is continuously held at the surface level within the
tank by the magne-ts 14 of the magnet float 14, 16.
The measuring tube 2 has a circular cross section.
The slider 2 has the shape of a circular cylinder, the
cylindrical surface of which is slidable along the inner
wall of the measuring tube 2 with only a small clearance.
The measuring tube 2 is sealed from the atmosphere by a
lower cap 10 and an upper cap 12. Two resistance elements
2~1, 26 are stretched along the central axis of the measur-
ing tube between the lower cap 10 and the upper cap 12 by
means of a spring 27 anchored to the upper cap 12a The
lower contacts 30, 32 of the resistance elements are
guided by means of a guideway 35 to the top of the tube
and then through the upper cap 12 to the outside. The
upper contact of the first resistance element 24 is also
guided through the upper cap 12 via the spring 27. The
upper end of the second resistance element 26 ends within
the measuring tube.
The slider 20 has a central bore or recess 22, through
which the resistance elements 2~, 26 extend. A first
sliding contact 31 is provided in the slider 20, which
10 engages the first resistance element 24. Further, a
second sliding contact 33 engages the second resistance
element 26. The sliding contacts 31, 33 are arranged in
a horizontaL plane correspondlng to the surface level of
the fluid in the tank.
A third sliding contact 34 is attached to the slider
20 for engaging the inner wa]l of the measuring tube 2 and
is electrically connected to the first and second sliding
contacts 31, 33. The wall of the measuring tube 2 forms
the circuit for the first and second sliding contacts 31,
20 33 via the third sliding contact 34, and has a correspond-
ing terminal 36 at the top of the measuring tube. Variable
conductors are thus provided within the measuring tube 2
and are connected to the outside via sliding contacts 31,
33.
The resistance elernents are formed of resistance wire
closely wound on a strip of insulating materialO This
allows a precise determination of the partial voltages
encountered and consequently provides a high measurement
accuracy.
A temperature sensor 50 is provided at the upper end
of the measuring tube and is connected to a temperature
sensing circuit 52. If for any reason a greater degree
of accuracy o~ the apparatus according to the invention
is desired, the temperature at the upper end of the
35 measuring tube may be measured locally by the sensor
50. Subsequently, the entire resistance of a resistance
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element is determined, e.g. in a preparatory measurement
step, and the average temperature in the whole Measuring
tube ascertained. From the local temperature at the
upper end of the measuring tu~e and the average tempera-
ture within the measuring tube, a linear temperat~redistribution as a function of position, and thereby a
corresponding linear function of the specific resistances
relative to position, can be calculated. This linear
function of the specific resistances according to position
can then ~e used to correct the determination of the li-
quid depth x in the Eirst method step. From the partial
voltage, measured in the first method step, the liquid
depth is obtained after a corrsponding correction, as the
measured partial voltage is no longer proportional to the
liquid depth x because of the positional variation of the
specific voltages. In practice, however, such a correction
can be omitted in most cases.
Fig. 2 shows a block circuit diagram of the appara-
tus according to the invention. A reference voltage is
applied/ during the first method step, to the two ter-
minals 28, 30 of the first resistance element 24 from a
reference voltage source 40 by closing a switch 41. A
partial voltage is generated between the first sliding
contact 31, provided on the slider 20 (not shown in Fig.
2, but see Fig. l), and the lower terminal 30 and this
is applied to the voltage measuring terminal 43 of a
measuring device 42. The measuring device 42 sends a
signal identi~ying the measured voltage to an analog/
digital transducer 46, which ~orwards a suitable signal
containing the measured information to a calculator 48.
The calculator 48 determines the li~uid depth x from
the measured voltage value and known total length l and
transmits a corresponding signal to a monitor 5Q.
In a second method step, the switch 41 is thrown, so
that the reference voltage is now applied to the lower
terminal 32 of the second resistance element 26. The
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electrical circuit continues through lower partial re-
sistance Rx of the second resistance element 26 to the
second sliding contact 33, which is located at the same
height as the first sliding contact. Then the circuit
continues to the resistance measurement terminal 44 of
the measuring device 42, which determines the value of
the partial resistance Rx. The measuring device 42
delivers a corresponding signal to the analog/digi-tal
transducer 46 which forwards a corresponding digital
signal containing the information to the calculator 48.
The calculator 48 calculates resistance Rx from the
known information for the resistance element 26, as
well as the previousl.y stored liquid depth x and from
this the average temperature within the filled zone which
the measured resistance Rx revea:Ls, is subsequently
calculated.
