Apparatus for fluid mass measurement

Apparatus for measuring the total mass of the fluid in a container, having pressure responsive, density determination means adapted to determine the localized density of the fluid as a function of pressure differential between two closely spaced, vertically separated measuring points in the fluid, means enabling the density determination means to determine the localized density values throughout the depth of the fluid, storage means for storing predetermined horizontal cross-sectional area values of the container as a function of depth, and computational circuitry responsive to the density determination means and the storage means for providing a total mass signal corresponding to the integral, over the depth of the fluid, of the product of localized density values times the corresponding area values.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for measuring the mass of a fluid in a 
container, e.g., oil in a tank. 
Storage, loading and delivery of large quantities of oil (and other liquids 
and powders) requires repeated determination of the mass of fluid being 
handled. In typical measuring devices, an element measures the 
temperatures at a number of regions in the container. Then the mass 
density in each region (corresponding to the temperature there) is 
multiplied times the volume of that region and the products are 
accumulated as the total mass. Fluid in such a tank typically includes a 
layer of water (e.g. water which has settled to the tank bottom), which 
affects the measurement of the total oil mass. 
SUMMARY OF THE INVENTION 
In general, the invention features apparatus for measuring the total mass 
of the fluid in a container, having pressure responsive, density 
determination means adapted to determine the localized density of the 
fluid as a function of pressure differential between two closely spaced, 
vertically separated measuring points in the fluid, means enabling the 
density determination means to determine the localized density values 
throughout the depth of the fluid, storage means for storing predetermined 
horizontal cross-sectional area values of the container as a function of 
depth, and computational circuitry responsive to the density determination 
means and the storage means for providing a total mass signal 
corresponding to the integral, over the depth of the fluid, of the product 
of localized density values times the corresponding area values. 
In preferred embodiments, the density determination means includes a 
vertically movable pressure transducer and the means enabling 
determination of density values throughout the fluid depth includes 
traverse means for moving the transducer throughout the depth of the 
fluid, and a level indicator for providing values indicative of the 
vertical location of the transducer in the fluid with respect to each of 
its pressure readings; the pressure transducer has two pressure sensors at 
two vertically spaced-apart positions and is arranged to provide signals 
corresponding to the difference in pressures at the two positions, the 
signals being indicative of the mean localized density of the fluid in the 
region between the two positions; the apparatus is adapted for use with 
fluids whose density varies with temperature variations over the depth of 
the fluid and the computational circuitry is adapted to determine the 
integral in steps each corresponding to a finite interval of depth, and 
the finite interval and the vertical distance between the spaced-apart 
positions of the pressure sensors being selected to be small compared with 
the magnitude of variations, over the depth of the fluid, of the 
cross-sectional area and the density of the fluid; the container holds an 
additional fluid vertically separate from the fluid to be measured, the 
additional fluid having an electrical conductivity different from the 
fluid to be measured, and the apparatus includes an element sensitive to 
electrical conductivity, the apparatus responsive to the element to 
exclude the mass of the additional fluid from the total mass signal; the 
computational circuitry is adapted, on the basis of differences in 
electrical conductivity, to measure oil, and to exclude water; the storage 
means is adapted to store the cross-sectional area values for different 
containers having respectively different geometric configurations; the 
storage means is adapted to store the localized density values; the 
computational circuitry has programmable arithmetic processing means for 
computing the total mass signal, and signal flow control circuitry 
connected to route the localized density signals and the stored 
cross-sectional area values to the arithmetic processing means; the 
computational circuitry also has fluid sensing means for delivering 
immersion signals indicative of when the density determination means is 
not immersed within the fluid, the signal flow control circuitry is 
adapted to route the immersion signals to the arithmetic processing means, 
and the arithmetic processing means is arranged to limit the total mass 
signal computation to positions at which the density determination means 
is immersed in the fluid; and the apparatus includes an element sensitive 
to the electrical conductivity of the fluid in which the density 
determination means is immersed, the signal flow control circuitry is 
connected to route signals from the element to the arithmetic processing 
means, and the arithmetic processing is arranged to limit the total mass 
signal computation to positions at which the density determination means 
is immersed in the fluid to be measured. 
The total mass is accurately and easily determined for a container of any 
configuration, without measuring temperature or requiring a priori 
knowledge of the dependence of density on temperature or depth the mean 
density in various regions is easily determined by the two sensors of the 
transducer; the mass of the measured fluid can be determined despite the 
presence of the additional fluid; and the mass measurement can be made 
easily for a variety of containers. 
Other advantages and features will be apparent from the description of the 
preferred embodiments and from the claims.

Structure 
Referring to FIG. 1, tank 10 is filled to level 12 with oil 14. Mass 
measuring system 16 includes measuring station 18 (positioned above the 
tank) and probe 20 suspended in the oil by gauging tape 22. 
Referring to FIG. 2, measuring station 18 has reel 24 for winding and 
unwinding tape 22 to raise and lower probe 20 in the tank. Tape 22 is 
wound and unwound over hub 28 so that the side 34 of tape 22 facing away 
from reel 24 is exposed to optical mark sensor 29 in housing 30. 
Referring to FIGS. 3, 4, and 5, tape 22 has a supporting layer 36 (e.g., 
steel), side 34 of which is imprinted with periodic marks 38 (e.g., every 
1/4") in a color which contrasts with the surface of layer 36. The other 
side of layer 36 (i.e., side 40 in FIG. 2) is coated with insulating layer 
42 (e.g., nylon or enamel) on which four silver electrical conductors 44 
are silkscreened, brushed or extruded. Both sides of tape 22 are coated 
with abrasive-resistant and electrochemically-resistant insulating layers 
46. 
Referring again to FIG. 2, optical mark sensor (level indicator) 29 
includes light emitting diode (LED) 52 and phototransistor 54, which are 
positioned (relative to each other and to side 34 of tape 22) so that 
light path 56 from the LED is reflected from the tape onto the 
phototransistor, causing a change in the phototransistor's output whenever 
a mark 38 is located at the point where light path 56 is being reflected. 
Referring to FIG. 6, probe 20 contains a differential pressure transducer 
60, a fluid sensor 62, and a conductivity probe 64, all connected by wires 
44 to measuring station 18. 
Pressure transducer 60 has a pair of pressure ports 66 (vertically 
separated by a distance D). Pressure ports 66 are pneumatically connected 
by tubes to a pressure transducer element 68 (semiconductor, inductive or 
capacitive), which is sensitive to the difference in pressure (and hence 
to the mean density of the liquid) between ports 66. Element 68 is powered 
over line 70 from voltage source 72 (in the measuring station) and 
provides an analog output voltage indicative of the pressure differential 
over line 74 to an analog-to-digital converter 61 (in measuring station 
18). 
In fluid sensor 62, an infrared LED 80 (powered over line 70) projects 
infrared radiation along path 82 inside transparent light conductor 84. 
Surface 86 reflects the radiation back through conductor 84 to 
phototransistor 90 whenever fluid sensor 62 is in air (and hence the 
relative refractive indexes of the air and the conductor cause internal 
reflection in the conductor), and phototransistor 90 delivers a signal 
over line 92 indicating that the sensor is not in fluid. Whenever sensor 
62 is in fluid, the relative refractive indexes at surface 86 permit the 
radiation to exit the conductor, along path 94, thereby interrupting the 
signal from phototransistor 90. 
Conductivity sensor 64 has a pair of spaced-apart electrodes 100, one of 
which is grounded, and the other of which is connected (through dropping 
resistor 102 and line 70) to voltage source 72, and (directly through line 
104) to measuring station 18. The current flowing through resistor 102 
(and hence the voltage on line 104) depends on the conductivity of the 
fluid (e.g., water or oil) surrounding electrodes 100. 
At the measuring station, the output of optical mark sensor 29 (powered 
from voltage source 72) is connected to a counter 111 which, each time it 
counts a preset number of marks (equal to the ratio of the step size of 
the numerical integration, explained below, to the separation of the 
optional marks 38), sends a signal by line 110 to I/O address port 112 
whose output provides an address to memory (storage means) 114 each time a 
signal is received over line 110. The addresses are provided from 
microcomputer 113 through microcomputer interface (signal flow control 
circuitry) 115. Memory 114 is also connected to the output of A/D 
converter 61 so that a digital differential pressure value (i.e., 
localized density value) is stored in memory 114 (each time a signal is 
delivered on line 110), at the address delivered from I/O address port 112 
to memory 114. In the computational circuitry at the measuring station, 
conventional signal conditioner 120 receives the signal from fluid sensor 
62 over line 92 and delivers a conditioned fluid signal through interface 
115 to microcomputer 113 to indicate when probe 20 is no longer immersed 
in the fluid being measured. Nor gate 122 (controlled from microcomputer 
113) selectably gates the signal from conductivity probe 64 through signal 
conditioner 124 to microcomputer 113 to indicate when probe 20 is in water 
rather than in oil. 
Microcomputer 113 can read stored data from memory 114 through interface 
115 from addresses determined by the I/O address port 112. 
Memory 114 also contains values for the areas enclosed by the tank at 
successive evenly-spaced levels above the bottom (which levels are 
separated by the same interval as the separation between levels in the 
fluid at which pressure difference data are recorded). Microcomputer 
(arithmetic processing means) 113 (working through microcomputer interface 
115) uses the pressure and area information stored in memory 114 and the 
known vertical interval between successive levels in the tank to calculate 
the mass of the oil in the tank, using the trapezoid rule of numerical 
integration: 
##EQU1## 
where .DELTA.y is the integration step size, A.sub.i is the 
cross-sectional area of the container at a level i .DELTA.y above the 
bottom of the tank, .DELTA.P.sub.i is the pressure difference between 
levels i .DELTA.y+D/2 and i .DELTA.y-D/2 above the bottom of the tank, D 
is the vertical separation of ports 66 of the pressure transducer, and g 
is the local acceleration of gravity, (approximately 9.81 m/sec.sup.2). 
Note that .DELTA.P.sub.i /gD is the mean density in an interval of 
thickness D. Within a vertical distance D/2 of the boundaries of the fluid 
being measured, .DELTA.P is extrapolated from adjacent values of .DELTA.P 
measured entirely within the fluid being measured. 
Operation 
To measure the mass of oil in the tank, the cross-sectional areas A(h) are 
stored in memory 114. Probe 20 is lowered to the bottom of the tank, and 
the reel is rotated to draw the transducer up through the fluid. As the 
transducer rises, each time one of the marks on the tape passes optical 
mark sensor 29, memory 114 is triggered to store the digital differential 
pressure value from pressure transducer 60. If water (which has a higher 
conductivity than oil) is encountered at any level, the conductivity 
signal informs the microcomputer, which then ceases the storage of 
pressure values until oil is again encountered. When the probe reaches the 
surface of the oil, fluid sensor 62 being no longer immersed, informs the 
microcomputer which then ceases the storage of pressure values. The 
microcomputer performs the calculation of the equation set forth above. 
The total mass is then displayed on conventional display elements 26 (FIG. 
2). No temperature measurements are required. 
Other Embodiments 
Other embodiments are within the following claims. E.g., the tank may be a 
vessel or well or any other container; the pressure transducer need not be 
of the differential type in which case the pressure differences can be 
calculated at the measuring station; LED 52 and phototransister 54 can be 
replaced by a Hall effect switch, and marks 38 can be magnetic so that the 
Hall effect switch "reads" the marks as tape 22 moves; optical sensor 29 
can be replaced by an encoding shaft and wheel (e.g., 4" circumference) on 
reel 24, whose angular position can be read continuously to indicate the 
corresponding vertical location of the transducer and the microcomputer 
can calculate the mass by continuous integration of the pressure 
difference function times a stored area function over the entire depth of 
the fluid; the oil may instead be any liquid or powder; the microcomputer 
can calculate the areas at each level from information about the container 
shape, rather than storing individual areas in advance; the microcomputer 
can be physically separated from the measuring station, so that the 
pressure data is first stored in the memory and later used to calculate 
the mass; and different area data for different tanks can be stored on 
different interchangeable memory elements so that the same measurement 
device can be easily used with different tanks.