Liquid level sensor device

A liquid level sensor device, particularly adapted for use with corrosive and hazardous liquids such as oil, for measuring the liquid level in a storage tank or vessel. A tubular device is placed vertically in a liquid storage tank and senses the liquid level by having a float embedded with magnets move vertically along the axis of reed switches encased in the tubular device. The device contains temperature sensors to measure the temperature of the tank fluid, and microprocessors that scan the reed switches, transmitting data through a connector in the device top to a nearby computer for recording and transmission to a control and monitoring location. The electrical data system inside the device utilizes only two conductive trace runs throughout the device length to maximize reliability while minimizing the copper used. Special mechanical and electrical assembly connectors inside the device permit the device to flex safely in turbulent liquid conditions, without breakage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to liquid level sensors and more particularly, to a 
liquid level sensor device adapted for use in a storage tank having a 
hazardous and explosive environment. 
2. Background 
Many industrial fluids are stored in large tanks prior to their 
distribution for use. It is often necessary to know the fill status of the 
fluids in the tanks, and for this purpose, liquid level sensors inserted 
into the tanks are used. 
The presently available liquid level sensor devices for industrial, fluid 
storage utilize electrical assemblies that are contained in one or more 
sealed tubes, and have a variety of external float mechanisms to activate 
switches on the electrical assemblies. The status of these switches and 
therefore, the liquid level, is communicated by wire from the top of the 
sensor device to any desired monitoring location. 
A typical use for these sensors is in the oil tanks and water tanks located 
near an oil well head. The sensors measure the levels of oil or water in 
the tanks deposited by the pump at the well head, and transmit the liquid 
level data to a local or remote monitoring unit. 
While the presently available liquid level sensor devices generally perform 
adequately, they are known to have serious disadvantages and potential 
problem areas as a consequence of their physical and electronic data 
system mechanizations. 
Since a sensor device usually is located in a turbulent environment inside 
a liquid storage tank, liquid movement in the vessel impinges on the 
device, creating substantial flexing of the device tube and the electrical 
assemblies inside the tube. In the prior art devices, the electrical 
assemblies on which are mounted sense switches, are generally arranged and 
joined end to end, with wire connectors and often switches connected 
across the assembly board ends. After a time, breakage of internal 
electrical components is observed near or at assembly joints. The device 
then has to be repaired or replaced by another, which is usually an 
expensive, time consuming procedure. 
Many electrical conductive traces are typically used in the prior art 
devices to connect and address the switches on the electrical assemblies. 
As an example, in U.S. Pat. No. 4,976,146, the liquid level sensor 
utilizes 31 traces to run from the bottom of the unit to its top assembly. 
This large number of traces requires large connectors and greatly reduces 
the reliability of the device, increasing connection/connector complexity 
and the probability of electrical failures. 
An additional problem area concerns detection of failed sensor switches 
inside the device. As yet, none of the prior art available liquid level 
sensor devices is known to incorporate a satisfactory method of 
determining which sensor switches have failed. The accuracy and 
reliability of the liquid level detection is then compromised. 
Thus, there exists a need for a liquid level sensor device that can 
withstand a turbulent liquid environment without internal breakage and has 
high measurement accuracy and reliability of operation. 
SUMMARY OF THE INVENTION 
The invention is a liquid level sensing device, particularly adapted for 
use in an explosive, hazardous environment. The relative position of 
liquid level, from full to empty is measured by having a float embedded 
with magnets move up or down along the axis of two staggered lines of reed 
switches encased in a tube. Inside the tube, are contained a number of 
sensor circuit board assemblies, which are connected mechanically and 
electrically from the top to the bottom of the tube. In addition to the 
two lines of reed switches and supporting circuitry, each circuit board 
assembly has its own microprocessor that scans and reports on the status 
of the reed switches. The topmost board assembly contains a master control 
microprocessor which addresses each of the sensor circuit board assemblies 
and communicates the data over an RS-485 serial communication bus to an 
external computer. The device incorporates a program for automatically 
scanning and determining which if any reed switches have failed, and 
reporting them for repair. Only two electrical conduction traces are used 
throughout to maximize contact reliability and also to reduce the amount 
of copper as compared with that needed for a parallel bus. 
The device incorporates mechanical and electrical connections for the 
circuit board assemblies which allow the tube and its internal assemblies 
to flex in a turbulent liquid without breakage of electrical components or 
contacts. 
The material used for the tube and float is selected for its ability to 
endure harsh environments, particularly that found at oil well sites. 
The device may be used with continual level sense switches from top to 
bottom, or with level sense switches only near the device top and bottom, 
without requiring any change to the device microprocessor programming. 
Accordingly, it is a principal object of this invention to provide a liquid 
level sensor that can withstand a turbulent liquid environment without 
internal device breakage. 
Another object is to provide a highly reliable liquid level sensor. 
Yet another object is to provide a liquid level sensor that incorporates a 
new, accurate method of liquid level detection which also detects faulty 
sensor switches for repair or replacement. 
Further objects and advantages of the invention will be apparent from 
studying the following portion of the specification, the claims and the 
attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring particularly to the drawings, there is shown in FIG. 1 a 
perspective view of a liquid level sensor device 1 according to the 
present, invention. The device 1 is shown in place vertically in a vessel 
of liquid with the device top cover 3 and data transmission terminals 7 
protruding above the top of the vessel, and a tube 9 containing rows of 
sense switches, extending to the bottom of the vessel. The tube 9 is 
attached to the vessel with standard mounting and sealing methods. 
The purpose of the liquid level sensor device 1 is to indicate the relative 
position of the liquid level by having a float 5, embedded with magnets, 
move up or down along the axis of reed switches encased in a tube 9. The 
proximity of the float to any of the reed switches causes those reed 
switches to close contacts, indicating the position of the float and thus, 
the level of the liquid in the vessel. 
The bottom of the tube 9 is expected to sit at "Level 0 inches" of the 
fluid to be measured, which is at the bottom of the vessel. 
Once scanned, along with liquid temperature information, the data is sent 
across an RS-485 serial computer communication bus, through terminals 7 at 
the top of the device 1 to a computer located nearby. 
Refer now to FIGS. 2 and 3 which are respectively, a cutaway perspective 
view of the invention device, and a plan view taken along line 3--3 of 
FIG. 1, showing a cross-section of the float 5. The device comprises a 
tube 9 sealed at its bottom end, a cap 3 which screws on the top of the 
tube 9, a float 5 which fits around 1 he tube 9, external wiring terminals 
7, and a quantity of circuit board assemblies connected together inside 
the tube 9. 
Inside the tube 9 are two types of circuit board assemblies; a top sensor 
circuit board assembly 13 and one or more lower sensor circuit board 
assemblies 15, 17. The quantity of the lower sensor circuit board 
assemblies depends on the height of the vessel containing the liquid whose 
level is to be measured. The top sensor circuit board assembly 13 is the 
control board. In addition to scanning its own reed switches 16 which are 
shown on its inside surface 13a, the top assembly processes configuration 
data, is responsible for power conditioning and distribution to all the 
sensor circuit board assemblies, and for communication with outside 
computers through an RS-485 serial computer-communications port. A 
connector 11 connects the top sensor circuit board assembly to the 
external transmission wires. 
The lower sensor circuit board assemblies 15, 17, are identical, each 
containing processor circuitry to scan its own sensor reed switches 16 and 
any temperature sense device 19 placed on the assembly. Part of a lower 
sensor circuit board 15 has been folded outward to display a view of its 
inward facing surface 15a showing placement of controls and sensor reed 
switches 16. 
Usually, the sensing circuit board assemblies extend to the bottom of the 
tube 9 and, depending on the depth of the liquid container vessel, may 
require numerous lower sensing circuit board assemblies connected end to 
end to reach the bottom or the tube. However, this is not a necessity for 
the device to operate. The user may need to have only bottom and top level 
portions of the vessel monitored. In that case, only enough lower sensing 
circuit board assemblies would be provided to cover the needed bottom and 
top level portions. Blank boards could be used, connecting the bottom 
board assemblies to the top board assembly. Considerable cost is saved 
thereby. 
The tube 9 and cap 3 are made of fiberglass material to withstand a 
corrosive crude-oil environment. The float 5 is made of Nitrile rubber to 
withstand a crude-oil environment, and has magnets 21 embedded in it, 
either in stud-magnet as shown in FIG. 3 or ring-magnet form. The external 
terminals 7 are standard, insulated wire grippers screwed into the side of 
the fiberglass tube 9, serving to secure the transmission wires to the 
sensor device 9. 
Refer now to FIG. 4 which is a cross-section of the tube 9 showing a 
temperature sensor 19 attached to a lower sensor circuit board assembly 
17. A temperature sensor 19 is shown near the bottom of FIG. 2 in the cut 
away front view. The temperature sensor 19 is a coiled strip of high 
thermal conductivity metal, such as copper, which presses against the 
inside wall of the tube 9. It provides a conduction path for the inside 
tube wall and hence the surrounding liquid temperature, to a temperature 
monitoring circuit mounted on the board assembly. The sensed temperature 
is processed by the microprocessor on the top sensor circuit board. 
Referring now to FIGS. 5, 6 and 7, these drawings illustrate the mechanical 
and electrical connections made between the circuit board assemblies, also 
showing the location of a microprocessor 14 and some of the other control 
circuit components. The drawings depict typical lines of reed switches 16, 
which are shown in this case, on one of the lower sensor circuit board 
assemblies 15. All the sensor circuit board assemblies have two lines of 
reed switches 16 or other magnetically sensitive switches on them in 
addition to various electronic control components. The locations of the 
reed switches are staggered with switches in one line overlapping switches 
in the other line. For accuracy of measurement, it is essential that the 
distance between centers of any two switches, located staggered side by 
side on the two lines, is kept at approximately 0.5 inches. 
The board assemblies are fastened to each other at overlapping ends using 
three threaded pins 23 which pass through cylindrical electrical insulator 
spacers 24 used to keep the board assemblies the correct distance apart. 
This arrangement keeps the joined portions rigid, so that they can not 
bend. It should be noted on FIGS. 5 and 6 that the lines of sensor reed 
switches on each board assembly are extended until the end of the lines of 
switches on one board overlaps the lines of switches on the opposite, 
joined board. Thus, the switches can detect the liquid level without a 
blank space at the assembly joint. 
The electrical connection between boards is made using plug 25 and 
receptacle 27 connectors which make a good connection and allow easy 
disconnection of the boards for maintenance or repair. The connectors 
25,27 are located in the area of the boards held rigidly apart by the 
three sets of screws 23 and spacers 24, near to one of the screws 23 and 
spacers 24. This ensures that the connectors will not bend during any 
flexing of the boards. 
It has been determined, that the above described type of electrical and 
mechanical connection, greatly diminishes the probability of electrical 
component or contact breakage on any circuit board assembly during 
moderate bending. This characteristic results in a considerable 
reliability advantage for the invention device, compared with prior art 
liquid level sensor devices which are more likely to experience electrical 
component breakage due to bending. 
In addition to having a microprocessor for local control on each sensor 
circuit board assembly, two staggered lines of reed switches are arranged 
along the board length to correspond with a given liquid level. The 
microprocessor on the top sensor circuit board assembly acts as a master 
controller for the liquid level sensor device. It performs the function of 
periodically obtaining reed switch status data from all the lower sensor 
circuit boards, including its own circuit board assembly, and transmitting 
the data to an external computer. 
Refer now to FIGS. 8 and 10 for the top sensor circuit board assembly, 
showing a functional block diagram of the components on the board, and a 
simplified flow chart of the operation of the master microcontroller 36. 
The major components on the top sensor circuit board assembly are: a 
microprocessor 36 that acts as a master control, controlling data 
acquisition on all circuit board assemblies in the device and handling 
communication with an external monitor; an I/O bus 38 connected to the 
microprocessor 36; an RS-485 connector interface 40 connected to the 
RS-485 line drivers 44 and thence to the microprocessor; a power supply 42 
for all board components, connected to the RS-485 connector interface; and 
the following components, all of which are connected to the microprocessor 
36: a matrix array of reed switches 46 for sensing the liquid level; a 
switch configuration selection circuit 48; a local temperature signal and 
conditioning circuit 52, a temperature signal conditioning circuit 50 for 
lower board assembly signals, and an interconnect 54 for connecting 
signals to and from a lower sensor circuit board assembly. 
The master control microprocessor 36 is programed to periodically and 
automatically address all sensor circuit board assemblies as shown in the 
flow chart of FIG. 10. After Start 80, the RS-485 buffer is parsed 82 and 
checked for the correct address 84. If the address is incorrect, the 
buffer is parsed again. If it is correct, the command is checked 86. The 
command validity 88 is next checked. If not, the buffer is parsed again 8. 
If OK, the microprocessor proceeds with the following steps 90: 
a) Read top sensor board reed switch matrix; 
b) Go to lower sensor boards in turn, 
c) Read switch matrix on each sensor board and 
d) Acquire thermal sense data; 
e) Read/program I/O data; 
f) Check for data errors. 
When this is done, the microprocessor then 92 a) enables the RS-485 path, 
b) transmits the data to an external remote terminal unit (RTU), and then 
c) disables the RS-485 path. 
The particular microprocessor software for the top and lower sensor circuit 
boards that directs the microprocessor to perform the foregoing and other 
steps, is considered to be integral with and a vital part of this 
invention. A separate patent application for this software, referencing 
this invention, is being filed at the earliest date. 
Refer now to FIGS. 9 and 11 for a lower sensor circuit board assembly, 
showing a functional block diagram of the components on the board, and a 
simplified flow chart of the operation of the local microcontroller 56. 
The major components of each lower sensor circuit board assembly are: a 
local microprocessor 56 that initiates and collects sensor switch data, an 
upper board connector interface 58 connected to the local microprocessor 
56, a power supply 60 for all board components, connected to the upper 
board connector interface 58, and the following components, all connected 
to the microprocessor 56: a matrix array of reed switches 62 for sensing 
the liquid level, an automatic addressing circuit 64 having an input from 
the upper board interface 58, a local temperature signal and conditioning 
circuit 52, a temperature sensor circuit 66 and temperature signal 
conditioning 68, and an interconnect 70 for connecting to other lower 
sensor circuit board assemblies. 
The local microprocessor 56 will perform the steps of scanning and reading 
the state of the reed switches upon receipt of a command from the master 
control microprocessor 36 on the top sensor circuit board assembly. A 
simplified flow chart of its operation is shown in FIG. 11. Upon receipt 
of command to Start 94, the buffer is parsed 96. A cheek 98 is made next 
to see if address data is received. If the cheek result is negative, the 
buffer is parsed again. Otherwise, the address data is checked for 
accuracy 100. If it is not the right address, the thermal data path is 
disabled 102 and the buffer is again parsed. When the address is found to 
be correct, the microprocessor proceeds to a) enable the thermal data 
path; b) scan the reed switch matrix and read switch status, and c) 
transmit the switch data to the top sensor circuit board assembly. 
In the proposed invention electrical configuration, the number of 
conductive traces has been minimized to two. This has been achieved by 
using a RS-485 serial communication bus. Since only two conductive lines 
are carrying information, rather than the twenty-four or so lines 
presently used in prior art devices, the invention configuration will 
yield better noise immunity and better overall system reliability, as 
compared with prior art devices. 
Each lower sensor circuit board assembly is terminated by an intelligent 
local controller, which determines the status of the rows of reed switches 
and reports this status to a master controller on the top sensor circuit 
board, using a universal digital word. This digital word is the same for 
all the circuit board assemblies: LIS sssssssss, 
where IS=assembly number in Hex, 
sssssssss=status of reed switches in one specified matrix. The above method 
is simple and universal for a multitude of different applications. Device 
adaptation to diverse applications does not require dramatic software 
modification. For example, if the liquid level sensor device is to be 
configured to measure only liquid levels at the top and bottom of a tank, 
the prior art level sensor devices would require complete change of 
communication protocol, whereas the invention device would not. The 
invention device method can automatically address any combination of 
assemblies. 
In addition to performing the functions of data collection from all 
assemblies and communicating the data to an external. computer or monitor, 
the master control microprocessor also determines the health status of the 
reed switches. It does this by utilizing an artificial intelligence and a 
fuzzy logic algorithm. The software algorithm statistically determines the 
rate of liquid change in the liquid vessel and, using a value of the rate 
of change, computes the prognosed liquid level for the next cycle of data 
acquisition. The master control compares the prognosed and actual liquid 
levels to determine the degree of probability of the next reed switch to 
be closed. Based on this criteria, the master control determines which 
switches are healthy and which switches are likely in default. Possible 
bad switches are looked at for two occurrences; switch closed and never 
open or switch open and never closed. A fault matrix is compounded from 
detected bad switches, giving the assembly location of the bad switches. 
This information can be used in maintenance and repair for identifying 
faulty assemblies and switches which need to be replaced. 
In maintenance and repair, the invention mechanical fastening method and 
electrical connectors for the sensor circuit board assemblies provide an 
easy way of disconnecting the assemblies. The spacers separating the 
assemblies are removed after unscrewing and releasing the fastening 
screws. The electrical connectors are simply disengaged. There is no need 
to de-solder any electrical components. To access the internal assemblies 
for troubleshooting and repair or replacement, it is only necessary to 
remove the top cap of the device by unscrewing it. No special tools are 
required. Maintenance and repair of the device are thus greatly 
facilitated by the invention mechanical and electrical configuration. 
In summary, the invention device is seen to have at least the following 
advantages over the presently available devices: 
1. The invention reliability is much higher due to the device assembly 
mechanical and electrical connector design which prevents electrical 
component or contact breakage due to assembly bending, and also due to the 
minimum number of traces (two) used throughout. 
2. Accuracy of level readings is greater, because of the methods of 
addressing the switches and determination of faulty switch locations. 
3. Maintenance troubleshooting and repair/replacement of device 
subassemblies or parts is much easier than is the case for most presently 
available liquid level sensor devices, because the invention assemblies 
can be easily unscrewed and disconnected from each other. 
4. The device adapts easily with little or no software modification to a 
multitude of diverse applications, including bottom and top level only 
measurements. 
From the above description, it is clear that the preferred embodiment of 
the liquid level sensor device achieves the objects of the present 
invention. Alternative embodiments and various modifications may be 
apparent to those skilled in the art. These alternatives and modifications 
are considered to be within the spirit and scope of the present invention.