Method and apparatus for mercury cell anode adjustment

A method and apparatus for obtaining signals from the anode buses of a mercury cell to provide automatic protection against short circuits, visual readouts, and computer information. A multiplexer is associated with each cell which scans the signals from the various bus bars. The scanned signals from each bus bar are sequential with one another and are converted to proportional voltage and anode bus current signals, which are transmitted to a console for decoding and processing to provide a visual readout and to a computer for processing and anode adjustment. In addition, each cell has an overcurrent and short circuit protection circuit based upon anode bus current. The instantaneous anode bus current for a given bus of a cell is compared with an average anode bus current for that cell and the anodes raised when the current exceeds the average bus current by a certain percentage.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus for adjusting 
anodes in an electrolytic cell. More particularly this invention relates 
to a method and apparatus for generating voltage and current signals for 
use in adjusting the anode-cathode spacing in mercury cells and also for 
automatically raising the anode with respect to the cathode to prevent 
overcurrent and short circuits. 
Electrolytic mercury cells have been used commercially in the production of 
chlorine and caustic by the electrolysis of brine for many years. In 
general these cells employ a metal cell container which slopes slightly 
downwardly from one end to the other, and which utilizes a cathode 
comprised of a moving stream of mercury on the bottom of the cell. A 
stream of brine flows on the top of the mercury cathode in the cell 
container. Anodes, fabricated either from graphite or metal, are secured 
to the top of the cell container and positioned in the brine above the 
mercury cathode. 
When a voltage is applied across the cell, current flows from the anode 
through the brine electrolyte to the cathode and causes electrolysis of 
the brine and the formation of gaseous chloride, which is removed from the 
cell, purified and stored. Elemental sodium, another product of the 
electrolysis forms an amalgam with the mercury cathode and is removed from 
the cell and processed to form a caustic solution. Regenerated mercury 
from the amalgam is recycled for use as a cell cathode. 
A plurality of electrolytic mercury cells are normally electrically 
connected in an electrical series circuit. Each cell usually has a 
plurality of anode buses which are electrically connected to the cathode 
side of an adjacent cell. In some instances the anodes of each cell may be 
divided into sets with each set of anodes of a given cell being separately 
adjustable to raise and lower the anodes of a given set with respect to 
the mercury cathode. Each anode set may be provided with one or a 
plurality of anode buses. With cells of this design, a motor is associated 
with each anode set to raise and lower the anodes of the set. 
According to another cell design, all the anodes of the cell are raised and 
lowered simultaneously. In this case, a motor is associated with the 
entire group of anodes to raise and lower the anodes. 
Whatever the design of the cell, the control of the inter-electrode 
distance between the anode and cathode is economically important. The 
inter-electrode distance should be as small as possible to reduce the 
wasteful consumption of energy. 
In addition to the problem of maintaining the optimum anode-cathode spacing 
in mercury cells, the problem of preventing short circuiting due to 
contact between the mercury cathode and anode is also of importance. Such 
short circuits may be caused by breakage of a graphite anode, by loosening 
of anode support posts, by changes in the thickness of mercury due to 
faulty flow control, or other causes which allow the anode to contact the 
flowing mercury cathode. The resulting short circuit causes an excessive 
flow of current in the anode and in the anode bus serving that anode, 
along with anode damage, overheating of the anode leads, loss in 
production of chlorine, excessive hydrogen in the chlorine, and other 
problems. In addition, with metallic anodes, a short circuit damages the 
active coating and the support structure which cannot be economically 
tolerated. 
Numerous techniques have been developed for adjusting the anode-cathode gap 
in electrolytic cells. See, for example, U.S. Pat. Nos. 3,574,073, which 
issued Aug. 6, 1971; 3,873,430, which issued Oct. 25, 1975; and 3,900,373, 
which issued Aug. 19, 1975. In general these patents describe a method of 
adjusting the anodes by transmitting current and voltage signals to a 
computer or to some type of visual readout or both. The operator, during 
manual control of the anode sets, can raise or lower a particular anode 
set through a motor control system until the desired operating condition 
is reached. Alternatively, these systems may utilize a digital computer 
which can perform various operations and actuate the motor control circuit 
to raise or lower the anode sets until the desired condition is reached. 
In addition, such patents as the aforementioned U.S. Pat. No. 3,574,073, as 
well as U.S. Pat. No. 3,844,913, issued Oct. 29, 1974, disclose systems 
for the automatic raising of the anodes or anode sets of a given cell upon 
the sensing of an overcurrent or short circuit condition. 
SUMMARY OF THE INVENTION 
The present invention relates to an improved system for obtaining voltage 
and current signals from an electrolytic cell and transmitting such 
signals to such items as a console for visual display, to a computer 
interface for permitting the computer to provide readouts of various 
parameters and automatically control the anode-cathode spacing, and to an 
overcurrent motor control circuit which will raise the anodes upon 
detection of an incipient short circuit. 
According to the present invention a multiplexer or scanner is associated 
with each cell. The multiplexer continuously and rapidly scans all bus 
bars of the cell with which it is associated and transmits current and 
voltage signals associated with each bus bar to a console and computer as 
well as to an overcurrent motor control circuit. 
In the overcurrent motor control circuit, the individual bus bar current 
signals in sequence from the multiplexer are compared with a signal 
representing the average bus bar current. In the event that the individual 
bus bar current signal exceeds the average bus bar current signal of the 
given cell by a predetermined amount, the raise mode of the motor control 
circuit is actuated causing the motor to raise the anode for a 
predetermined time period upon the receipt of each signal pulse. The motor 
will continue to operate until the individual bus bar current signal drops 
to a value equal to or less than the average bus bar current signal. 
The use of a scanner or multiplexer with each cell enables the current and 
voltage signals to be transmitted to the console in a manner which easily 
permits the various readings to be visually displayed on an oscilloscope 
with the readings for each of the anode buses of the given cell being 
visible at the same time. 
In addition, the voltage drop across the cell bottom bus joint connection 
for each anode bus can be scanned using the multiplexer associated with 
each cell to provide a readout of the voltage drop associated therewith.

DETAILED DESCRIPTION 
The present invention is particularly applicable for use in connection with 
a plurality of electrically interconnected mercury cells as typically 
found in a cell room. FIG. 1, discloses a typical arrangement wherein 64 
mercury cells 2 are electrically connected in series with two rows of 
cells, 32 cells in a row. Each cell 2 is designated in the cell room by a 
different number for reference purposes as indicated on the drawing with 
the subscript to reference numeral 2 indicating the particular cell 
number. 
Each cell 2 may be provided with one or a plurality of anode sets 4, each 
set containing one or a plurality of anodes 6. As shown in FIG. 1, one 
arrangement is 10 anode sets 4 with five anodes per anode set. The anode 
sets 4 of a given cell 2 are electrically connected in parallel to the 
cathode of a preceeding cell by anode buses 8. Each anode set 4 may have 
one or a plurality of anode buses 8. As shown, each anode set 4 of FIG. 1 
has two anode buses 8.sub.a and 8.sub.b with the subscript number 
indicating the particular anode set 4 with which a particular anode bus 8 
is associated. Thus 8.sub.10b for example refers to the second or "b" 
anode bus 8 associated with the tenth anode set 4. 
Each anode 6 is provided with at least one anode post 12, and, according to 
FIGS. 1 and 4, preferably with two anode posts 12 arranged in two parallel 
rows. One of the anode buses 8 is connected to each row of anode posts in 
a given cell 2. The other end of the anode bus 8 is connected to a 
terminal at the metallic bottom 14 of the cell 2 at a point nearest its 
associated anode 6. Current from plant supply (not shown) is conveyed to 
each of the anode buses 8 of cell 2.sub.1 and then to anode posts 12 
associated with the particular anode bus 8. Current from the anode posts 
12 of cell 2.sub.1 passes to the anodes 6 of that cell 2.sub.1, through 
the electrolyte (not shown), the mercury amalgam (not shown) to the bottom 
14 of cell 2.sub.1. 
Each of the anode buses of cell 2.sub.2 carry current from the bottom of 
cell 2.sub.1, to their associated anode posts 12, through the anode posts 
12, the anodes 6, electrolyte, and mercury amalgam to the bottom of cell 
2.sub.2. In a similar matter current is carried to all of the cells 2 in 
the cell room that are electrically connected together. 
Each of the anode sets 4 have a motor 16 (see FIG. 4) associated therewith 
which when operated causes all of the anodes 6 of that anode set 4 to be 
raised or be lowered with respect to the cathode. Thus in the embodiment 
shown in FIG. 1, there would be 640 motors, one motor being associated 
with each of the ten anode sets each of the 64 cells. 
As an alternative, in some arrangements, there may not be distinct movable 
anode sets within a cell, but rather all the anodes of a cell may be 
raised or lowered simultaneously by the use of a single motor. The present 
invention is applicable to both arrangements. 
Each anode bus 8 of each cell is tapped as indicated in FIG. 1. This is 
accomplished by providing spaced terminals or taps 18 and 20 along each 
anode bus 8. The subscripts used in connection with the spaced terminals 
18 and 20 indicate the particular bus that is being tapped. For example, 
terminal 18.sub.1a indicates terminal 18 associated with the first anode 
bus 8.sub.1a of the first anode set 4.sub.1. The terminals 18 and 20 of 
each anode bus are used to generate a direct current millivolt signal 
proportional to current flow. A volt signal proportional to voltage drop 
across an anode set 4 associated with a given anode bus for a given cell 
is generated by using terminal 20 of the next cell. For example, the 
signal for the voltage drop for Cell No. 1, reference number 2.sub.1, in 
FIG. 1, associated with anode bus 8.sub.1a and the anode set 4.sub.1, is 
generated by using signals from terminal 20.sub.1a of the bus 8.sub.1a of 
cell 2.sub.1 and terminal 20.sub.1a of bus 8.sub.1a of cell 2.sub.2. 
As seen in FIG. 4, the input signals from terminals 18 and 20 may be 
temperature compensated by an appropriate electrical circuit such as a 
thermistor circuit as shown and described in U.S. Pat. No. 3,900,373, 
issued Aug. 19, 1975, to R. W. Ralston, Jr. and which is incorporated 
herein in its entirety. The input signals from terminals 18 and 20 may 
also be filtered by filter 21. 
The system in the present invention as shown in FIG. 2 includes the use of 
a multiplexer 22 in association with each cell 2 which receives signals 
from all the terminals 18 and 20 of all the bus bars 8 of its associated 
cell simultaneously. The output from the multiplexer at any given time is 
two signals, one from each of the terminals 18 and 20 on one particular 
bus 8. The multiplexer continuously scans the bus bar signals so that one 
of its outputs is all the signals from the terminals 18 of all buses 8 of 
its associated cell in sequence, and another output is all the signals 
from the terminals 20 of all the bus bars 8 of its associated cell in 
sequence. 
As shown in FIG. 2, all the multiplexers 22 are driven by a master address 
system 24 that is synchronized with the power frequency. The master 
address system also drives the overcurrent decoder 26, the signal decoding 
system 28, the computer interface 30, and supplies the synchronizing pulse 
for an oscilloscope which is part of the readout and oscilloscope section 
32 of a console 34. The signals from each multiplexer 22 pass through an 
associated amplifier system 36 (See FIG. 3) where the multplexer signals 
are buffered and a differential amplifier is used to obtain a signal 
proportional to the bus kiloamperes. The current signal is supplied to a 
motor control circuit 38 where it is compared to the average bus current 
of the cell and will cause the raise mode of motor operation to actuate if 
any instantaneous current signal from a given bus 8 exceeds the average 
bus current of the cell by a predetermined amount. In the case where there 
is more than one motor per cell, the current signals associated with the 
buses of a given cell are supplied to the motor overcurrent decoder 26, 
which in conjunction with the signals from the master address system 24, 
electronically selects which of the motors should be operated to raise the 
anodes associated therewith. With this arrangement, only the anodes 
associated with the bus which indicated the overcurrent signal will be 
raised. 
The signal decoder 28, in addition to performing various calculations 
electronically such as averaging cell voltage and current, also samples 
and holds the respective signals until called for by either the manual 
anode selector or by the computer. 
FIG. 3 shows in more detail the electronic detail of the system of the 
present invention as applied to two adjacent cells, for example, cells 
2.sub.2 and 2.sub.3 (Cell No. 2 and Cell No. 3 of FIG. 1). The signals 
from each tap 18 and 20 of each bus 8 for a given cell 2 are fed into the 
multiplexer 22 associated with that particular cell 2. The multiplexer 22 
may be considered a plurality of individual switches 40, one switch 
associated with each of the taps 18 and 20 of each bus 8. The multiplexer 
22 is solid state to permit a "fast" scanning rate. In the embodiment 
shown in FIG. 3, the multiplexer 22 has two output channels 42 and 44, 
output channel 42 for the signals from the taps 18 and output 44 for 
signals from the taps 20. 
The multiplexer, driven by the master address system 24, closes 
simultaneously the switch 40.sub.181a associated with the tap 18, on the 
first bus of the first anode set and switch 40.sub.201a associated with 
tap 20 on the first bus of the first anode set, permitting such signals to 
pass through their respective multiplexer outputs 42 and 44 to the 
amplifier system 36. The same switches 40.sub.181a and 40.sub.201a on all 
the multiplexers of all the cells are closed at the same time by the 
master address system 24 (FIG. 2). 
The master address system 24 serves to scan all of the bus signals by 
opening the contacts 40 associated with terminals 18 and 20 on the first 
bus 8 of the first anode set and closing the switches 40.sub.181b and 
40.sub.201b associated with the second bus of the first anode set 
permitting those signals to pass through outputs 42 and 44 simultaneously 
to the amplifier system 36. This process continues until the signals from 
each bus 8 have been scanned sequentially, whereupon the scanning process 
starts over again. The rate of scanning, related to the power frequency, 
is at "fast" speed, which herein means a speed such that all channels of 
the multiplexer are scanned in 1/60 of a second. Other fast speed scan 
rates which are multiples of the power frequency may be used. For example, 
if the power frequency is 60 Hz, the rate of scanning of all channels 
could also be 1/30, 1/120 or 1/360. 
Thus, for the system shown, the multiplexer 22 will address sequentially 20 
sets of two inputs. If desired, another switch, or channel may be provided 
on the multiplexer which may be used for zero reference and calibration, 
and which would be scanned sequentially after the last bus tap signals. 
The amplifier system 36 is shown in more detail in FIG. 4 which shows the 
electronic circuit in simplified form as applied to one bus 8 of Cell No. 
2 and the equivalent bus 8 of Cell No. 3. The signals from taps 18 and 20 
pass through their associated switches 40 in the multiplexer 22 and are 
buffered by individual unity gain, differential amplifiers 46 and 48. The 
outputs from the amplifiers 46 and 48 are simultaneously amplified and 
converted by differntial amplifier 50 to a single-ended voltage signal, 
referenced to cell common, and proportional to the bus kiloamperes (bus 
current). The output 52 of the differential amplifier 50 is transmitted to 
a contact 54 of a cell select relay 56. As will be noted, each cell 2 has 
an associated cell select relay 56. The relay output 58 of the current 
signal from at least a plurality of cell select relays 56 are connected to 
a single isolation amplifier 60. The cell common 62 for each cell, to 
which the single-ended voltage proportional to current is referenced is 
also transmitted through a contact 64 of the cell relay 56 of its 
associated cell and the output 66 fed to isolation amplifier 60. The 
isolation amplifier 60 converts the high common-mode voltages to 
single-ended signals referenced to the console common, which is preferably 
earth potential. The cell common for each cell is preferably referenced 
back by a separate line to tap 18 of the middle bus of a given cell, for 
example, tap 18 of either bus 8.sub.5b or 8.sub.5a of the arrangement 
shown in FIG. 1. 
The output from amplifier 48, i.e., the buffered signal from tap 20 of the 
bus bar 8, is also fed to a contact 68 of its associated cell relay 56, 
the output 70 of which is fed to an isolation amplifier 72. The output 
signal from the amplifier 48 associated with the next successive cell is 
also fed to a contact 74 of the cell select relay 56 associated with a 
given cell with the output 75 fed to the isolation amplifier 72. Isolation 
amplifier 72 converts the signals to a single-ended voltage, reference to 
the console common, and proportional to the voltage drop associated with a 
given anode set and one of its associated buses. Depending upon the 
particular configuration of a given bus, the signals from amplifier 46 may 
be used for obtaining the anode set voltage signals. 
A cell select circuit 76, which may either be manually or computer 
operated, causes the actuation of the cell relay coil 78 to close the 
contacts 54, 64, 68, and 74 of a given relay when the cell with which that 
relay is associated is selected. Upon the closure of the contacts of any 
cell select relay 56, the voltage and current signals for all the buses 8 
of that cell are passed to the console for decoding, readout, and signal 
processing. 
If there are a relatively large number of cells that are connected in 
series, it may be desirable to sectionalize the signals from the cells by 
sectionalizing relays (tree relays). For example, with the arrangement 
shown in FIG. 1 which depicts a total of 64 cells, there may be eight 
sectionalizing relays, one for cells 1-8, another for cells 9-16, etc. All 
signals from each individual cell select relay associated with cells 1 to 
8 would pass through a single sectionalizing relay before passing to the 
isolation amplifiers. In addition, there may be more than one set of 
isolation amplifiers 60 and 72. In the specific embodiment mentioned above 
wherein there are 64 cells, those may be two sets of isolation amplifiers 
60 and 72, one set for the signals from cells 1 to 32 and the second set 
for the signals from cells 33 to 64. All the isolation amplifiers 60 and 
72 may be separated from the readout, decoding and signal processing 34 by 
a suitable relay. With such an arrangement, when a given cell is selected, 
the individual cell select relay 56 must be actuated, the sectionalizing 
relay with which that cell is associated must be activated, as well as the 
relays associated with the set of amplifiers associated with that 
particular sectionalizing relay. 
The current signals, i.e., the output from differential amplifier 50, from 
all the buses are also fed to an overcurrent circuit 80 for automatic 
prevention of overcurrent and short circuits. Each cell has its own 
individual overcurrent circuit 80. The overcurrent circuit 80 includes an 
averaging circuit 82 which continuously averages the instantaneous 
individual bus current signals for the cell to obtain an average cell 
anode bus current. This average bus current signal is then transmitted to 
a comparator 84. The instantaneous current signals from amplifier 50 are 
transmitted to the comparator 84 which serves to compare the instantaneous 
current signal of the buses 8 sequentially with the average bus current. 
If the instantaneous current signal of any given bus bar is determined to 
be higher than the average bus current by a predetermined percentage, the 
comparator 84 will send a signal to the overcurrent interlock 86. 
An adjustment circuit 85 is associated with the comparator 84 so that the 
percentage by which the instantaneous current signal must exceed the 
average anode bus signal before a signal is sent to the interlock 86 may 
be varied. The range of adjustment may be such that the comparator 84 will 
send a signal to the interlock when the instantaneous current is between 
100 to 200 percent of the average current. The preferred range is between 
100 and 150 percent with 130 percent being the desired set point in many 
cases. It is to be noted that the average anode bus current is 
proportional to total cell current or system load current. Therefore, the 
set point for comparison with the instantaneous anode bus current 
automatically changes as the total cell current changes. 
In the event there is more than one motor associated with a given cell such 
that one anode set can be raised independently of the others, each motor 
will have its own motor control circuit 88. A logic system is provided to 
determine which one of the motor control circuits should be raised 
depending upon which anode bus has indicated a high current. The signals 
from the interlock 86 are a series of pulses that are time related to the 
individual bus inputs. These pulses are simultaneously sent to a series of 
logic elements 90, one associated with each motor control circuit 88. The 
motor overcurrent decoder 26, run by the master address system 24, decodes 
the signal information and supplies a signal to the appropriate logic 
element 90. When an individual logic element 90 receives a signal from 
both the interlock 86 and decoder 26, a signal is transmitted to the motor 
control 88 to actuate the raise mode of the motor control and operate the 
motor 16. 
Due to the fact that each signal pulse transmitted to the motor control 
circuit is of a very small time duration, it is desirable that the motor 
control include a timing circuit 92. This timing circuit determines the 
length of time that the raise mode of the motor control circuit will be 
actuated upon receiving a single pulse from the logic element 90. The 
timing circuit may be adjustable up to several seconds. If, after the time 
interval, the motor control circuit continues to receive signals from the 
logic element 70, the motor will continue to operate. The motor will 
constantly raise the particular anode set until the current signal from 
the particular anode bus decreases until it reaches the set point of the 
comparator 82. 
In the event that only one motor is associated with a given cell, and all 
the anodes of the cell are raised simultaneously, the logic elements 90 
and motor overcurrent decoder 26 are not needed, and only one motor 
control circuit 88 is necessary. In such a case, the signal from interlock 
86 would pass directly to the motor control circuit 88 including a timing 
circuit 92, and function as explained above. 
In some instances it is desirable to disable the overcurrent motor raise 
circuit 80. One such instance is where, because of load changes, the load 
in the cell becomes so low that a relatively small change in current in 
one of the anode buses would result in actuation of the raise mode of the 
motor control. For this purpose, a low load current interlock circuit 94 
may be provided. This circuit 94 compares the average anode bus current 
with a set point, and if the average anode bus current is below the set 
point, an output signal 96 will be sent to the interlock 86 preventing the 
signal from the comparator 86 from being transmitted to any of the motor 
control systems 88. The set point should be adjustable, and may be a 
percentage of the rated current flow through an individual anode bus. For 
example, the set point should be adjustable between a range of 0 to 30 
percent of the total rated anode current. For example, if 137 kiloamperes 
is normally applied to each cell unit through 20 parallel buses, then each 
anode bus would carry 6.8 kiloamperes. The set point should then be 
between 0 and 2.14 kiloamperes. Normally, it is desirable to have the set 
point at 10 percent, which in the example just given would be 0.68 
kiloamperes. The set point adjustment may be accomplished by using a 
single turn potentiometer. The average anode current signal may be derived 
from the output of the averager 82. 
It is also desirable to disable the overcurrent motor raise circuit 80 when 
the cell switch is closed, i.e., when the cell is short circuited by the 
cell shorting switch. This may be accomplished in one of three ways 
depending upon whether the cell construction is such that a pair of cells 
are shorted by a single shorting switch, or as each cell has its own 
shorting switch. 
In the case where a pair of cells is shorted by a single shorting switch, 
the arrangement is usually such that the switch is physically associated 
with one cell and causes the current to pass from the anode buses of the 
preceeding cell to the anode buses leading to the next succeeding cell. 
The switch is normally associated with an even numbered cell. Thus, 
referring to FIGS. 1 and 4, with the switch associated with cell number 2, 
then the closing of the switch will complete a circuit from the anode 
buses 8 of Cell No. 1 to the cell bottom 14 of Cell No. 2, completely 
bypassing the anode buses 8 of Cell No. 2. In this instance, the low load 
current interlock system described above can be utilized for disabling 
when the cell shorting switch is closed. However, appropriate circuitry 
must be added so that sensing of low current by the buses of the cell with 
which the switch is associated will not only disable that cell's overload 
motor raise circuit 80, but will also disable the overload motor raise 
circuit of the preceeding cell by sending a signal to its interlock 86. 
In the case where each cell has its own shorting switch, it is necessary to 
provide a different arrangement for preventing operation of the motor 
control circuit 80. This is due the fact that with such an arrangement, 
the taps 18 and 20 will be located in a portion of the bus where current 
passes through the bus even though that cell is short circuited. In this 
case, a low voltage interlock circuit 98 may be provided. When the cell 
switch is closed the cell voltage is very low, equal to the bus and switch 
voltage drop and usually below 1.0 volts. To disable the overload motor 
raise circuit in such a case, a signal is obtained from the common on that 
particular cell and the common on the next cell and filtered and time 
delayed. This signal is compared to an adjustable bias which is adjusted 
to a fixed set point, and which, if the voltage signal falls below it, 
will send a signal 100 to the interlock 86 which will prevent the signal 
from the comparator 84 from being transmitted to any of the motor controls 
88. The set point adjustment may include a single turn potentiometer which 
has an adjustment range of 0 to 5 volts. 
In the case where each cell has its own shorting switch a third arrangement 
can be used for preventing operation of the motor control circuit 80. The 
cell switch position may be mechanically or electrically sensed and an 
electrical signal generated when the switch is closed and sent to the 
interlock 86 to prevent the signal from the comparator from being sent to 
any of the motor controls 88. 
A third condition under which the overload motor circuit should be disabled 
is when the anode-cathode gap becomes too large. This could result from a 
failure in the system such that the motor controls raise an anode set or 
all anodes past the point as which the signals from the comparator 84 
should cease. As an excessive anode-cathode gap results in high voltage, a 
high voltage interlock circuit 102 may be provided to disable the 
overcurrent motor raise circuit 80. In this case, a voltage signal, 
obtained as mentioned above in connection with the low voltage interlock 
circuit, is compared to an adjustable bias, which is adjusted to a fixed 
set point. If the voltage signal exceeds the set point, a signal 104 is 
sent to the interlock and prevents the signal from the comparator from 
being transmitted to any of the motor control systems 88. The set point 
adjustment may include a single turn potentiometer and be adjustable 
between 4.5 and 6.5 volts. 
The fourth condition for which it is desirable to disable the overcurrent 
motor raise circuit 80 is when no load tap changes or switching is being 
made at the rectifier which usually results in considerable disturbance on 
the cell electrical system. Such disturbances could cause an impulse 
signal which would result in the activation of the overcurrent motor 
control when not necessary. To disable the motor control circuit under 
this condition, the equipment used for rectifier switching and no-load tap 
changing may provide interlock contacts to circuit 106. The closure of any 
of the interlock contacts serves to send a signal 107 to all the 
interlocks 86 for each cell which will prevent the signal from the 
comparator 84 from being transmitted to the motor control circuits 88. 
This signal, which is transmitted to all of the cell overcurrent 
interlocks 86, is adjustable to remain on for a set period of time. This 
time adjustment may vary from about 0 to about 5 seconds. Preferably the 
time delay for the signal to turn off is 3 seconds. 
Another feature of the present invention is the determination of the 
millivolt drop across the cell bottom bus joint connection. Each anode bus 
8 is connected to the cell bottom of the preceeding cell by a suitable 
connection. This connection has its own voltage drop and requires a 
certain amount of power. As time passes, these joints become worn and 
consume a greater and greater amount of power. At some point, due to the 
high cost of power, it becomes economical to clean or replace the 
connections. The present invention makes use of the multiplexer 22 of each 
cell 2 to scan a signal which may be used to provide a readout of the 
millivolt drop across the bus joint connection. 
The bus joint signal feature is shown in FIG. 4 which shows a tap 110 
connected to the cell bottom 14 adjacent the end of the bus 8 which 
connects with the anodes of the next succeeding cell. Each bus 8 of every 
cell 2 has a tap 110 associated with it. The signals from each of the taps 
110 are filtered and connected to the multiplexer 22 with a single output 
from the multiplexer being transmitted to a differential amplifier 112 
where the signal is buffered. The output from the amplifier 112 is 
transmitted to a differential amplifier 114. The output from the amplifier 
46 associated with tap 18 is also transmitted to the differential 
amplifier 114. The differential amplifier 114 converts the signals to a 
single-ended voltage signal, reference to cell common, and proportional to 
the millivolt drop across the cell bottom bus joint. The output from the 
differential amplifier 114 is transmitted to a contact 116 of the cell 
select relay 56. The corresponding output 118 from all, or at least a 
plurality of relays 56 are transmitted to a single isolation amplifier 
120. The output 66 from the contact 64 of the cell relay 56 conveying cell 
common is also transmitted to the isolation amplifier 120. The isolation 
amplifier converts the differential signals at high common-mode voltage to 
a single-ended signal, referenced to the console common and proportional 
to the voltage drop of the bus joint connection. If sectionalizing relays 
and a plurality of isolation amplifiers 120 are used as explained above, 
the output from differential amplifier 112 associated with a given cell 
would pass through its individual cell select relay 56, the sectionalizing 
relay associated with that particular cell, and the isolation amplifier 
112 associated with that particular sectionalizing relay. 
As an alternative, for the purpose of obtaining the bus joint voltage, the 
signal from tap 18 may pass through a separate filter circuit, a separate 
channel on the multiplexer, a separate buffer amplifier, and be connected 
to differential amplifier 114. This is in place of using the signal from 
amplifier 46 as one of the inputs to the differential amplifier 114 and 
permits heavy filtering to improve the bus joint voltage readout without 
affecting the current signals. 
As explained above in connection with the voltage and current signals, each 
multiplexer scans all of the signals on each bus bar, one bus bar at a 
time, so that the various bus bar signals are fed sequentially through the 
multiplexer to the amplifier system. Thus, the signal from tap 110 on a 
given bus bar will be fed through the multiplexer at the same time the 
signal from tap 18 of that bus is being fed. Thus when cell select 76 
actuates a given relay 56, the bus joint voltage signals from each anode 
will be fed sequentially through the isolation amplifier 120 into the 
console. 
Within the console, or signal decoding system 28, the bus joint signals are 
decoded and processed by conventional electrical circuitry so that a 
readout of the bus joint voltage drop of any given anode bus can be 
displayed or transmitted to the computer if desired. The display may be 
either analog or digital as desired. 
The master address system 24 may include a multi-vibrator clock 
synchronized to the line frequency. As the line frequency is usually 60 
Hz, the clock cycle should be such that it runs a binary address signal 
generator at a rate which will address all the inputs of the multiplexer 
and then repeat at the rate of 60 per second. Other scan rates may be used 
such as 30, 120, or 360 repeats per minute. An address checking circuit 
may also be included for testing for failure in the address. Upon 
detection of address failure, an indicator light and/or an external 
audible alarm may be actuated. The master address system also drives the 
signal decoding system 28 and the motor overcurrent decoder 26. 
Referring to FIG. 2, the cell and anode select may be operated by a 
keyboard or pushbuttons on the console during manual operation or by the 
computer 124 for automatic operation. Switches 126 are provided to change 
from one mode of operation to the other. Once a particular cell and anode 
are selected, the appropriate cell select relay is actuated and the signal 
decoder 28 operates the sample and hold circuits to select the proper 
readout signals for display. 
The multiplexer, which transmits the signals from the various anode buses 
of a given cell in sequence, permits all the bus current signals and all 
the anode set voltage signals to be displayed as separate traces on an 
oscilloscope. In addition, the bus current signal trace can be displayed 
on the oscilloscope along with a trace representing the average current. 
Also, an electrical circuit can be utilized to calculate the anode set 
coefficient and average cell coefficient and display the signal on the 
oscilloscope. The anode set voltage signals for a given cell may also be 
displayed as a signal trace on the oscilloscope along with a trace of cell 
voltage. The average bus current, cell voltage, and cell coefficient 
traces may contain an anode bus select (marker) pulse to indicate on the 
oscilloscope which bus is being monitored by the digital meter when 
operating in the manual mode. 
The computer, receiving signals through the computer interface 30 can be 
used to provide for automatic adjustment of the anode-cathode gap. One 
such method is the use of the voltage coefficient to obtain the optimum 
spacing. The voltage coefficient Vc is calculated according to the 
formula: 
EQU Vc = (V-D)/(KA/M.sup.2) 
where V is the measured voltage for the electrolytic unit such as the anode 
set; D is the decomposition voltage for the electrolysis being conducted 
and KA/M.sup.2 is the current density in kiloamperes per square meter of 
cathode surface below the anode set. In the electrolysis of sodium 
chloride in a mercury cell for producing chlorine the value for V is about 
3.1. Thus, utilizing the voltage and current signals obtained from the 
multiplexer, the computer can calculate the voltage coefficient for any 
given anode set. The use of the computer for anode adjustment is described 
more fully in U.S. Pat. Nos. 3,873,430 and 3,900,373 which are 
incorporated herein by reference in their entirety. 
When the system is on manual control by switches 126, often a given cell 
and anode bus is selected, the manual raise and lower switches 128 may be 
used by an operator to actuate the appropriate motor control circuit. 
By virtue of the above described arrangement, a relatively simple system is 
utilized for obtaining voltage and current signals from each of the anode 
buses of a given cell. These signals may be used to operate an overcurrent 
motor raise circuit, may be visually displayed on an oscilloscope or 
analog, or digital meter and provide signals for a computer to 
automatically adjust the anode-cathode spacing.