Abstrict In this inductive flow meter a pair of electrodes are disposed
on opposite sides of a tube through which flows the fluid being
measured. A magnetic field is generated across the tube, the strength
of which increases and decreases linearly with time. The voltage
developed across the electrodes is scanned by first and second sample
and hold circuits operative respectively near the beginning and
near the end of each increasing or each decreasing branch of the
magnetic field cycle. A difference circuit obtains the mean value
of the difference between the sampled voltages; this value very
accurately indicates the flow rate, since the described circuitry
compensates for various system error voltages.
Claims BACKGROUND OF THE INVENTION
1. Related Application
The present application is a continuation-in-part of the (Dec.
10 1976) inventors' copending application, Ser. No. 749331 now
abandoned. 2. Field of the Invention
The invention relates to an inductive flow meter comprising at
least one solenoid generating an alternating magnetic field in the
flow region and a pair of electrodes connected with an evaluation
circuit.
3. Description of the Prior Art
Such flow meters operate under the Faraday induction law, according
to which an electrical voltage is generated in response to movement
of the charge carriers in a magnetic field. In practice, a solenoid
provides a magnetic field which extends across a tube through which
the liquid to be metered flows. Two electrodes are provided at diametrically
opposite sides of the tube; the voltage to be evaluated is taken
from these electrodes. This voltage U.sub.net is proportional to
the magnetic flux density B, the flow rate v and the pipe diameter
d, and this can be expressed by the following formula:
It is taken for granted that the vectors of the voltage U.sub.net,
of the flux density B and of the flow rate v are perpendicular relative
to each other.
The practical implementation of such flow measurements is quite
difficult in spite of the simple physical principle because the
measuring voltage U.sub.net is distorted by a series of interference
voltages. If the solenoid is operated with alternating current,
the following types of interference voltages occur:
1. Inductive interference voltages which are caused by the inductive
coupling of the magnetic coils with the electrode circuit.
2. Capacitive interference currents by the capacitances between
the electrodes and the magnet system. These interferences are dictated,
inter alia, by the conductivity between the electrodes (e.g., conductivity
of the liquid, degree of contamination).
3. Polarization voltages caused by different potentials formed
when the liquid contacts the tube walls or the electrodes.
4. Vagrant parasitic currents generated e.g., by the induction
in the liquid. These parasitic currents, as a rule, have the frequency
of the mains supply concerned of 50 Hz or 162/3 Hz.
These voltages are superimposed on the active voltage U.sub.net,
so that to obtain high measurement accuracy of the wanted signal
it is absolutely imperative to eliminate the different interference
voltages.
In addition to the conventional flow meters operated at 50 Hz,
there is another type of prior art inductive flow meter which uses
a solenoid that generates a d.c. field timed (i.e., connected and
disconnected periodically) at a low frequency. The time slope of
the magnetic current approximately corresponds to a rectangular
function. Each switch operation is accompanied by transient effects
which must end before the measurement can be made. The measurement
of the active voltage is, therefore, made on a short-time basis
by a difference evaluation of the momentary values which are determined
during the switch-on and switch-off times of the magnetic field.
The interference d.c. voltage, which may be a thousand times the
active voltage, is separated from the supply voltage so that the
difference of two voltage levels of nearly equal height is formed.
It is difficult to obtain an accurate evaluation with this approach.
The use of a timed d.c. field involves the advantage that the inductive
and capacitive interference voltages are zero. When using a sine
alternating field, it is very difficult to separate the inductive
and capacitive interference voltages from the wanted signal. On
the other hand, it has the advantage that by using an a.c. field,
the polarization voltages are eliminated.
It is an object of the present invention to provide an inductive
flow meter of the type described, which uses a very simple evaluation
circuit wherein the wanted signal can be separated from all interference
voltages so as to obtain relatively great accuracy. At the same
time, d.c. voltage drifts caused by the requisite amplifiers or
by the polarization voltages can be compensated for.
SUMMARY OF THE INVENTION
To achieve this objective, the present invention utilizes a specific
evaluation circuit and a specific curve shape (e.g., triangular)
and frequency of the magnetic current.
In the evaluation circuit, a first voltage sample and hold circuit
is so controlled that in each period of the electrode voltage a
value in the vicinity of the upper peak value is sampled and held
in storage. A second voltage sample and hold circuit is so controlled
that in each period of the electrode voltage a value in the vicinity
of the lower peak value is sampled and held in storage. The output
signals of both the voltage scanning memories are supplied to difference
amplifier the output of which is connected to a measuring instrument
via an average value former.
Due to the fact that the peak values in both the half periods are
retained, stored and compared with each other, the exact value for
the active voltage is obtained at the measuring instrument. The
voltage at the output of the difference amplifier is constant as
long as there is no drift. In case of a drift, a rectangular voltage
does exist at the output of the difference amplifier and its arithmetical
mean value is the real net voltage. Therefore, it is easily possible
to determine the active voltage from the output voltage of the difference
amplifier.
It is a prerequisite for the elimination of the inductive and capacitive
interference voltages and of the vagrant parasitic currents that
the solenoid be operated at a sufficiently low frequency. This should
be lower than the frequencies of the industrial alternating voltages
which are 50 Hz or 162/3 Hz. Preferably, the frequency of magnetization
should be at about 1 Hz. The magnetization is carried out by excitation
of the solenoid with a constantly rising and constantly falling
current, so that there is no need to wait for transient effects
to terminate before taking the measurements.
To determine the sample points during the magnetization period,
the inventive circuitry advantageously includes a multi-stage frequency
divider, connected with the a.c. supply, together with a waveshape
generator connected to the solenoid via an amplifier circuit having
a high-impedance output, the outputs of individual stages of the
frequency divider being combined in two AND-gates each of which
controls one of the voltage scanning memories.
Thus, both synchronization of the sample points as well as excitation
of the waveshape of the magnetic field are realized by dividing
the power frequency. Since it is necessary that the magnetic field
have a very low frequency, and since the power frequency (at 50
Hz) has a half wave duration of 10 msec, the power frequency offers
a time increment sufficiently fine for establishing the scanning
rate. The selection of scanning increments may be performed by a
suitable combination of the outputs of the frequency divider, using
logical switches responsive to specific frequency divider output
signal combinations.
Preferably, the electrodes are connected via d.c. blocking capacitors
to respective terminals of a common input amplifier. The blocking
capacitors prevent d.c. voltages from reaching the evaluation circuit.
Higher frequency interference voltages are blocked by a low-pass
filter interconnected between the input amplifier and the voltage
scanning memory, the limit frequency of this filter being below
16 Hz.
An extremely advantageous magnetic field curve is obtained by connecting
the solenoid to a source of current that rises and falls linearly
with time. Such a triangular current flow causes the inductive interference
voltages and the capacitive interference currents always to be of
rectangular waveform. The amplitudes of the interference factors
are constant. By controlling the voltage scanning memories in such
a way that the two sample points always lie on the same rising or
falling branch of the ramp voltage, the constant interferencesum
voltage is eliminated by obtaining the difference between the values
at the two scanning points.
Description What is claimed is:
1. An inductive flow meter comprising a current source and at least
one solenoid generating in the flow region an alternating magnetic
field and a pair of electrodes connected to an evaluation circuit,
characterized in that the current source provides to the solenoid
a current linearly rising and falling in time, so that the generated
magnetic field and the resultant electrode voltage both alternately
increase and decrease linearly in time, that the evaluation circuit
has a first voltage sample and hold circuit controlled to sample
and store, during each period of the electrode voltage, a value
in the vicinity of the upper peak value and a second voltage sample
and hold circuit to sample and store during each period of the electrode
voltage, a value in the vicinity of the lower peak value, and that
the evaluation circuit has a difference amplifier and a mean value
former, the output signals of both voltage sample and hold circuits
being supplied to said difference amplifier the output of which
is connected to said mean value former, the output of said mean
value former being connectable to a measuring instrument.
2. An inductive flow meter according to claim 1 characterized
in that said current source comprises a frequency divider, connectable
to an a.c. power source, a waveshape generator to which said frequency
divider is connected, an amplifier circuit having a high-impedance
output connected between said waveshape generator and said solenoid,
two AND-gates, the outputs of the frequency divider being combined
in said two AND-gates each of which controls one of said voltage
sample and hold circuits.
3. An inductive flow meter according to claim 1 and having a common
input amplifier, characterized in that the electrodes are connected
via one blocking capacitor each with a respective one terminal of
said common input amplifier, the output of said common input amplifier
being supplied to said first and second voltage sample and hold
circuits.
4. An inductive flow meter according to claim 3 and having a low
pass filter, characterized in that said low pass filter is connected
between the output of the input amplifier and the sample and hold
circuits, said low-pass filter having a cut-off frequency which
is below 16 Hz.
5. An inductive flow meter according to claim 1 characterized in
that said current source is an integrator that converts a rectangular
voltage into a ramp voltage having alternately positive and negative
gradients.
6. An inductive flow meter according to claim 1 characterized in
that the voltage sample and hold circuits are controlled so that
the two sample points always lie on the same rising or falling branch
of the solenoid current.
7. An inductive flow meter according to claim 1 wherein the electrode
voltage is fed to an interference suppression circuit comprising:
first and second differentiator circuits connected in series, said
second differentiator circuit producing an output if the second
derivative of the electrode voltage exceeds a certain level, and
circuit means, responsive to the output of said second differentiator
circuit, for disconnecting the output of said difference amplifier
from said mean value former when said output of said second differentiator
circuit exceeds said certain level.
8. An inductive flow meter according to claim 1 wherein the output
of said difference amplifier is fed to an interference suppression
circuit comprising:
an a.c. coupled amplifier receiving said output from said difference
amplifier,
a rectifier for rectifying the output of said a.c. coupled amplifier
so as to provide an inhibit signal if the alternating portion of
said difference amplifier output exceeds a certain value, and
switch means for disconnecting the output of said difference amplifier
from said mean value former in response to occurrence of said inhibit
signal.
9. An inductive flow meter of the type wherein a magnetic field
is generated across a non-magnetic tube in which there is flowing
the fluid to be measured, and having a pair of diametrically opposed
electrodes situated on said tube along a line that crosses said
magnetic field, the improvement comprising:
solenoid drive means for producing a magnetic field across said
tube that alternately increases and decreases linearly with time,
first means for sampling and holding the voltage sensed across
said electrodes near the start of each increasing or each decreasing
time portion of said magnetic field,
second means for sampling and holding the voltage sensed across
said electrodes near the end of each same increasing or decreasing
time portion of said magnetic field, and
voltage difference means for obtaining the mean value of the difference
between the voltages sampled by said first and second sampling and
holding means.
10. An inductive flow meter according to claim 9 wherein said solenoid
drive means includes a solenoid situated to produce a magnetic field
across said tube perpendicular to the line through said opposed
electrodes,
triangular wave circuitry for generating a triangular wave current
at a frequency substantially below that of the a.c. power supply
mains, and connected to supply said triangular wave current to said
solenoid, and
gate circuitry, connected to said triangular wave circuitry, for
producing first and second control signals respectively near the
beginning and near the end of the same alternate half cycles of
said triangular wave current, said first and second control signals
being connected respectively to enable said first and second sampling
and holding means.
11. An inductive flow meter according to claim 10 further comprising:
an amplifier, said electrodes being connected via d.c. blocking
capacitors to an input of said amplifier, and
a low-pass filter connecting the output of said amplifier to the
inputs of both said first and second sampling and holding means.
12. An inductive flow meter according to claim 9 wherein the voltage
sensed across said electrodes is fed to an interference suppression
circuit comprising:
first and second differentiator circuits connected in series, said
second differentiator circuit producing an output if the second
derivative of the sensed voltage exceeds a threshold level, and
circuit means, responsive to the output of said second differentiator
circuit, for disconnecting said sampled voltages from said means
for obtaining the mean value when said output exceeds said threshold
level.
13. An inductive flow meter according to claim 9 wherein said voltage
difference means comprises a difference amplifier receiving said
sampled voltages, and a mean value former receiving the output of
said difference amplifier, and wherein the output of said difference
amplifier is fed to an interference suppression circuit comprising;
an a.c. coupled amplifier receiving said output from said difference
amplifier,
a rectifier for rectifying the output of said a.c. coupled amplifier
so as to provide an inhibit signal if the alternating portion of
said difference amplifier output exceeds a certain value, and
switch means for disconnecting the output of said difference amplifier
from said mean value former in response to occurrence of said inhibit
signal.
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Description
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BRIEF DESCRIPTION OF THE DRAWINGS
A preferred working example of the invention is explained more
fully hereinafter with reference to the drawings wherein:
FIG. 1 shows the schematic design of an inductive flow meter which
may be operated in accordance with the present invention.
FIG. 2 shows an electrical block diagram of an evaluation circuit
according to the invention.
FIG. 3 shows different voltage curves illustrating the elimination
of the voltage drift.
FIG. 4 shows different voltage diagrams to demonstrate the elimination
of inductive and capacitive interference voltages.
FIGS. 5 and 6 show electrical block diagrams of alternative, optional
circuitry for interference suppression.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in FIG. 1 a measuring tube 10 of non-magnetic material
is provided with a magnetic coil or solenoid 11 which generates
a magnetic field passing through tube 10. Inside the tube there
are provided two electrodes 12 13 which are diametrically opposed.
The imaginary line connecting the electrodes 12 13 cuts perpendicularly
across the magnetic field. Electrodes 12 13 are connected to the
input of an evaluation circuit 14. The measured voltage is read
out at a meter or like instrument 15 connected to the evaluation
circuit 14. To generate the magnetic field, the magnetic coil 11
is connected to the control circuit 46 which impresses on the magnet
winding a current having a specific waveshape. In addition, the
control circuit 46 establishes the sample points used by the evaluation
circuit 14.
FIG. 2 shows a block diagram of the evaluation circuit 14 and of
the control circuit 46. The input lines of the evaluation circuit
14 coming from the electrodes 12 13 are connected via blocking
capacitors 16 17 with an input terminal of an amplifier 18. The
output of the amplifier 18 is connected, via a low-pass filter 19
having a cut-off frequency below 16Hz, to two voltage scanning memories
each including an amplifier 20 21. At the input to each such amplifier,
there is provided a holding capacitor 22 23. Control of the sample
points is performed by one or the other of a pair of switches 24
25 operated by the control circuit 46. Switches 24 25 are indicated
as mechanical switches to simplify the illustration. In a practical
implementation of the circuitry, electronic switches would be used.
The output lines of the voltage sample and hold circuit amlifiers
20 21 are connected to the inputs of a difference amplifier 26
the output of which feeds a mean value former 27. The output of
the mean value former 27 is the output of the evaluation circuit
14 and is supplied to the indicator 15.
The control device 46 is operated in synchronism with the 50 Hz
power frequency. To this effect, the input of a multistage frequency
divider 28 is connected to receive a clock signal at the power frequency.
A rectangular voltage having a frequency of 50/64 Hz=0.78 Hz is
obtained at one output of the binary frequency divider 28. This
rectangular voltage is supplied to an integrator 30 consisting of
a capacitive-feedback-coupled amplifier. A triangular voltage is
generated at the output of the amplifier 30 which is supplied to
a driver amplifier 31 having a high output resistance and being
capable of impressing on the coil or solenoid 11 a triangular current
with a frequency of 0.78 Hz.
The control of switches 24 and 25 is performed via a pair of AND-gates
32 and 33. In FIG. 2 each of the inverting inputs of the gates
32 and 33 is indicated by a dot; the other inputs are non-inverting.
At the inputs of the gates 32 and 33 the outputs of the frequency
divider 28 are so combined that the output signal of each gate 32
33 exists only in a short time interval within each period of the
triangular magnetization current.
The frequency divider 28 could be also considered a counter, and
the outputs associated with different counts can be supplied to
a digital-to-analog converter which would take the place of the
integrator 30.
For a better understanding and with reference to FIG. 3 it is
first supposed that switch 24 is always closed on a short-time basis
by the gate 32 at the upper peak value of the triangle current,
and that the switch 25 is closed by the gate 33 at the lower peak
value of the triangular voltage. It must be taken into consideration
that the temporal development of the active voltage U.sub.net generated
at the electrodes 12 and 13 is equal to the time variation of the
triangular current generated at the output of the amplifier 31.
To explain how the interference influences are eliminated due to
slow changes of the direct voltage potential, suppose for the moment
that inductive and capacitive couplings do not occur or are negligible,
and that stray parasitic currents from external sources are eliminated
by the low-pass filter 19. The remaining interference influences
are polarization voltages which change very slowly in time, and
the drift of the input amplifier 18.
In FIG. 3 curve (a) shows the development of the active voltage
signal U.sub.net. This active voltage signal is superimposed on
the interference voltage U.sub.drift shown in (b), composed of the
polarization voltages and the amplifier drift, and changing very
slowly in time. The combined signal, including both the interference
voltage and the active voltage, which occurs at the output of the
amplifier 18 is illustrated by curve (c) of FIG. 3. The sample points
are shown in (d). These are the times at which the indicated switches
24 and 25 are closed briefly to feed into the voltage sample and
hold circuits 20 or 21 the combined signal amplitude values. These
values are retained until the next closure of the switch concerned.
In FIG. 3 (e) the resultant voltage levels are shown for the two
sample and hold circuits 20 and 21 if the input signal develops
as shown in (c). It is apparent that stepped curves are formed at
the outputs of the two voltage sample and hold circuits 20 and 21
which are offset with respect to each other as to amplitude and
time.
In FIG. 3 (f) the temporal development of the output signal of
the mean value former 27 is shown. The difference between the output
voltages of the amplifier 20 and 21 varies about the mean value
39 in the form of a rectangular curve. This mean value is obtained
in the arithmetic means value former 27 which is, for instance,
an RC-element. The mean value 39 is proportional to the active voltage
U.sub.net. It is readily evident that the rectangular curve in FIG.
3 (f) disappears and becomes the constant voltage 39 if there is
no time variable interference voltage such as shown in (b). In such
a case, the difference between the upper and the lower peak value
of the signal to be evaluated always is constant.
Due to the temporal change of the interference direct voltage,
the frequency of the magnet current cannot be substantially below
about 1 Hz. Thus, it is still possible that inductive and capacitive
interference voltages exist which will alter the measured result.
In FIG. 4 (a), the temporal development of the magnetization current
I is indicated as corresponding to the time variation of the flux
density B and of the wanted signal U.sub.net. The inductive interference
voltage, as known, is:
and, accordingly, it is proportional to the time derivative of
the magnetization current. The magnetization current being a triangular
function, the inductive interference voltage becomes a rectangular
function. The same applies to the capacitive interference current
I.sub.C, fo which the formula
is applicable. The two interference factors, which are of equal
phase and rectangular, can be combined into the interference voltage
U.sub.noise shown in FIG. 4 (b).
The combined active-interference signal is shown in FIG. 4 (c).
This results from superimposing the triangular active voltage U.sub.net
on the rectangular interference voltage U.sub.noise. To extract
only the active portion from the combination signal shown in FIG.
4 (c), sampling points 42 43 shown in FIG. 4 (d) are situated so
that the evaluation of the combined signal always occurs only on
the rising branch 40 or only on the descending branch 41. In the
instant case, the rising branch 40 has been chosen. The inputs of
gates 32 and 33 (FIG. 2) are switched so that the output signals
42 of the gate 32 always occur 50 msec after the lower reversal
point of the triangle voltage, and so that the output signals 43
of the gate 33 always occur 50 msec before the upper reversal point
of the triangle voltage. This ensures that the scanning is performed
only on the rising branch 40 of the total signal. It is evident
that subsequently, when the difference between voltages 44 and 45
is obtained, the interference voltage drops out because at both
sample times it is always of equal amplitude.
The use of a triangular magnetization current is advantageous because
the inductive and capacitive interference influences are rectangular
and can be eliminated by a suitable selection of the scanning sample
points. Moreover, the triangle voltage ensures that transient effects
are avoided during the remagnetization. The triangle shape of the
magnetization current can be implemented easily, and it can be derived
from the power frequency, it being possible to perform the time
control of switches 24 and 25 with the same control circuit 46.
FIG. 5 shows a first embodiment 50 of an interference suppression
circuit that compensates for interference signals which do not linearly
increase or decrease in time. The measurement signal from the electrodes
12 13 is supplied via the low-pass filter 19 to a first differentiator
circuit 51 and thereafter to a second differentiator circuit 52.
The differentiators 51 52 each consist, in a manner known per se,
of an operational amplifier 51a, 52a having a feedback resistor
51b, 52b and an input capacitor 51c, 52c.
The output of each differentiator circuit 51 52 is the time derivative
of the input signal to that circuit. Thus, the output of the second
differentiator circuit 52 corresponds to the second derivative of
the electrode measurement signal supplied from the filter 19. If
no interference voltages exist, so that the measurement signal increases
or decreases linearly, the first derivative signal at the output
of the circuit 51 will be constant, and the second derivative signal
at the output of the circuit 52 will be zero. (Brief transients
will occur at the output of the circuit 52 in correspondence with
the knees or turning points of the triangular or trapezoidal measurement
signal.)
If a non-linear interference voltage is superimposed on the measurement
voltage, the second derivative of the combined signal will not be
zero. In such case, the circuit 52 will produce an output signal
indicative of the presence of such a non-linear interference voltage.
This signal is supplied to one input of a differential amplifier
53 which receives at its other input a reference voltage set by
a potentiometer 53a. This reference voltage establishes a threshold
value which must be exceeded by the second derivative signal from
the circuit 52 before the amplifier 53 will produce an output. The
output of the differential amplifier 53 is supplied via a diode
54 having an output filter capacitor 55 to the control element of
a switch 56 that is connected in the output line from the difference
amplifier 26. For simplicity of illustration, the switch 56 is shown
as a mechanical switch. In practice, an electronic switch such as
a field effect transistor would be used. Connected between the switch
56 and the mean value former 27 is a voltage sample and hold memory
57 consisting of a holding capacitor 57a and an amplifier 57b.
If the output of second differentiator circuit 52 surpasses the
set threshold value, the switch 56 will be opened so as to interrupt
signal flow from the amplifier 26 to the mean value former 27. The
last signal amplitude present just before opening of the switch
56 will be stored in the voltage sample and hold memory 57 so that
the indicating instrument 15 connected to the mean value former
27 will not change its indication during the time of interruption
of the switch 56. When the amplitude of the interference signal
decreases below a certain value, the differential amplifier 53 will
no longer produce an output signal. The switch 56 then will close,
after a slight delay for discharge of the capacitor 55. The difference
amplifier 26 then will again be connected to provide an input to
the mean value former 27.
A light-emitting diode 59 provides a visual indication that an
interference voltage is present. To this end, the output from the
differential amplifier 53 and diode 54 also is supplied to a transistor
58 which then turns on current to the diode 59.
The transients which occur at the output of the second differentiator
circuit 52 in correspondence with the knees of the triangular or
trapezoidal measurement signal may exceed the threshold established
at the differential amplifier 53. These transients will be filtered
by the capacitor 55. However, even if they should cause opening
of the switch 56 it will be for only a short time duration, and
this will not effect the measurement indicated by the indicating
instrument 15 since the last prior measurement signal will still
be provided to the mean value former 27 by the sample and hold memory
57.
An alternative embodiment of an interference suppression circuit
is shown in FIG. 6. This circuit 60 makes use of the fact that the
output of the difference amplifier 26 (shown in FIG. 3 (f)) contains
an alternating voltage component only if an interference signal
is present. The amplifier 26 output is a constant or direct voltage
if no interference signal is present. In the case of a slowly varying
interference signal (drift), the output of the difference amplifier
26 contains a rectangular voltage superimposed on the effective
dc signal. If interference voltages of higher frequency are present,
they will cause the superposition of a corresponding higher frequency
component on the output of the difference amplifier 26.
In the embodiment of FIG. 6 the alternating voltage component,
present at the output of the difference amplifier 26 and indicative
of an interference signal, is a.c. coupled via a capacitor 61 to
an amplifier 62. The output of the amplifier 62 is rectified by
a rectifier 63 the output of which is fed to one input of a difference
amplifier 64. A reference voltage is supplied via a potentiometer
64a to the other input of the amplifier 64 so that the potentiometer
64a functions to establish a threshold value.
Similar to the example of FIG. 5 a diode 65 and a filter capacitor
66 are connected between the output of the difference amplifier
64 and the control element of the switch 56. Occurrence of an interference
signal of sufficient magnitude to overcome the threshold set by
the potentiometer 64a likewise causes the switch 56 to open. The
last prior signal level from the difference amplifier 26 continues
to be supplied to the mean value former 27 via the sample and hold
memory 57 so that the indicated signal level is unchanged. Again,
a light-emitting diode 59 gives a visual indication of the presence
of the interference voltage.
The interference suppression circuit of FIG. 6 has the advantage
that both slow drifts as well as high frequency interferences may
be recognized and eliminated. The circuit 60 will handle voltages
up to the threshold level set for the amplifier 64. The circuits
50 and 60 both have the benefit that the evaluation system will
not supply incorrect values in the presence of interference.
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