Magnetic flowmeter with empty tube detection

A magnetic flowmeter has a tube for supporting fluid flow and a circuit for indicating when the tube is empty. The circuit includes an amplifier connected to two electrodes in the tube and a variable resistor. The amplifier produces an oscillating signal when the tube is empty. The circuit is calibrated for a particular fluid by finding a range of resistances for the variable resistor at which the circuit provides an accurate indication that the tube is empty, and storing information in the processor indicating a resistance in the range for the fluid.

BACKGROUND OF THE INVENTION 
This invention relates to magnetic flowmeters with empty tube detection 
circuitry. 
Magnetic flowmeters measure the rate of flow of a process fluid through a 
tube. Magnetic coils mounted on opposite sides of the tube produce a 
magnetic field perpendicular to the direction of fluid flow in the tube. 
Electrodes placed in the tube measure a resulting current in the fluid 
that is perpendicular to both the direction of fluid flow and the magnetic 
field. A processor converts the output of the electrodes to a measure of 
the rate of fluid flow. 
Some magnetic flowmeters are equipped with a circuit that detects the 
presence of fluid in the tube and prevents the flowmeter from measuring a 
non-zero flow rate when the tube is empty. One such circuit, manufactured 
by Fischer and Porter, produces an oscillating signal to indicate that the 
tube is full. The oscillating signal disappears when the tube is empty. 
SUMMARY OF THE INVENTION 
In general, the invention features a magnetic flowmeter with a circuit that 
produces an oscillating signal when the flowmeter tube is empty. The 
circuit receives the output of two electrodes mounted in the tube. 
Preferred embodiments of the invention include the following features. 
The circuit comprises an amplifier with a non-inverting input that receives 
an output of each electrode and an output of a variable resistor. The 
variable resistor and other resistors may be arranged to provide a fixed 
negative feedback path and a variable positive feedback path to the 
amplifier. 
The variable resistor is set to a resistance causing the output of the 
amplifier to oscillate when the tube is empty and causing the output of 
the amplifier to not oscillate when an electrode is at least partially 
immersed in fluid. The calibration may be based on the conductivity of the 
fluid. 
The circuit may also include a rectifier connected to the output of the 
circuit and a processor providing an indication that the tube is empty 
based on the output of the circuit. The indication may include a digital 
signal. 
The circuit is not limited for use with magnetic flowmeters, but may be 
employed with any tube that carries fluid. 
In general, in another aspect, the invention features a method for 
calibrating an empty tube detection circuit for a flowmeter, of the type 
described above. The method includes finding a range of resistances of the 
variable resistor at which the circuit provides an accurate indication 
that the tube is empty, and storing information in the processor 
specifying a resistance in the range for a particular fluid. 
Preferred embodiments of this aspect of the invention include the following 
features. 
An identifier for the fluid and a resistance in the range are stored in a 
lookup table. A user sets up the circuit for a particular fluid by 
specifying the fluid to the processor. The processor sets the variable 
resistor to the stored resistance for the fluid. 
Alternatively, a characteristic of the particular fluid is measured, and 
the characteristic and a resistance in the range are stored. The 
characteristic includes a resistance of the variable resistor below which 
the circuit erroneously indicates an empty tube condition. A user sets up 
the circuit by filling the tube with fluid and causing the processor to 
measure the characteristic of the fluid. The processor then sets the 
variable resistor to the stored resistance for the characteristic. 
In some embodiments, characteristics for several different fluids are 
measured, and a relation between the characteristics and a resistance in 
the range is stored. 
Advantages of the invention include the following features. 
The circuit accurately indicates that the tube is empty over a large range 
of process fluid conductivities. An empty tube signal from the circuit 
causes the processor to zero the flow rate, preventing any erroneous flow 
measurements from being displayed to the user. 
This is accomplished without providing an alarm indicating to the user that 
the tube is empty and that any displayed flow measurements may be 
erroneous. The flowmeter thus does not require a system for disabling the 
alarm when the user already knows that the tube is empty, or simply does 
not need to be alerted to the tube's empty condition. 
Because both electrodes must be uncovered before an empty tube is 
indicated, the presence of an air bubble at one electrode does not produce 
a false empty tube signal. 
Rectifying the output of the circuit requires the processor only to 
distinguish a low signal from a high signal in determining whether the 
tube is empty. This takes less processor time than identifying a 
particular frequency output from the amplifier, and prevents errors when 
this frequency is low. The capacitors in the rectifier also absorb some 
noise from the system and decrease the likelihood of errors. 
The flowmeter circuitry is easily calibrated at the factory for a range of 
fluids with differing conductivities. As a result, an end user need only 
select a flowmeter appropriate for his particular process fluid, without 
specifying the conductivity of the fluid. The user prepares the flowmeter 
for use by simply activating a calibration feature on the flowmeter when 
the tube is full, e.g., by pushing a button on the processor. This 
procedure does not require the user to empty the tube after it has been 
filled with fluid--a procedure that is not usually performed during flow 
measurement. This is in marked contrast to existing schemes that require 
the user to perform complex calibration procedures, e.g., by filling the 
tube, adjusting certain readings, then emptying the tube and re-adjusting 
the readings. 
In addition, the user can easily reactivate the calibration whenever the 
conductivity of the process fluid changes, or the electrodes become 
fouled. Alternatively, the circuit may re-calibrate itself at periodic 
intervals to monitor changes in the fluid and tube characteristics. This 
is possible because the empty tube measurement is accurately performed 
independently of whether there is fluid flow in the tube, or whether a 
flow measurement is being made. In addition, because the amplifier 
produces an oscillating output only when the tube is empty, the signal in 
no way disturbs the output of the electrodes when the tube is full. This 
prevents the circuit from introducing errors into the processor's flow 
measurements. The circuit is thus versatile and accurate under varying 
conditions. 
The calibration performed at the factory is a simple procedure that 
requires finding a threshold variable resistance at which the circuit 
oscillates when the tube is full, and storing either a formula or a 
look-up table in a processor that indicates an appropriate variable 
resistance setting at each threshold. In addition, this calibration 
technique is not specific to the circuit described above, but may be 
performed with any circuit that provides an empty tube signal. 
Other features and advantages of the invention will become apparent from 
the following description, and from the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1 and 2, a magnetic flowmeter 10 has a tube 12 for 
supporting fluid flow along its longitudinal axis 14. A pair of magnetic 
coils 16, 18 mounted on opposite-sides of the tube create a magnetic field 
perpendicular to the axis of the tube. Electrodes 20, 22 are disposed on 
opposite sides of the interior of the tube, along a line perpendicular to 
axis 14 and the magnetic field. A surface of each electrode is in contact 
with the process fluid when the tube is full. 
Magnetic coils 16, 18 and electrodes 20, 22 are each connected to a 
processor 24 that controls the current through the coils and converts the 
output of the electrodes to a flow rate measurement, as well as performing 
other functions. 
Referring to FIG. 3, processor 24 includes an empty tube detection circuit 
50 connected to each electrode 20, 22. The resistance (2R) between each 
electrode and the tube (which is grounded) depends on the conductivity of 
the process fluid, the size and placement of the electrodes, and the level 
of impurities on the inside of the tube. The preferred embodiment of 
circuit 50 described below is designed for process fluid conductivities 
ranging from at least 1.5 to 325 .mu./cm. 
Each electrode is connected to a non-inverting input 52 of an operational 
amplifier 53 via a variable resistance positive feedback path 54. The 
amplifier also has a fixed resistance negative feedback path 55. When the 
tube is empty (i.e., both electrodes are completely uncovered), the 
resistance (2R) between the electrodes and the tube is high, and the 
positive feedback to the amplifier exceeds the negative feedback and the 
output of the amplifier oscillates. When the tube is full (i.e., one or 
both electrodes are at least partially immersed in fluid), and the 
resistance (2R) between the electrodes and the tube is relatively low, the 
negative feedback exceeds the positive feedback to the amplifier and the 
output no longer oscillates. The presence of an oscillating output thus 
signals to the processor that the tube is empty. 
The negative feedback path includes a 10 kOhm resistor 66 connecting an 
inverting input 64 of the amplifier to ground. The output of the amplifier 
is fed back to the inverting input of the amplifier via a 191 kOhm 
resistor 68. The amplifier is biased by a supply voltage and capacitor 62 
in a conventional manner. 
The positive feedback path includes 2200 pF capacitors 56, 58 connecting 
each electrode to the non-inverting input of the amplifier. The capacitors 
prevent DC signals from coupling to the electrodes, for example, the flow 
measurement signal from the electrode. As a result, the electrodes act 
essentially as resistors, and the magnitude of the coil current and the 
presence or absence of fluid flow in the tube do not disturb the empty 
tube measurement. The outputs of the capacitors are additionally connected 
to a 1 MOhm resistor 60. 
The output of the amplifier is fed back to the non-inverting input 52 of 
the amplifier via subcircuit 70. Subcircuit 70 couples the output of the 
amplifier to ground via a 9.09 kOhm resistor 72 connected in series to a 1 
kOhm resistor 74. These resistors attenuate the output of the amplifier to 
prevent a large signal from being coupled back to the electrodes. 
The output of resistor 72 is coupled to the non-inverting input of the 
amplifier by a variable resistor 76 connected in series with a 4700 pF 
capacitor 78. Capacitor 78 together with resistor 60 prevent a DC output 
of the amplifier from coupling back to the non-inverting input. The 
amplifier is thus stable at DC. 
Variable resistor 76 includes eight resistors 80.sub.1, . . . ,80.sub.8 
connected in series and having varying resistances. For example, in the 
embodiment shown, resistors 80.sub.1 to 80.sub.8 have values of 0.590, 
1.40, 3.32, 7.87, 18.7, 44,2, 105 and 249 kOhm, respectively. 
Each resistor 80.sub.1, . . . ,80.sub.8 is connected in parallel to a 
switch 82.sub.1, . . . ,82.sub.8 coupled to a multiplexer 84. Multiplexer 
84 receives digital signals from the processor, and varies the resistance 
of resistor 76 by selectively opening or closing the switches. The value 
of the resistance is 250 Ohms when all the switches are on, and 431 kOhms 
when all the switches are off. As described in more detail below, the 
minimum resistance of the variable resistor is lower than the resistance 
(R.sub.min) at which the circuit will oscillate when the tube is full. In 
addition, the maximum resistance of the variable resistor exceeds the 
highest resistance (R.sub.max) at which the circuit will not oscillate 
when empty. 
The output of amplifier 53 is also connected to a rectifier 86 which 
provides a DC signal 88 to a CMOS inverter. The inverter provides a 
digital signal to the processor indicating whether the tube is full or 
empty. The rectifier includes a diode 90 connected in series to a 10 .mu.F 
capacitor 92, 10 nF capacitor 94 and a 1 MOhm resistor 96, all connected 
in parallel. 
In operation, the processor causes the magnetic coils 16, 18 to produce an 
alternating magnetic field in the tube 12 (FIG. 1). This causes electrodes 
20, 22 to supply an AC signal to the amplifier. Above a threshold 
frequency, capacitors 56, 58, 78 act as open circuits, supplying a 
positive feedback to the amplifier that is proportional to R/(R+R.sub.S) 
(where R.sub.S is the resistance of variable resistor 76, and where the 
effect of resistor 60 has been ignored). When this positive feedback 
exceeds the negative feedback arriving at inverting input 64, the 
amplifier produces an oscillating output. When the positive feedback is 
less than the negative feedback, the output of the amplifier is a low 
voltage DC signal. 
Before the flowmeter is shipped to an end user, a technician calibrates the 
flowmeter circuitry using the method shown in FIG. 4. When the tube is 
empty, and the resistance between the electrodes and the tube is high, the 
technician instructs the processor to set the variable resistance to a low 
value (e.g., the minimum value of 250 Ohms) (step 200). This causes the 
positive feedback to the amplifier to exceed the negative feedback, and 
the output of the amplifier oscillates. The oscillating output of the 
amplifier is converted to a high DC signal by the rectifier, and the 
inverter converts this signal to a digital logic High signal indicating to 
the processor that the tube is empty. This triggers the processor to 
interrupt the flow measurement, and indicate to the user that the tube is 
empty. 
The technician then gradually increases the variable resistance until the 
positive feedback no longer exceeds the negative feedback input to the 
amplifier, and the output of the amplifier stops oscillating (step 202). 
The DC signal at the output of the rectifier is now a low signal, which 
the inverter converts to a logic Low signal indicating that the tube is 
full. To avoid this error during use, the technician must set the variable 
resistance at a value below this threshold resistance (R.sub.max). 
Next, the technician fills the tube with a particular process fluid, e.g., 
by connecting the tube to a pipeline (step 204). This causes the 
resistance between the electrodes and the tube to decrease (in relation to 
the conductivity of the fluid) and prevents the output of the amplifier 
from oscillating. The technician then continues his calibration by 
decreasing the variable resistance until the positive feedback to the 
amplifier exceeds the negative feedback, and the circuit resumes 
oscillating (step 206). To prevent an erroneous empty tube reading based 
on this signal, the variable resistance must be set to a value (R.sub.S) 
that exceeds this threshold resistance (R.sub.min) but remains below 
R.sub.max (step 208). 
The technician then repeats steps 200 through 208 for another process 
fluid, having another fluid conductivity. He then either stores in the 
processor appropriate values of R.sub.S for each fluid, or he derives an 
equation relating R.sub.S to R.sub.min and stores this equation in the 
processor. One setting that works well for process fluid conductivities 
between 1.5 .mu./cm to 325 .mu./cm is: 
EQU R.sub.S =63+1.707R.sub.min (1) 
Conductivities that vary greatly from those given above may require a 
different formula or a variable resistor with a different range of 
resistances to be employed. It is sufficient to derive R.sub.S based only 
on R.sub.min since R.sub.max is related to R.sub.min. 
An end user then chooses a flowmeter, calibrated according to the process 
described above, that is appropriate for the particular process fluid the 
user wishes to measure. The user then sets up the flowmeter by indicating 
to the processor the type of fluid to be measured. If a lookup table is 
used, the processor matches the fluid type to its corresponding resistance 
R.sub.S stored in memory, and sets the variable resistance to that value. 
Alternatively, if the processor stores an equation in the form of Equation 
(1), the user first ensures that the tube is full before activating the 
processor for calibration. The processor turns all switches off so that 
the variable resistor is at a maximum value. Because this resistance 
exceeds R.sub.min, the amplifier does not oscillate and the processor 
indicates a full tube. The processor then gradually decreases the variable 
resistance until the amplifier oscillates and an empty tube signal is 
generated. The value of the resistance that causes the empty tube signal 
is set to R.sub.min. The processor then calculates R.sub.S from R.sub.min 
based on Equation (1). 
The user re-activates the calibration procedure whenever the type of 
process fluid changes, or when the electrodes become fouled. 
Alternatively, the processor automatically re-calibrates the circuit at 
set intervals. 
Other embodiments are within the following claims. 
For example, the values of various components in circuit 50 are varied to 
accommodate process fluids with different conductivities. Alternatively, 
values of capacitors 56, 58, 78 are decreased to 1500 pF to create a 
circuit less sensitive to low frequencies. In other embodiments, the 
capacitance of the capacitors exceeds 200 pF to increase the sensitivity 
of the amplifier. 
Instead of using resistors 72, 74, the output of the amplifier can be 
attenuated by decreasing the supply voltages to the amplifier or limiting 
the amplifier's output swing with zener diodes. 
Temperature correction circuits can be connected to the circuit to 
compensate for fluctuations in the temperature of the process fluid.