A mass flowmeter of the Coriolis-type in which fluid to be metered is conducted through a flow tube which is coiled to define a helix having a pair of identical measuring loops forming a double loop, on either side of which is an isolation loop. The fluid is admitted into the input of one isolation loop and is discharged from the output of the other isolation loop. The helix is concentric with a support structure having at one end a flow inlet to which the isolation loop input is affixed, and at the other end a flow outlet to which the isolation loop output is attached. A rigid bar parallel to the axis of the helix is joined to the junction of the measuring loops as well as to the respective junctions of each measuring loop and its associated isolation loop whereby the isolation loops then function as decoupling springs to effectively isolate the bar and the double measuring loop from external forces.

BACKGROUND OF INVENTION 
1. Field of Invention: 
This invention relates generally to mass flowmeters, and more particularly 
to a Coriolis-type meter in which the fluid to be metered is conducted 
through a helically coiled flow tube that includes a double loop that 
functions as a tuning fork whose tines are free to vibrate in phase 
opposition so as to torsionally oscillate as a function of mass flow. 
2. Status of Prior Art: 
A mass flow rate meter is an instrument for measuring the mass of a fluid 
flowing through a conduit per unit time. Most meters for this purpose 
measure a quantity from which the mass can be inferred, rather than 
measuring mass directly. Thus, one can measure the mass flow rate with a 
volumetric flowmeter by also taking into account pressure, temperature and 
other parameters to compute the mass. 
A Coriolis-type mass flowmeter provides an output directly proportional to 
mass flow, thereby obviating the need to measure pressure, temperature, 
density and other parameters. In this type of meter, there are no 
obstacles in the path of the flowing fluid, and the accuracy of the 
instrument is unaffected by erosion, corrosion or scale build-up in the 
flow sensor. 
In the Roth U.S. Pat. No. 3,132,512, a Coriolis-type mass flowmeter is 
disclosed in which a flow loop vibrating at its resonance frequency is 
caused to oscillate about a torque axis which varies with fluid flow in 
the loop. This torsional oscillation is sensed by moving coil transducers. 
The Cox et al. U.S. Pat. Nos. 4,127,828 and 4,192,184 show a Coriolis-type 
meter having two U-shaped flow loops arranged to vibrate like the tines of 
a tuning fork, torsional oscillation of these loops in accordance with the 
mass of the fluid passing therethrough being sensed by light detectors. In 
the Smith U.S. Pat. No. 4,222,338, electromagnetic sensors provide a 
linear analog signal representing the oscillatory motion of a U-shaped 
pipe. Electromagnetic sensors are also used in the Smith et al., U.S. Pat. 
No. 4,492,025, in which the fluid whose mass is to be measured flows 
serially through two parallel U-shaped pipes which together operate as the 
tines of a tuning fork. 
Because a double-loop Coriolis-type meter functions as a tuning fork, the 
minimum power required to oscillate the two loops occurs at their natural 
frequency. When the two loops vibrate as a tuning fork with respect to an 
anchor at the junction of the two loops, they will alternately draw 
together to a minimum spacing and then separate to a maximum spacing; 
hence the angular velocity vector for one loop will always be opposite to 
the angular velocity vector for the other loop. And because the flow 
through the two loops is the same, the loops will be subjected to opposing 
torques by reason of the opposite angular velocity vectors. As a 
consequence, the two loops are caused alternately to twist toward and away 
from each other. 
A double-loop tuning fork configuration also provides a more stable 
operation than two single loops in parallel relation, for the mass flow is 
common to both loops and does not depend on evenly splitting the flow 
between the two loops. This results in a dynamically balanced pair of 
loops and a substantially decreased sensitivity to external vibratory 
forces. 
However, because the loops of the tuning fork are anchored at their center 
which is the junction of the two loops as well as the inlet and outlet 
ends, such anchoring strongly inhibits deflection of the loops. As a 
result, velocity sensors of the type used in the prior art are not 
sufficiently sensitive to provide an adequate signal for mass flow 
measurement. 
To overcome this drawback, the Herzl U.S. Pat. No. 4,747,312 discloses a 
mass flowmeter of the Coriolis type in which the fluid to be metered is 
conducted through a pipe which is coiled to form a double loop. The pipe 
is anchored on a stationary frame at its inlet and outlet ends and also at 
its center which is the junction of the two loops to define a tuning fork 
in which the identical loops on either side of the anchored center 
function as tines that are free to vibrate as well as to twist. 
An electromagnetic driver mounted at the vertex of the double loop is 
electrically energized to cause the loops to vibrate, in phase opposition, 
at the natural frequency of the tuning fork. The fluid passing through the 
double loop is subjected to Coriolis forces, thereby causing the vibrating 
loops to torsionally oscillate in accordance with the mass flow of the 
fluid. Capacitance sensors are symmetrically mounted on the respective 
loops to yield signals having a difference in magnitude and phase that 
depends on the amplitude of the torsional oscillations, these signals 
being applied to a differential amplifier whose output is proportional to 
the mass flow of the fluid. 
The Herzl double-loop meter exhibits serious defects in meter performance. 
In the Herzl meter, the double loop is supported on a stationary frame in 
which the double loop is anchored at three points; namely, at its inlet 
and outlet end and also at the junction of the two loops forming the 
double loop. The double loop is therefore highly sensitive to forces from 
external sources that may give rise to vibration of the frame or produce 
torsional or bending moments. 
SUMMARY OF INVENTION 
In view of the foregoing, the main object of this invention is to provide a 
Coriolis-type mass flowmeter of the double-loop type which operates 
efficiently, reliably and accurately. 
More particularly, an object of this invention is to provide a mass 
flowmeter of the above type which makes use of a double loop that 
functions as a tuning fork and is excited into vibration at its natural 
resonance frequency, the double loop being joined at three points to a 
rigid bar which is free to float relative to the support structure for the 
meter, thereby effectively isolating the double loop from external forces. 
Also an object of the invention is to provide a meter of the above type in 
which the helically coiled flow tube defines not only a double loop but 
also an isolation loop on either side of the double loop which functions 
as a spring to isolate the bar and the double loop from external forces. 
A significant advantage of the invention is that the meter is insensitive 
to torsional, vibrational and bending forces from external sources applied 
to the support structures therefor, and therefore affords accurate mass 
flow readings under adverse operating conditions. 
Also an object of the invention is to provide a flowmeter of the above type 
in which the double loop and the isolating loops are defined by a single 
length of tubing without joints. This makes possible practical 
applications where joints, crevices or a second wetted material are 
interdicted. 
Yet another object of the invention is to provide a mass flowmeter of the 
Coriolis-type in which the helix which defines the double loop and the 
isolation loops on either side of the double loop has an isolation loop 
input and an isolation loop output that both extend along the longitudinal 
axis of the helix, thereby reducing the effect of external forces on the 
calibration of the meter and rendering the meter self-draining when 
mounted in a vertical pipeline. 
Briefly stated, these objects are attained in a mass flowmeter of the 
Coriolis-type in which fluid to be metered is conducted through a flow 
tube which is coiled to define a helix having a pair of identical 
measuring loops forming a double loop, on either side of which is an 
isolation loop. The fluid is admitted into the input of one isolation loop 
and is discharged from the output of the other isolation loop. The helix 
is concentric with a support structure having at one end a flow inlet to 
which the isolation loop input is affixed, and at the other end a flow 
outlet to which the isolation loop output is attached. 
A rigid bar parallel to the axis of the helix is joined to the junction of 
the measuring loops as well as to the respective junctions of each 
measuring loop and its associated isolation loop whereby the isolation 
loops then function as springs to effectively isolate the bar and the 
double loop from external forces. The measuring loops act as the tines of 
a tuning fork and are excited to vibrate in phase opposition. The fluid 
passing through the double loop is subjected to Coriolis forces, thereby 
causing the vibrating loops to torsionally oscillate in accordance with 
the mass flow of the fluid. Sensors mounted on the oscillating loops yield 
signals having a difference in magnitude and phase that is a function of 
the torsional oscillations. These signals are applied to a differential 
amplifier whose output is proportional to the mass flow of the fluid.

DETAILED DESCRIPTION OF INVENTION 
Referring now to FIG. 1, there is shown a mass flowmeter of the 
Coriolis-type according to the invention which includes a flow tube, 
generally designated by numeral 10, formed of a single length of stainless 
steel tubing or other material that is non-reactive to the fluid being 
metered and is capable of withstanding the pressure of the fluid. 
Tube 10 is coiled into a helix constituted by a series of loops, the pair 
of adjacent identical measuring loops 10A and 10B in the helix creating a 
double loop, on either side of which are isolation loops 10C and 10D. 
The helix is concentric with a cylindrical support structure 11 having at 
one end a flow inlet 12 and at the other end a flow outlet 13. Affixed by 
welding or other means to flow inlet 12 is the input to isolation loop 10C 
of the flow tube into which the fluid to be metered is admitted, and 
affixed to flow outlet 13 is the output of isolation loop 10D from which 
the fluid is discharged. Thus the input and output of the helix are 
colinear with and extend along its longitudinal center line axis X. This 
permits the helical flow tube to be self-draining when the flowmeter is 
interposed in a pipeline 14 which is vertical, rather than horizontal, as 
shown. 
While mass flow may be measured in either flow direction, for purposes of 
illustration, the left end of flow tube 10 is treated as the flow input to 
be coupled to the upstream side of pipeline 14, and the right end of the 
flow tube is treated as the flow output to be coupled to the downstream 
side of the pipeline. 
The junction J.sub.1 of measuring loops 10A and 10B which form the double 
loop is joined to the midpoint of a rigid bar 15 which is parallel to the 
longitudinal axis X of the helix. The junction J.sub.2 of measuring loop 
10A and isolation loop 10C associated therewith is joined to one end of 
bar 15, while the junction J.sub.3 of measuring loop 10B and its 
associated isolation loop 10D is joined to the other end of this bar. Thus 
the double loop is connected at its three junction points to bar 15 which 
is resiliently supported by the isolation loops and is therefore free to 
float relative to support structure 11. 
The double loop 10A-10B configuration in which its three junction points 
are connected to rigid bar 15 functions effectively as a tuning fork whose 
tines are constituted by the identical measuring loops. These tines are 
caused to vibrate in phase opposition at the natural resonance frequency 
of the fork. When fluid flows through the vibrating measuring loops, loops 
10A and 10B are then subjected to opposing Coriolis force torques, and are 
concurrently caused to twist alternately toward and away from each other. 
Hence the loops not only vibrate in phase opposition, but they also 
oscillate torsionally in opposing directions. 
The measuring loops are driven to vibrate as the tines of a tuning fork by 
an electromagnetic driver formed by a permanent magnet 16 attached to the 
vertex of loop 10A and a cooperating coil 17 attached to the vertex of 
loop 10B. Coil 17 is excited by a drive power source 18 to cause the 
magnet to be alternately attracted to the coil and repelled thereby at a 
rate corresponding to the natural resonance frequency of the fork, thereby 
causing loops 10A and 10B to swing back and forth in phase opposition. 
Two capacitance sensors S.sub.1 and S.sub.2 are mounted on measuring loops 
10A and 10B of the double loop. The structure and function of these 
sensors and their placement on the loops may correspond to the sensor 
arrangement in the Herzl U.S. Pat. No. 4,747,312. However, the invention 
is not limited to such sensors, and other known sensors used in 
conjunction with tuning-fork loop arrangements in Coriolis-type mass 
flowmeters may be used including strain gauge sensors, to sense the 
torsional oscillations of the vibrating double loop. 
Inasmuch as each loop vibrates back and forth and oscillates torsionally, 
the spacing between the plates of the capacitance sensor varies to an 
extent determined by the vector resultant of the vibratory and torsional 
movements. The change in capacitance experienced by each sensor is 
converted into a corresponding voltage signal by connecting the capacitor 
to a direct-current voltage source in series with a current limiting 
resistor, in a manner to be later explained. 
The signal voltage from sensor S.sub.1 is applied to a preamplifier 21 and 
from sensor S.sub.2 to a preamplifier 20. The output of preamplifier 21 is 
connected to the negative input of a differential amplifier 22 through a 
fixed resistor 23 in series with a variable gain-control resistor 24. The 
output of preamplifier 20 is connected to the positive input of 
differential amplifier 22 through a fixed resistor 25. The output of 
differential amplifier 22 which represents the difference between the 
amplitudes of the sensor signals is applied to a microprocessor 26. 
The output of preamplifier 21 is also applied through a fixed resistor 27 
to the input of a summing amplifier 28 to which is also applied through a 
fixed resistor 29 the output of preamplifier 20. Hence the output of 
summing amplifier 28 is the sum of the sensor signals, and this is applied 
to another input of microprocessor 26. 
Microprocessor 26 on the basis of the sum and difference signal data 
entered therein, calculates the mass flow rate of fluid flowing through 
the flow loop to provide a digital value representing the mass flow rate. 
This is displayed on visual indicator 30. 
As shown in FIGS. 2, 3 and 4, in a practical embodiment of the invention, 
rigid bar 15 is joined at the three junction points by clamping rigid bar 
15 thereto by means of a clamping strip 15B which is screwed onto the bar, 
the bar having notches therein to accommodate the tubing. 
It will be seen in FIG. 2 that the ends of the cylindrical support 
structure 11 are secured to the opposing side walls of a box-like case 19 
to enclose the helix within the box. The box is preferably light-weight, 
for its main purpose is to protect the meter from harsh environmental 
conditions. Only the lower half section of the case is shown. 
Rigid bar 15 which is joined to junction points J.sub.1, J.sub.2 and 
J.sub.3 of the double loop 10A-10B (junction J.sub.1 being the midpoint, 
junction J.sub.2 being the upstream point and junction J.sub.3 being the 
downstream point) is elastically connected to the ends of the cylindrical 
support structure 11 by isolation loops 10C and 10D which are integral 
extensions of the associated measuring loops and behave as decoupling 
springs to isolate the bar and the measuring loops from external forces. 
The isolation loops are adapted to take no energy from the flowmeter 
structure or the user's pipeline structure. And the isolation loops are 
designed to have a stiffness which is a function of the mass of the 
floating assembly and the frequencies and accelerations applied by both 
internal and external vibrations. These isolation loops afford positional 
transition from the ends of the measuring loops 10A and 10B to the 
installation connections. 
The three junction points which are rigidly connected to each other by bar 
15 are permitted to float freely in a frictionless manner relative to the 
meter support structure. This condition, together with the positioning of 
the installation centerline coincident with the loop centerline renders 
the meter insensitive to torsional, vibrational and bending forces applied 
to the support structure. 
While there has been shown and described a preferred embodiment of an 
improved Coriolis-type flowmeter in accordance with the invention, it will 
be appreciated that many changes and modifications may be made therein 
without, however departing from the essential spirit thereof.