Abstrict A flow meter for the registration of pulsating liquid flows, and
preferably for registration of small blood flows. The flow is measured
by means of a differential pressure measurement over a first flow
resistance (I) having a resistance R.sub.1. The flow meter includes
a high pressure part in the form of a flow resistance (II) and a
lower pressure part in the form of a flow resistance (III), and
these have the resistances R.sub.2 and R.sub.3 respectively. The
resistances R.sub.1 R.sub.2 and R.sub.3 are selected so as to adjust
the pressures on each side of the differential pressure meter in
phase with each other independent of the frequency of the flow pulsations.
Each resistance has a corresponding modulus of elasticity, and the
resistances and moduli of elasticity are so related as to fulfill
certain formulas.
Claims I claim:
1. A flow meter for the registration of pulsating liquid flows,
preferably for registration of small blood flows, the flow being
measured by means of a differential pressure measurement over a
first flow resistance (I) with the resistance R.sub.1 and the flow
meter comprising, besides the flow resistance I, also a high pressure
part in the form of a flow resistance (II) and a lower pressure
part in the form of a flow resistance (III) with the resistances
R.sub.2 and R.sub.3 respectively, wherein the resistances R.sub.1
R.sub.2 and R.sub.3 are selected so as to adjust the pressures on
each side of the differential pressure meter in phase with each
other independent of the frequency of the flow pulsations, the flow
resistances (I), (II) and (III) with the resistances R.sub.1 R.sub.2
and R.sub.3 respectively, having moduli of elasticity k.sub.1
k.sub.2 and k.sub.3 respectively, said factors being adapted to
fulfill the following formula
in which 1/k.sub.1 is approximately equal to zero, that is the
flow resistance material in the main is inelastic.
2. A flow meter according to claim 1 including an outer receptacle
and a differential pressure meter mounted therein, both the outer
receptacle and the interior of the pressure meter containing an
inert not electrically conductive liquid having a low viscosity.
3. A flow meter according to claim 2 wherein the inert liquid
has another density than water.
4. A flow meter according to claim 2 or 3 wherein the outer receptacle
has an elastic wall portion which alters its shape with volume alterations
so that the pressure in the outer receptacle remains constant.
5. A flow meter according to claim 2 or 3 wherein the outer receptacle
also contains a smaller quantity of gas which compensates for the
volume alteration of the liquid by which the pressure in the outer
receptacle remains approximately constant.
6. A flow meter according to claim 1 2 or 3 wherein the high pressure
part (II) in the main has a volume which is equal to that of the
low pressure part (III).
7. A flow meter according to claim 6 wherein the equal volume
of the high pressure part and low pressure part (II, III) has been
obtained by means of intermediate pieces with a large and a smaller,
respectively, inner volume, the pressure part originally having
the largest volume moreover having been reduced by means of a pierced
insert mounted in a coupling tube of that part, which tube extends
to the pressure meter, said insert preferably having a good heat
conductivity.
8. A flow meter according to claim 1 2 or 3 wherein the desired
resistances are obtained by means of intermediate pieces which have
depressions for membranes in contact with the flow, the pressure
of which should be measured.
9. A flow meter according to claim 8 wherein the membranes transferring
the pressure from the flow path to the differential pressure meter
are double-walled, the membrane wall contacting the blood vessel
being a part of the flow resistance (I).
10. A flow meter according to claim 8 wherein the membranes are
elastic and at measurement of the blood flow the membrane walls
positioned in the blood flow also consist of a blood-minded material,
such as for instance siliconized-rubber.
11. A flow meter according to claim 1 2 or 3 wherein the flow
meter is for measurement of blood flow and wherein the inner surface
of flow resistance (I) comprises a blood-minded material, for instance
silicon-rubber, and is shaped without unevenesses damaging to the
blood during the flowing thereof.
12. A flow meter according to claim 11 wherein flow resistance
(I) forms a mountable part thereof.
13. A flow meter according to claim 2 wherein the receptacle is
heat insulating and has a reflecting surface.
14. A flow meter according to claim 2 wherein the inert liquid
in the outer receptacle has a temperature buffer capability.
15. A flow meter for the registration of pulsating liquid flows,
preferably for registration of small blood flows, the flow being
measured by means of differential pressure measurement over a first
flow resistance (I) with the resistance R.sub.1 and the flow meter
comprising, besides the flow resistance (I), also a high pressure
part in the form of a flow resistance (II) and a low pressure part
in the form of a flow resistance (III) with the resistances R.sub.2
and R.sub.3 respectively, and wherein the resistances R.sub.1
R.sub.2 and R.sub.3 are selected so as to adjust the pressures on
each side of the differential pressure meter in phase with each
other independent of the frequency of the flow pulsations the flow
resistances (I), (II) and (III), with the resistances R.sub.1 R.sub.2
and R.sub.3 respectively, having moduli of elasticity k.sub.1
k.sub.2 and k.sub.3 respectively which are adapted to fulfill the
formula if in which 1/k.sub.1 is approximately equal to zero, that
is the flow resistance material in the main is inelastic.
Description The present invention relates to a flow meter for registration
of fluctuating liquid flows, preferably for registration of small
flows, for instance blood flows in connection with animal experiments
at which the flow meter measures the flow by means of differential
pressure measurement over a fixed flow resistance and therefore
comprises a high pressure part and a low pressure part.
At the registration of pulsating blood flows on the artery or vein
part up till now generally electromagnetic flow meters have been
used. These, however, are inconvenient to use for the measurement
of small flows, for instance in the order of (<10 ml/min) on
account of an all too great base line drift which causes a very
poor accuracy at said flow quantities. On the vein part also droppers
are used as blood flow meters, but the accuracy is very poor.
The purpose of the present invention is to bring about an exact
and continuous flow registration of a pulsative liquid flow independent
of the frequency of the pulsations, above all within the small-flow
range and not only a possibility of registration of the average
flow of pulsatile flow.
According to the invention this in the main is achieved in that
the resistances R.sub.1 R.sub.2 and R.sub.3 of the flow meter and
its moduli of elasticity k.sub.1 k.sub.2 and k.sub.3 (reference
numerals according to FIG. 1) are adjusted according to the formula
in which 1/k.sub.1 approximately equals to 0 that is, the material
of the flow resistance material is inelastic.
In the following the invention will be more exactly described with
reference to the accompanying drawings in which
FIG. 1 schematically illustrates a flow meter according to the
invention,
FIGS. 2 and 3 show calibration curves for two types of flow meters
according to the invention,
FIG. 4 is a sectional view of a flow meter according to the invention,
and
FIG. 5 is a sectional view on the line V--V in FIG. 4 whereas
FIGS. 6 and 7 show an alternative construction which is particularly
suited for blood flow measurement for clinical purpose.
The main principle of the flow meter according to the invention
is schematically illustrated in FIG. 1. It consists of a main tube
(I) conducting the blood flow (Q) through the flow meter. The pressure
drop across the tube I is measured by means of a differential pressure
meter 4 having practically no "displacement" (National
Semiconductor Corp., LX1601DD) via tubes II and III, the former
being shaped with an inner conduit of a smaller size for a purpose
which will later be described. It is important that the pulse pressure
at the entrance of the tube II is transferred to each side of the
differential pressure meter without any considerable phase shift.
This means that the time constant (T.sub.2) for the tube II shall
be identical with the time constant (T.sub.3) for the tubes I+III
which can be brought about by adopting of suitable values for the
three tube resistances R.sub.1 R.sub.2 and R.sub.3 and the moduli
of elasticity k.sub.1 k.sub.2 and k.sub.3 thereof in accordance
with the following calculation. Since the time constants are independent
of the flow, the calculation refers to a situation with the average
flow to be zero which is obtained by temporary closing of the outlet
(FIG. 1). The pulse pressure then will cause smaller flows q.sub.1
q.sub.2 and q.sub.3 through the three tubes I, II and III of the
flow meter. p.sub.0 p.sub.1 p.sub.2 and p.sub.3 indicate the pressures
in the positions in FIG. 1 .DELTA.V.sub.1 .DELTA.V.sub.2 and .DELTA.V.sub.3
the pressure inducing volume alterations of the tubes I, II and
III, and k.sub.1 k.sub.2 and k.sub.3 the moduli of elasticity thereof.
By that, dp=k.sub.1 dV; dp.sub.2 =k.sub.2 dV.sub.2 ; dp.sub.3 =k.sub.3
dV.sub.3 and dV.sub.1 /dt=q.sub.1 -q.sub.3 ; dV.sub.2 /dt=q.sub.2
; dV.sub.3 /dt=q.sub.3.
The relationship between flow, resistance and pressure drop for
the three tubes can be expressed as follows:
If 1/k.sub.1 .apprxeq.0 (that is, the tube I is rigid) the above
equations after Laplace's transformation, will give the following
solutions: ##EQU1## Thus, if k.sub.2 and k.sub.3 are identically
shaped, the relationship T.sub.2 =T.sub.3 is valid when R.sub.2
=R.sub.1 +R.sub.3. As appears, this will apply for all s, that is
for all frequencies.
The flow meter is designed according to the above rule. The relationship
k.sub.2 =k.sub.3 was obtained by shaping the tube II (outer tube)
and the tube III in equal size and of the same material. The relationship
R.sub.2 =R.sub.1 +R.sub.3 was obtained by mounting the smaller conduit
within the tube II with a resistance equal to R.sub.1. It is obvious
that the pressure-sensitive portion 5 of the differential pressure
meter must operate without any considerable displacement, a rule
which is fulfilled by means of the seclected piezo-resistive differential
pressure meter (National Semiconductor Corp., LX1601DD). The tubes
II and III are filled with a low-viscous fluid, i.a. in order to
avoid pockets of trapped air. In order to also avoid short-circuiting
and by that a damage of the pressure meter, the interior thereof
is filled with said quite inert electrolyte-free fluid FC (40-85)
(BM Brand inert fluorochemical Liquids Chemical Division 3M Company).
With this design in the main any phase shift between P.sub.2 and
P.sub.3 (FIG. 1) did not arise, as appears from the abscence of
pulse pressure oscillations at zero-flow with outer closure. The
pressure meter was hermetically enclosed together with said inert
liquid in order to avoid damages by contact with electrolytes and
interference by change of the outer temperature. In this way the
inert liquid protects the laser-trimmed resistances positioned on
the outside of the pressure meter.
The size of the resistance in the tube I, R.sub.1 (and therewith
also R.sub.2) can be varied within a wide range. The selected value
is dependent on the range within which the flow is to be measured
and on the upper limit of the acceptable arterial blood pressure
drop across the tube and on the sensitivity and precision of the
differential pressure meter. R.sub.1 must be as small as possible
in order to limit the pressure drop across the flow meter. This
above all applies to measurement of blood flow. With the present
construction of the selected differential pressure meter, intended
for blood flow measurement, flows within the range <50 ml/min
can be correctly measured for a pressure drop across the flow meter
of <10 mm/Hg.
In FIGS. 2 and 3 are shown calibration curves for two types of
a blood flow meter, a first one having a relatively high R.sub.1
for measurement in the lower flow range (the length of the tube
I being 85 mm, the inner diameter being .apprxeq.1.15 mm), and a
second one having a lower R.sub.1 for larger flows (the length of
the tube I being 85 mm, the inner diameter being 1.75 mm). The blood
flow (for calibration purpose measured by means of graduated glass)
is ticked off relatively to the output signal (U.sub.Q) from the
differential pressure meter. Along the abscissa is also ticked off
the pressure difference across the tube I(p.sub.0 -p.sub.1). It
appears that the non-linearity for blood is very modest. This fact
also indicates that viscosity alternations in the blood within these
speed ranges are of less importance. The present construction of
the flow meter provides a base line drift/hour <2% of the deviation
at full scale, corresponding to a maximum pressure drop of 10 mm
Hg across the flow meter is standardized circumstances with a relatively
constant ambient temperature and constant blood viscosity.
The flow meter according to FIGS. 4 and 5 may be provided with
an outer respectacle 6 in which the differential pressure meter
4 is mounted. The liquid via the inlet 7 enters a somewhat widened
portion 8 which is shielded from the high pressure side II by means
of a very thin movable membrane 9 preferably of siliconized latex
rubber for blood flow measurement. The tube, corresponding to I,
extends from the widened portion 8 as an arch (not shown in the
drawing) to a similar widened portion 10 which is shielded from
the low pressure side III via a similar membrane II. The liquid
flows out of the flow meter via the outlet 12 which is connected
with the widened portion 10. The whole flow passage from inlet to
outlet 12 preferably is moulded in one unit. At the feeding of blood
flow this moulding should be made in a blood-minded material without
sharp unevenesses damaging the blood-corpuscles.
The use of the flow meter for blood flow measurement for clinical
purposes, for instance blood flow measurement at kidney dialysis
or blood flow measurement in the use of a heart-lung-machine etc.,
will require another design considering requirements for sterility
and requirements for an easy interchangeability of the material
contacting the blood vessel (possibly of disposable type). FIG.
6 illustrates how such a flow meter can be constructed. The flow
resistance (1) shown in FIG. 7 is designed as an easily interchangeable
unit, the inner surface of which contacts the blood, is covered
with a thin layer of silicon rubber. The counterpart of the membrane
9 and 11 in FIG. 4 has double walls, the membrane wall contacting
the blood vessel being comprised in the flow resistance portion
(1) and the other membrane wall being attached to the intermediate
pieces 15 and 16 in FIG. 6 by means of an O-ring 23. The intermediate
pieces are screwn on the top by means of nuts 24 and sealed against
the top by means of an O-ring 25. The flow resistance (I) is easily
mountable by means of a fastening device (26) and can therefore
be easily replaced, at which the flow resistance (I) may be disposable.
Differential pressure meters having a great accuracy of measurement
may be sensitive partly to temperature variations and partly to
outer and inner influence of ion-containing solutions, such as for
instance ordinary water or blood. In order to eliminate these problems
both the outer receptacle 6 and the interior of the pressure meter
is provided with the inert liquid (FC 40-85). By means of suitable
positioning of the flow meter it is avoided that liquid is mixed
with blood at a possible membrane damage since the suggested inert
liquid is many times heavier than water and blood.
Pressure alterations caused by temperature variations in the outer
receptacle 6 may be eliminated either in that the respectacle has
an elastic wall portion 13 which alters its shape with volume variations
so that the pressure in the outer receptacle 6 remains constant.
The outer receptacle 6 has an opening 14 which without resistance
permits an alteration of the shape of the wall portion 13. This
solution is shown in FIG. 4. Alternatively the outer receptacle
6 besides a liquid, may contain a smaller quantity of gas. Since
the gas is easily compressible the pressure may be maintained approximately
constant despite the volume alteration of the liquid with the temperature.
This solution is shown in FIG. 6.
The inert liquid in the pressure meter alters its volume with temperature
variations causing different expansion of the membranes 9 and 11
so that a substantial base line drift may arise. In order to eliminate
this effect when one pressure part of the pressure meter has a considerably
larger volume than the other pressure part thereof, the total space
of the high pressure part II and the space of the low pressure part
III have been made equal large by volume.
The equal volumes of the high pressure and low pressure part spaces
II, III preferably is obtained by means of intermediate pieces 16
15 with a large and a smaller inner volume, respectively, and in
that the originally relatively much larger volume of the low pressure
part space III moreover has been reduced by means of a pierced insert
17 mounted in the own low pressure coupling tube of the pressure
meter 4 said insert 17 preferably having a good heat conductivity
in order to obtain an equilibrium temperature as fast as possibly.
In order to bring about a constructively simple and simultaneously
effective design desired resistances may be obtained by means of
intermediate pieces 16 15 having depressions 18 19 for membranes
9 11 actuated by the flow the pressure of which is to be measured.
In order to avoid at blood flow measurement the formation of blood
pockets where the blood may be stopped and clot on peril of emboli
and cause cleaning problems the membranes 9 11 may be arranged
in the blood flow so that they are continuously flushed by the flowing
blood and that sharp edges and a turbulence-generating shape is
avoided. The membranes should be thin, elastic and made of a material
standing blood contact.
Components being parts of the flow meter but less important for
the invention have not been shown or described. However, it should
be mentioned that the inserts 16 15 are screwn on the tube of the
pressure meter and that they have sealing rings 20 21 for sealing
against surrounding walls. The differential pressure meter 4 is
connected with an electrical power source and a registration apparatus
(not shown) by means of a cable 22. As shown in the drawing the
two flow meters in FIGS. 4 and 6 illustrate two different ways to
fix the pressure meter.
The flow meter according to the invention preferably may be used
for measuring other flows than blood flows, in other words, the
invention is not limited to the embodiments exemplified above and
shown in the drawings but can be modified within the scope of the
following claims.
Finally it should be mentioned that the liquid in the receptacle
6 preferably has a temperature buffer effect. In order to prevent
that radiation heat causes a temperature drift the outer receptacle
may be heat insulated and have reflecting surfaces.
The illustrated and described flow meter as mentioned is suitable
for the measurement of fluctuating fluids of all kinds, i.e. also
gasoline. The flow meter for instance may be used for continuous
gasoline flow measurement in vehicles at which the obtained flow
signal, by means of an analog divider, may be divided with a signal
proportionel to the speed of the vehicle, by which the consumption
of gasoline may be continuously registered as consuption per way
unit.
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