Abstrict Mass-flow meter with temperature sensors, designed for measuring
mass flow rates of liquids or gases, containing a heat conductive
tube, through which the mass flow to be measured occurs and of which
the beginning and the end are kept at the same temperature, as well
as one or more means to raise the temperature in the tube and means
to measure the temperature of the mass in the tube in different
places. In or on the tube one or more facilities are applied with
which the zero-offset of the means to measure the temperature can
be eliminated in a physical instead of an electronic way.
Claims I claim:
1. A mass-flow meter with temperature sensors adapted for measuring
mass flow rates of liquids or gases, having:
(a) a heat conductive tube through which the mass flow to be measured
occurs and of which the beginning and the end are kept at the same
temperature,
(b) means to raise the temperature in the tube,
(c) means to measure the temperature of the mass in the tube at
different places including a plurality of temperature sensors placed
symmetrically in relation to the temperature-raising means, characterized
in that
(d) said means to raise the temperature including electrically-responsive
means fixed to the tube for modifying the temperature profile along
the tube such that when the tube with an immobile mass is heated
said temperature sensors show a temperature difference equal to
zero.
2. A mass-flow meter according to claim 1 characterized in that
the electrically-responsive means comprises a heating element in
the middle of the tube, said heating element comprising two parts
to be controlled separately, said parts being located respectively
downstream and upstream of the middle.
3. A mass-flow meter according to claim 2 characterized in that
the temperature measuring means comprise thermocouples having a
hot and a cold part.
4. A mass-flow meter according to claim 3 characterized in that
the heat conductive tube is U-shaped, the heating element is mounted
around the tube, approximately at the middle of the curve of the
U-shaped, the distance between both legs of the U is just equal
to the distance between the hot and cold parts of the thermocouples,
the thermocouples being clasped between both legs of the U-shaped
tube.
5. A mass-flow meter according to claim 3 characterized in that
the hot and cold sides of the thermocouples are about 2.5 mm apart.
6. A mass-flow meter according to claim 1 characterized in that
said electrically-responsive means comprise adjacent heating coils
with a center tap.
Description The invention relates to a mass-flow meter with temperature sensors,
designed for measuring mass flow rates of liquids or gases, containing
a heat conductive tube, through which the mass flow to be measured
occurs and of which the beginning and the end are kept at the same
temperature, as well as one or more means to raise the temperature
in the tube and means to measure the temperature of the mass in
the tube in different places.
BACKGROUND OF INVENTION
Such a mass-flow meter is known, for instance from U.S. Pat. No.
4100801. The principle on which the operation of this known mass-flow
meter is based is as follows. The gas or liqid (the medium) of which
the mass flow rate is to be measured, flows through a metal, heat
conductive tube, of which the beginning and the end are kept at
the same temperature. In the middle of the tube, inside or outside,
a heating element is put, with which the medium and the tube are
heated. On both sides of the heating element, temperature sensors
are put, symmetrical in relation to this element.
When the medium in the tube does not flow, such a temperature profile
occurs, when heated by the heating element, that in the middle of
the heatiang element the temperature is at its maximum. Without
heat loss to the environment, the tube temperature decrease is linear,
as a function of the distance to the middle mentioned. Without heat
loss, or with a heat loss to the environment which is symmetrical
in relation to the middle mentioned, the temperature profile along
the tube is symmetrical in relation to the middle mentioned.
The temperature sensors, placed symmetrically in relation to the
heating element, in case of heating an immobile medium, show, in
theory, a temperature difference equal to zero.
When the medium in the tube does flow, when heating in the way
mentioned above takes place, a sensor upstream from the heating
element will show a lower temperature than sensor which is placed
symmetrically in relation to the first, downstream from the heating
element. The difference in the temperature measured by both sensors
is a measure for the speed of flow of the medium and therefore for
the mass flow through the tube.
The mass-flow meter with temperature sensors, according to the
principle mentioned above, shows a number of disadvantages.
Air currents on the outside of the tube can disturb the temperature
profile along the tube wall, and thus can have a negative influence
on the accuracy and reproduceability of a measurement. The phenomenon
of air currents leading to a changed temperature profile is called
external convection. External convection can be counteracted by
isolating the tube thermally, for instance by wrapping it in a synthetic
foam.
Another disadvantage is the internal convection, occuring in the
form of gravity-induced convection flows in the medium itself, which
can occur when the parts of the tube on both sides of the heating
element cannot be oriented symmetrically in relation to the direction
of gravity. Internal convection also provides cause for additional
temperature gradients along the tube surface, and thus for extra
inaccuracy in measuring the mass flow. This problem, which occurs
especially when the mass-flow meter with temperature sensors is
applied in spacecraft, can be counteracted by putting a loop course
in the tube, because of which the medium is forced to turn around
the flow direction at least once. A description of a mass-flow meter
with loop-shaped tube is given in the mentioned U.S. Pat. No. 4100801.
Internal convection as well as external convection are manifest
in the occurrence of so-called zero-offset, by which is meant the
phenomenon that when the tube, with an immobile medium, is heated,
in variance with that which one would expect from theory, temperature
sensors applied symmetrically in relation to the heating element
show a temperature difference not equal to zero. Internal and external
convection are not the only causes for the occurrence of a zero-offset.
Other causes for the occurrence of a zero-offset are, among others,
a difference in the temperature at the beginning and end of the
tube, with an otherwise exact symmetry of the temperature sensors
in relation to the heating element, a positioning of the temperature
sensors not exactly symmetrical in relation to the middle of the
tube, a positioning of the heating element not exactly in the middle
of the tube and inhomogeneities in the tube wall.
In the known embodiments of the mass-flow meter with temperature
sensors, the problem of occurring zero-offset is avoided by compensating
for the zero-offset electronically. The disadvantage of this is
the inherent need for cost-raising additional electronic equipment
and/or components for reading the temperature sensors.
SUMMARY OF INVENTION
Purpose of the present invention is to provide a mass-flow meter
with which the so-called zero-offset can be eliminated physically.
This purpose is achieved, according to the invention, by applying
one or more facilities in or on the tube, with which the temperature
profile along the tube can be regulated so, that when the tube,
with an immobile medium, is heated, temperature sensors placed symmetrically
in relation to the heating element show a temperature difference
equal to zero.
These facilities can either consist of additional heating elements
with which the temperature at the beginning or the end of the tube
can be set, or consist of one or more cooling bodies (so-called
Peltier-elements) or consist of a mechanical construction against
the tube wall with which the effect of a lengthwise varying wall
thickness can be simulated. An example of such a mechanical construction
is a rail against the tube wall, along which a metal heat conductor
can be moved and fixed.
In a preferred embodiment of a mass-flow meter according to the
invention, the facility that regulates the temperature profile along
the tube consists of a heating element in the middle of the tube,
which element consists of two parts, to be steered separately, which
parts are located respectively on both sides of the middle, downstream
and upstream, in other words a heating element that is doubly constructed.
By using a doubly constructed heating element in the middle of the
tube, instead of a single heating element, the strict construction-technical
requirement of very accurate positioning of the heating element
in the middle of the tube, does not have to be met. Applying a double
heating element also hardly raises the cost compared to a single
heating element, because the latter mostly consists of a resistance
coil, wound around the tube, whereas in order to make a double heating
element, one only has to supply the mentioned resistance coil with
an additional power supply wire.
A double constructed heating element in mass-flow meters according
to the invention distinguishes itself from earlier applied doubly
constructed heating elements in mass-flow meters, of which the functioning
is based on a circuit in which the heating elements also function
as temperature sensors, because for both functions the same temperature-sensitive
resistance coils are used, as described in the U.S. Pat. No. 4487062
just because with mass-flow meters according to the type described
there, the possibility to compensate for the zero-offset physically
with the doubly constructed heating element, is absent. Another
distinction between a doubly constructed heating element in a mass-flow
meter according to the invention and the type described in the U.S.
Pat. No. 4487062 is caused by the position and the construction
of the heating element. A doubly constructed heating element according
to the invention should be placed, because of the measuring principle,
in or very near the middle of the tube, whereas with the type according
to the American patent, positioning at a substantial distance from
the middle of the tube is a necessary condition for the functioning
of the otherwise basically different measuring principle.
The sensitivity of a mass-flow meter according to the invention
is determined by the sensitivity of the temperature sensors, for
which in known mass-flow meters temperature-dependent resistance
wire is used. The more heat is dissipated through the heating element,
the greater the temperature difference measured by the temperature
sensors becomes, and therefore the sensitivity of the mass-flow
meter.
The mass-flow meters used in practice are sometimes dimensioned
so that the temperature induced by the heating element in the middle
of the tube can rise to more than 100.degree. C. above the environmental
temperature. This can cause the following disadvantages. Volatile
liquids can start boiling, which renders a measurement impossible.
Some liquids and gases can dissociate with a temperature rise as
great as the one mentioned. The measuring accuracy can be influenced
in a negative way when the specific heat of the medium to be measured
is strongly temperature dependent. With very low flow rates, as
a result of the expansion of the medium, a flow occurs, the so-called
thermosyphon-effect. If the order of magnitude of this flow equals
that of the mass flow to be measured, the measuring error becomes
unacceptably great. Elimination of all these disadvantages is only
possible by drastically lowering the temperature induced by the
heating element in the medium, and therefore a lowering of the temperature
difference measured by the temperature sensors.
The practical possibilities of a mass-flow meter which is equipped,
according to the invention, with a doubly constructed heating element
in the middle of the tube are notably widened when the temperature
sensors consist of thermocouples or, more specific, a thermopile.
A thermopile consists of a great number of thermocouples, in serial
connection, which are wound around a holder in such a way, that
all `hot` sides of the thermocouples are located on one side of
the holder, and all `cold` sides of the thermocouples are located
on the other side of the holder.
When, instead of temperature dependent resistance wire, thermocouples
are used as temperature sensors, a much smaller temperature rise
of the medium in the middle of the tube will suffice, a sufficient
linearity of the sensor signal at an ample range of mass flow rate
values is realised, sufficient sensitivity of the temperature sensors
is obtained, and the sensibility for the influence of the temperature
of the environment of the tube is at an acceptable low level.
The advantages of a thermopile are still enlarged when the temperature
of the tube downstream of the heating element, placed in the middle,
is measured with the hot side of the pile, and the temperature of
the tube upstream is measured with the cold side of the pile. Such
a sensor configuration is possible with an U-shaped tube, with which
the doubly constructed heating element is put on the tube in the
middle of the curve of the U-shape, and of which the distance between
both legs is just equal to the distance between the hot and the
cold part of the thermopile, so that the thermopile can be clasped
between both legs of the U-shaped tube.
In an example of a mass-flow meter according to the invention,
with U-shaped tube, doubly constructed heating element and thermopile,
the applied thermopile has the characteristic that the cold and
hot sides are about 2.5 mm apart. Both legs of the U-shaped tube
run parallel, with an in-between distance of about 2.5 mm. Between
the legs, a thermopile is put along the total length or a part thereof.
With this configuration, the temperature difference between two
points of the tube, in a symmetrical position upstream and downstream
in relation to the heating element put in the middle of the tube,
is measured again and again. All measured temperature differences
are, inherent in the use of a thermopile, summed, with which the
highest possible signal is obtained. The beginning and end of the
tube, both `feet`, are constructed of a solid piece of metal, with
the purpose of keeping the temperatures at the beginning and end
of the tube the same, as well as possible. The solid piece of metal
mentioned is thermally coupled tightly to a metal box which makes
up the casing of the isolation. With this, one achieves that the
temperatures of the beginning and end of the tube are equalised
with the outside temperature as well as possible. Both legs of the
U-shaped tube are assembled symmetrically within the metal box,
the rest of the space within is filled with isolating material.
In the upper part of the legs, symmetrically around the top, a doubly
constructed heating element is put.
SUMMARY OF DRAWINGS
The invention will now be explained further with the help of a
drawing in which
FIG. 1 shows a diagram of the mass-flow meter according to the
invention;
FIG. 2 shows a graphical representation of the ideal course of
the temperature of the medium in the tube in FIG. 1 for the situation
in which the medium is immobile as well as the situation in which
it flows;
FIG. 3 shows a graphical representation of a possible course of
the temperature of the medium in the tube in FIG. 1 in case the
medium is immobile and the situation is not in accordance with the
theoretical ideal;
FIG. 4 is a side view of a preferred embodiment of a mass-flow
meter with U-shaped tube, doubly constructed heating element and
thermopile.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows the metal heat-conductive U-shaped tube 1 through
which the liquid or gaseous medium to be measured flows, and the
metal casing 2 of the mass-flow meter. Beginning 3 and end 4 of
the tube, the `feet`, are thermally coupled to the metal casing
2. Around the middle M of the tube, a doubly constructed heating
element is put in the shape of a resistance wire, wound about the
tube, with three connection points for power supply, so that the
left and right part, respectively 5 and 6 of the heating element,
can be steered separately. The temperature of the outside of the
tube is measured, upstream and downstream, by two temperature sensors
7 respectively 8 which are symmetrical in relation to the middle
M. The arrows in beginning and end of the tube show the direction
of the flow of the medium.
When the medium does not flow and there are no heat losses of the
medium to the environment, then, when heated in point M, in the
theoretically ideal situation, a temperature profile arises in the
tube wall, as shown by the dashed, straight lines in FIG. 2. When
sensors 7 and 8 are put in exactly symmetrical positions, the temperature
in both places will be the same.
When the medium flows, under otherwise similar (ideal) circumstances,
the temperature profile in the tube wall will develop in accordance
with the unbroken curve in FIG. 2. In simple and well-known way,
one can deduce that the speed of flow of the medium is proportional
with the temperature difference T'8-T'7.
In FIG. 3 the temperature profile, in solid lines, in the tube
wall is drawn for an immobile medium in case for whatever reason
the temperature at the end of the tube 4 is lower than at the beginning
3 or that the right leg would be or seem shorter than the left
(for instance if the heating element is not put exactly in the middle).
It is obvious that in this situation, the temperatures at the sensors
7 and 8 are no longer the same, but a so-called zero-offset T7-T"8
has occurred. This zero-offset influences the accuracy of the measuring
in an unacceptably negative way.
By dissipating more heat in the right part 6 of the doubly executed
heating element than in the left part 5 one can recover the ideal
temperature profile (left part unchanged, right part dashed line
in FIG. 3) and thus eliminate the zero-offset T7-T"8. One can
easily see that other non-ideal temperature profiles than the one
from the example in FIG. 3 can also be corrected in the same simple
way, with the help of the doubly constructed heating element.
FIG. 4 shows an example of a mass-flow meter according to the invention
with U-shaped aluminum tube 1 and a doubly constructed heating element
with parts 5 and 6 in a side view. Between the legs of the tube,
a thermopile 9 is put along the total length or a part thereof.
With this configuration, each time the temperature difference is
measured between two points of the tube, in a symmetrical position,
upstream and downstream, in relation to the heating element put
in the middle of the tube. All measured temperature differences
are, inherent in the use of a thermopile, summed, with which the
highest possible signal is obtained. The beginning 3 and end 4 of
the tube, both `feet` are constructed of a solid piece of aluminum,
with the purpose of keeping the temperatures at the beginning and
end of the tube the same, as well as possible. The solid piece of
aluminium mentioned is thermally coupled tightly to an aluminum
box (of which only the bottom 13 is shown in the Figure) which makes
up the casing of the isolation. With this, one achieves that the
temperatures at the beginning and end of the tube are equalised
with the outside temperature as well as possible. Both legs of the
U-shaped tube are assembled symmetrically within the metal box,
the rest of the space within is filled with isolating material.
In the upper part of the legs, symmetrically around the top, a doubly
constructed heating element, with parts 5 and 6 is put. |