Abstrict For temperature compensation in a thermal mass flow meter with
heated and unheated electric resistors which are interlocked to
a bridge, a variable temperature-independent electric resistor is
connected in parallel to the bridge. The sum of the current through
the bridge and of the current through the resistor connected in
parallel to the bridge functions as measuring signal.
Claims I claim:
1. A temperature compensated arrangement for measuring the mass
flow rate of a fluid, comprising:
first and second temperature-dependent resistors placed so as to
be in thermal contact with the flowing medium to be measured, one
of which first and second resistors is electrically heated, and
the resistance value of the other of which provides an indication
of its temperature, and hence the temperature of the fluid;
third, fourth and fifth temperature-independent resistors, one
of which is variable and used to set an excess temperature of the
heated resistor in relation to the medium temperature, another of
which affects the temperature dependency of a bridge current, the
first through fifth resistors being coupled in so as to form a bridge
circuit having a diagonal;
a regulating circuit coupled to the diagonal of the bridge, the
regulating circuit including a differential amplifier and a transistor;
a sixth variable temperature independent resistor coupled in parallel
with the bridge; the third resistor having a fixed value for affecting
the temperature dependency of a bridge current; and
means for measuring the sum of the current through the bridge and
the current through the sixth resistor as a measuring signal indicative
of mass flow rate.
2. A temperature compensated arrangement for measuring the mass
flow rate of a fluid, comprising:
a first temperature-dependent resistor that is electrically heated
placed so as to be in thermal contact with the flowing medium to
be measured and forming a first of four legs of a bridge circuit;
a second temperature-dependent resistor, also placed in thermal
contact with the flowing medium to be measured, the resistance value
of which is related to its temperature, and hence the temperature
of the fluid, said second resistor being in circuit in a second
of the four legs of the bridge circuit;
a third temperature-independent resistor having a fixed value in
series circuit with said second resistor in the second leg of the
bridge circuit for affecting the temperature dependency of a bridge
current;
a fourth variable temperature-independent resistor for setting
an excess temperature of the heated resistor in relation to the
medium temperature, the fourth resistor being in circuit in a third
of the four legs of the bridge circuit;
a fifth temperature-independent resistor in circuit in a fourth
leg of the bridge circuit, the bridge circuit having a diagonal
between first and second nodes thereof, the first node being at
a junction of said first and fifth resistors and the second node
being at a junction of said third and fourth resistors;
a regulating circuit coupled to the diagonal of the bridge, the
regulating circuit including a differential amplifier and a transistor;
a sixth variable temperature independent resistor coupled in parallel
with the bridge, such that it is coupled at one end thereof to said
first resistor, and at the other end thereof to said fifth resistor;
and
means for measuring the sum of the current through the bridge and
the current through the sixth resistor as a measuring signal indicative
of mass flow rate.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to temperature compensation of a thermal
mass flow meter.
2. Description of the Prior Art
Thermal anemometers are used to measure the mass flow rate of a
fluid, especially a gas at changing temperature. It is known to
provide an anemometer having two temperature-dependent electric
resistors, usually in the form of thin-film resistors or wires,
connected together with two temperature-independent electric resistors,
one of which is variable, in a bridge. An electric regulating circuit
controls the operational voltage of the bridge in such a manner
that one of the temperature-dependent resistors is electrically
heated to a constant temperature that is in excess of the temperature
of the unheated temperature-dependent resistor and of the temperature
of the medium to be flow measured. This makes it possible to measure
the mass flow of fluids fairly reliably even if the fluid temperature
is changing. The change of the bridge current is a measure of the
mass flow of the flowing fluid.
A thermal anemometer for measuring the mass flow of air with improved
measuring accuracy is described in DE-OS No. 28 43 019 and in the
publication "DISA-Information 22 1977 pp. 5 to 14".
The bridge contains, in addition to the two temperature-dependent
electric resistors, three temperature-independent electric resistors,
two of which are variable resistors. An excess temperature of the
heated resistor in relation to the unheated temperature-dependent
resistor is set with the one variable resistor R.sub.UT at a typical
medium temperature. A difference in the temperature coefficients
of the electric resistance of the two temperature-dependent electric
resistors and a positive temperature dependency of the coefficient
of heat transfer from the heated resistor to the fluid is compensated
by using the other variable resistor R.sub.TK, which can also be
a resistance network. The temperature dependency of the excess temperature
in relation to the medium temperature is varied with this electric
resistor. The bridge current provides a measuring signal indicative
of mass flow rate.
However, this known arrangement for temperature compensation has
disadvantages. There is an unavoidable spread (difference) e.g.
in temperature characteristics among various units of the temperaturedependent
electric resistors used as sensors and in thermal conductivity of
their respective holders. Therefore, the resistor R.sub.TK must
be determined individually for each measuring device by measuring
the temperature characteristic of the measuring signal in order
to obtain exact temperature compensation. However, a change of resistor
R.sub.TK necessarily changes the set excess temperature at a typical
medium temperature, which must then be reset by changing the resistance
value of R.sub.UT. This balancing process must be performed several
times, if necessary, and is quite time-consuming.
SUMMARY OF THE INVENTION
The present invention provides an arrangement for temperature compensating
a thermal mass flow meter in a manner that requires as little time
as possible. More specifically, it provides a mass flow meter arrangement
in which two temperature-dependent resistors are in thermal contact
with the flowing medium to be measured. One of the resistors is
electrically heated and the other measures the temperature of the
medium. The two temperature-dependent resistors are connected in
circuit with three additional temperature independent resistors
to form a bridge circuit. One of the three temperature independent
resistors (4) is a variable resistor that is used to set an "excess"
temperature of the heated resistor in relation to the temperature
of the medium. A second (3) of the three temperature independent
resistors has a fixed value and affects the temperature dependency
of the bridge current. To the diagonal of the bridge circuit is
connected a regulating circuit which includes a differential amplifier
and a transistor.
An additional variable temperature-independent electric resistor
(8) is coupled in circuit in parallel to the bridge. The sum of
the current through the bridge and of the current through the additional
resistor (8) connected in parallel to the bridge is used as measuring
signal indicative of mass flow rate.
BRIEF DESCRIPTION OF THE DRAWINGS
The sole FIGURE is a schematic diagram of the temperature compensation
arrangement according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The presently preferred embodiment of the invention is shown in
schematic form in the sole FIGURE. The anemometer includes a heated
temperature-dependent resistor 1 which is exposed to the cooling
action of the flowing medium to be measured and an unheated temperature-dependent
electric resistor 2 which is also in thermal contact with the flowing
medium to be measured. Resistor 2 acts as a temperature sensing
element. Resistors 1 and 2 are connected in circuit with three other
resistors 3 4 and 5 all of which are temperature-independent,
to form a bridge circuit. The diagonal of the bridge is formed between
a pair of nodes, the first node of which is between resistors 1
and 5 and the second node of which is between resistors 3 and 4.
To the bridge diagonal is connected a control circuit which includes
a differential amplifier 6 and a transistor 7 which controls the
operational voltage of the bridge. Temperature independent resistor
4 is a variable resistor which is used to set an "excess"
temperature of heated resistor 1 at a typical medium temperature.
Resistor 3 which is not variable, affects the temperature coefficient
of the bridge current.
An additional variable resistor 8 is placed in parallel with the
bridge. The sum of the current through the bridge and through resistor
8 provides a measuring signal indicative of mass flow rate. Resistor
8 does not effect the excess temperature set by resistor 4. The
current flowing through resistor 8 is a function of the resistance
value of heated resistor 1 and therefore its temperature. Resistor
8 influences the temperature dependency of the measuring signal.
A lowering of the resistance value of resistor 8 has qualitatively
the same effect on the temperature dependancy of the signal as lowering
the resistance value of resistor 3. This eliminates the need which
would otherwise exist to reset of the excess temperature by adjusting
resistor 4 thereby saving time.
The temperature compensation occurs in the following manner. The
resistance value of resistor 3 is selected such that each bridge
is overcompensated regardless of the differences between units of
the sensors and of their respective holders. The desired excess
temperature of the heater resistor at a typical medium temperature
is set by adjusting the resistance value of variable resistor 4.
The measurement of the temperature characteristic of the measuring
signal determines the resistance value required for resistor 8 for
temperature compensation. A resetting of the excess temperature
is then no longer necessary, so that the time loss is considerably
reduced.
While this invention has been described in connection with what
is presently considered to be the most practical and preferred embodiment,
it is to be understood that the invention is not limited to the
disclosed embodiment, but, on the contrary, is intended to cover
various modifications and equivalent arrangements included within
the spirit and scope of the appended claims. |