Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2001-03-30
2003-10-28
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06637264
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a mass flowmeter comprising a hollow conduit of a heat-conducting material for transporting in a determined flow direction a fluid with a mass flow rate to be measured, a first temperature-sensitive resistor element at a first position in thermal contact with this conduit for supplying heat to said fluid, a temperature sensor and measuring and control means connectable to the resistor element and the temperature sensor.
DESCRIPTION OF THE RELATED ART
Such a mass flowmeter is known from the U.S. Pat. No. 4,984,460, wherein a first temperature-sensitive resistor element wound round a conduit tube is incorporated in a first bridge circuit, which further comprises two resistor elements which function respectively as temperature sensor for determining the ambient temperature and as setting resistor for setting a temperature of the conduit tube at the position of the first resistor element through heat dissipation in this resistor element. The known mass flowmeter further comprises a second temperature-sensitive resistor element which is wound round the conduit tube and which is incorporated in a second bridge circuit, which likewise further comprises two resistor elements which function respectively as temperature sensor for determining the ambient temperature and as setting resistor for setting a temperature of the conduit tube at the position of the second resistor element through heat dissipation in this resistor element. The bridge circuits are connected to a control unit which arranges that the difference in temperature between the two resistor elements wound round the conduit tube and the ambient temperature is roughly equal to a value set using the setting resistors. The mass flow rate of a fluid flowing through the conduit tube is determined in the known device from the difference in energy supplied to the first and second resistor elements wound round the conduit tube.
Because use has to be made of two bridge circuits for operation of the mass flowmeter known from the U.S. patent, the difference in temperature at the position of the first and second resistor elements wound round the conduit tube does not always equal the value zero, which results in an inherent limitation to the sensitivity of this mass flowmeter. A further drawback of the use of two bridge circuits, in addition to inherent stability problems, ensues from the relatively large number of components required for these circuits, this having a cost-increasing effect.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a mass flowmeter with a simple principle of operation, wherein it is possible in principle to suffice with one bridge circuit.
It is also an object to provide a mass flowmeter with a greater range than the known mass flowmeter.
It is a further object of the invention to provide a mass flowmeter with which the mass flow rate of a fluid can be measured more quickly and accurately than with a prior art mass flowmeter.
The said objectives are also stated in the European Patent Application EP 0467430A. A known, conventional flowmeter is also described herein—see column 1, lines 33-48. Stated as drawbacks of this known flowmeter are—see column 1, lines 49-56-: ‘Non-linear relationship between the flow and the sensor-output, showing a small inclination and an inflection point in the range of small flow, wherein the flow rate approaches zero, thereby indicating the decreased sensitivity of the sensor in that range’. In column 2, line 31 up to and including column 4, line 5 and referring to the drawings 1-3 accompanying that patent application, there is explained in detail what a conventional thermal-type flowmeter looks like, and in column 4, line 31 and further, referring to
FIGS. 4
,
5
and
6
, there is explained how a thermal-type flowmeter can be constructed to overcome the said drawbacks and be highly sensitive, even in the range of small flows, and be capable of generating a high sensor output over the full range of flow rates, with no inflection point.
These objectives are achieved, and other advantages gained, with a mass flowmeter of the type stated in the preamble, wherein according to the invention the temperature sensor is provided in thermal contact with the conduit at a second position upstream in relation to said first position, and the measuring and control means are adapted to maintain a constant difference in temperature at said first and second positions.
The invention is based on the surprising insight that the mass flow rate of a fluid flowing through a hollow conduit can be derived in reliable manner from the energy which must be supplied at said second position to maintain the difference in temperature at said first and second positions at a constant value.
In an embodiment of a mass flowmeter according to the invention the temperature sensor is a second heat-sensitive resistor element.
In a favourable embodiment the first and the second heat-sensitive resistor element have the same temperature coefficient and these elements are incorporated in a bridge circuit, wherein the resistance of the second resistor element is greater at a determined temperature than the resistance of the first resistor element. Otherwise than in the prior art device, the output signal of the bridge circuit in this embodiment, which is a measure for the mass flow rate to be measured, is wholly independent of the temperature of a medium flowing through the conduit.
In a preferred embodiment the bridge circuit is a wheatstone bridge, the output of which is connected with a feedback loop to the top of the bridge.
In a subsequent embodiment a mass flowmeter with wheatstone bridge comprises a third temperature-sensitive resistor element identical to the first temperature-sensitive resistor element at a third position downstream in relation to said second position in thermal contact with this conduit for supplying heat to the fluid. By choosing identical first and third resistor elements and heating the temperature thereof to the same constant value above the value measured by the temperature sensor, the output signal of the wheatstone bridge is, other than in the preceding embodiments, equal to zero if the mass flow rate for measuring is zero, so that it is not necessary to correct for an offset signal. If the conduit is moreover configured in this embodiment such that heat dissipated in the first and third resistor elements can disappear via convection, conduction or radiation without influencing the value measured by the temperature sensor, the sign of the output signal of the wheatstone bridge moreover gives information about the flow direction of the fluid through the conduit.
In a following embodiment a mass flowmeter with wheatstone bridge comprises an additional temperature-sensitive resistor element identical to the second temperature-sensitive resistor element at a fourth position downstream in relation to said second and first position in thermal contact with this conduit for measuring the temperature of the fluid. This embodiment provides the advantage that the value of the temperature measured by the first temperature sensor can be replaced by an average of the values measured by the first and second temperature sensor, so that inaccuracies in the measured value of the temperature resulting from temperature gradients on the conduit are to a large extent averaged out. If the conduit is moreover configured in this embodiment such that heat dissipated in the first resistor element can disappear via convection, conduction or radiation without influencing the values measured by the temperature sensors, the sign of the output signal of the wheatstone bridge moreover gives information about the flow direction of the fluid through the conduit.
In a very favourable embodiment a mass flowmeter with wheatstone bridge comprises a third temperature-sensitive resistor element identical to the first temperature-sensitive resistor element at a third position downstream in relation to said second position in thermal contact with this conduit for
Boer Hendrik Jan
Jouwsma Wybren
Lötters Joost Conrad
Berkin B.V.
Lefkowitz Edward
Thompson Jewel V.
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