Abstrict A fluid flow meter using a pair of temperature-variable resistance
elements as detectors. Both elements are exposed to the fluid flow
and supplied with a series of current pulses. The pulses supplied
to one element, the reference element, are of very short duration
so as to have no appreciable heating effect; the pulses supplied
to the other element are of longer duration and are controlled so
as to maintain this element at a fixed temperature differential
above the other element, which remains at fluid temperature. The
pulse width of the longer duration pulses is a measure of the flow
rate of the fluid. Since the relationship between heat dissipated
in the element and fluid flow varies with fluid temperature, the
voltage pulses across the reference element are used as a measure
of fluid temperature to compensate for this variation in calculating
the exact fluid flow rate.
Claims What is claimed is:
1. A fluid flow meter comprising:
means defining a path for fluid;
a pair of detector elements exhibiting temperature-variable resistivity
and positioned to contact fluid in said path;
a pair of constant current sources each being in series with one
of the detector elements via a switching means;
a pair of peak voltage detectors connected one to each detector
element to sense the maximum voltage across that element;
a differential amplifier having inputs connected to each peak voltage
detector;
a pulse generator connected to the output of the differential amplifier
producing a series of pulses having a pulse width proportional to
the difference in voltage across the peak voltage detectors;
means coupling the output of the pulse generator to one of the
switching means;
a monostable circuit connected to the output of the pulse generator
to provide a series of short pulses;
means coupling the output of the monostable circuit to the other
of said switching means;
whereby the width of pulses from said pulse generator is a measure
of the flow rate of fluid.
2. A meter as set forth in claim 1 wherein said detector elements
are identical.
3. A fluid flow meter as in claim 2 further including processing
means coupled to the output of said pulse generator to provide a
display of said flow rate.
4. A fluid flow meter as in claim 3 further including a voltage-to-frequency
converter connected to the peak voltage detector which responds
to said other detector element, the output of said voltage-to-frequency
converter being connected to said processing means whereby the peak
voltage detector supplies a measure of the fluid temperature to
compensate for variations in the displayed result.
Description This invention relates to a fluid flow meter and, in particular,
a fluid flow meter of the non-obstructive thermodynamic type in
which the energy necessary to replace the heat lost from a transducer
in contact with the fluid is measured as an indication of the fluid
flow.
Such flow meters are described in Canadian Patent No. 1187719
issued May 28 1985 in the names of Petrov and Goldstein. This patent
uses a pair of nickel foil sensors mounted flush with the inside
wall of a pipe to be contacted by the fluid flowing through the
pipe. A third sensor, used as a heater is mounted close to but electrically
insulated from one of the sensors. The pair of sensors are connected
in a bridge circuit and a closed loop completed from the bridge
output via an amplifier to the heater to maintain one of the sensors
at a fixed temperature differential above the temperature of the
other. At zero flow rate of the fluid the bridge is balanced. As
fluid flows through the pipe, heat is drained away from one of the
sensors and the additional power required to bring the bridge back
into balance is a measure of the flow rate. Since the relationship
between the heat dissipated in the sensor and fluid flow varies
with temperature of the fluid a separate measurement of fluid temperature
is made and supplied to a processor to provide appropriate compensation
to the measured flow rate to provide an accurate indication of the
flow rate.
Although this known flow meter provides a useful and accurate flow
meter with a fast response time it has a disadvantage that it is
analog in operation and requires the provision of separate analog-to-digital
converters for signal processing. Further, the superimposed heater
and sensor element must be accurately aligned and results in a slower
response time than using the sensor by itself.
The present invention relates to a fluid flow meter of the general
type discussed above in which the detecting elements are energized
by pulses of current and the output signal is in digital form for
immediate processing. It is of compact form, economical in construction
and has a fast response time of the order of 50 .mu.sec.
Specifically, the invention relates to a fluid flow meter, comprising:
means defining a path for fluid and a pair of elements exhibiting
a temperature-variable resistance positioned to contact fluid in
the path. A separate current supply is provided for each element,
the current supply to one element providing sufficient energy to
maintain it at a higher temperature than the other element. Means
are provided to compare the voltage across each element and to control
the energy supplied to the one element by its current supply to
maintain the one element at a fixed temperature difference above
the other element, whereby parameters of the current give a measure
of the flow rate.
In its method aspect, the invention relates to a method of determining
fluid flow through a pipe comprising: positioning a pair of elements
having temperature-variable resistance to contact fluid flowing
in the pipe; supplying separate trains of current pulses to the
elements; varying the width of pulses in the current supplied to
one of the elements to maintain its temperature a fixed amount above
the temperature of the other element; and utilizing the pulse width
of the current supplied to the one element as a measure of the flow
rate of the fluid.
A specific embodiment of the invention will now be described in
conjunction with the accompanying drawings, in which:
FIG. 1 is a cross-sectional view of the flow meter; and
FIG. 2 is a schematic diagram of the control circuit.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The structure of the flow meter 10 is shown in FIG. 1. A block
11 of material such as epoxy, has two identical nickel foil resistance
temperature detector elements 12 and 13 mounted on a polyamide film
on one side thereof. The block is adapted to be inserted in a pipe
wall so that the detector elements contact the fluid flowing along
the inner pipe wall without obstructing it and with shoulder 14
providing a sealing surface. Terminals 15 and 16 are connected to
one edge of each element with the opposite edge of each element
being grounded.
The general operation of such thermodynamic detectors is that sufficient
power is supplied to detector element 12 to maintain a constant
temperature differential between detector element 12 and detector
element 13. Detector element 13 is always maintained at the temperature
of the fluid. As the liquid flow increases, more power is necessary
to maintain this differential, due to the greater amount of heat
lost from detector element 12 to the liquid. The power dissipated
in detector 12 is, thus, a measure of the flow rate.
FIG. 2 shows the drive circuitry of the flow meter of the present
invention. Element 12 is connected to a constant current source
17 by means of a fast acting switch 28. Switch 28 is controlled
by a series of pulses from a pulse width modulator 31 via conductor
21. Element 13 is connected to a further constant current source
18 via a further fast acting switch 29. Switch 29 is controlled
by a series of narrow pulses from a monostable circuit 20. The constant
current sources are commercially available semiconductor circuits
and the fast acting switches can be of any conventional type.
Thus, element 13 is supplied with a series of narrow current pulses
and element 12 is supplied with another series of current pulses,
which need not be at the same repetition rate. The pulses on conductor
21 have a varying pulse width which, generally, will be greater
than the pulse width of the pulses from monostable circuit 20. Each
current pulse supplied to elements 12 and 13 produces a voltage
pulse across the detector element which is sensed by two peak detector
circuits. The peak detector circuit for element 12 consists of diode
22 and a parallel circuit of capacitor 23 and resistor 24 connected
to ground. Similarly, the peak detector for element 13 consists
of diode 25 and a parallel circuit of capacitor 26 and resistor
27 also connected to ground. The peak voltage across each of the
elements 12 and 13 appears on capacitors 23 and 26 and is compared
in differential amplifier 30 which provides a difference voltage
indicative of the difference in the peak voltage readings across
elements 12 and 13. The difference voltage from amplifier 30 is
used to control a pulse width modulator 31 which, in turn, supplies
the variable width pulse train on conductor 21.
The pulses from monostable circuit 20 are sufficiently narrow to
avoid any significant heating of element 13. The wider pulses on
conductor 21 do provide a significant heating effect on element
12 and, hence, the amplitude of the peak voltage across element
12 differs from that across element 13 and the closed loop including
differential amplifier 30 functions to maintain this voltage at
a constant difference.
As discussed in Canadian Patent No. 1187719 the power required
to maintain a constant temperature difference between elements 12
and 13 is not only a function of fluid flow, but also a function
of fluid temperature. A measure of fluid temperature is the voltage
across capacitor 26. This voltage is supplied to a voltage-to-frequency
converter 32 to provide an output signal which may then be used
to obtain a temperature-compensated measurement of the difference
in temperature, the difference in temperature being representative
of fluid flow.
In the preferred embodiment the signal from voltage to frequency
converter is fed to a microprocessor 33 which uses this information,
as well as the basic flow information in the pulse-width modulator
output, to determine fluid flow. The duty cycle is determined by
measuring pulsewidth with the help of an internal or external clock.
This is in contrast to known circuits using DC analog drive for
element 12 which require an analog to digital conversion prior to
flow computation.
The circuit of this invention has the additional advantage that
the power dissipated in element 12 is linearly related to the duty
cycle of I.sub.1 :
where D is the duty cycle of the output of the pulse width modulator,
I.sub.1 and R.sub.A (resistance of element 12) are assumed constant.
The assumption that the resistance R.sub.A is constant is valid
under conditions of steady state, constant flow temperature and
very large loop gain. Previously known circuits using variable amplitude
current drive to heat element 12 operate by deriving the square
of the measured current to obtain a measure of power, and hence
flow.
Various changes in the disclosed embodiment will be clear to those
skilled in the art. Typically, the temperature difference between
the sensors is maintained at 5.degree. C. giving an output signal
even when the flow rate is zero. |