GB2042190A - Flow monitoring unit - Google Patents

Abstract

A flow monitoring unit comprises a venturi tube 10, the output of which is fed to transducing and linearising means to provide an electrical output linearly proportional to the flow rate. As shown, the pressure difference generated by the venturi tube 10 is supplied to a transducer 20 the output of which is fed to a lineariser 30 comprising a diode/resistor chain. The output of the lineariser is supplied to a voltage- controlled oscillator 40 which produces a frequency proportional to the flow rate. In a second embodiment (Figures 2 4 not shown) a non-linear variable resistor constitutes said transducing and linearising means. The resistor is short-circuited by a movable column of mercury in a cylinder, the ends of which are coupled to the venturi tube. <IMAGE>

Description

SPECIFICATION Flow monitoring unit The present invention relates to systems for monitoring flow rates and in particular the flow rates of heat transfer fluids in heating systems.

In domestic heating systems and especially in district heating systems it is often necessary to measure the flow of the heat transfer fluid which is usually water. This enables the amount of heat supplied to be calculated and thus the cost to the consumer to be determined. In prior art systems the flow is measured by positive displacement rotor meters since such meters have moving parts, however, they suffer from the disadvantage that breakages and malfunctions are liable to occur.

According to the present invention there is provided a flow monitoring unit comprising a venturi tube arranged to be coupled to a flowing fluid and to generate a pressure difference dependent on the rate of flow, and transducing and linearising means coupled to the venturi tube to generate an electrical output substantially linearly proportionate to the flow rate.

There may be provided a transducer followed by a separate linearising element. In this case the linearising element is preferably constituted by a diode/ resistor chain.

Alternatively said transducing and linearising means may be constituted by a variable resistor.

The unit may also comprise an oscillator connected to the output of the linearising means and arranged to produce a frequency proportional thereto.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings of which: Figure 1 shows a diagrammatic view of the first embodiment of a flow monitoring unit according to the present invention; Figure 2 shows a diagrammatic view of a second embodiment of the invention; and Figures 3 and 4 show part of the embodiment of Figure 2 at different flow rates of fluid passing through the venturi.

Referring now to Figure 1 a venturi tube 10 is coupled to a pipe in which the flow m is to be monitored. The pipe may be standard hot water copper pipe. The venturi tube may monitor flows of up to ten gallons per minute with a turn-down ratio of 10:1 generating a pressure difference of typically 40 cm water gauge at maximum flow. The pressure difference is applied to a pressure transducer 20 which generates an electrical output dependent on the pressure difference. The transducer comprises a stainless steel diaphragm 21 with a strain gauge 22 and a strain gauge amplifier 23, and gives an output of above one volt at maximum pressure difference.

Since the pressure difference produced by venturi tube is not linear but varies generally with the square of the flow rate m, the signal VI generated by transducer 20 is not linear; that is Vl = F(m).

Accordingly the transducer is connected to a lineariser element 30 which operates to generate internally a function (F) -1 such that the overall transfer function of elements 10,20 and 30 is unity and the voitage output V2 is linearly proportional to flow rate m. The function F is determined by bulk calibration. The function (F) ~1 is generated internally by a diode/resistor chain.

The output of lineariser element 30 is connected to an oscillator 40 which produces a frequency linearly proportional to the output of the lineariser element.

Thus the above embodiment provides a system for monitoring flow with no moving parts and producing an output linearly dependent on the flow rate.

Various modifications may be made to the above embodiment. Transducer 20 may be replaced by any suitable pressure transducing arrangement for example a bellows/solenoid type. In another modification it is possible for the linearising function (F) -1 to take into account small non-linearities in the voltage controlled oscillator 40.

Referring now to Figure 2 there is shown a second embodiment of the present invention. The flow monitoring unit comprises a venturi tube 10 to which a cylinder 52 is connected at each end by respective tubes 53 and 54. The cylinder contains a quantity of mercury which is represented in the drawings by the shaded areas 55. A register 56 extends through the cylinder 52 along its longitudinal axis thereof and the part of the resistor surrounded by the mercury 55 is short-circuited. The variable resistor is connected to an oscillator circuit 40. Figure 3 shows the level of mercury at a flow rate A and Figure 4 shows the level of mercury at a different flow rate B. The flow rate B may be arranged to be higher or lower than the flow rate A. At a flow rate m, the resistor has a resistance Ram and the oscillator circuit 40 has a frequency of fam as indicated in Figure 2.

The flow meter will normally be arranged so that the flow rate B is higher than the flow rate A. In this case, a unit increase in the flow rate will generate an increased pressure difference in the flow meter and hence an increased head of mercury 55 in the cylinder 53 which will produce a unit change in the resistance of the resistor 56. It is clear that a special geometric arrangement of the resistor is needed to accommodate the fact that the height of mercury does not vary linearly with fluid flow.This is ensured by the formula:

where RA iS the overall resistance of the resistor when no part of it is surrounded by mercury, R5 jS the resistance of the resistor at flow rate B, hA is the head of mercury above a datum line corresponding to zero flow at flow rate A, and h5 is the head of mercury above said datum line at flow rate B.

The position of the datum line is arbitrary and one could arrange for the cylinder to be full of mercury at zero flow and for the resistor to be increasingly exposed and hence its resistance increased (proportional to flow rate) with increasing flow rate.

Since the output of a venturi meter generally follows a square law, a resistor is needed whose resistance is substantially proportional to the square root of the distance from one end, as defined by the free surface of the mercury column reiative to the datum line. Thus in this embodiment the resistor 56 in cylinder 52 constitutes both transducing and linearising means.

The resistor 56 which may be profile machined from stock, film evaporated or etched, specially wound or screen printed and fired, forms part of an electronic circuit in conjunction with the oscillator 40 such that the clock frequency produced is proportional to the resistance appearing at the terminals of the resistor.

Instead of a mercury column any contact element may be used which is relatively movable to the resistor to selectively short-circuit parts thereof and the position of which is dependent upon the output of the venturi tube. If this contact element introduces additional non-linearities, these may also be compensated by the resistor.

The flow monitoring units may be used in other applications than heating systems. For example they may be used to monitor the flow of fluids in chemical plant or in food manufacturing apparatus.

Claims (12)

1. Aflowmonitoring unit comprising aventuri tube arranged to be coupled to a flowing fluid and to generate a pressure difference dependent on the rate of flow, and transducing and linearising means coupled to the venturi tube to generate an electrical output substantially linearly proportional to the flow rate.

2. A flow monitoring unit according to claim 1, wherein there are provided separate transducing and linearising elements.

3. Aflow monitoring unit according to claim 2, wherein the linearising element is constituted by a diodefresistor chain.

4. A flow monitoring unit according to claim 3, wherein the output of the linearising element is connected to an oscillator which produces a frequency substantially linearly proportional to said flow rate.

5. A flow monitoring unit according to any of claims 2,3 or 4, wherein the transducing element comprises a diaphragm and strain gauge means.

6. Aflow monitoring unit according to any of claims 2,3 or 4, wherein the transducing element comprises bellows and a solenoid.

7. A flow monitoring unit according to claim 1 comprising a variable resistor which constitutes said transducing and linearising means.

8. A flow monitoring unit according to claim 7, wherein said resistor is short-circuited by a relatively movable contact element the position of which is dependent upon the pressure difference generated by said venturi tube.

9. Aflow monitoring unit according to claim 8, wherein said movable contact element is constituted by a column of mercury in a cylinder, the ends of cylinder being coupled to the venturi tube.

10. Aflowmonitoring unit according to any of claims 7, 8 or 9, wherein the resistance of the resistor is substantially proportional to the square root of the distance from one end.

11. A flow monitoring unit according to any of claims 7 to 10 wherein the resistor is connected to an oscillator which produces a frequency substantially linearly proportional to said flow rate.

12. Aflowmonitoring unit substantially as herein described with reference to Figure 1 or Figures 2,3 and 4 of the accompanying drawings.