GB2050660A

GB2050660A - Flowmeters

Info

Publication number: GB2050660A
Application number: GB8014642A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-05-15
Filing date: 1980-05-02
Publication date: 1981-01-07
Also published as: ATA360479A; DE3010921A1; AT360777B

Abstract

A flowmeter, particularly for use in calorimetry, measures the rate of flow of a medium by a turbine-wheel flow pick-up device (6) having pulse transmitter system (IG), the rate of pulse output being proportional to the rate of flow. To adapt the flowmeter for microprocessor-controlled data collecting systems with the minimum of hardware and a degree of accuracy limited only by the pick-up device (6), the output line (25) of the pulse transmitter (IG) is connected directly or via a pulse height matching amplifier (V) to at least one input pin (31) of a digital input port (28) of a microprocessor (16), the state of the output line (25) being read by the microprocessor (16) through the digital input port (28) and tested at a frequency at least twice the maximum frequency of the pulse transmitter (IG) to detect changes in that state, the number of changes being counted by a software-type counter to determine the rate of flow. The flowmeter is also used for measuring characteristics of heat transporting systems. <IMAGE>

Description

SPECIFICATION Flowmeters This invention relates to a flowmeter, particularly for calorimetry purposes, of the kind wherein the quantity of flow of a medium per time unit is measured by means of a turbine-wheel flow pick-up device which comprises a pulse transmitter system whereof the number of output pulses per time unit is proportional to the quantity of flow of the medium per time unit.

The general term "turbine-wheel flow pick-up device" here includes all flow pick-up devices wherein a flowing medium drives a turbine wheel of any kind or description, irrespective of whether this may be a single - or multijet paddle wheel, a Woltmann's sail-wheel, or any other kind of sirnilar device.

All conventional, commercially available flowmeters fall basically into two classes, namely those which operate wholly mechanically and those which comprise a pulse transmitter system.

In wholly mechanical flow-meters the revolutions of the turbine wheel are transmitted directly to a mechanical wheeler roller-type counter device which, in view of the mechanical transmission, must be located in the immediate vicinity of the turbine wheel. Consequently this kind of device is not suitable for long range transmission of the flow-meter readings. Flowmeters of the kind comprising a pulse transmitter system, on the other hand, allow in principle a transmission over distances of any desired length of the measured value, projected on the number of pulses within the considered time period. Normally the pulses are counted by means of an electro-mechanical wheeler roller-type counting device.The pulse transmitter system may be wholly electronically operated, e.g. by a periodic pulse generation on the part of the turbine wheel, either by induction or by interception of a light beam directed at a photo-electric cell. However there are also many cases where pulse transmitter systems are provided and where the turbine wheel is adapted either magnetically to actuate a Reed contact, or to drive a contact device, either by direct mechanical means or by means of a magnetic clutch. The periodic making and breaking of the contact can then easily be converted into an electrical pulse sequence.

Strictly speaking the above described conventional flowmeter devices measure primarily the volume of flow. However, in view of the direct relationship M=p.V between flow mass, or quantity M and flow volume V by p, i.e. the density of the flowing medium, every turbine-wheel flow pick-up device can also be used to measure flow mass or quantity. The so-called pulse valency, specifies how many pulses correspond to, for example, one litre or one kg. of the flowing medium.

The above described flowmeters are also used for calorimetry purposes, for measuring amounts of heat, by application of the following physical relation formula:

wherein Q is the amount of heat transported in time interval At=t2-t1 by the heat carrying medium, Q(t) the heat power output of the heat carrying medium at the time t, Cp the specific thermal capacity and p the mass density of the heat carrying medium, V(t) the rate of flow, i.e. the amount of heat carrying medium volume flow per time unit at point tin time, and AT the temperature difference between the feed flow of the heat carrying medium and its return flow.

The product Cp. p is usually expressed as a thermal coefficient k. In practice this multiplication by constant factors is mostly taken care of by appropriate scaling of the data collected. In the case of the heat carrying medium being water-which is most frequently used in practice-mass density and specific thermal capacity have the unit value so that here- numerically speaking-the corresponding multiplication can be dispensed with altogether.

In so far as this description, for the purpose of characterising the metering or measuring process, refers to multiplication processes, this means multiplications which are necessary at least for dimensional reasons in accordance with the basic value equation of the measuring process.

Figure 1 is a schematic lay-out for calorimentric measuring in accordance with the above quoted physical formula. A temperature probe 2 (e.g. a thermocouple) is arranged in the feed pipe 1 to a heat consumer 3 (e.g. a radiator), to ascertain the feed temperature Tv of the heat carrying medium. Another temperature probe 5 is arranged in the return flow pipe 4 which ascertains the return flow temperature TN of the heat carrying medium, said return flow pipe 4 further containing a turbine-wheel flow pick-up 6 with a pulse transmitter system IG providing a pulse output 25 at a frequency which is proportional to the rate of flow, i.e. to the amount of medium flowing through the pipe per time unit V:The temperature probes 2 and 5 and the flow pick-up 6 are connected to an electronic circuit 7 which functions as a calculator unit and multiplies the rate of flow, i.e. amount of flow per time unit, by the temperature difference AT between feed flow temperature Tv and return flow temperature TN. The output of this logic circuit provides a pulse frequency which is proportional to the amount of heat Q which is transported per time unit. As a general rule these pulses are counted by an electro-mechanical counting device of the revolving wheel or roller type. The counter reading-after multiplication by an appropriate scaie factor-represents the amount of heat lost or gained, in the time interval considered, between the temperature measuring points Tv and TN.

The main drawback of this heat measuring method according to Figure 1 is that the electronic circuit 7 must be properly calibrated and adjusted, or tuned, whilst owing to the analog circuits accuracy is comparatively low, especially when the temperature difference AT is small.

In view of the ever increasing importance of automatic data collection in process-measuring technology and with microprocessor-controlled data collecting systems (the so-called data loggers) there is an ever growing demand for computer-adapted, or computerised flowmeter systems, particularly for calorimetry purposes.

The generally increasing awareness of energy consumption has brought the need for exact measuring and analysis of multivalent, especially solar, heating equipment into the foreground of general public interest. Owing to the complexity of such equipment and the great number of measuring points involved accurate thermal baiance assessments can only be obtained at an economically acceptable cost level by means of m icro-processor-controlled data collecting systems.

In principle it is possible to use a lay-out according to Figure 1 also for microprocessorcontrolled data collecting systems and to feed the pulse output 33 of the electronic logic circuit 7 through a counter into the microprocessor.

However, considerably more accurate results are obtained by means of separate pick-ups for flow rate, i.e. amount of flow per time unit V on the one hand and temperature difference AT between feed and return flow temperatures on the other and assigning the necessary work of multiplying V by AT to the micro-processor to ascertain the amount of heat.

In the past basically two methods have been used for feeding the flow rate information provided by the pulse transmitter system to the microprocessor for further processing: According to the first of these known methods, as shown in Figure 2, the pulses issued by the pulse transmitter system of the turbine-wheel flow pick-up 6 are converted by a frequency/voltage transformer 9 into an analog voltage proportional to the amount of flow per time unit, which via multiplexer 11, after conversion by means of an analog-digital converter 12, is in a form suitable for feeding into microprocessor 1 6 via the digital input ports 13, 14, 1 5 and direct feeding to the microprocessor databus 17.The voltage provided by the feed and return flow temperature sensors 2 and 5 is also applied via multiplexer 11 to the input port 1 9 of the analog-digital converter. The free multiplexer input ports 21 or 20 may be connected with further flow pick-ups with associated frequency/voltage transformers and/or feed and return flow temperature sensors. The drawback of this method is that each flow pick-up requires its own frequency/voltage transformer and an additional multiplexer input port. Measuring accuracy is markedly impaired by the frequency/voltage transformer, especially for low rates of flow and/or brief but frequent flow periods.

The second o; the known methods for microprocessor-controlled data collecting systems is illustrated in Figure 3. It differs from the first method according to Figure 2 only with regard to the method of ascertaining flow volume.

According to Figure 3 the output pulses 25 of the pulse transmitter system IG associated with the flow pick-up 6 are counted by means of an electronic counter 26. The digital word output 27 of the counter representing the counter reading or content, is fed via digital input ports IP 22, 23, 24 and databus 1 7 to the microprocessor 1 6 for further processing. This method has the advantage that the error or deviation based on the degree of accuracy of the flow pick-up itself is subjected to virtually no magnification by the subsequent chain of measurement links. On the other hand, it requires a very great outlay in hardware which is a distinct disadvantage as compared with the method according to Figure 2. An electronic counter is needed for every rate of flow which is to be measured, e.g. to eight dicimal digits.For a BCD counter this means 8 x4=32 bits, occupying eight 4-bit input ports. In other words, multiple heat measuring operations carried out by application of this method become highly lineintensive with complex circuitry.

It is the aim of the present invention to provide a flowmeter for microprocessor-controlled data collection systems which for a minimum outlay of hardware achieves a degree of accuracy which is limited only by the accuracy of the flow pick-up itself.

According to the present invention, in a flowmeter of the kind set forth the pulse output line of the pulse transmitter system is connected, either directly or through a pulse-height matching amplifier, to at least on input pin of a digital input port of a microprocessor or of an LSI interface module containing a microprocessor, the state of this output line being read by the microprocessor through the digital input port and tested at a frequency which is at least double the value of the maximum possible flow pick-up pulse frequency, and the number of changes in the state of the output line is determined by a software-type counter whereof the counter reading represents the rate of flow, having due regard to the pulse valency of the medium.

The great advantage of the flowmeter according to this invention resides in the extremely simple hardware and in the fact that its accuracy depends solely on the flow pick-up itself.

Whilst for realising eight flowmeters, each with eight decimal digits, the arrangement according to Figure 3, for example requires 64 4-bit input ports, the new arrangement according to Figure 4 needs only two such 4-bit ports or one 8-bit port.

In a further development of the invention the flowmeter can also be used for measuring amounts of heat by multiplication of the flow volume per time unit of the heat carrying medium by the difference between the heat carrying medium feed and return flow temperatures and by the mass density and specific thermal capacity of the heat carrying medium, in which event at least one of the multiplications necessary to take into account the pulse valency of the medium is carried out by the said microprocessor.

The invention is hereinafter more specifically described with reference to an embodiment schematically illustrated by way of example in Figures 4 and 5 of the accompanying drawings.

Figure 4 shows the general lay-out of the component parts of the flowmeter according to this invention and Figure 5 the associated voltage/time curves.

In Figure 4 the pulse-ouput line 25 of a turbine-wheel flow pick-up 6 with pulse transmitter system IG is connected, either directly or through a pulse-height matching amplifier V, with one input pin 31 of a digital input port IP (e.g. an Intel 8255 Input Port) 28 of a microprocessor 16. The pulse-height matching amplifier V may, if required also be a passive fader (voltage divider). It is also conceivable to connect several inputs of the digital input channel IP.

Likewise, an LSI interface module may be used wherein the digital input channel IP and the riiicroprncessor MP are integrated (e.g. of the kind made and sold by the firm Intel under specification no. UPI-41 A). The state of this output line 25 is read through the digital input channel IP at a frequency provided by the strobe line 29 which is at least double the value of the maximum possible flow pick-up pulse frequency.

The number of changes in the state of the output line is determined by a software-type counter whereof the counter reading represents the rate of flow with due regard to the pulse valency characteristic of the flowing medium.

Figure 5 illustrates the time curves of the various impulses. U1 represents the voltage curve at output 25 of the pulse transmitter system IG, of the flow pick-up 6. In view of the positive logic applied in this case, the upper voltage level corresponds to logic 1 and the iower to logic 0.

U2 represents the pulses of the strobe line 29 which determines the reading, or feed frequency of the digital input port IP by the microprocessor MP. Each change in the voltage state of the output line 25 can be detected by the microprocessor MP by appropriate programming and used for the incrementation of a softwaretype counter. For example, detection of a change of state can occur by an EXCLUSIVE-OR forming between the result of the present and the inmediately preceding test of the state timed by the strobe line 29 frequency as plotted U 3a.

Detection of the change of state is delayed relative to testing of the state by the corresponding calculating time td in the microprocessor MP. Conceivably the microprocessor could also be pre-programmed in such a way that it uses only every change of state from 0 to 1, according to curve U 3b, or every change of state from 1 to 0, according to U 3c, to drive the counter. In both of these last two mentioned cases the number of changes of state detected per second is equal to the pulse frequency of U1, in the case of U 3a on the other hand, the number of changes of state detected is twice as high as the pulse frequency of U1. This must be taken into account in ascertaining or calculating the pulse valency for the software counter.

Claims

1. A flowmeter, particularly for calorimetry purposes, of the kind wherein the quantity of flow of a medium per time unit is measured by means of a turbine-wheel flow pick-up device which comprises a pulse transmitter system whereof the number of output pulses per time unit is proportional to the quantity of flow of the medium per time unit, in which the pulse output line of the pulse transmitter system is connected directly, or through a pulse-height matching amplifier, with at least one input pin of a digital input port of a microprocessor or of an LSI interface module containing a microprocessor, the state of this output line being read by the microprocessor through the digital input port and tested at a frequency which is at least double the value of the maximum possible flow pick-up pulse frequency and the number of changes in the state of the output line is determined by a software-type counter whereof the counter reading represents the rate of flow having due regard to the pulse valency of the medium.

2. A flowmeter according to claim 1 , for measuring amounts of heat by multiplying the flow volume per time unit of the heat carrying medium by the difference between heat carrying medium feed and return flow temperatures, and by the mass density and specific thermal capacity of the heat carrying medium in which at least one of the multiplications necessary to take into account the pulse valency of the medium is carried out by the said microprocessor.

3. A flowmeter of the kind set forth, substantially as described herein with reference to and as illustrated by Figures 4 and 5 of the accompanying drawings.