GB2212277A - Gas flow meter - Google Patents

Abstract

A gas mass flow meter comprises means for measuring mass flow at low flow rates and velocity at high rates with an overlap region in which the outputs from the two means are combined to provide the gas density. The latter is then combined with the velocity to give mass flow at high rates. A thermal type flow meter is used to measure low flow rates, a vortex flowmeter to measure velocity. The outputs are combined in a microprocessor. Temperature may be monitored and used to apply a correction without waiting for flow in the overlap region to re-occur. A proportion of the total flow may be passed through the measuring means which may form part of a domestic gas meter.

Description

GAS FLOW METERS This invention relates to gas flow meters particularly, but not exclusively, to gas meters suitable for use in measuring the consumption of fuel gas, such as natural gas, supplied to a building from a gas distribution system.

The present invention stems from work aimed at producing a compact domestic gas meter capable of monitoring a wide range of gas flow rates, but the invention is not confined to domestic gas meters.

According to one aspect of the invention a gas meter comprises first measuring means capable of measuring mass flow rate over a first, low flow range of gas flow, and second measuring means capable of measuring flow velocity over a second, high flow range of gas flow, the first and second ranges overlapping with each other in an overlap region, or having a common upper and lower limit respectively, and means for computing an amount representative of the gas density at a predetermined flow rate in said overlap region, or at said common limit, said amount being used as a basis for calculating mass flow rate in the second range from a measurement of flow velocity in the second range.

Thus, we use a first technique to measure mass flow rate in the first range, and a second technique to measure gas velocity in the second range, and the measurement of gas velocity is converted to a value of mass flow by basing the figure for gas density on a density figure computed from the measurements in the overlap region of both mass flow and velocity. The use of two techniques enables a wide overall range of gas flows to be measured.

Of course, we do not exclude the possibility that the figures for mass flow may be presented as volumes at standard conditions, but most preferably an indication of mass flow is provided. In the case of a domestic gas meter a display 'of the mass flow measurement is preferably provided, conveniently a liquid crystal display.

One advantage of this method of metering is that it can take some account of changes in gas density which result from changes in gas composition.

Preferably the arrangement is such that an up to date value for the supply gas density is computed each time ' that the flow rate coincides with said predetermined flow.

Preferably the first technique for measuring mass flow is a thermal technique whereby a proportion of the metered flow is subjected to a heating means, and the resulting rise in temperature of the gas is measured by suitable temperature monitoring means.

In a convenient arrangement a heating element is positioned in a meter passage through which is caused to pass a proportion of the metered flow, and the temperature of the gas in the meter passage is measured both upstream and downstream of the coil in order to measure the rise in temperature of the gas caused by the coil. The power supplied to the coil is arranged to be kept constant.

The element is preferably connected to a battery supply in the specific case of a domestic gas meter.

We consider that two D size batteries, such as Every Ready Gold Seal (Trade Mark) can provide a suitable compact power source for the coil, and can also be used to power any associated electronics, as will be discussed hereafter.

We prefer to use thermistors as the temperature sensing elements responsive to the upstream and downstream temperatures, although other accurate, stable temperature measuring means could be used. Such thermistors need to be sealed against moisture to ensure long term stability.

An advantage of using such a thermal mass flow measuring technique for measuring low flow rates is that one of the temperature sensing means can be used to provide a temperature measurement which can sometimes be utilised as the basis for a temperature correction factor in computing the mass flow measurement in the high flow range.

A further advantage of the thermal mass flow measuring technique is that the measuring unit can be made very compact, since the heating element and temperature sensors can be accommodated in a small tube.

In a preferred embodiment we measure gas velocity in the second range by a vortex shedding technique.

When a bluff body is positioned in a fluid stream vortices are shed alternately by opposite edges of the bluff body, and the frequency of generation of vortices is generally proportional to that fluid velocity.

Accordingly by measuring the frequency of vortex production the fluid velocity can be determined.

Various ways have been proposed for detecting the vortices such as ultrasonics, piezo resistors or piezo ceramics. Whilst any of these ways may be employed, we prefer to use piezo ceramics.

The invention will now be further described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic cross-sectional view of the heating and temperature sensing elements of a thermal mass flow meter device arranged in a meter passage of a domestic gas meter unit in accordance with the invention; Figure 2 is a graph showing the rise in temperature in the gas flowing through the meter passage of Figure 1; Figure 3 is a graph showing how the density of different gas compositions varies with temperature at constant pressure; Figure 4 shows schematically an arrangement for measuring gas velocity using a vortex shedding technique; and Figure 5 is a block diagram of the electronic circuit used with the sensor of Figure 4.

Figure 1 shows a meter passage 1 through which a known proportion of the total gas flowing through the inventive meter unit is caused to pass. A suitable heating element 2 is positioned in the passage 1 and is arranged to be fed with a constant current by a suitably stabilised current supply source fed by the two D-size batteries. Upstream and downstream temperature sensors are provided by thermistors 3 and 4 respectively to monitor the gas temperature which is plotted in Figure 2 using as abscissa the distance in Figure 1 along the meter passage 1.

The Equation E = MsCh T applies where: E = rate of electrical energy input M = mass flow rate C = specific heat capacity of the gas A T = temperatures difference Thus the mass flow rate M is inversely proportional at T as measured by the difference in readings from thermistors 3 and 4 when E is maintained constant.

Whilst in principle this technique for measuring mass flow can be extended to high flow rates this places heavy demands on the energy requirements to power heater 2, bearing in mind that there is a practical lower limit to the temperature difference AT that can be measured accurately with the thermistors 3, 4, and that we are using a compact battery supply for the heater 2.

We therefore use a different flow measuring technique for high gas flow rates. We use a velocity measuring technique. It will be appreciated that M = pV where p= gas density V = gas velocity Thus, providing that the density p of the gas is known it is possible from measurements of the gas velocity to compute the mass flow rate M. In accordance with the invention, we arrange that in an overlap region of gas flow in which the thermal and velocity measuring techniques are both valid we use both techniques in order to obtain a measure of the gas density, which measure can then be used at a higher gas flow as a basis for computing the mass flow rate from velocity measurements Thus, we use the equation E = (pV) CAT to determine p at a predetermined mass flow or predetermined velocity in the region of overlap. This value of p at the predetermined measuring point we shall term po.The computed value of po is stored in a suitable memory and preferably it is arranged that the stored value of po is updated each time that the varying flow rate becomes equal to the predetermined amount. This value of po can then be used when the flow rate enters the higher region as the basis for converting velocity measurements to mass flow measurements.

Figure 3 shows curves of how the gas density p varies with temperature T for three different gas compositions a, b and c. It will be seen that, although the gas composition will not normally be known or measured, a measurement of po at the measuring point combined with a measurement of the gas temperature To will enable a correction to be made to the value of p currently being used in the velocity to mass flow conversion calculation, to take account of any change in the temperature T from the measured value of the temperature To of the gas at the time at which the measurement of po was last made and stored.

Therefore, in order to enable such a correction for temperature change to be made we measure the temperature of the gas passing through the velocity metering passage. Conveniently the measurement is made using one or both of the thermistors 3, 4 from the thermal metering unit. It will be appreciated, however, that the gas which flows through the meter passage in which a velocity measurement is made need not also pass through the meter passage 1. That is, the two meter passages in which measurements are made need not be in series with each other, rather the two meter passages may be arranged in parallel with each other.

Although the precise composition of the gas will not generally be known the shapes of the curves, such as a, b, c, over the range of temperature T encountered in practice will be sufficiently similar to enable a correction to be made to the value of gas density p starting from the combination of values po, To (shown in Figure 3 for the composition b), in the circumstance that the flow rate has been maintained at a relatively high level over a substantial period so that it has not been possible recently to update the stored value of po.

The correction to po necessitated by a temperature change from To can be achieved by storing the general shape of the curves shown in Figure 3.

Either a series of curves may be stored, or a single curve shape may be stored and the most appropriate curve can be generated based on the measurements of po, To.

It will be appreciated that one advantage of using the thermal technique described for low flow measurements is that temperature sensors 3, 4 are available for use in correcting the conversion of velocity-measurements to mass flow at high flow rates.

Since the passage 1 can be of small dimensions, the thermal device can be accommodated in a meter of small size.

In order to measure gas velocity in the upper range of gas flow we prefer to use a vortex shedding technique. An arrangement for this is shown schematically in Figure 4. A bluff body in the form of a strut 5 extends transversely across a tube 6 defining a metering passage 7 through which is caused to flow a proportion of the total gas flow from the pressure governer associated with the meter. A piezo-electric pressure sensor 8 in the form of a plate is positioned downstream of the strut to detect the pressure changes in the gas associated with the vortices shed alternately from opposite edges of the strut.

Plate 8 comprises two layers of electrically polarized ceramic sandwiching a thin strip of brass, and bending of the ceramic causes stress induced charge generation. Due to the alternating pattern of vortices, the ceramic is caused to oscillate to generate an alternating voltage which is monitored electrically. A block diagram of the monitoring circuit connected to the output leads of the piezo plate 8 is shown in Figures 5.

The various functions of the gas meter are conveniently implemented by a dedicated microprocessor with associated integrated circuit units.

Such circuitry is used to control and analyse the thermal metering unit of Figure 1, to analyse the signals from the velocity metering unit of Figure 4, to control the sampling of measurements to compute po at the measuring point, storing po, correcting the working value of po for any changes in temperature T, and outputting the computed values of mass flow.

A suitable display such a liquid crystal display is preferably provided for displaying the computed values of mass flow.

The thermal and vortex devices described facilitate a compact design which is rugged, has a low power consumption, a long maintenance free life and is capable of providing mass flow measurements over a wide range of flows.

Whilst we have described a unit which is intended for use as a domestic gas meter, a modified unit employing the same principles is envisaged for metering of other fuel gas supplies. Indeed, the invention can be used to measure the flow of gases other than fuel gases.

Claims (8)

1. A gas meter comprising first measuring means capable of measuring mass flow rate over a first, low flow range of gas flow, and second measuring means capable of measuring flow velocity over a second, high flow range of gas flow, the first and second ranges overlapping with each other in an overlap region, or having a common upper and lower limit respectively, and means for computing an amount representative of the gas density at a predetermined flow rate in said overlap region, or at said common limit, said amount being used as a basis for calculating mass flow rate in the second range from a measurement of flow velocity in the second range.

2. A gas meter as claimed in claim 1 comprising a display means adapted to display the mass flow measurement.

3. A gas meter as claimed in claim 1 or claim 2 in which the arrangement is such that an up-to-date value for the supply gas density is computed each time that the flow rate coincides with said predetermined flow.

4. A gas meter as claimed in any of the preceding claims in the which means for measuring mass flow in the first range uses a thermal technique whereby a proportion of the metered flow is subjected to a heating means, and the resulting rise in temperature of the gas is measured by suitable temperature monitoring means.

5. A gas meter as claimed in claim 4 in which a heating element is positioned in a meter passage through which is caused to pass a proportion of the metered flow, and the temperature of the gas in the meter passage is measured both upstream and downstream of the coil in order to measure the rise in temperature of the gas caused by the coil, the power supplied to the coil being arranged to be kept constant.

6. A gas meter as claimed in claim 5 suitable for domestic use, in which the element is connected to a battery supply.

7. A gas meter as claimed in any of the preceding claims so arranged as to measure gas velocity in the second range by a vortex shedding technique, in which a bluff body is positioned in a gas stream whereby vortices are shed alternately by opposite edges of the bluff body, and the frequency of generation of vortices is measured and used as an indication of the gas velocity.

8. A gas meter substantially as described with reference to the accompanying drawings.