DESCRIPTION

The present invention relates to a method for calibrating a gas meter and a gas meter calibrated according to the method.

Gas meters are commonly used to measure the flow rate of gas flowing through the cross-section of a tube, in which they are installed so as to form the total volume of gas which has flowed through the tube over time.

In this context, there is perceived the need to provide precise and reliable measurements with respect to the tolerance ranges concerning the measurement errors allowed by the standards applicable with respect to legal metrology.

To this end, each gas meter is typically specifically configured to measure the flow rate exclusively of a gas or an admixture of gases having a predetermined composition so as to minimize the measurement errors relating to this gas or gas admixture.

The main disadvantage of this prior art is that it is not possible to use a single gas meter for measuring a wide variety of gas or gas admixtures, as required, however, by practical and commercial requirements .

This problem is experienced by each combustible gas user: residential, commercial, industrial, etc. Various attempts have been made to solve this problem by seeking to contain the measurement errors within the limits fixed by the applicable standards, such as, for example, the Directive 2014/32/EU (Measuring Instruments Directive) , more commonly known in the metric/legal context by the acronym "Directive MID".

The problem has evident difficulties, further resulting from the need to limit the number of additional components which affect the cost and the complexity of the gas meters.

The technical problem addressed by the present invention is to provide a method for calibrating a gas meter and a gas meter calibrated according to the method which is structurally and functionally configured to at least partially overcome one or more of the disadvantages set out with reference to the indicated prior art .

In the context of this problem, an object of the present invention is to provide a gas meter which allows the flow rate of any admixture of a liquefied petroleum gas to be measured, regardless of the percentage of butane and/or propane and/or other component of the admixture. Another object of the present invention is to provide a gas meter which operates in a reliable and secure manner within a wide range of temperatures and flow rates of the gas. This problem is at least partially solved and these objects are at least partially achieved by the invention by means of a method for calibrating a gas meter and a gas meter which is calibrated according to this method which are constructed according to one or more of the appended claims.

It will be appreciated that the method for calibrating a gas meter according to the present invention comprises: determining an error of the gas meter in relation to a measurement of a flow rate of butane and an error of the gas meter in relation to a measurement of a flow rate of propane with respect to a reference flow rate; calculating a mean error between the error in relation to the measurement of the flow rate of butane and the error in relation to the measurement of the flow rate of propane; determining correction constants as a function of the mean error; and assigning the correction constants to the gas meter in such a manner that the gas meter is calibrated in order to supply a measurement of a flow rate of a liguefied petroleum gas which contains any percentage of butane and/or propane flowing through the gas meter.

It may be noted that, in this context, the term "mean error" is advantageously intended to be understood to mean a mean between the error relating to the measurement of the flow rate of butane and the error relating to the measurement of the flow rate of propane. It will be understood that, in some embodiments, the mean can be replaced by a different position index, such as, for example, the median. It will be appreciated that, advantageously, the mean error between the error relating to the measurement of the flow rate of butane and the error relating to the measurement of the flow rate of propane allows the meter to be calibrated in an intermediate situation between the two limit cases of an admixture of LPG which is composed substantially only of butane or substantially only of propane .

In this context, it must be observed that, over the entire range of flow rates to be measured, the difference between the measurement errors of the flow rate of butane and the corresponding measurement errors of the flow rate of propane always remains less than the value of the tolerance range allowed by the applicable standards for the measurement errors. In particular, on average, the difference between the mean error relating to butane and the mean error relating to propane is substantially 2%. Therefore, the mean between the mean error for butane and the mean error for propane is maintained equidistant from the mean error for butane and the mean error for propane at substantially 1%. In this manner, it is ensured that, for any percentage of butane and/or propane in the LPG admixture to be measured, the meter is calibrated so as to provide a measurement of the flow rate with an error within the tolerance range allowed by the applicable standards.

Preferably, there is provision for determining an error of the gas meter in relation to a measurement of a flow rate of air with respect to the reference flow rate, and determining the correction constants as a function of a difference between the error in relation to the measurement of the flow rate of air and the mean error .

Advantageously, there is provision for determining the error in relation to the measurement of the flow rate of butane, the error in relation to the measurement of the flow rate of propane and the error in relation to the measurement of the flow rate of air at a plurality of reference flow rates so as to determine the correction constants uniquely for each of the reference flow rates.

According to another advantageous aspect, the plurality of reference flow rates comprise six reference flow rates. It will be understood that the number of reference flow rates may be different from six, may be at least six or may be exactly six.

In some preferred embodiments, the plurality of reference flow rates comprises: a first reference flow rate, preferably equal to substantially 0 m ³ /h; a second reference flow rate, preferably equal to substantially 0.016 m ³ /h; a third reference flow rate, preferably equal to substantially 0.25 m ³ /h; a fourth reference flow rate, preferably equal to substantially 0.6 m ³ /h; a fifth reference flow rate, preferably equal to substantially 2.4 m ³ /h; and a sixth reference flow rate, preferably equal to substantially 6 m ³ /h.

In one aspect, there is provision for determining the error in relation to the measurement of the flow rate of butane and the error in relation to the measurement of the flow rate of propane at a plurality of reference temperatures so as to determine the correction constants uniquely for each of the reference temperatures .

Advantageously, the plurality of reference temperatures comprise three reference temperatures. It will be understood that the number of reference temperatures may be different from three, may be at least three or may be exactly three.

In some preferred embodiments, the plurality of reference temperatures comprise: a first reference temperature which is preferably between -30°C and -20°C, even more preferably equal to substantially -25°C; a second reference temperature which is preferably between +15°C and +25°C, even more preferably equal to substantially +20°C; and a third reference temperature which is preferably between +45°C and +65°C, even more preferably equal to substantially +55°C.

It is thereby possible to calibrate the gas meter so that it is operational in a continuous temperature range which extends from a minimum temperature (for example, -25°C) to a maximum temperature (for example, +55°C) .

According to another advantageous aspect, there is provision for determining the error in relation to the measurement of the flow rate of air at a reference temperature.

In one aspect, this reference temperature is unique. Preferably the reference temperature for measuring the flow rate of air is between +15°C and +25°C, even more preferably it is equal to substantially +20°C. It will be appreciated that, advantageously, the value of the reference temperature for measuring the flow rate of air corresponds to the value of the second reference temperature for measuring the flow rate of butane and propane.

Advantageously, there is provision for determining the correction constants uniquely for a plurality of reference flow rates and reference temperatures, compiling lookup tables for the correction constants determined in this manner as a function of the reference flow rates and reference temperatures, and assigning to the gas meter the lookup tables in such a manner that they are accessible for automatic consultation by the gas meter in the course of the measurement of a flow rate of gas flowing through the meter.

It may be noted that, in this context, the term "lookup table" is intended to be understood to be an information data structure, generally an array, which allows the association of any permissible combination of data being input (ranges of interest of flow rate and temperature) with a corresponding configuration of data being output (the correction constants) .

It will be appreciated that the lookup tables allow the replacement of runtime calculation operations with a simpler automatic consultation operation by the meter, advantageously reducing the processing times. It must be observed that, advantageously, there is provision for permanently assigning the correction constants to the gas meter. This solution prevents any change of the correction constants, preventing any accidental modifications of or attempts to tamper with the gas meter following the calibration of the meter itself.

In one aspect, the present invention relates to a gas meter which is calibrated to supply a measurement of a flow rate of a liquefied petroleum gas which contains any percentage of butane and/or propane flowing through the meter itself according to the abovementioned method.

In one aspect, the gas meter comprises a sensor for measuring an instantaneous flow rate of the gas flowing through the meter itself

It may be noted that, in this context, the term "instantaneous flow rate" is intended to be understood to mean the value of the flow rate which is read by the measurement sensor and which is intended to be corrected subsequently by means of a compensation algorithm of the meter.

Advantageously, the gas meter further comprises a processing unit which is operationally connected to the sensor for measuring the instantaneous flow rate and storage means which are accessible to the processing unit. The storage means keep the correction constants stored and comprise instructions which can be carried out by the processing unit which, when they are carried out, cause the processing unit to interpolate (preferably in a linear manner) the correction constants so as to determine a compensation coefficient as a function of the instantaneous flow rate and the temperature of the gas flowing through the gas meter, and to multiply with each other the instantaneous flow rate and the compensation coefficient in order to supply a measurement of the flow rate of the liquefied petroleum gas flowing through the meter.

In this manner, the firmware of the meter is configured to carry out a compensation algorithm for measuring the flow rate of the gas flowing though the meter.

In one aspect, the storage means include a non-volatile memory and the correction constants are stored permanently in the nonvolatile memory.

In some preferred embodiments, the correction constants are stored in the form of lookup tables and the instructions which can be carried out by the processing unit, when they are carried out, cause the processing unit to extract the correction constants to be interpolated from the lookup tables as a function of the instantaneous flow rate and the temperature of the liquefied petroleum gas flowing through the gas meter. Three lookup tables are preferably provided, each lookup table being defined at a specific reference temperature. In this manner, the meter is suitable for supplying a measurement in a wide range of flow rates and temperatures of the gas. Advantageously, the gas meter includes means for measuring the temperature of the gas flowing through the meter. According to another advantageous aspect, the processing unit is operationally connected to the means for measuring the temperature of the gas so as to be able to extract the correction constants as a function of the measured temperature from the lookup tables and subsequently to interpolate them.

In a preferred embodiment, the sensor for measuring the instantaneous flow rate is of the ultrasonic type. This solution, with respect to the conventional mechanical measuring means, allows any mechanical anomalies connected, for example, with wear phenomena, to be prevented to the advantage of the service-life of the meter and the stability of the measurements over time. Furthermore, the ultrasonic sensor ensures maximum accuracy, silence and reduced weight.

The features and advantages of the invention will be better appreciated from the following detailed description of a preferred though non-limiting embodiment thereof which is illustrated by way of non-limiting example with reference to the appended drawings, in which:

- Figure 1 is a schematic view of the gas meter according to the invention;

- Figure 2 represents lookup tables according to the invention;

- Figure 3 represents a table for calculating the values of the lookup tables of Figure 2;

- Figures 4 and 5 represent curves relating to measurement errors of the flow rate of the meter of Figure 1 of butane and propane, respectively;

Figure 6 represents a diagram of a consultation of the lookup tables of Figure 2.

In the Figures, there is generally designated 1 a gas meter, preferably of the type supplied by battery and suitable for domestic use.

The gas meter 1 comprises a sensor 2 for measuring an instantaneous flow rate Q of the gas flowing though the meter itself.

In a preferred embodiment, the sensor 2 is of the ultrasonic type, preferably comprising an ultrasonic tube which is configured to receive a flow of the gas to be measured.

Advantageously, the gas meter 1 further comprises a processing unit 3 and storage means 4 which are accessible to the processing unit 3. The storage means 4 comprise a compensation algorithm which can be carried out by the processing unit 3 which, when it is carried out, causes the processing unit 3 to calculate a measured flow rate Qm of the gas flowing through the meter 1 which is defined according to the relationship: Qm = K * Q where K denotes a compensation coefficient.

Therefore, the instantaneous flow rate Q set out by the sensor 2 is used as a measurement variable in order to supply the measured flow rate Qm.

It may be noted that, in this context, the term "measured flow rate" is intended to be understood to be a value of the flow rate of the gas flowing through the meter 1 which is obtained from the value of the instantaneous flow rate Q which is read by the sensor 2 and which is subjected to a correction or compensation which takes into account the temperature of the gas and any systematic errors and/or offset errors which are connected with the production process of the meter 1 and, in one aspect, with the production process of the ultrasonic tube of the meter itself.

Preferably, the gas meter 1 comprises means 5 for measuring a temperature Tf of the gas flowing through the meter itself. In one aspect, the means 5 for measuring the temperature Tf are integrated in the ultrasonic tube.

In this context, the compensation coefficient K serves to correct the instantaneous flow rate Q which is read by the sensor 2 as a function of the temperature Tf and/or as a function of any systematic errors and/or offset errors which are connected with the production process of the meter 1 and, in one aspect, with the production process of the ultrasonic tube of the meter itself.

In one aspect, the compensation coefficient K is determined from a plurality of correction constants K_T as a function of the instantaneous flow rate Q and the temperature Tf of the gas flowing through the gas meter 1.

Advantageously, the correction constants K_T are specific for each gas meter 1 and are assigned to the gas meter 1 during the calibration so as to overcome any systematic errors and/or offset errors which are connected with the production process of the meter 1 and/or of the ultrasonic tube of the meter itself.

In some preferred embodiments, the calibration of the gas meter 1 provides for the meter 1 to be assigned different correction constants K T corresponding uniquely to different reference flow rates Qr and different reference temperatures Tr so as to calibrate the meter 1 for a wide range of use.

In one aspect, the storage means 4 include a non-volatile memory in which there are stored the correction constants K T which result from the calibration of the meter 1.

According to another advantageous aspect, the compensation algorithm provides for the processing unit 3 of the meter to determine the compensation coefficient K for interpolation (preferably, by linear interpolation) of the correction constants K_T as a function of the instantaneous flow rate Q and the temperature Tf of the gas flowing through.

Preferably, once the calibration of the gas meter 1 is completed, the correction constants K_T are not able to be changed or modified in any manner.

It must be observed that, advantageously, the gas meter can be calibrated in order to measure a gas to be selected from a natural gas and a liquified petroleum gas (LPG for short) . In one aspect, the correction constants K T assigned to the meter are tabulated in one or more lookup tables and, depending on the case, the calibration provides for one or more lookup tables specific to measuring natural gas or measuring LPG to be assigned to the meter 1.

Preferably, if natural gas is being measured, there is provided a single lookup table while, if LPG is being measured, there are preferably provided three lookup tables LUT1, LUT2, LUT3 which are shown in Figure 2.

If LPG is being measured, therefore, there is advantageously provision for updating the compensation coefficient K by means of a compensation algorithm which makes use of the three lookup tables LUT1, LUT2, LUT3.

With reference below to measuring LPG, each of the three lookup tables LUT1, LUT2, LUT3 is defined at a specific reference temperature Tr which, in a preferred embodiment, is equal to substantially -25°C, +20°C and +55°C.

It must be observed that there are preferably exactly three lookup tables for measuring LPG. It will be understood that, in some embodiments, the number of lookup tables for measuring LPG may be less than three (therefore, one or two lookup tables) so as to simplify the consultation procedure, or may be greater than three

(for example, four, five or six lookup tables) so as to provide a more accurate measurement.

The correction constants K T are set out in each of the three lookup tables LUT1, LUT2, LUT3 for different reference flow rates Qr. In one aspect, it may be noted that a single set of different reference flow rates Qr which is identical for each of the three lookup tables is provided.

In one aspect, the correction constants K_T are calculated and fixed uniquely during the calibration of the gas meter 1 on the basis of data and measurements carried out in laboratories on one or more reference meters. There is preferably provided a reference meter for each reference flow rate Qr, therefore, for example, six reference meters are provided in the case of six reference flow rates Qr.

In general terms, the correction constants K_T are formed as a function of an error of the gas meter 1 in relation to a measurement of a flow rate with respect to a reference flow rate Qr .

In practice, with reference to the table of Figure 3, the correction constants K_T are formed by the difference Aair/lpg between a mean error Elpg of the meter 1 with respect to the measurement of a flow rate of LPG and a mean error Eair of the meter 1 with respect to the measurement of a flow rate of air, as described in greater detail below.

As indicated above, it may be noted that, in this context, the term "error" is intended to be understood to be an error (preferably a mean error) of the meter 1 with respect to the measurement of a flow rate of a gas flowing through the meter 1 as defined with respect to a reference flow rate Qr measured by a reference meter.

Advantageously, the mean error Elpg of the meter 1 in relation to the measurement of a flow rate of LPG is calculated using means between a mean error of the meter 1 in relation to the measurement of a flow rate of butane and a mean error of the meter 1 in relation to the measurement of a flow rate of propane.

Since the main components of any admixture of LPG are butane and/or propane, in this manner there is an intermediate situation between the two limit cases of an LPG admixture substantially composed only of butane or substantially composed only of propane.

It must be observed that, over the entire range of flow rates to be measured, the difference between the flow rate measurement errors of butane and the corresponding flow rate measurement errors of propane always remains below the value of the tolerance range permitted by the applicable standards (for example, MID standard) . As set out by a comparison between the curves relating to the errors for butane in Figure 4 and for propane in Figure 5, on average the difference between the mean error of butane and the mean error of propane is substantially 2%. Therefore, the mean between the mean error for butane and the mean error for propane is kept equidistant from the mean error for butane and the mean error for propane at substantially 1%. It is thereby ensured that, for any percentage of butane and/or propane in the admixture of LPG to be measured, the meter is calibrated so as to provide a measurement of the flow rate with an error which is within the tolerance range permitted by the standards applicable.

It may be noted that, in this context, the term "curve" is intended to be understood to be the whole of the errors of the gas meter 1 in relation to the measurement of the different reference flow rates Qr (preferably 0 m ³ /h, 0.016 m ³ /h, 0.25 m ³ /h, 0.6 m ³ /h, 2.4 m ³ /h and 6 m ³ /h) at a given reference temperature Tr.

In one aspect, the correction coefficients K_T set out in the lookup tables LUT1, LUT2, LUT3 of Figure 2 are calculated from the mean errors Elpg, Eair which are tabulated in Figure 3.

The table of Figure 3 sets out the following parameters (from left to right) : the reference flow rates Qr with which the errors are measured; the mean errors Elpg for LPG at the three different reference temperatures Tr of -25°C, +20°C and +55°C; the mean errors Eair for air at the reference temperature Tr = 20°C; the differences Aair/lpg between the mean errors Elpg for LPG at the different reference temperatures Tr and the mean errors Eair for air at the different reference flow rates Qr .

For each of the reference flow rates Qr, there is provision for calculating the correction constants K_T from the difference Aair/lpg between the mean errors Elpg for LPG at the different temperatures Tr and the mean errors Eair for air according to the following relationship:

K_T = 1/ (l+Aair/lpg/100) where Aair/lpg denotes the difference as a percentage error between the curve for air and the curve obtained from the mean between the mean curve for butane and the mean curve for propane at each of the reference temperatures Tr and flow rates Qr indicated in the table of Figure 3.

It may be noted that, in this context, the term "mean curve" is intended to be understood to be the curve obtained from the mean of the values collected during the different measurements.

Advantageously, given the technical and constructive characteristics thereof, the gas meter 1 is suitable for measuring both natural gas and LPG. It will be understood that, preferably, the calibration of the meter provides correction constants K_T and relative lookup tables which are different and specific for selectively measuring natural gas or LPG, respectively.

According to another advantageous aspect, the calibration of the meter 1 provides for selecting an operating mode for the meter 1 which may be selected from an operating mode exclusively with natural gas and an operating mode exclusively with LPG. Preferably, the default operating mode is the operating mode with natural gas. In one aspect, the default operating mode with natural gas can be modified (in order to instead select the operating mode with LPG) one single time in the life of the meter 1 during the calibration and then can no longer be modified.

In some preferred embodiments, the calculation of the measured flow rate Qm in the operating mode with natural gas provides for multiplying the instantaneous flow rate Q by the compensation coefficient K obtained directly by the linear interpolation of the correction constants K_T with respect to a lookup table for natural gas while the calculation of the measured flow rate Qm in the operating mode with LPG provides for multiplying the instantaneous flow rate Q by the compensation coefficient K which is updated by means of the compensation algorithm which makes use of the lookup tables LUT1, LUT2, LUT3 for LPG.

With reference below to the operating mode with LPG, for each measurement there are preferably used the correction constants K T of only two lookup tables selected from the three lookup tables LUT1, LUT2, LUT3 as a function of the temperature Tf of the gas. In practice, the ultrasonic tube, by providing the temperature Tf of the gas, allows a temperature range of interest to be established. Consequently, the two lookup tables which correspond to the upper extreme and lower extreme of the above-mentioned range are considered.

In the example of Figure 6, the temperature Tf measured by the ultrasonic tube is equal to 30°C. In this case, the lookup tables to be considered are the ones corresponding to +20°C (lower limit of the range of interest) and +55°C (upper limit of the range of interest) .

It will be appreciated that, in any case, the temperature range of interest is limited - in a downward or upward direction in accordance with the value of Tf - by the reference temperature of +20°C. Therefore, the compensation algorithm carries out in each case the linear interpolation of the correction constants K T of the lookup table LUT2 (with respect to the reference temperature Tr = +20°C) . The linear interpolation of the correction constants K_T of the lookup table LUT2 provides a first compensation constant with respect to the reference temperature Tr = +20°C.

Preferably, the calculation of the first compensation constant is the first operation carried out by the compensation algorithm in the course of the compensation of a measurement of the gas flow rate, regardless of the temperature Tf.

Afterwards, the compensation algorithm carries out a second linear interpolation. The second linear interpolation involves the correction constants K T of the lookup table LUT1 (with respect to the reference temperature Tr = -25°C) if the range of interest is between -25°C and +20°C and otherwise the second linear interpolation involves the correction constants K_T of the lookup table LUT3 (with respect to the reference temperature Tr = +55°C) if the range of interest is between +20°C and +55°C. The second linear interpolation thereby provides a second compensation constant . At this point, there are two compensation constants (the first compensation constant and the second compensation constant) corresponding to two different temperatures. The compensation coefficient K which serves to multiply the instantaneous flow rate Q in order to supply the measured flow rate Qm will therefore be given by the linear interpolation of the first compensation constant and the second compensation constant. The invention thereby solves the problem proposed while achieving a number of advantages, including:

- measuring LPG in all the mixed versions thereof between butane and propane,

- measuring LPG in a wide temperature range, - conformity with the applicable standards with respect to legal metrology .