Calibration method of a temperature sensor in a gas burner

Abstract

The object of the present invention is a calibration method of a thermoresistance temperature sensor, comprising the following steps in sequence: - Step 1: waiting for a cooling time t1, following the burner switching off, so as to cool the temperature monitored by the thermoresistance to values close to those of the environment temperature Ta; - Step 2: waiting for a stabilisation time t2, acquiring a plurality of measurements within said stabilisation time t2 and calculating the average of said plurality of measurements to obtain the "temperature average Tf / resistance average Rf / environment temperature average Ta" values; - Step 3: calculating the resistance value R(Ta) in environment temperature conditions Ta and subsequently calculating the correct resistance value Rr at a reference temperature Tr; - Step 4: calibrating the thermoresistance, according to the following 15 "resistance / temperature" conversion formula: T = (R - Rr) ∙ K + Tr

Description

[0001] CALIBRATION METHOD OF A TEMPERATURE SENSOR IN A GAS

[0002] BURNER

[0003] DESCRIPTION

[0004] The object of the present invention is a calibration method of a temperature sensor, particularly useful to be used in fully premixed burners of condensing boilers for the production of hot water for civil uses.

[0005] Without any limiting intent, the method herein described is advantageously suitable for the calibration of temperature sensors adapted to verify the flame presence in a premixed gas burner, both during the ignition step and during the steady-state operation step, especially in burners fed with pure hydrogen or fuel gas with a high hydrogen content (for example having a hydrogen volume content equal to or greater than 98%).

[0006] In fact, it is known that the hydrogen flame is not an ionising flame, if not at the highest power steady states: in case of hydrogen combustion, it is therefore not possible to monitor the flame presence using a conventional ionisation sensor (as it usually happens with the traditional methane or other light hydrocarbon burners), but it is necessary to resort to other solutions, including the use of temperature sensors that measure the temperature within the combustion chamber of the boiler to discriminate whether the flame has ignited or extinguished.

[0007] In addition to the aforementioned verification of the flame presence, the temperature sensors may be used also to control further characteristic parameters of the combustion of a fully premixed burner, such as for example the correct air / gas ratio as a function of the working power, in order to have a more stable flame and reduce the risk of flashback at low powers and at flame ignition or detachment at high powers.

[0008] Among the various types of temperature sensors, the thermoresistance ones have some characteristics that make them particularly suitable for use in the sector of thermo-sanitary appliances.

[0009] Such type of sensors (among which the positive temperature coefficient PTC or the negative temperature coefficient NTC sensors may be included) uses the temperature variation of the metal oxides of the electrical resistance thereof to determine the temperature of the flame.

[0010] In other words, when the temperature of the resistance element increases, the electrical resistance increases linearly: such value of the electrical resistance therefore changes as a function of the environment temperature wherein the sensor is immersed and such linearity in the temperature variation is a peculiarity that may also be used inside the combustion chamber of a boiler for the necessary checks and adjustments.

[0011] As is known, the measuring characteristic of a thermoresistance temperature sensor may be expressed with the following “resistance / temperature” conversion formula:

[0012] T = (R - Rr■ K + Tr wherein:

[0013] T = the temperature in °C of the flame in operating conditions (i.e. when the burner is on);

[0014] R = the resistance in of the temperature sensor in operating conditions (i.e. when the burner is on);

[0015] Rr = the resistance in of the temperature sensor at a reference environment temperature Tr (for example 20 °C);

[0016] K = an amplification factor of the temperature sensor that may be expressed with the following formula: JZ _ _ 1

[0017] CRT ■ Rr where CRT is a characteristic coefficient of the temperature sensor, defined experimentally and dependent exclusively on the geometry and material of the sensor used.

[0018] Consequently, once the type of thermoresistance temperature sensor has been established, the only relevant variable in the “resistance / temperature” conversion is represented by the parameter Rr, i.e. the resistance value at a reference environment temperature.

[0019] In the thermoresistance sensors, such value Rr is typically comprised between 20 and 60 and therefore it appears clear that, in order to correctly apply the conversion formula mentioned above, it is necessary to accurately measure said value Rr before actually using the sensor, in order to avoid errors in the estimation of the temperature: for example, an error of 5 in the measurement of the value Rr may result in a deviation of even more than 15% in reading the temperature of the flame.

[0020] Currently, the actual measurement of the resistance value Rr of the sensor is carried out in a laboratory, using instruments such as a reference temperature meter and a resistance meter: however, this is a laborious and expensive operation, not compatible with the serial production processes of the thermoresistance temperature sensors.

[0021] The object of the present invention is to solve the drawbacks of the prior art, by providing a calibration method for a thermoresistance temperature sensor that is faster and cheaper than those of the state of the art.

[0022] Another object of the present invention is to provide a calibration method for a thermoresistance temperature sensor through means normally present in the thermo-sanitary appliance whereon it is implemented, in particular in a condensing gas boiler.

[0023] A further object of the present invention is to provide a calibration method for a thermoresistance temperature sensor that is particularly suitable for monitoring the flame of a fully premixed burner, fed with pure hydrogen or fuel gas with a high hydrogen content (for example having a hydrogen volume content equal to or greater than 98%).

[0024] These and other objects, which shall appear clear hereinafter, are achieved with a calibration method of a thermoresistance temperature sensor according to the independent claims.

[0025] Other objects may also be achieved by means of the additional features of the dependent claims.

[0026] Further objects of the present invention shall be better highlighted by the following description of a preferred embodiment, in accordance with the patent claims and illustrated, purely by way of a non-limiting example, in the annexed drawing tables, in which:

[0027] - Figure 1 represents a qualitative diagram showing the trend of the temperature value “T” (placed on the ordinate axis) with respect to the time “s” (placed on the abscissa axis), as read by a thermoresistance temperature sensor not subjected to the calibration method according to the invention;

[0028] - Figure 2 shows a qualitative diagram similar to that of Figure 1, in which the temperature value “T” (placed on the ordinate axis) with respect to the time “s” (placed on the abscissa axis) is read by a thermoresistance temperature sensor subjected to the calibration method according to the invention.

[0029] The features of a preferred variant of the calibration method according to the invention are now described using the references in the figures.

[0030] The above calibration method of a thermoresistance temperature sensor (hereinafter abbreviated to “thermoresistance”) is based on the calculation of the value Rr through components that are normally already present in the product in which the burner is installed.

[0031] The product in question is typically a condensing boiler for the production of hot water for civil uses, whose constituent elements are well-known to the man skilled in the art; for what is relevant here for the purposes of this description, it is sufficient to say that such boiler comprises at least:

[0032] - one or more thermoresistances for monitoring the flame presence and / or for measuring the temperature of the flame in order to regulate other combustion parameters,

[0033] - one or more temperature sensors for regulating other combustion parameters, such as for example the air / gas ratio of the mixture feeding the burner, - a control and regulation electronic device (hereinafter “control board”), adapted to receive and process the signals provided by the aforementioned thermoresistances and temperature sensors.

[0034] For simplicity of description and in accordance with the example of the attached figures, hereinafter reference shall be made to a thermoresistance adapted to monitor the flame presence of a premixed gas burner: however, as said above, the thermoresistance may also be used for different purposes, connected to the control and regulation of further parameters of the combustion of a gas containing pure hydrogen or a high hydrogen content (for example having a hydrogen content by volume equal to or greater than 98%)

[0035] Preferably said one or more thermoresistances for monitoring the flame presence are not in contact with the body of the burner, but are positioned so as to be hit by the flames coming out of the openings of the burner body. Such type of temperature sensors must be able to withstand the temperatures that develop inside the combustion chamber which, in case of a hydrogen-fed burner, result in a temperature of the flame varying between approximately 800 °C to 1,200 °C, depending on the excess air value and the operating power of the burner.

[0036] In the preferred variant of the invention, the control board adapted to receive the signals and process them to carry out the calibration method is the same board normally present on the product, appropriately programmed to carry out the operations dedicated to the calibration of the thermoresistance. However, such control board could also consist of an additional board.

[0037] For the sake of brevity, hereinafter the description of the invention shall assume that all the receipt and processing capabilities reside in the boiler control board 1 but, more generally, “control board” means the entire set of data processing elements, united or distributed in multiple subsets, which are necessary to implement the method according to the invention.

[0038] The calibration method of at least one thermoresistance according to the invention shall be now described with reference to the various sequential steps of which it is composed. Step 1: waiting for a cooling time interval tl.

[0039] As mentioned, the method according to the invention provides for calculating the resistance value Rr of the thermoresistance directly inside the boiler, in order to calibrate the thermoresistance so that it correctly measures the temperature value of the flame in accordance with the aforementioned “resistance / temperature” conversion formula:

[0040] T = (R - Rr) ■ K + Tr

[0041] To this end, Step 1 of the method provides for waiting for a minimum time tl, following the burner being turned off, so as to cool the temperature monitored by the thermoresistance to values close to those of the environment temperature (i.e. to a temperature value of approximately 20 °C, which is substantially the temperature in the combustion chamber with the burner off).

[0042] Generally, said time interval tl (hereinafter “cooling time tl”) lasts between 1 and 20 minutes, preferably between 3 and 5 minutes.

[0043] Step 2: waiting for a stabilisation time interval t2 and measurement of the resistance and temperature values.

[0044] After the cooling time interval tl of Step 1 has elapsed, the method according to the invention provides for waiting for a time interval t2 (hereinafter referred to as “stabilisation time t2”) during which a series of temperature and resistance values, necessary to act as reference values for the continuation of the thermoresistance calibration method, are acquired.

[0045] During such stabilisation time t2, a plurality of measurements are therefore acquired and the average of these is calculated in order to obtain the following quantities:

[0046] - the average of the temperature Tf measured by said thermoresistance,

[0047] - the average of the resistance Rf measured by said thermoresistance,

[0048] - the average of the temperature Ta of the air within the combustion chamber with the burner off, also known as “environment temperature Ta”.

[0049] Such environment temperature Ta is measured with one or more reference sensors, all of which are already present in the boiler; by way of a non-limiting example, said one or more temperature sensors may comprise one or more sensors among:

[0050] - thermocouple located in the proximity of the burner,

[0051] - sensor positioned in the mixing chamber of the combustion air and fuel gas (in particular gas with a high hydrogen content) feeding the burner.

[0052] In such case, in order for the temperature sensors to measure reliable reference values of the environment temperature Ta, during the stabilisation time t2 the boiler fan is activated at a known and constant speed to ensure uniformity of the temperature.

[0053] Still by way of a non-limiting example, said one or more temperature sensors for measuring the environment temperature Ta may further comprise one or more sensors between:

[0054] - sensor positioned in the combustion fume exhaust duct,

[0055] - sensor located outside the boiler, through the measurements whereof the temperature value Ta may be derived with good reliability, through known formulas and interpolations.

[0056] Said one or more temperature sensors for obtaining a value of the environment temperature Ta may also comprise one or more sensors placed in the primary circuit of the boiler.

[0057] In such case the fan and the circulation pump of the boiler are activated at known and constant speeds, while the three-way diverter valve is switched to the “domestic water heating DHW” operating mode, so as to ensure that the temperatures read by the sensor may be attributable, with a good degree of approximation, to the temperature Ta of the air in the combustion chamber.

[0058] The duration of Step 2 of the method according to the invention, i.e. the duration of the stabilisation time t2, is typically comprised between 5 and 120 seconds, preferably between 10 and 30 seconds.

[0059] Step 3: calculation of the resistance value R(Ta) at environment temperature Ta and subsequent calculation of the correct resistance value Rr at the reference temperature Tr.

[0060] Once the above values “average temperature Tf measured by the thermoresistance / average resistance Rf measured by the thermoresistance / average temperature Ta measured by a reference sensor” during the Step 2 have been acquired, the following Step 3 of the method according to the invention begins with the calculation of the resistance R(Ta) of the thermoresistance, i.e. the correct resistance value in conditions of environment temperature Ta.

[0061] The boiler control board processes such values using the following formula:

[0062] Rf R(~Ta) “ 1 + CRT - (Tf - Ta) in which, as mentioned above, the parameter CRT is a fixed coefficient once the specific type of thermoresistance has been chosen.

[0063] Once such value R(Ta) has been obtained, the Step 3 continues with the calculation of the resistance value Rr at the reference temperature Tr (i.e. the reference environment temperature, for example 20 °C), in accordance with the following formula:

[0064] R(Ta)

[0065] Rr = - - - -

[0066] 1 + CRT ■ (Ta-Tr)

[0067] The calculation of such correct resistance value Rr of the thermoresistance may be represented by the following formula, which incorporates the two previous formulas:

[0068] Rf

[0069] D — _

[0070] [1 + CRT ■ (Ta - Tf)] ■ [1 + CRT ■ (Tf - Ta)]

[0071] Step 4: calibration of the thermoresistance.

[0072] Once the correct resistance value Rr of the thermoresistance has been obtained in Step 3, it is possible to proceed with the calibration of the thermoresistance according to the “resistance / temperature” conversion formula previously seen:

[0073] T = (R - Rr) ■ K + Tr

[0074] In figure 1 an example of measurement of the temperature value read by a thermoresistance not correctly calibrated is shown: it is noted how, within the stabilisation time t2, a significant temperature difference At is found between the values of the temperature of the flame Tf and the environment temperature Ta, which instead should basically overlap for the reasons explained above.

[0075] Such deviation At may also be greater than 15% and result in an error in the order of 40-50 °C, with all the possible consequences, in terms of safety and operation of the boiler (especially of a boiler comprising a hydrogen-fed burner), that such reading errors may cause.

[0076] In figure 2, the identical example of measurement of the temperature value of a thermoresistance already depicted in figure 1 is shown, this time however, after the application of the calibration method according to the present invention.

[0077] In this case, there is a substantial identity between the values of the temperature of the flame Tf and the environment temperature Ta within the stabilisation time t2, or in any case the deviation is very small, less than 2% (i.e. at most 5 °C), ensuring a reliable operating precision of the thermoresistance subjected to calibration according to the invention.

[0078] From the above description it is clear how the method of the invention is able to achieve the underlying objects, in particular that of making the calibration of a thermoresistance temperature sensor significantly faster and cheaper than the systems currently used in a laboratory according to the prior art.

[0079] This without compromising the reliability of the calibration, as schematically represented in the graphs of figures 1 and 2.

[0080] Obviously, the method according to the invention may also be carried out in a laboratory during the configuration and calibration step of the product, however, while maintaining the advantage of using sensors and components already present in the boiler, i.e. without the need to resort to external instrumentation such as a reference temperature meter and a resistance meter.

[0081] Furthermore, such method may be carried out and / or repeated periodically during the life of the product, for example to obviate possible miscalibrations and drifts over time of the thermoresistance or following the replacement of the sensor for maintenance operations carried out by a technician.

Claims

CLAIMS1. Calibration method of a thermoresistance temperature sensor, said thermoresistance being adapted to measure the temperature of the flame produced by a premixed gas burner of a thermo-sanitary appliance, in particular a condensing gas boiler, said boiler comprising at least:- one or more of said thermoresistance temperature sensors,- one or more additional temperature sensors for the regulation of other combustion parameters,- a control board adapted to receive and process the signals provided by said thermoresistance temperature sensors and temperature sensors for carrying out said method, such calibration method of said thermoresistance comprising in sequence at least the following steps:- Step 1: waiting for a cooling time tl, following the switching off of said burner, so as to cool the temperature monitored by the thermoresistance to values close to those of the environment temperature Ta, said environment temperature Ta being the temperature of the air in the combustion chamber of said boiler with the burner off;- Step 2: waiting for a stabilisation time t2, acquisition of a plurality of measurements within said stabilisation time t2 and calculation of the average of said plurality of measurements to obtain the following values:- average of the temperature Tf measured by said thermoresistance,- average of the resistance Rf measured by said thermoresistance,- average of the environment temperature Ta measured by said further temperature sensors;- Step 3: calculating the resistance value R(Ta) in environment temperature conditions Ta and subsequently calculating the correct resistance value Rr at a reference temperature Tr;- Step 4: calibration of the thermoresistance, according to the following “resistance / temperature” conversion formula:T = (R - Rr) ■ K + Tr wherein:- T is the temperature of the flame in ignition conditions of the burner;- R is the resistance of the thermoresistance in ignition conditions of the burner;- Rr is the resistance of the thermoresistance at a reference temperature Tr, as calculated following said Step 3;- K is an amplification factor of the thermoresistance, that may be expressed by the following formula:JZ _ _ 1CRT ■ Rr where CRT is a characteristic coefficient of the thermoresistance, dependent on the geometry and on the material of said thermoresistance.

2. Calibration method according to claim 1, characterised in that in said Step 3 the resistance value R(Ta) is calculated by said control board according to the following formula:Rf R(~Ta) “ 1 + CRT - (Tf - Ta) while the correct resistance value Rr at the reference temperature Tr is calculated according to the following formula:R(Ta)Rr = -1 + CRT ■ (Ta-Tr) the calculation of said correct resistance value Rr at the reference temperature Tr in said Step 3 may be also obtained in a single step with the formula:RfD — _[1 + CRT ■ (Ta - Tf)] ■ [1 + CRT ■ (Tf - Ta)]3. Calibration method according to claim 1, characterised in that in said Step 1 the cooling time tl lasts between 1 and 20 minutes, preferably comprised between 3 and 5 minutes.

4. Calibration method according to claim 1, characterised in that in said Step 2 the stabilisation time t2 lasts between 5 and 120 seconds, preferably comprised between 10 and 30 seconds.

5. Calibration method according to claims 1 and 4, characterised in that in said Step 2 said one or more temperature sensors, adapted to carry out the plurality of measurements of the environment temperature Ta in order to calculate the average thereof, comprise temperature sensors normally present in the boiler, including at least one of:- thermocouple located in the proximity of the burner,- sensor positioned in the combustion air and fuel gas mixing chamber,- sensor positioned in the combustion fume exhaust duct,- sensor located outside the boiler,- sensor located in the primary circuit of the boiler.

6. Calibration method according to claim 5, characterised in that when said one or more temperature sensors comprise at least one between:- thermocouple located in the proximity of the burner,- sensor positioned in the combustion air and fuel gas mixing chamber, during said stabilisation time t2 of Step 2 the fan of the boiler is activated at known and constant speed.

7. Calibration method according to claim 5, characterised in thatwhen said one or more temperature sensors comprise at least one sensor located in the primary circuit of the boiler, during said stabilisation time t2 of Step 2 the fan and the circulation pump of the boiler are activated at known and constant speeds and said boiler is in “domestic water heating” mode.

8. Calibration method according to any claims 1 to 7, characterised in that it is carried out directly in the boiler by means normally available in said boiler.

9. Calibration method according to claim 8, characterised in that it is carried out and / or repeated periodically during the life of the boiler.

10. Thermo- sanitary appliance, in particular a condensing gas boiler, comprising at least:- a fully premixed burner, fed with pure hydrogen or fuel gas with a high hydrogen content,- one or more thermoresistance temperature sensors for measuring the temperature of the flame produced by said burner,- one or more additional temperature sensors for the regulation of other combustion parameters,- a control board adapted to receive and process the signals provided by said thermoresistance temperature sensors and said further temperature sensors for carrying out the calibration method of the thermoresistance according to the method of claims 1 to 9.

11. Use of a thermoresistance temperature sensor, comprising:- calibrating said sensor by a calibration method according to any of claims 1 to 9,- monitoring with said sensor the flame of a fully premixed burner, fed with pure hydrogen or fuel gas with a high hydrogen content.

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