Calibration of temperature sensors in weathering devices by contactless temperature measurement

p0001The present invention relates to a method for calibration of temperature sensors, such as are used in particular in apparatuses for the artificial weathering of samples. Such temperature sensors are known under the names black panel sensor, black standard sensor or a white standard sensor. The invention further relates to a device for carrying out the calibration method.

p0002In apparatuses for artificial weathering of material samples the lifetime of materials should be estimated, which are exposed to natural weather conditions during their use and thus deteriorate under climatic influences such as sunlight, solar heat, humidity and the like.. To obtain a good simulation of natural weather conditions, the spectral energy distribution of the light generated in the device must be possible that correspond to the natural solar radiation, for which reason used in such devices as sources Xenon spotlights. An accelerated aging test of the materials will be achieved mainly through an opposite to the natural conditions greatly intensified irradiation of samples by the aging of the sample is accelerated. Thus, can be a relatively short time a prediction of the long-term aging of a material sample.

p0003The majority of the investigated in artificial weathering samples consist of polymer materials. These weather-related deterioration caused by the UV component of solar radiation substantially. The case drain primary photochemical processes, so the absorption of photons and the generation of excited states or free radicals, are independent of temperature. In contrast, the subsequent reaction steps with the polymers or additives may be temperature dependent such that the observed aging of the materials is also temperature dependent. The degree of temperature dependence on the material and the observed change in properties dependent.

p0004To take account of this fact, the room temperature and / or the sample temperature is kept constant generally in the artificial weathering of polymeric materials. The constant and knowing the temperatures are due to the temperature dependence of aging needed to the results of various Bewitterungsläufe compare them to.

p0005Since it is difficult to measure the sample temperature of the test the material samples directly, temperature sensors are used in weathering whose measured temperature is used as a measure of the sample temperature. For example, can be used as such a temperature sensor is used, a black panel sensor. In the document<patcit id="pcit0001" dnum="EP0320209A2"><text>EP 0320209 A2</text></patcit> describes an exposure apparatus having a weathering chamber, in which a xenon lamp is provided as light source for emitting light having a predetermined intensity. Within the weathering chamber is a cylinder-symmetrical sample holder frame which is rotatable around the light source. From this sample holder frame be worn under investigation material samples and black panel sensors. The material samples and black panel sensors are thus exposed Alike the radiation field of the light source and the other, set within the weathering conditions. In order to control the sample temperature within certain limits and in order to equalize inside the weathering chamber, an additional air flow into the weathering is initiated, the sweeps past cylindrically symmetrical with respect to the light source to the sample holder frame and held therein material samples and black panel sensors. The airflow leads here from some of the heat of the material samples and black panel sensors. This can be exploited for a temperature control by the temperature measured by the black panel sensors temperature is used as a control signal for the strength of the introduced into the weathering airflow.

p0006The black panel sensors, black standard sensors and white standard sensors are used as temperature sensors are designed so all that they comprise a metal plate having, in operation of the light source facing the painted surface and on the metal plate on the reverse side thermally coupled temperature-dependent electric component. The electrical component is usually formed here by a temperature-dependent resistor as a platinum resistor (commercial designations Pt100 or Pt1000) and is connected to an electrical measuring transducer.

p0007A black standard sensor has in detail a side black lacquered steel plate (1 mm thick), one on the uncoated back side thermally coupled Pt100 or Pt1000 resistance, one on the back applied and enclosing the platinum resistor plastic plate made of PVDF (polyvinylidene fluoride) and a cover plate made of stainless steel on. A white standard sensor is constructed in a similar manner, with the difference that the facing in the operation of the light source surface is painted white. A black panel sensor is in contrast to the black standard sensor from both sides of a metal plate blackened without PVDF insulation. The temperature-dependent resistor is applied to the back without surrounding insulation.

p0008In weathering devices according to the current standard, black standard or black panel sensors are used to specify a black standard temperature for each weathering. The black standard temperature is an upper limit for the range of interest of the surface temperature of the material sample. In addition, often a white standard sensor is used, the temperature measurement provides a lower limit of this range. Thus, the sample temperature can be limited and it can optionally be taken as first approximation for the sample temperature is the arithmetic average of the measured temperatures.

p0009Before commissioning, the temperature sensors described above need to be calibrated. The black listed and white standard sensors and black panel sensors are calibrated at the moment the so-called Kontaktthermometrieverfahren, with one standard sensor is compared with a measuring sensor. Two identical sensors are adjacent on a hot plate with the black (or white) coating on the heat plate. Alternatively, both sensors can be placed in an oil or water bath. The standard sensor has previously been calibrated in an oil or water bath by means of a standard thermometer as a mercury thermometer and thus provides a PTB standard (Physikalisch-Technische Bundesanstalt) traceable temperature standard. It will now be on the hot plate (or the oil - or water) variable temperatures produced. Then, an output signal of the measuring transducer, in which the platinum resistor to calibrate the temperature sensor is integrated, calibrated with the measured with the standard temperature sensor.

p0010One disadvantage of this conventional calibration method is that the calibration is carried out under exclusion of influencing factors xenon radiation, humidity and air movement, but precisely determine these factors in reality the surface temperature in weathering or weathering. Contact problems with the electrical temperature sensors and minor differences in structure, which can lead under real conditions to temperature differences, are not recognized.

p0011Another disadvantage of the conventional Kontaktthermometrieverfahrens is that the standard temperature sensor used in this case must be equipped completely as the to be calibrated temperature sensor and calibrates itself as such. He must therefore have in accordance with the structure described above one at the back of the metal plate is thermally coupled platinum resistance including the necessary electrical measuring transducer. To be used as a standard temperature sensor can, he needs to be calibrated in an oil or water bath. The provision of the standard temperature sensor thus presents itself as quite expensive.

p0012The document <patcit id="pcit0002" dnum="US5707146A"><text>US-A-5, 707, 146</text></patcit> describes a method for calibration of thermocouples in which radiation from a modulated light source is focused on the tip of the thermocouple, the emissivity and temperature of the tip of the thermocouple are measured by first and second pyrometer, and the temperature measured by the first and second pyrometers emissivity and temperature are compared with the measured temperature of the thermocouple.

p0013It is an object of the present invention, a method for calibrating an insertable in weathering temperature sensor such as a black panel sensor, a black standard sensor or a white standard sensor, specify which can be performed with reduced effort and under conditions such as those prevailing in the operation of a Bewitterungsgerätes inside the weathering chamber ,

p0014This object is achieved by the features of independent claim 1. Advantageous developments and embodiments of the method according to the invention are indicated in the dependent claims.

p0015The calibrated temperature sensor is designed as a black panel sensor, black standard sensor or white standard sensor. The temperature sensor is such that it has a contained in an electrical circuit such as a temperature-dependent electric component measuring transducer. This device is a temperature-dependent resistor, particularly a platinum resistance.

p0016An essential idea of the present invention is that a non-contact temperature measurement within a B during calibration is ewitterungsgeräts performed. This contactless temperature measurement is carried out from the surface of the reference temperature sensor.

p0017The contactless temperature measurement is performed by is that in step c) the blackbody radiation of the surface measured and from this the temperature. This can be done using a commercially available pyrometer at least in the measuring range corresponding to the temperature range of interest calibrated. With the pyrometer, a measurement spot is fixed on the surface of the reference temperature sensor and detects the light emitted from this spot in the corresponding solid angle blackbody radiation, thus determining the surface temperature below a certain defined angle.

p0018The present invention relates to the case that the emissivity of the surface of the to be calibrated, the temperature sensor is not known. Practical experience with coating materials of black standard sensors or black panel sensors has indeed shown that, in particular, have emissivities such black coatings which are particularly sought after for use as coatings in black standard or black panel sensors due to their great resistance to aging, which are not exactly known. Before the emissivity is determined in a complex manner, the inventive method can be carried out in such cases so that a reference temperature sensor is used with a surface having a known emissivity. In process step b) is produced both on the surface of the to be calibrated, the temperature sensor and on the surface of the reference temperature sensor under equal conditions of variable temperature, so that it can be assumed that the surface temperatures of the two sensors are equal. In process step c) then need only be measured without contact, the temperature of the surface of the reference temperature sensor and, with this measured temperature then step d), the output of the electrical circuit of the platinum resistor to calibrate the temperature sensor to be calibrated. If desired, in parallel also the blackbody radiation to calibrate the temperature sensor are measured, so that by comparing the measurements of blackbody radiation at the reference temperature sensor and the calibrated temperature sensor, the emissivity of the surface of the latter can be determined.

p0019Since due to the unknown emissivity of the surface of the to be calibrated temperature sensor, a reference temperature sensor must be used, this however does not have the complicated structure of the standard temperature sensor used in the conventional Kontaktthermometrieverfahren. The reference temperature sensor to be used herein for example, requires no thermally coupled temperature-dependent resistance element at the back, together with associated circuitry. Instead, the reference temperature sensor can only consist of a metal plate, the one or both sides with the desired finish (black or white) is coated with a known emissivity. Such a constructed reference temperature sensor represents a traceable to a PTB standard temperature standard for this variant of the method according to the invention.

p0020The inventive method also makes it possible that it can be carried out under conditions such as prevail, typically within the weathering chamber of an operating weathermeter. This allows for calibration of sensors as well as black and white standard black panel sensors, while they are being acted upon by the occurring during operation influencing factors such as light radiation from the Xenon lamp, air flow and moisture. These factors determine in reality, the surface temperature of the sensors.

p0021The ruling in a weathering conditions for a largely determined by the light radiation from the xenon radiator. In order to provide approximately the same conditions at the calibration method according to the invention, can thus be provided in method step b) the variable temperature cause by subjecting the surface of the temperature sensor and that of the reference temperature sensor with a radiation corresponding to the solar spectrum, in particular the radiation of a xenon light source. Furthermore, it can be provided to cause a similar movement of air to the ambient air in the immediate vicinity of the surfaces of the sensors, as produced in weathering. It can accordingly be generated a flow of air which sweeps past parallel to the surfaces of the sensors thereto.

p0022The above conditions are thus provided by the calibration procedure is performed in an apparatus for the artificial weathering of samples.

p0023The present application also does not describe by way of example to the invention associated devices for implementing the method according to the invention, according to two different variants. In a first variant, a positioning device is provided, which has means for receiving and holding the temperature sensor to be calibrated and possibly the reference temperature sensor, and means for holding the non-contact temperature measuring sensor, in particular of the pyrometer comprises. To carry out the calibration process then has to be ensured that the sensor to be calibrated temperature heat is supplied, preferably. By applying radiant energy from a xenon radiation source In a second variant a commercial weathering device remodeled so that the inventive method can be carried out therein. Provided that there are means for receiving and holding the temperature sensor to be calibrated and possibly the reference temperature sensor within the weathering chamber for this purpose, these means can also be provided by the sample holder frame. Further, means for mounting a non-contact temperature sensor (such as a pyrometer) on an outer wall of the weathering chamber needed as well as at least one opening in the outer wall, so that the pyrometer can be aligned with the temperature sensors held in the weathering chamber.

p0024The invention will be explained in more detail with reference to the drawings, in which exemplary embodiments are shown. Show it:<dl id="dl0001"><dt>Fig. 1</dt><dd>a longitudinal section through a sensor to be calibrated to the standard black;</dd><dt>FIG. 2a, b, c</dt><dd>a not belonging to the invention positioning device as part of an apparatus for performing the calibration process in a perspective point of view (a), in a side view (b) and in a front view (c), and </dd><dt>Fig. 3</dt><dd>a partial view of the interior of a angepass- to perform the calibration th weathering apparatus.</dd></dl>

p0025In the <figref idrefs="f0001">Fig. 1</figref> is a black standard sensor shown 10, as is typically used in devices for the artificial weathering for monitoring the temperature of the to be assayed sample of material. Such a black standard sensor 10 is thereby supported in the immediate vicinity of the material samples with the same geometric orientation as this in respect of the xenon radiation source. The black standard sensor 10 is in the<figref idrefs="f0001">Fig. 1</figref> in a longitudinal section along a through the center, ie, the platinum resistor 3 extending plane shown.

p0026The black standard sensor 10 has a rectangular stainless steel plate 1, which is provided on one of its major surfaces with a matte black finish 2 or black coating. While the intended use of the black standard sensor 10 as well as the same during calibration, the black paint 2 of xenon radiation source 7 is turned. On the back of the stainless steel plate unblackened 1 a platinum resistor 3 is mounted, is being ensured for optimum thermal coupling. Moreover, a heat insulating plastic plate 5 made of PVDF (polyvinylidene fluoride) is applied to the backside of the stainless steel plate. 1 The plastic plate 5 has at its center on one of the stainless steel plate 1 facing recess 5A, in which the platinum resistor 3 is. a stainless steel plate 1 remote up to the surface of the plastic plate includes at 5 extending implementation 5B of the recess 5A. The implementation of Figure 5B is used for supplying the electrical lead wires 4 to the platinum resistor 3. The lead wires 4 of the platinum resistor 3 is connected to a suitable electrical circuit in which the resistance value of the platinum resistance 3 is translated into a current signal of 4-20 mA. For the calibration of the black standard sensor 10 is applied with the light radiation 7a of xenon radiation source 7, which in the<figref idrefs="f0001">Fig. 1</figref> is not shown to scale with respect to the black standard sensor 10th As will be explained, the calibration process can be carried out within a Bewitterungsgerätes with the xenon contained therein radiation source 7th In addition, an air stream 9 is produced which flows past in close proximity to the surface of the metal plate 1 on this, with the air stream 9 can be produced by a blower. Alternatively, or additionally, the black standard sensor can also be placed in a rotational movement around the xenon radiation source 7, so that the perpendicularly directed in this case to the plane of air flow exists only in a relative movement between the black standard sensor 10 and the ambient air.

p0027The contactless temperature measurement is performed by a commercially available, calibrated pyrometer 8, which has a spectral sensitivity range of 8-14 microns, which here in the <figref idrefs="f0001">Fig. 1</figref> of not belonging to the present invention, a case is shown that the contactless temperature measurement of the temperature sensor to be calibrated is carried out itself. The pyrometer 8 detects the light emitted by a spot with a diameter between 10 and 20 mm on the blackened surface of the stainless steel plate 1 into a solid angle 8a black body radiation and calculated from this taking into account the emissivity of the black paint finish 2, the surface temperature. For the present invention alone authoritative event that the emissivity of the black paint 2 to calibrate the temperature sensor 10 is not known, in the immediate vicinity of the unknown temperature sensor 10, a reference temperature sensor is arranged with a black paint with a known emissivity and by this means the pyrometer 8 blackbody radiation measured. With the calculated therefrom, surface temperature of the platinum resistor 3 or the output signal of the electrical (Transmitter) circuit connected to it is calibrated.

p0028The measurement of the temperature independent per se actual calibration process is known in the art. As mentioned above, the platinum resistor (Pt1000 Pt100 o.) 3 is connected via the lead wires 4 to a suitable electrical circuit. This may for example be a commercially available programmable measuring transducer circuit, as commercially available under the Internet address http://www.sensor-interface.de/dscp80.html example of the company Intelligent Instrumentation under the name "DSCP80 module" stating of a block diagram is described in more detail.

p0029Before starting the calibration, a resistor with a suitable measuring bridge is first determined. For the platinum resistance Pt100 or Pt1000 table works that represent the relationship between the measured resistance and the temperature exist. These curves can be approximated by polynomials. An analytical solution for the polynomial the Pt100 curve does not exist, which is why often counted when calculating t (R) only until the term of 2nd order. The resistance is translated to the programmable measuring transducer into a current signal 4-20 mA (or a digital signal), the lower value of 20 ° C and the upper value corresponds to 180 ° C. This thus preprogrammed measuring transducer is then calibrated with the process of the invention, in which embodiment means to calibrate actually a setting and adjustment of the measuring transducer to the measured with the pyrometer. There are thereby substantially sequentially two temperature values ​​of the surface temperature was adjusted by controlling the radiation power of the xenon radiation source 7th For example, the radiation power is initially set so that the pyrometer with a lower value of 60 ° C is measured. By means of an offset control of DSCP80 transmitter this is then adjusted so that it is also indicative of a temperature of 60 ° C. Then, the radiation power is increased, so that the pyrometer indicates a top value of 120 ° C. By means of a gain control of DSCP80 transmitter this is then adjusted so that it is also indicative of the temperature of 120 ° C. This process is to be performed several times in succession iteratively under certain circumstances. However, it has been shown in practice that is already sufficient, the first setting and that only two more temperature values ​​should be approached for review. But then deviations are larger observed 1 ° C, the calibration procedure must be performed again.

p0030For carrying out the process according to the invention, there are the following two ways in principle. Firstly, an appropriate positioning device can be provided in a non-for invention related manner, on the one hand to be calibrated temperature sensor and the reference temperature sensor and on the other hand, the pyrometer in a reproducible manner relative to each other can be positioned. then a radiation source must be brought into the vicinity of the temperature sensors for the calibration, whereby a weathering apparatus can be brought into the vicinity of the positioning device to that end. On the other hand, the inventive method can be performed by an existing weathering device converted appropriately.

p0031In the <figref idrefs="f0002">Figures 2a, b</figref> and <figref idrefs="f0003">c</figref> is shown from various angles, the first-mentioned variant. This is a positioning device 20 which has two parallel horizontal rails 14, on which two vertical parallel rails are mounted 15 at one end. On the horizontal rails 14, a platform 13 is mounted, which can be displaced along the rails 14 and fixed at the desired position on it. On the platform 13, two rectangular receiving blocks 11 are arranged side by side such that they. In guide channels which extend parallel to the rails 14, in the direction of the rails 14 are slidably The receiving blocks 11 each on its upper side rectangular recesses, into which the temperature sensors can be inserted. The receiving blocks 11 can be fixed at any desired position on the platform. 13

p0032At its upper end, the vertical rails 15 are connected by a horizontal rail 16th On the upper side of the horizontal rail 16, a clamping device 12 can be secured for a pyrometer in two positions in which the pyrometer is alignable with each one of the two positioned in the receiving blocks 11 temperature sensors such as in particular in the<figref idrefs="f0003">Fig. 2c</figref> you can see. The clamping device 12 consists essentially of a tubular cavity, in which the pyrometer can be inserted from one side and is clamped therein.

p0033Regarding the latter variant for implementing the method according to the invention an existing device for the artificial weathering of material samples is such modified or adapted to the inventive method can be performed with it. This essentially means that are provided for the temperature sensors within the Bewitterungsgerätes on one of the inner walls of receiving and supporting portions. In the<figref idrefs="f0004">Fig. 3</figref> is shown a perspective view of a portion of the interior of an artificial weathering apparatus. On the horizontal lower inner wall of the calibrated temperature sensor and the reference temperature sensor are positioned side by side. In located in the longitudinal direction of the temperature sensors vertical inner wall two openings are formed, through which the fixed external pyrometer is alignable to the reference temperature sensor. In this case, it suffices in principle, if only one opening in the housing wall is formed, since the calibration is sufficient in principle a non-contact temperature measurement of the reference temperature sensor. In addition, since a reference temperature sensor is present due to unknown emissivity of being calibrated temperature sensor, only the temperature must be measured at this and calibrated to be calibrated temperature sensor. Preferably, however, two openings are formed, so that from the start the possibility of being able to target both provided for receiving the temperature sensors receiving areas with the pyrometer externally mounted. The pyrometer can be secured from the outside with a latch or the like..