Calibration of temperature sensors in weathering devices by contactless temperature measurement

The present invention relates to a method for calibrating temperature sensors, as described in particular in devices used for the artificial weathering of samples will. Such temperature sensors are available under the designations Black panel sensor, black standard sensor or white standard sensor known. The invention further relates to a Apparatus for carrying out the calibration method.

In apparatuses for artificial weathering of material samples is the lifetime of materials can be estimated, constantly in its application to natural weather conditions are exposed and thus under climatic Influences such as sunlight, solar heat, moisture and the like. deteriorate. A good simulation of natural Weather conditions to obtain the spectral must Energy distribution of the light generated in the device possible that correspond to the natural sunlight, for any reason in such devices as radiation sources Xenon lamps are used. An accelerated aging test of the materials is essentially determined by a compared to the natural conditions greatly intensified Irradiation of the samples obtained by the aging of the Sample is accelerated. Thus can be determined by relatively short time a prediction of the long-term aging of a material sample.

A majority of the investigated in artificial weathering Samples consist of polymer materials. these the deterioration due to weathering mainly caused by the UV component of solar radiation. The that have occurred in primary photochemical processes, ie the absorption of photons and the generation of excited states or free radicals, are independent of temperature. By contrast, the subsequent reaction steps with the polymers or its additives depends on temperature, so that the observed Aging of the materials is also temperature dependent. The degree of temperature dependency on the material and the considered property change depends.

To take account of this fact is generally in the artificial weathering of polymeric materials which Room temperature and / or the sample temperature kept constant. The constant and knowing the temperatures are necessary because of the temperature dependency of aging, the results of various Bewitterungsläufe among themselves to be able to compare.

Since it is difficult to be examined, the sample temperature of Material samples to measure directly, be in weathering Temperature sensors are used, which are measured Temperature is used as a measure for the sample temperature becomes. For example, a black panel sensor, such as Temperature sensor to be used. In the document EP 0320209 A2 describes a weathering, having a weathering chamber, in which a Xenon lamp as a light source for emitting light having a predetermined intensity is provided. Within the weathering chamber There is a cylindrically symmetric specimen holder frame, which is rotatable around the light source. Of this are sample holder frame, both to be examined samples of material and black panel sensors worn. The material samples and the black panel sensors are therefore under the same Conditions the radiation field of the light source and the other, set within the weathering conditions exposed. To the sample temperature within certain To control borders and order inside the weathering chamber to equalize, in addition, an airflow is introduced into the weathering, the cylindrically symmetrical with respect to the light source to the sample holder frame and held therein material samples and black panel sensors sweeps past. The airflow leads this part the heat from the material samples and black panel sensors. This can be exploited for a temperature control by measured by the black panel sensors as temperature Control signal for the strength of the introduced into the weathering is airflow used.

The black panel sensors used as temperature sensors, Black and white standard sensors Standard sensors are all constructed so that they in a metal plate with a Operation of the light source facing surface and lacquered a thermally coupled to the metal plate on the reverse side exhibit temperature-dependent electrical component. The electrical component is as a rule by a temperature-dependent resistor as a platinum resistor (Commercial designations Pt100 or Pt1000) formed and is equipped with an electrical measuring transducer connected.

A black standard sensor has a one-sided in detail black painted steel plate (1 mm thick), one on the uncoated back side thermally coupled Pt100 or Pt1000-resistance, a coating applied to the back side 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 corresponding manner, with the difference that in the operation of the light source facing 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 without surrounding Insulation applied to the back.

In weathering devices according to the current standard are black standard or black panel sensors used for each Weathering process a black standard temperature state can. The black standard temperature provides an upper limit for the area in question of the surface temperature the material sample. In addition, often, a white standard sensor used whose temperature measurement a lower Limit of this range provides. Thus, the sample temperature be limited and it may optionally as a first approximation for the sample temperature of arithmetic Average of the measured temperatures are accepted.

Before commissioning the above have described Temperature sensors are calibrated. The Schwarzund said are white standard sensors and black panel sensors currently calibrated at the so-called Kontaktthermometrieverfahren, wherein in each case a standard sensor having a measuring sensor compared becomes. Two identical sensors are adjacent on a hot plate with the black (or white) Coating on the heat plate. Alternatively, can also both of the sensors are placed in a oil or water bath. Of the Standard sensor is previously in an oil or water bath using a standard thermometer as a mercury thermometer calibrated and thus provides a traceable PTB standard (Physikalisch-Technische Bundesanstalt) 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 the to be calibrated platinum resistance The temperature sensor is integrated with the temperature sensor with the standard measured temperature calibrated.

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

Another disadvantage of the conventional Kontaktthermometrieverfahrens is that the standard temperature sensor used in this case itself completely as to be calibrated equipped measuring temperature sensor and calibrated as such have to be. It must therefore in accordance with the above-described Structure coupled thermally one at the back of the metal plate Platinum resistance including the necessary electrical have measuring transducer. To a standard temperature sensor to be able to be used, it must be in a Oil or water to be calibrated. The provision the standard temperature sensor is thus to be quite consuming represent.

It is an object of the present invention, a A method for calibrating a temperature sensor, in particular an insertable in weathering temperature sensor as a black panel sensor, a black standard sensor or a white standard sensor, indicate that with can be carried out less effort. Especially is intended to enable such a calibration method that an optionally for use in a reference or standard temperature sensor simpler design or even omitted entirely can be. Another object of the calibration is that it should be adapted under Conditions are executed can, as in operation a Bewitterungsgerätes inside the weathering chamber prevalence.

These objects are achieved by the features of the independent claim 1 dissolved. Advantageous developments and embodiments of the inventive method are in the Subclaims. Also, devices for Carrying out the process indicated.

The calibrated temperature sensor can in itself conventionally be constructed, ie as a black panel sensor, Black standard sensor or white standard sensor to have previously described construction. In general Form the temperature sensor is arranged such that it a in an electrical circuit as a measuring transducer contained temperature-dependent electrical component having. This component is preferably a temperature-dependent Resistance, particularly a platinum resistance.

An essential idea of ​​the present invention the fact that during calibration a contactless temperature measurement is carried out. This contactless temperature measurement can either be calibrated directly from the surface of Temperature sensor or of the surface of a reference temperature sensor be performed. As will be explained will be, this depends on a precise definition of to calibrate a material property of the surface of the Temperature sensor from.

In the method according to the invention is in detail<sl><li>a) on a surface of the to be calibrated, the temperature sensor and where applicable, at a reference temperature sensor, a variable temperature generated</li><li>b) the temperature of the surface of the to be calibrated, the temperature sensor or of the reference temperature sensor contactlessly measured and</li><li>c) an output signal of the electric circuit with the measured Temperature calibrated.</li></sl>

The contactless temperature measurement is preferred dimensions in that in step b) the blackbody radiation measured the surface and from this the temperature is determined. This can with a commercial pyrometer take place, which at least in the the temperature range of interest appropriate measuring range is calibrated. With the pyrometer is under a certain defined angle a spot on the surface of the unknown temperature sensor fixed or of the reference temperature sensor, and of this spot in the corresponding solid angle detects emitted blackbody radiation and from the Surface temperature determined.

If the contactless temperature measurement by measuring the Blackbody radiation carried out, so is this principle requires that the emissivity or the emissivity of the surface material known to calibrate the temperature sensor is because the blackbody radiation emitting Surface to calibrate the temperature sensor not having characteristics of an ideal black body (ε = 1) and the emissivity ε is always less than 1. Especially must also known, the angular dependence of the emissivity be, as preferred dimensions, as mentioned above, in the a given solid angle radiated blackbody radiation is detected by a pyrometer.

In this respect, the emissivity of the surface to be calibrated the The temperature sensor is known, which can be emitted by it measured blackbody radiation and from the surface temperature be calculated. This surface temperature then, the output signal of the electric circuit, in of the platinum resistor is to be calibrated. This would thus have the great advantage that a calibrated Reference temperature sensor constructed in essentially the same as the calibrated temperature sensor, not to would come use.

However, it may also be the case that the emissivity the surface not to calibrate the temperature sensor is known. Practical experience with coating materials black standard sensors or black panel sensors has in fact shown that in particular those black paint, because of their great resistance to aging particularly sought after for use as coatings for Black standard or black panel sensors are emissivities own that are not precisely known. Before the emissivity is determined in a complex manner, which can present invention Process carried out in such cases, such be that a reference temperature sensor with a surface is used with a known emissivity. In process step a) is to be calibrated, both on the surface of Temperature sensor and on the surface of the reference temperature sensor Alike a variable Temperature generated so that be assumed be that the surface temperatures of the two sensors is equal to are. In process step b) then needs only the temperature the surface of the reference temperature sensor contactlessly are measured and this measured temperature then in step c) the output of the electrical circuit of the platinum resistor to be calibrated the Temperature sensor to be calibrated. If desired, can parallel also the blackbody radiation of be measured to be calibrated temperature sensor, so that by comparing the measurements of blackbody radiation at the reference temperature sensor and the temperature sensor to be calibrated, the emissivity of the surface of the latter can be determined.

If due to the unknown emissivity of the surface to calibrate the temperature sensor a reference temperature sensor must be used, it has, however, not the complicated structure of the conventional in which Kontaktthermometrieverfahren used standard temperature sensor on. The reference temperature sensor to be used herein for example, requires no thermal docked temperature-dependent resistive element to the Back together with associated circuitry. Instead, the Reference temperature sensor only of a metal plate exist that one or both sides desired with paint (Black or white) with a known emissivity coated is. Such a constructed reference temperature sensor provides for this variant of the invention Method traceable to a PTB standard one Temperature Standard represents.

The inventive method also makes it possible that it can be carried out under conditions such as are typically within the weathering of the operation located weathering apparatus prevail. this makes possible a calibration of the black and white standard sensors and Black panel sensors as they occur from the operation Influencing factors such as light radiation from the xenon lamp, Airflow and moisture are applied. These factors determine in reality the surface temperature the sensors.

The ruling in a weathering conditions to a significantly by the light radiation from the xenon radiator certainly. In order for the calibration method according to the invention provide approximately the same conditions, can thus be provided in step a) the variable Temperature by exposing the surface of the temperature sensor and optionally that of the reference temperature sensor with the solar spectrum corresponding radiation in particular the radiation of a xenon light source cause. Furthermore it can be provided a similar Air movement of ambient air in the immediate vicinity of causing surfaces of the sensors as in weathering is produced. It can accordingly creates an air stream are coupled to parallel to the surfaces of the sensors These sweeps past.

The above conditions can be ideally provided by be by the calibration in or in Conjunction with a device for the artificial weathering is carried out of samples.

The present invention relates accordingly further to apparatuses for carrying out the method according to the invention, according to two different variants. At a first variant is provided a positioning device, said means for receiving and holding the to be calibrated, Temperature sensor and optionally of the reference temperature sensor and means for holding the contact- measured temperature sensor, in particular of the pyrometer, having. To carry out the calibration must then be ensured that the temperature sensor to be calibrated Heat is supplied, preferably by applying energy radiation from a xenon radiation source. In a second variant is a commercial weathering device converted, so that the inventive method can be carried out therein. There are for this purpose means for receiving and holding the temperature sensor to be calibrated, and, optionally, of the reference temperature sensor to provide, within the weathering chamber, said means can also be given by the sample holder frame. Furthermore, means for attaching a noncontact Temperature sensor (such as a pyrometer) on an outer wall of the needed weathering chamber and at least one opening in the outer wall, so that the pyrometer to in the weathering chamber held temperature sensors can be aligned.

The invention is described with reference to the Drawings in more detail, in which embodiments are shown. Show it:<dl tsize="13"><dt>Fig. 1</dt><dd>to be calibrated is a longitudinal section through a Black standard sensor;</dd><dt>FIG. 2a, b, c</dt><dd>a positioning as part of a Device for implementing the calibration method in a perspective 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 the Implementation of the calibration method adapted Weathering apparatus.</dd></dl>

In FIG. 1, a black standard sensor 10 is shown, as typically in devices for artificial weathering for monitoring the temperature to be examined of Samples of material is used. Such a black standard sensor 10 is in this case in the immediate vicinity the material samples with the same geometric orientation as this supported with respect to the xenon radiation source. The black standard sensor 10 is shown in FIG. 1 in a Longitudinal section along a through the center, ie, the platinum resistor 3 extending plane represented.

The black standard sensor 10 has a rectangular stainless steel plate 1 which on one of its main surfaces with a black paint or black coating 2 provided is. While the intended use of Black Standard sensor 10 as well as during the calibration thereof is the black paint 2 of xenon radiation source 7 faces. On the back of the stainless steel plate unblackened 1 is a platinum resistor 3 mounted, is provided for optimum thermal coupling. Furthermore is on the back side of the stainless steel plate 1, a heat-insulating Plastic plate 5 made of PVDF (Polyvinylidene fluoride) applied. The plastic plate 5 has at its center one of the stainless steel plate 1 facing Recess 5A, in which the platinum resistor 3 is. Adjoining the recess 5A, an up to the the stainless steel plate 1 facing away from the surface of the plastic plate 5 extending implementation to 5B. The implementation 5B serves to supply the electrical lead wires 4 to the platinum resistor 3 via the lead wires 4 is the Platinum resistor 3 with a suitable electrical circuit connected, 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 with the will Light rays 7a of xenon radiation source 7 acted upon, which in Fig. 1 is not drawn to scale with respect to the illustrated black standard sensor 10th As will be explained will be, the calibration procedure within a can Bewitterungsgerätes with the xenon radiation source contained therein 7 carried out. In addition, a stream of air 9 generated, the close to the surface Metal plate 1 flows past this, where the airflow 9 can be generated by a fan. Alternatively, or in addition, the black standard sensor also in a rotational movement be offset by the xenon radiation source 7, so that the vertically directed in this case to the image plane Air flow only in a relative movement between the Black standard sensor 10 and the ambient air is.

The contactless temperature measurement is performed by a commercially available, Calibrated pyrometer 8 that a spectral Sensitivity range of 8-14 <i>μ</i>m owns. The pyrometer 8 detected by a spot with a diameter between 10 and 20 mm on the blackened surface of the stainless steel plate 1 emitted in a solid angle 8a blackbody radiation and calculated from this considering the emissivity the black paint 2, the surface temperature. For In the event that the emissivity of the black paint of 2 to calibrating the temperature sensor 10 is not known, in the immediate vicinity of the unknown temperature sensor 10 a reference temperature sensor with a matte black finish arranged with a known emissivity and from this measured by the pyrometer 8 blackbody radiation. With the calculated from surface temperature is the Platinum resistor 3 and the output of its related electrical (Transmitter) circuit calibrated.

The measurement of the temperature independent per se actual Calibration procedure is known in the art. As already mentioned, the platinum resistor 3 (Pt100 o. Pt1000) via the lead wires 4 to a suitable electrical Circuit. This can, for example, a commercially be available programmable measuring transducer, such as those from the company Intelligent Instrumentation commercially available under the name "DSCP80 module" and at the Internet address http://www.sensorinterface.de/dscp80.html stating a block diagram is described in detail. This prior art is hereby included in the disclosure of the present application.

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

For carrying out the process according to the invention, there is In principle, the following two possibilities. Firstly, a suitable positioning device are provided, on the one hand to be calibrated temperature sensor and the reference temperature sensor and on the other hand, the pyrometer positioned in a reproducible manner relative to each other can be. then you have a radiation source for calibration are brought into the vicinity of the temperature sensors, for which purpose also an exposure apparatus in the Near the positioning can be accommodated. On the other hand The inventive method can thus performed be by an existing weathering in is suitably modified.

In the Figures 2a, b and c is the first-mentioned variant shown from various angles. It deals , this is a positioning device 20, the two parallel horizontal rails 14 having at which at its fixed a vertical end two parallel rails 15 are. On the horizontal rails 14 is a Platform 13 is placed running along the rails 14 moved and fixed to the desired position on these can be. On the platform 13 are two rectangular Receiving blocks 11 arranged side by side such that they in guide channels which extend parallel to the rails 14, toward the rails 14 are movable. The receiving blocks 11 each on its upper side rectangular recesses, into which the temperature sensors can be inserted. The receiving blocks 11 may at any desired position are attached to the platform. 13

At its upper end, the vertical mounting rails have been 15 connected by a horizontal rail 16 together. can on the upper side of the horizontal rail 16 a clamping device 12 for a pyrometer in two positions are attached, in which the pyrometer each have a both positioned in the receiving blocks 11 temperature sensors can be aligned as shown particularly in FIG. 2c is seen. The clamping device 12 consists essentially of a tubular cavity, in which the pyrometer be inserted from one side and clamped therein can.

Regarding the latter variant for carrying out the The inventive method is an existing device for artificial weathering of material samples such rebuilt or adapted to the inventive method can be carried out with it. This essentially means that within the Bewitterungsgerätes on one of the Interior walls receiving and supporting portions for the temperature sensors underfoot. In Fig. 3 is a perspective View of a portion of the interior of an artificial Weathering apparatus shown. On the lower horizontal Inner wall are to be calibrated and temperature sensor the reference temperature sensor positioned adjacent one another. In the located in the longitudinal direction of the temperature sensors vertical Inner wall are formed two openings through which the externally fastened pyrometer on each one of the two Temperature sensors can be aligned. It ranges in Principle from when only one opening in the housing wall is formed, as in the calibration principle, a contactless Temperature measurement at a sensor is sufficient. Either there is a temperature sensor to be calibrated with a known emissivity in the apparatus, so only needs at this temperature are measured. If due unknown emissivity of the unknown temperature sensor Additionally, a reference temperature sensor is present, so only the temperature of this must be measured and on to be calibrated temperature sensor to be calibrated. Preferably, however, two openings are formed so that from exclude the possibility of the externally mounted Pyrometer both provided for receiving the temperature sensors to aim at receiving areas. The pyrometer can from the outside with a latch or the like. fixed will.