Method for the in-situ calibration of a thermometer

Abstract

The present disclosure relates to a method and apparatus for in-situ calibration and/or validation of a thermometer having a temperature sensor and a reference element composed at least partially of a material that undergoes a phase transformation at a phase transformation temperature, wherein the material remains in the solid phase in the phase transformation, the method including detecting and/or registering a measured value from the temperature sensor; detecting and/or registering a reference variable of the reference element; detecting the occurrence of the phase transformation based on a change of the reference variable; ascertaining a phase transformation time at which the phase transformation occurs; determining a sensor temperature using the temperature sensor at a measurement time that has the shortest time separation from the phase transformation time; and comparing the sensor temperature with the phase transformation temperature and/or determining a difference between the sensor temperature and the phase transformation temperature.

Claims

1. A method for in-situ calibration and/or validation of a thermometer, the method comprising: providing a thermometer having a temperature sensor and a reference element composed at least partially of a material that undergoes a phase transformation at at least one phase transformation temperature within a temperature range of operation of the thermometer, wherein the material remains in the solid phase in the phase transformation; detecting and/or registering a measured value as a function of time using the temperature sensor; detecting and/or registering a characteristic physical or chemical reference variable as a function of time from the reference element; detecting the occurrence of the phase transformation based on a change of the reference variable; ascertaining a phase transformation time at which the phase transformation occurs; determining a sensor temperature from the measured value obtained using the temperature sensor at a measurement time that has the shortest time separation from the phase transformation time; and comparing the sensor temperature with the phase transformation temperature and/or determining a difference between the sensor temperature and the phase transformation temperature.

2. The method of claim 1, wherein the material is a ferroelectric material, a ferromagnetic material or a superconducting material.

3. The method of claim 1, wherein the characteristic physical or chemical variable is a crystal structure, a volume, or a dielectric, electrical or magnetic property of the material.

4. The method of claim 1, wherein the temperature is ascertained based on a comparison of the measured value obtained using the temperature sensor at the measurement time with a temperature sensor characteristic line or curve.

5. The method of claim 1, wherein a model of dynamic heat flow is used for determining the difference between the sensor temperature and the phase transformation temperature.

6. The method of claim 4, wherein the difference between the sensor temperature and the phase transformation temperature indicates a change of the temperature sensor characteristic line or curve.

7. The method of claim 5, wherein, based on the model of dynamic heat flow, a time correction value is ascertained after which the reference element and the temperature sensor have the same temperature, and wherein the time correction value is used for determining the difference between the sensor temperature and the phase transformation temperature.

8. The method of claim 5, wherein, based on the model of dynamic heat flow, a temperature correction value is ascertained, which is present at a determinable point in time between the reference element and the temperature sensor, and wherein the temperature correction value is used for determining the difference between the sensor temperature and the phase transformation temperature.

9. The method of claim 5, wherein the model of dynamic heat flow is a parametric model.

10. An apparatus for in-situ calibration and/or validation of a thermometer, the apparatus comprising: a temperature sensor; a reference element composed at least partially of a material in which at least one phase transformation occurs at at least one phase transformation temperature within a operating temperature range of the temperature sensor, wherein in the case of which phase transformation the material remains in the solid phase, and an electronics unit configured to: register a measured value as a function of time using the temperature sensor; register a characteristic physical or chemical reference variable as a function of time from the reference element; register the occurrence of the phase transformation based on a change of the reference variable; ascertain a phase transformation time at which the phase transformation occurs; determine a sensor temperature from the measured value obtained using the temperature sensor at a measurement time that has the shortest time separation from the phase transformation time; and compare the sensor temperature with the phase transformation temperature and/or determine a difference between the sensor temperature and the phase transformation temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be explained in greater detail based on the appended drawing. Equal elements of the apparatus are provided with equal reference characters. The figures of the drawing show as follows:

(2) FIG. 1 shows a schematic representation of a thermometer having a temperature sensor and a separately contacted reference element for determining and/or monitoring temperature of a flowing liquid according to state of the art;

(3) FIG. 2 shows an illustration of a first embodiment of the method of the present disclosure; and

(4) FIG. 3 shows an illustration of a second embodiment of the method of the present disclosure, taking into consideration a dynamic model of the heat flow; and

(5) FIG. 4 depicts a flowchart of a method of the present disclosure.

DETAILED DESCRIPTION

(6) FIG. 1 presents a schematic illustration of a thermometer 1 with a protective tube 2 and an electronics unit 4 according to state of the art. The portion of the protective tube 2 toward the liquid 5 is referred to also as the sensor head 3. The internal volume of the sensor head 3 is filled with an, especially electrically insulating, filler 6, especially a cement. Furthermore, there are arranged in the interior of the sensor head 3 a temperature sensor 7 and a reference element 8, each of which is contacted, especially electrically contacted, by means of two connection wires, 9, 10 and connected with the electronics unit 4. Temperature sensor 7 is, for example, a resistance element or a thermocouple. Reference element 8, in turn, is composed at least partially of a material in which at least one phase transformation at least of second order occurs at least one predetermined phase transformation temperature within a temperature range relevant for operation of the apparatus. The number of needed connection wires 9, 10 for contacting the reference element 8 and the temperature sensor 7 can vary, depending on type of measuring principle applied. In the shown embodiment, the temperature sensor 7 and the reference element 8 are arranged mutually separated within the sensor head 3. They can, however, likewise directly contact one another and be, for example, soldered together.

(7) FIG. 2 illustrates how the temperature sensor 7 can be calibrated and/or validated by means of the reference element 8 based on the method of the invention. The upper diagram represents the course of a characteristic physical or chemical variable G used for detection of the phase transformation. If a phase transformation occurs in the reference element 8, this is accompanied in the illustrated example by an abrupt change of the variable G. The point in time, at which the abrupt change of the variable is detected, is the phase transformation point in time t.sub.ph at which the reference element 8 has the phase transformation temperature T.sub.ph.

(8) The lower diagram is the sensor temperature T ascertained by means of the temperature sensor 7 as a function of time t. For calibration and/or validation of the temperature sensor 7 based on the reference element 8, a measurement point at time t.sub.m is ascertained, which has the shortest time separation from the phase transformation point in time t.sub.ph. The sensor temperature T.sub.m corresponding to the measurement point at time t.sub.m is compared with the phase transformation temperature T.sub.ph and, in the case of a difference ΔT=T.sub.m(t.sub.m)−T.sub.ph(t.sub.ph) above a predeterminable limit value, the thermometer 1 can be automatically calibrated and/or validated and/or a report concerning the occurrence of a difference generated and/or output.

(9) For a high accuracy of measurement, it must be assured that the temperature sensor 7 and the reference element 8 are ideally at all times in thermal equilibrium. In order to achieve this, usually various measures are performed of which some are listed in the following, by way of example: 1. Temperature sensor 7 and reference element 8 are arranged symmetrically within the sensor head 3, especially symmetrically relative to an imaginary axis extending in the longitudinal direction of the protective tube 2 through a center of the protective tube 2. 2. Temperature sensor 7 and reference element 8 are as thermally well coupled as possible (e.g., soldered). 3. Support substrates, in given cases, applied for the temperature sensor 7 and/or the reference element have essentially the same thermal conductivity. 4. Temperature sensor 7 and the reference element 8 are embodied in such a manner that they have essentially the same thermal capacitance. 5. The filler 6 and/or partitions (not shown) arranged in the region of the sensor head 3 are formed in such a manner that they assure isotropic and/or homogeneous heat movement within the sensor head 3. 6. All components of at least the sensor head 3 are embodied in such a manner that they have an as high as possible thermal conductivity. 7. The connection wires 9, 10 are so embodied that heat conduction occurring via the connection wires 9, 10 is minimum, and preferably is essentially the same via each connection wire 9, 10.

(10) Even with greatest care with reference to the manufacture of a thermometer 1, however, different cases can occur in which the temperature sensor 7 and the reference element 8 are at least at times not in thermal equilibrium and, correspondingly, are exposed to different temperatures. This can in turn lead to considerable errors and/or measurement inaccuracies in the case of a calibration and/or validation of the temperature sensor 7 by means of the reference element 8.

(11) In order the to be able to cope with this problem, in an additional embodiment of the method of the invention a dynamic model of the heat flow, or heat movement, is taken into consideration, such as illustrated, by way of example, in FIG. 3. The model is, in such case, adapted to the specific application of the thermometer 1 and takes into consideration, for example, in given cases, present heat flow, temperature or also temperature rate of change of the liquid 5 or the environment of the thermometer, the thermal conductivities and/or heat capacities of the particular materials utilized for the thermometer 1, or components, geometric dimensions within the thermometer, and/or the immersion depth of the thermometer in the particular liquid.

(12) FIG. 4 shows an embodiment of a method of employing a dynamic model of the heat flow according to the present disclosure. The following description is for the case in which the thermometer 1 is applied for determining the temperature of a flowing liquid 5. The thermometer 1 is, in such case, in contact with the flowing liquid 5 in such a manner that the temperature sensor 7 is arranged in the region facing against flowing liquid 5 and the reference element 8 in the region facing in the flow direction of the liquid 5. In this case, upon a temperature change from a first T.sub.1 to a second T.sub.2 temperature, the temperature sensor 7 reaches the second temperature T.sub.2 always at an earlier point in time than the reference element 8.

(13) If at a point in time t.sub.ph, the occurrence of a phase transformation is detected, the temperature of the reference element 8 corresponds, at this point in time, to the phase transformation temperature T.sub.ph. From a direct comparison of the phase transformation temperature T.sub.ph with the sensor temperature T.sub.m of the temperature sensor 7 at a measurement point in time t.sub.m, which has the shortest time separation from the phase transformation point in time t.sub.ph, however, no correct information for a calibration and/or validation of the temperature sensor 7 by means of the reference element 8 can be derived, since, as above described, because of the flow of the liquid 5, the temperature reigning at the site of the reference element 8 lags the temperature reigning at the site of the temperature sensor 7.

(14) The dynamic model is embodied, for example, to provide a suitable correction value, for example, a temperature correction value ΔT.sub.dyn or a time correction value, which correction value takes into consideration the influence of the inhomogeneous heat flow, or heat movement, within the thermometer 1, especially within the sensor head 3. The model is applicable for the case of an at least at times and/or partially inhomogeneous temperature field, which is caused by a liquid (for example, for application in a flowing liquid) or which is caused by the environment of the thermometer (for example, heat removal).

(15) The particular correction value is used then for determining a difference present, in given cases, between the sensor temperature T.sub.m and the phase transformation temperature T.sub.ph. For example, the difference is determined based on one of the two Equations, ΔT=T.sub.mdyn(t.sub.m−Δt.sub.dyn)−T.sub.ph(t.sub.ph), or ΔT=T.sub.m(t.sub.m)−ΔT.sub.dyn−T.sub.ph(t.sub.ph). Alternatively, the correction values can also, in each case, be suitably added.