Patent Application
20090044593
The present invention relates to a method according to the preamble of Claim 1 for calibrating a relative-content measurement.
The invention also relates to an apparatus intended to apply the method.
There is an increasing need on the market for hygrometers with a measurement result that would be as accurate as possible and a calibration procedure that would be according to a standard and traceable. The term traceability refers to a calibration procedure, in which the calibration operations are precisely documented while the actual calibration event produces documents for later examination.
In the prior art, calibration vessels for making a precise calibration are available, in which a predefined relative content is created in saturated solutions of various salts. Non-saturated salt solutions, glycerol solutions, and hydrochloric acid solutions are also known for this purpose. The problem with these solutions has been that the correction of both a gain error and an offset has required at least two measurement stages in different calibration vessels. Each calibration stage requires a settling time, which typically varies from a few minutes to half an hour, so that in the worst case making a full calibration will require an hour's extra work. Because the measuring devices are intended for field use and calibration can take place even daily, the time used for calibration is significant. Because typically individual measuring devices are involved and the calibration vessel must be changed during calibration, it is not economically worthwhile to automate multi-vessel calibration.
In particular, automation is made difficult and expensive by the fact that the calibration vessel must be tight. This means that robotized multi-vessel calibration using this manner of implementation will be extremely expensive and unsuitable for field calibration.
A rapid field calibration method is known from U.S. Pat. No. 5,033,284, in which the temperature of a sensor element is measured and the temperature of the sensor element is deviated from the ambient temperature in order to achieve the calibration points.
A drawback with this technique is that the use of the method gives information only on the offset error. If a change takes place in the gain of the sensor, the method will not detect it. In addition, this manner of calibration is not traceable, as the content in the environment is not measured and thus not documented. Though the publication refers to a possibility of two-point calibration, this requires both heating and cooling of the sensor. Cooling in turn demands the use of an expensive cooling element.
The invention is intended to eliminate defects of the arising in the solutions described above and for this purpose to create an entirely new type of method and apparatus for the calibration of the measurement of a relative content.
The invention is based on the sensor being brought, typically in a single closed measurement vessel, to the partial pressure Pw of a known gas, at which it is heated and both the temperature of the sensor and the value proportional to the relative content are measured at the increased temperature.
Thus in the invention, for example, a known partial pressure of a gas is created in a measurement vessel, for example, by means of a saturated or unsaturated solution of salt, glycerol, or acid solution. The measuring device is first allowed to reach a state of equilibrium with the ambient temperature in the gas space in connection with this solution, in order create one calibration point. After this, the sensor, such as a humidity sensor, is heated, using an internal heating resistor, to at least one increased temperature, at which both the temperature and the reading of the relative humidity are measured and recorded. Thus additional calibration points can be created. If the source of the humidity and the temperature measurement are traceable, these new reference points will also be traceable.
More specifically, the method according to the invention is characterized by what is stated in the characterizing portion of Claim 1.
The apparatus according to the invention is, in turn, characterized by what is stated in the characterizing portion of Claim 11.
Considerable advantages are gained with the aid of the invention.
The invention also has preferred embodiments, by means of which it is possible to implement gain and offset calibration automatically, for example, after the workday. Thus the user need only place the measuring device in one calibration station and take the fully calibrated device with them at the next work shift. The sensor thus need not be transferred from one calibration vessel to another.
Using the method, it is possible to avoid the time-consuming use of several calibration vessels and to achieve an easy situation corresponding to full laboratory calibration entirely automatically.
With the aid of some embodiments of the invention, it is possible to achieve traceable multi-point calibration with minimum equipment alternations and user's work.
With the aid of preferred embodiments of the invention, a simple and rugged solution for field calibration is achieved with very little work by the user.
In the following, the invention is examined with the aid of examples and with reference to the accompanying drawings.
Relative humidity is defined as follows
RH=Pw/Pws(T)*100%, in which
Pw=partial pressure of water
Pws(T)=saturation partial pressure of water, which is a function of temperature.
Pws(T) is well known in the literature. In a closed vessel Pw remains at a constant magnitude. By heating the sensor, Pws(T) increases, in which case RH decreases. The example below, relating to
In the first measurement, the temperature of the sensor (20° C.) has been the same as the ambient temperature and the relative humidity seen by the sensor has then been 97.6%, which defines a saturated solution of calcium sulphate (K2SO4). Once the sensor has been heated to 25 degrees, the relative humidity seen by the sensor changes to the value 72.5 while the saturation partial pressure of water Pws/(T), obtained through the temperature measurement, is correspondingly the value 23.385 hPa. In the table and in
The measurement data can be used to correct the reading of the device, in other words to correct both the offset and the gain errors. The correction can be linear, or based on a higher-degree correction algorithm.
The data of the calibration operation can be recorded in the memory of the device and possibly output. The data can include the measurement data prior to calibration and the measurement data corrected using the calibration data.
The apparatus can be used to monitor the rate of change and temperature values of the humidity/dew-point/partial pressure of gas and to automatically record information if the rates of change are below a predefined limit.
According to the invention, the sensor is heated in the calibration situation at such a low power that, in practice, it does not affect the temperature of the measurement vessel or of the gas surrounding it. The heating effect is typically less than 5 mW/° C.
The construction of the sensor should be such that the sensor measuring temperature is in very good thermic contact with the sensor measuring relative humidity. Very typically the element measuring temperature is also used as the heating element.
The sensor can be implemented, for example, as a laminar structure, in which a planar temperature sensor is placed in the immediate vicinity of a planar capacitive polymer sensor.
According to one preferred embodiment of the invention, the gas content in the calibration situation is determined with the aid of a separate precision measuring head.
Thus, according to the invention, either
Alternative additional operations include:
Recording measurement data before or after in the device being calibrated. A deviation between the before/after data can be used as diagnostics information. A large difference will cause an alarm and possibly a recommendation to send the device for servicing.
The source of humidity used can be water or a water solution, which is absorbed in felt or some other water-absorbing material, so that there is no free water. It is also possible to use at least three temperatures (ambient temperature and at least two higher temperatures). The deviation of the third calibration point after the offset and sensitivity corrections is used as diagnostics information. An excessive deviation will cause an alarm and possibly a recommendation to send the device for servicing.
The output of the humidity sensor can be, for example, relative humidity, the partial pressure of a gas, a dew point, or some other quantity, as long as the sensor signal is approximately proportional to the humidity. The methods described assume that the partial pressure of the gas remains of a constant magnitude in the vicinity of the sensor during the calibration process.
According to
The calibration event thus consists of a settling time, which is from several minutes to tens of minutes. The change in temperature can be implemented over a few minutes.
According to one preferred embodiment, the sensor's 2 heating resistor 3 is the same as the sensor's 2 temperature measuring resistor.
In the measuring apparatus, there can also be a separate temperature measuring device or arrangement, independent of the sensor 2 being calibrated, by means of which the sensor's temperature measurement offset can be eliminated. This temperature measurement is preferably traceable. The separate precision temperature-measuring device can be integrated, for example, in the measuring vessel 10, preferably in the gas space, in the vicinity of the sensor 1.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20050530 | May 2005 | FI | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FI2006/000157 | 5/16/2006 | WO | 00 | 11/15/2007 |
