WO1985005447A1

WO1985005447A1 - Device for measuring a physical magnitude

Info

Publication number: WO1985005447A1
Application number: PCT/CH1985/000085
Authority: WO
Inventors: Kurd G. GRÖNINGER
Original assignee: Thalmond Anstalt
Priority date: 1984-05-24
Filing date: 1985-05-21
Publication date: 1985-12-05
Also published as: EP0183734A1

Abstract

New measuring device comprised of a light source (1), a sensor element (2) and a photo-element (3). Light wave guides (4, 4') take the light to and from the sensor element (2). The sensor element (2) contains elements which go through a change of colour as a function of the physical magnitude. Due to this change of colour, it is possible to measure the change of the physical magnitude. Since this measuring principle may be applied to various physical magnitudes, it is possible to make without any particular difficulty integrated sensors enabling to simultaneously detect measuring data in the most reduced space.

Description

Device for measuring a physical quantity

The invention relates to a device for measuring a physical quantity according to the preamble of claim 1. Such a device is e.g. known from "Laser and Optoelectronics", No. 3/1983, pages 226 to 234. There, a temperature sensor is described which consists of an optical waveguide with atoms of rare earths doped therein, in particular neodymium and europium. The dopants can be excited to radiate luminescence by short-wave light, the intensity of which is temperature-dependent. Optical fibers doped with europium and neodymium also show temperature-dependent attenuation. For such a temperature sensor, two alternately pulsed LEDs are used, which emit at two different wavelengths. The light is divided between the undoped reference fiber and the doped fiber sensor via a fiber coupler and measured with two detectors. By using two LEDs of different wavelengths and forming the ratio, you get a measurement that is independent of changes in the intensity of the LED. An example is a 5% neodymium-doped optical waveguide with a length of 15 cm. With the aforementioned temperature sensors, measurements between 50 ° C and 250 ° C with a resolution of 0.1 ° C are possible.

The known measuring device has the major disadvantage that a complex technology is required to measure the physical quantity, here the temperature, and that the measuring range is limited to temperatures well above room temperature. In particular, measurements must either be carried out via luminescence or then two alternately pulsed LEDs with two different wavelengths are required in order to convert one to be able to carry out casual temperature measurement. Another major disadvantage is that the actual sensor is too long to be able to carry out measurements in places that are difficult to access and in a confined space. In addition, due to the measurement principle mentioned above, measurements of physical quantities other than temperature are not possible. A thorough research of the literature known in the field of fiber optics has shown that a different measuring principle must be used for the measurement of different physical quantities, which means an integration of sensors for the simultaneous measurement of different physical quantities such as pressure, temperature, humidity, air velocity , very difficult, so not almost impossible. On the other hand, there is an integrated measurement for many process engineering. Control systems of extraordinary importance.

The object of the invention is therefore to provide, on the basis of a device for measuring a physical quantity of the aforementioned type, a simpler and more economical one, which is based on a measuring principle which is simultaneously suitable for many, very different physical quantities.

The invention has the enormous advantage that several physical quantities can now be recorded using the same measuring principle, which enables the various sensors to be integrated in a very small space. This creates unprecedented possibilities for control problems, such as in the drying of bulk goods, for example animal feed, where only the detection of the relative humidity and the temperature on both the dried goods and the exhaust air results in optimal control of the drying system. In such systems, a fully automatic, computer-controlled process line can also require the measurement of the moisture, temperature and bulk density of the wet material entering the dryer, whereby the measured values must be recorded at the same location. Another The measurement of the compressed air for the brakes of trucks represents a game. In order to achieve an optimal regulation of the compressed air, the simultaneous measurement of the relative humidity, the temperature and the pressure is absolutely necessary. The invention has made it easy to measure these sizes even in the smallest of spaces and in hard-to-reach places. Further advantages of the invention result from the following description. There, the invention is explained in more detail using an example shown in the drawing. It shows:

1 shows an embodiment of the measuring device, and

Fig. 2 shows the absorption spectrum of CoCl ₂ depending on the relative humidity.

1 shows a basic arrangement of a measuring device. It consists of a light source 1, a light-emitting diode, a sensor element 2 and a photo element 3. The sensor element 2 is connected on one side to the light source 1 via an optical fiber 4, and on the other side to the photo element 3 via an optical fiber 4 ' , a photodiode or a phototransistor. The light source 1 is mounted on a base 5, the photo element 3 on a base 5 ', and connected via a cable 6 to a voltage supply or a signal amplifier. The light source 1 and the photo element 3 are coupled to the optical waveguides 4 and 4 'using a light-conducting paste 7.

2 shows the absorption spectrum of CoCl ₂ as a function of the relative humidity. The wavelength λ in nm is shown on the abscissa, and the transmission coefficient T in% is shown on the ordinate. The curve α is more relative at 35%

Humidity, the curve α _{z recorded} at 100% relative humidity. From the figure it can be seen that there is a strong change in the absorption as a function of the relative at about 650 nm Moisture there. This property is now used for the measurement. For this purpose, the light source 1 should emit monochromatic light of 650 nm and the photo element 3 should be particularly sensitive to this spectral range. There are a number of other moisture sensitive salts, such as CoBr ₂ ,

CoNO ₃ , NiCl ₂ , Ni (NO ₃ ) ₂ , NiSO ₄ , LaCl ₃ , CuCl ₂ , Cu (NO ₃ ) ₂ ,

CuSO ₄ or Er (NO ₃ ) ₃ . The associated sensor element 2 preferably consists of a cylindrical body made of a water vapor permeable and hydrophobic plastic which contains one of the salts mentioned above in crystal form. Another sensor element 2 consists of a cylindrical body made of a cellulose derivative such as cellulose acetate, cellulose propionate or cellulose acetate butyrate, in which one of the salts is dissolved. Instead of one of the salts, certain dyes are also sensitive to moisture and suitable for the measuring device described.

The measuring method itself is based on the moisture-dependent color change of the salt. With cobalt chloride, for example, this color change is a consequence of the ligand exchange on the Co ²⁺ ion. With sufficient presence of water molecules, it is coordinated octahedral with the cobalt chloride, otherwise the cobalt (II) ion is directly octahedral with the chloride. Since the cobalt chloride with the H ₂ O-coordinated complex primarily absorbs shorter wavelengths, the color lies in the red spectral range. In the dry state, light of longer wavelengths in particular is absorbed, so that the crystal assumes the complementary color blue. There are many shades of color between the two color states, since the change in coordination is based on a statistical distribution and mixed co-ordinations with the two ligands are also known. A similar mechanism of light absorption applies to the other salts. The observed swellability of the film at very high humidity

- over 95% relative humidity - has no influence on the measurements, since it is a reversible and therefore reproducible quantity. Only condensed water running down could change the uniform salt concentration in the film. However, a coating of a water vapor-permeable and hydrophobic plastic such as propylene can prevent free water from forming in the film. In order to achieve a continuous color transition across the entire moisture range, the striking color transition is set, for example, at around 50 to 60% relative humidity. For this purpose, the water vapor concentration in the salt is reduced by the strongly hygroscopic salts such as LiCl and M gCl ₂ -. An optimal color change at a relative humidity of 55% was achieved with a cellulose hydrate film (cigarette paper) of 50 µm thick and a coloring mixture of 71.5 volume percent saturated CoCl ₂ . 6H ₂ O solution and 28.5 volume percent saturated MgCl ₂ solution.

The sensor element 2 can also be a film made of a water vapor-permeable and hydrophobic plastic such as propylene, or of a cellulose derivative such as paper, cellulose hydrate or cellophane, so that it can also be used at high temperatures (e.g. 200 ° C).

The change in transmission of the light through the carrier 2 follows the Lambert-Beer law as the relative humidity decreases:

l = l _o . e ^{- Acd}

in which

I = light intensity after absorption I _o = light intensity of the incident light c = concentration of the salt, here of chloride-coordinated cobalt d = thickness of the water vapor permeable body (film) A = constant of the material properties of the body permeable to water vapor

The constant A is also dependent on the wavelength of the incident light. In order to obtain a linear function between the color change and the relative humidity, a logarithmic amplifier is now connected downstream of the photoelectric element 3. This amplifier is built into the base 5 'of the photodiode 3 (Fig. 1). However, a different construction of the measuring device than that shown in FIG. 1 is also possible. For this purpose, the light emitted by the light source 1 is pulse-modulated by means of a modulation circuit and converted into a DC voltage signal by a demodulation circuit after the photoelectric element 3. This ensures that the relative humidity is measured completely independently of the influence of stray light. In this case too, it is expedient to install the circuits in the form of integrated circuits in the base 5 'or 5' of the light source 1 or the photoelement 3. This enables a very compact design, so that the measuring device can be used without problems even in very confined spaces.

However, there are also substances such as dehydrodianthrone that change color depending on pressure or temperature. A detailed description of the so-called thermochromism and piezochromism can be found in Angew. Chemistry, 70th year 1958, No. 1 pages 14 to 20, which is why it is not discussed further here. As follows from this article, the dehydrodiathrone is excellent for the above measurement method to measure pressure or temperature. Thermochromism has generally been found in aromatically substituted ethylenes. In these cases, the sensor element 2 itself consists of a heat-conducting or pressure-sensitive body in which the substances are contained.

Other thermochromic substances are known from j. Amer. chem.Soc. 76 (1954), pages 4134 to 4136. Label list

1. Light source

2. Sensor element

3. Photo element 4,4 '. Optical fiber 5.5 '. base

6. Cable

7. light-conducting paste

α ₁ , α ₂ transmission curves

Claims

1. Device for measuring a physical quantity with at least one light source (1) that generates monochromatic light in the visible range, a sensor element (2) that is located between at least two optical fibers (4, 4 '), and at least one photo element (3 ), characterized in that the sensor element (2) experiences a color change depending on the physical size, such that the change in intensity of the monochromatic light is a measure of the change in the physical size.

2. Device according to claim 1, so that the sensor element (2) consists of a water-vapor-permeable body in which salts or dyes are dissolved, which undergo a color change depending on the moisture or water activity.

3. Device according to claim 1, so that the sensor element (2) consists of a thermally conductive body in which substances are dissolved which undergo a color change depending on the temperature.

4. The device according to claim 1, characterized in that the sensor element (2) consists of a pressure-sensitive body in which dyes are dissolved, which experience a change in color depending on the pressure.

5. The device according to claim 2, characterized in that the body consists of a water vapor permeable and hydrophobic plastic or of a cellulose derivative, and the moisture-sensitive salt from one of the salts CoCl ₂ , CoBr ₂ , CoNO ₃ , NiCl ₂ , Ni (NO ₃ ) ₂ ,

NiSO ₄ , LaCl ₃ , CuCl ₂ , Cu (NO ₃ ) ₂ , CuSO ₄ or Er (NO ₃ ) ₃ .

6. The device according to claim 3, characterized in that the body made of a heat-conducting and hydrophobic plastic and the temperature-dependent material made of Co (NO ₃ ) ₂ . 2 hexamethylenetramine. 10 H ₂ O or consists of dehydrodianthrone or diflavyen.

7. The device according to claim 4, that the pressure-sensitive body consists of a plastic and translucent plastic and that the dye consists of a piezochromic, aromatically substituted ethylene.