CS197742B1 - Method of making the electric moisture-measuring sensor - Google Patents

Description

The present invention relates to a method of manufacturing an electric hygrometer sensor.

One possibility for measuring the humidity of the environment is to use sensors to convert the humidity into an electrical signal. In particular, LiCl-based sensors coated with a suitable binder on a substrate, powdered carbon sensors coated with a suitable binder on a substrate, or sensors based on Al 2 O 3 based alumina are currently produced.

LiCl and powdered carbon sensors are moisture sensitive due to volume changes in the layers. However, their disadvantages are long response times to moisture change, instability of its calibration curve and strong dependence on sensor temperature.

More advantageous properties have a sensor made on the basis of porous alumina Al2O3, in which the adsorption of water on the surface of the pores and thus changes the electrical parameters. However, this is a surface process rather than a volumetric process and therefore the response time of these sensors is much shorter. The achievable long-term calibration stability and the measured resistance of these sensors depend on the technology of preparation of the porous layer of aluminum oxide Al2O3 and their artificial aging. At present, best results are achieved with sensors manufactured according to the Stover process, where the Al2O3 aluminum oxide layer is produced on clean polished

9 7 7 4 2 of aluminum by anodic oxidation with an alternating current of approximately 12 mA / cm ³ electrolyte of 50% sulfuric acid H ₂ SO ₄ (pa) at 33 ° C for 25 minutes. After washing with distilled water, the resulting layer is immediately exposed to artificial aging, the so-called sealing, which takes place in boiling redistilled water for about 30 minutes and a semipermeable top electrode is formed in a suitable manner by vacuum evaporation, for example. The disadvantage of the sensor produced in this way is that during operation, the adsorbed water affects the Al ₂ O ₃ alumina and its gradual hydration, resulting in the formation of α-alumina monohydrate in the pores, the so-called bohmite, which leads to irreversible structural changes sensor and its properties.

These drawbacks are overcome by a method of manufacturing an Al ₂ O ₃ -aluminium oxide electric moisture sensor made according to the Stover process, which is subjected to artificial aging, known as sealing, followed by passivation according to the invention. It is based on the fact that the surface of the porous Al ₂ CO 3 layer is passivated by wetting in an aqueous solution of 0.01 to 2 moles of sodium phosphate, for example NaH ₂ PO 4. H ₂ O at temperatures ranging from 35 to 100 ° C and for 10 to 60 minutes.

An advantage (of the method of manufacturing the sensor according to the invention) is that the sensor thus produced has a significantly better long-term stability and the sensor has a greater sensitivity in the area of low moisture dew points.

Said effects and advantages of the invention result from the fact that the passivation of the surface of Al 2 O 3 alumina significantly reduces the rate of its reaction with the adsorbed water, thereby suppressing the major factor causing changes in the sensor structure and its properties.

The method according to the invention is explained in more detail by way of example with reference to the accompanying drawings, in which Fig. 1 shows the calibration stability of the sensor without passivation of the Al2O3 alumina surface; Fig. 2 shows the calibration stability of the sensor according to the invention;

Calibration curve 6.9. Fig. 1 shows the waveform of the sensor signal as a function of the dew point of the humidity sensor produced according to the Stover process without the passivation of the Al 2 O 3 alumina surface.

Calibration curve 2.12. FIG. 1 shows the same dependence of the same sensor measured at three months.

Calibration curve 6.9. Fig. 2 shows the waveform of the sensor signal as a function of the dew point of the moisture meter of the electrical moisture meter produced by the method of the invention. Calibration curve 2.12. Fig. 2 shows the same dependence of the same sensor measured at three months.

Temperature of the sensors when measuring all dependencies at 20 ° C.

The electrical hygrometer sensor of FIG. 3 consists of an aluminum backing sheet 1 which also forms one sensor electrode, a base layer 2 of porous Al2O3, and an insulating varnish layer 3 which reinforces the insulation at the connection point of the lead to the second electrode formed by a semipermeable metal layer 4.

Example

An electrical hygrometer sensor made according to the Stover process is passivated by wetting in an aqueous solution of 0.1 mol of sodium phosphate NaH 2 PO 3. H ₂ O at 90 ^Q C for 10 minutes.

Due to the good stability of the sensor made according to the invention, it can be used to monitor the time changes in humidity of, for example, buildings, stored goods and the like. Thanks to the rapid response to moisture change (τ <1 sec), this sensor can also be used to monitor fast processes, for example in continuous dryers in the food industry.

Claims (1)

SUBJECT A process for the manufacture of an electric hygrometer sensor based on anodic oxidation in an electrolyte with anodic aging, with artificial aging, characterized in that its porous layer of Al2 O3 alumina is passivated by wetting in an aqueous solution of 0.01 to 2 moles of sodium phosphate, for example NaH2PO4. H 2 O at a temperature of 35 to 100 ° C for 10 to 60 minutes.