JPS6330987Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for measuring the absolute pressure (absolute value of pressure expressed in bars) of air. BACKGROUND OF THE INVENTION Absolute air pressure measuring devices are necessary for various applications, for example for pressure measurement in the intake pipes of internal combustion engines or for correcting pressure values in fuel injection systems. To date, therefore, transmitters have always been used to measure the deflection of a membrane evacuated on one side, for example in the form of a barometric vacuum box with a deflection detector or a membrane with a stretch-measuring strip. This transmitter has the disadvantage of a complicated structure and difficult operation. Furthermore, since pressure differences are always measured, the measurement results are affected by vacuum fluctuations on one side.

The device of the present invention having the features set forth in Claim 1 of the Utility Model Registration has the advantages that it is relatively simple to manufacture and easy to operate compared to known devices, and can measure the absolute value of air pressure. has. In this case, unlike the transmitters used up to now, there is no need for mechanical transformations that would make the measurement results uncertain. The result of the measurement is a current that depends on the magnitude of the air pressure, and this current value can be easily processed.

Advantageous embodiments and improvements of the device according to the invention are possible by means of the measures set forth in claims 2 to 10 of the utility model registration. It is particularly advantageous if the device has a heating conductor and a temperature-dependent resistance. This is because the measurement result is temperature dependent and the temperature of the device must therefore be kept constant for accurate measurements.

The device according to the invention is based on the principle of a polarographic sensor as described, for example, in DE-A-2711880. This sensor operates in the diffusion-limited current range and makes it possible to measure the oxygen concentration (thermodynamic concentration according to the Nernst equation) in a gas mixture, for example the exhaust gas of an internal combustion engine.
The diffusion barrier provided by this sensor generally has pores with a diameter greater than 1 μm. Such a pore size is larger than, or at least comparable to, the mean free path of an oxygen molecule, so that the diffusion-limited current generated depends on the concentration of oxygen, but not on its absolute pressure. On the other hand, when the diffusion layer has pores with a diameter smaller than the mean free path of oxygen molecules, it has become clear that the diffusion current depends on the governing absolute pressure of oxygen and is directly proportional to this. The absolute pressure of oxygen or air with a constant oxygen content can be measured using a standard device.
This kind of diffusion is the so-called Knudsen diffusion.

Next, the present invention will be explained with reference to the drawings.

The device has a substrate 1 made of stabilized zirconium dioxide as ionically conductive material, which has a thickness of about 1 mm and supports electrodes 2 or 3 on two large surfaces, respectively. This electrode 2
and 3 are porous and consist of platinum or a mixture of platinum and zirconium dioxide. The electrode 2 is covered with a microporous diffusion layer 4, the pore diameter of which is 0.1 μm.
It is as follows. This porous diffusion layer consists of zirconium dioxide with yttrium oxide as a stabilizing additive and further contains 40% by weight of titanium dioxide. Such a porous layer and its method of manufacture are described in DE-A-2945020. On the side of the diffusion layer 4 opposite the electrode 2, a heating conductor 5 made of platinum or, in particular, a platinum-zirconium dioxide mixture is printed. Furthermore, a temperature-dependent measuring resistor 6, preferably made of platinum, is arranged on the same side of the diffusion layer 4.

A voltage of approximately 1 V is applied to electrodes 2 and 3 between points e and f, with electrode 2 serving as a cathode and electrode 3 serving as an anode.
There is a current measuring device 7 in the current circuit, which can be calibrated with air pressure measurements or whose measured values can be used for control purposes. points a and b
A voltage is applied to the heating conductor 5 via the heating conductor 5, and this voltage is controlled depending on the temperature of the diffusion layer 4. The resistance value of the temperature-dependent measuring resistor 6, to which a voltage is likewise applied via points c and d, serves as a control value, for example by using this resistor in a bridge circuit. The advantage of such a specially installed measuring resistor (the heating resistor can optionally be formed as a temperature-dependent resistor) is that this measuring resistor 6 can be adjusted separately for each pressure measuring device, thereby making it possible to This makes it possible to compensate for variations in the diffusion layer pore size that affect the limiting current. This adjustment takes place, for example, by stripping off the resistive material with a laser beam to increase the resistance.

To protect against oxidation and mechanical damage, the resistance traces 5 and 6 can be coated in a known manner with a protective layer made of glass, for example. This protective layer is not shown. The four small vertical surfaces are hermetically covered with glass to avoid errors in the measurement results.

To measure air pressure, the device is first set up by applying voltages to points a and b and c and d.
Must be heated to a constant temperature above 600°C. At this time, the voltages applied to a and b are controlled by the resistance value generated in the resistor 6 as described above.
A voltage of about 1V is applied to points e and f. Although the device only reacts to oxygen due to its mechanism, it can nevertheless be used to measure air pressure since the air has a constant oxygen concentration. Air, together with its oxygen content, enters the porous electrode 2 (cathode) through the pores of the diffusion layer 4, passes through this, and finally reaches the solid electrolyte-electrode-gas three-phase boundary. Reduction to oxygen ions takes place at this three-phase boundary, which ions migrate through the solid electrolyte 1 and are oxidized again to elemental oxygen at the electrode 3 (anode). Since the diffusion layer 4 serves to immediately convert each arriving oxygen molecule into oxygen ions and send them out,
The three-layer boundary is always dominated by zero oxygen concentration.
Furthermore, the selection of the pore size ensures that the diffusion layer is such that the diffusion-limiting current generated by oxygen ion transport through the solid electrolyte 1, which is read by the measuring device 7, dominates the absolute pressure and is directly proportional to this pressure. act. With this device it is possible to measure air pressures from 0.1 to about 10 bar.

[Brief explanation of the drawing]

The drawing is a perspective view of the device according to the invention. 1... Substrate, 2, 3... Electrode, 4... Diffusion layer,
5... Heating conductor, 6... Measuring resistor, 7... Current measuring device.

Claims (1)

[Claims for Utility Model Registration] 1. An oxygen ion conductive solid electrolyte block 1, which supports an anode 3 and a cathode 2 exposed to the air to be measured, and is capable of applying a constant voltage to the electrodes. A device for measuring the absolute pressure of air, characterized in that the cathode 2 supports a porous diffusion layer 4, the pores of which have a diameter such that Knudsen diffusion of oxygen molecules takes place. 2. The device according to claim 1, wherein the oxygen ion conductive solid electrolyte block 1 is made of stabilized zirconium dioxide. 3. The device according to claim 1 or 2, wherein the anode 3 and the cathode 2 are made of conductive metal or ceramic material. 4 Perovskite such that the anode 3 and cathode 2 are mainly composed of platinum, other platinum metals, platinum alloys, gold, gold alloys, silver, silver alloys, La-Co mixed oxides, or two or more of the above components The device according to claim 3 of the utility model registration claim, comprising a mixture of. 5. The device according to any one of claims 1 to 3, wherein the porous diffusion layer 4 is made of ceramic containing aluminum oxide, magnesium spinel, or zirconium dioxide. 6. The device according to claim 5, in which the porous diffusion layer 4 comprises zirconium dioxide with stabilizing additives and 20 to 60% by weight of titanium dioxide. 7. The device according to claim 5 or 6 of the utility model registration, wherein the diameter of the hole is 0.1 μm or less. 8. The device according to any one of claims 1 to 7, wherein the porous diffusion layer 4 supports a heating conductor 5 to which a heating voltage can be applied on the side opposite to the cathode 2. 9. The device according to any one of claims 1 to 8, wherein the porous diffusion layer 4 supports a temperature-dependent resistor 6 for heating control on the side opposite the cathode 2. 10 The porous diffusion layer 4 supports on the side opposite the cathode 2 a heating conductor made of a temperature-dependent resistance material, which conductor serves at the same time for heating and heating control. The device according to any one of the items.