JPS5814031A - Sensor for amount of steam - Google Patents

Abstract

PURPOSE:To provide a steam amount sensor which is excellent in responseness and reproducibility and is suitable for a continuous control of humidity, by mounting foliated gold electrodes, whese one face contacts the air, and lead wires, connected to the electrode, on a piezo-electric body. CONSTITUTION:Gold electrodes 2 and 2' are mounted to centers of the surface and the back of a crystal segment 1, acting as a piezo-electric body, so that they are positioned facing and opposite to each other, and a surface, making no contact with the crystal segment 1 of the gold electrodes 2 and 2', is exposed so that it makes direct contact with the air. Lead wires 3 and 3', consisting of a corrosion resisting metal such as stainless steel, rhodium, is led out from the gold electrodes 2 and 2', and are secured to a base 4 such as glass. A vibrating voltage is applied to the electrodes 2 and 2' to detect a resonance frequency, and this causes measuring of a steam amount in measuring gas with a precision and high-responseness. The device, consisting of a corrosion resisting meterial, is excellent in reproducibility and is suitable for continuous use.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a sensor for measuring the amount of water vapor in a gas that has excellent responsiveness and reproducibility and is particularly suitable for measurements aimed at continuous control of humidity.

Various methods have been used to measure the amount of water vapor in the air, but all of them are responsive.

None of them were satisfactory in terms of all characteristics such as reproducibility, reliability, ease of maintenance, and changes over time.

For example, if a conventional wet and dry bulb thermometer is well maintained, there is little change in performance over time and it is possible to measure humidity relatively easily and accurately. Therefore, it is not suitable for measuring humidity in a small space. In addition, it has a slow response and cannot be easily miniaturized.
Since the indicated value cannot be easily converted into an electrical signal, it is completely unsuitable for measurement aimed at controlling humidity.

In this regard, recent capacitive or electrical resistance humidity sensors that use metal oxides (aluminum oxide, etc.) or electrolytes (lithium chloride, etc.) are small, lightweight, and easy to handle, and the measured values are converted into electrical signals. Efforts are being put into development from the souls that can be obtained.

However, these sensors have insufficient reproducibility due to the adsorption phenomenon of the resistor and also have large changes over time.
In terms of reliability, it has not reached the level of practical use. More recently, piezoelectric resonators (crystal resonators) that vibrate through thickness shear
forming a thin film of a hygroscopic substance on at least one surface of the
A humidity sensor that detects humidity from a change in the resonant frequency of the piezoelectric resonator due to a change in humidity has also been published (Japanese Patent Application Laid-Open No. 52-55580). However, this humidity sensor has a slightly faster response than previous sensors, but attempts to measure and continuously control the amount of water vapor in the engine's intake air in order to improve the characteristics of the internal combustion engine. The response characteristics disclosed in the above-mentioned publication are insufficient for application to cases where extremely fast response is required, such as in cases where extremely fast response is required. Also, it is difficult in practice to obtain compatible products.

The present invention has been made in view of these points, and it is an object of the present invention to provide a water vapor amount sensor that has excellent responsiveness and reproducibility and is suitable for measurements aimed at continuous control of humidity, which particularly requires quick responsiveness. That is. The present inventor conducted various experiments in order to obtain a water vapor amount sensor that satisfies the above-mentioned objectives. Conventionally, it was thought that a moisture-absorbing substance must be applied or pasted to a humidity sensor, but as a result of various experiments, unexpected experimental data that could not have been predicted by engineers in the field was obtained. The present invention was completed on the basis of this experimental data and has achieved its intended purpose.The present invention will be clarified by describing embodiments of the present invention in detail below with reference to the drawings.

FIG. 1 is a front view of a water vapor amount sensor as an embodiment of the present invention, and FIG. 2 is a plan view of the water vapor amount sensor of FIG. 1. Corresponding parts in FIG. 1 and FIG. 2 are given the same reference numerals. In the figure, reference numeral 1 denotes a crystal piece as a piezoelectric body that generates a piezoelectric phenomenon, and foils for applying an oscillating voltage to the crystal piece 1 as a piezoelectric body are respectively placed at the center of the front and back surfaces of the crystal piece l. Gold electrodes 2 and 2/ are arranged in opposing pairs, and the surfaces of these gold electrodes 2 and 2' that are not in contact with the crystal piece 1 are exposed so as to be in direct contact with the outside air. . Therefore, the crystal blank 1 and the pair of gold electrodes 2, 2' have the same shape as the crystal blank and electrodes in a known crystal resonator. The gold electrode 2
, 2' lead out lead wires 8.8' made of a corrosion-resistant metal (for example, stainless steel, rhodium, etc.), and these lead wires 8.8' are connected to the base 4 made of insulating glass or the like.
Their relative positions are fixed by Each of the lead wires 8, 8' has a coil spring shape with a small diameter at the part that contacts the corresponding gold electrode 2, 2', and is designed to sandwich and support the crystal piece 1 together with the gold electrode 2, 2'. is being done.

The water vapor amount sensor of the present invention configured as described above operates as follows. The inventor placed the water vapor amount sensor of the present invention in a gas to be measured, applied an oscillating voltage from the outside through the lead wires 8, 8/, and conducted an experiment in which the resonance frequency of the water vapor amount sensor at that time was measured. As a result of this experiment,
It has become clear that there is a certain correspondence relationship with extremely good response and reproducibility between the change in the amount of water vapor in the gas to be measured and the transition value of the resonance frequency of the water vapor amount sensor.

The reason why there is such a constant relationship between responsiveness and reproducibility is that the sensor of the present invention does not contain any hygroscopic substances, so it does not absorb or release water vapor in such a large amount that it delays the response. It is thought that the transition of the resonance frequency is caused by an adsorption phenomenon at the interface between the sensor's electrode and the outer surface of the crystal piece, which are in the same phase, and the gas to be measured, which is in the gas phase, and water vapor at this interface affects the sensor.

Therefore, the water vapor m (for example, relative humidity) in the gas to be measured can be accurately determined from the resonance frequency of the water vapor amount sensor of the present invention. The ability to accurately measure the amount of water vapor using a sensor that does not contain any hygroscopic material was something that had never been expected in this field, and is an epoch-making result. The characteristics of the water vapor amount sensor of the present invention are as follows: A device having the same shape as the present invention, and using a silver electrode instead of a gold electrode (as a crystal piece:
The graph in FIG. 8 shows the results of a simulation using an AT-cut crystal piece with MHz specifications. In the graph of Figure 8, the vertical axis is the amount of transition of the resonant frequency -Δf (:Hz), the horizontal axis is the relative humidity [%], and the measurement point is a fixed humidity point specified by JI8 (a saturated aqueous solution of salts). relative humidity of a gas that coexists with and is in equilibrium). Note that this characteristic is a measurement result at an air temperature of 25°C. The water vapor amount sensor of the present invention has much better response than the conventional sensor described in the above-mentioned publication (conventionally, it took about 20 seconds to several minutes for 100% response7, but the present invention (approximately 0.5 seconds or less) and reproducibility (±1%). This is considered to be due to the following reasons. In other words, the conventional sensor described above is
The hygroscopic resin is placed on a small piezoelectric resonator to form a uniform thin film (although it is difficult to form such a thin film in order to prevent variations in the product). The hygroscopic resin absorbs water vapor and it takes time to release it, so the response is delayed, and in a high humidity environment, the hygroscopic resin contains a large amount of water and becomes sticky.
Since dust and the like tend to adhere to the sensor, hysteresis increases especially when measuring in high humidity areas due to the influence of the attached dust and the like. Further, due to restrictions on physical properties, the operating temperature range is also limited to approximately 0 to 50°C. On the other hand, since the device of the present invention does not have a hygroscopic resin that causes the above-mentioned response delay and hysteresis, it has an extremely fast response and hardly causes hysteresis even in a high humidity and high temperature region. Furthermore, since the apparatus of the present invention does not have a thin film of hygroscopic resin that is difficult to uniformly mass-produce, products with consistent quality can be easily mass-produced and are highly compatible.

This is a very important advantage for sensors used under harsh conditions. Furthermore, in general, conventional electric humidity sensors have a tendency for the indicated value to increase over time even when the humidity is constant when measuring in a high humidity region. Although this phenomenon has not yet been fully analyzed, it is presumed that it is caused by some kind of electrochemical reaction occurring near the electrodes in the sensor that has adsorbed a large amount of water. Conventionally, as a countermeasure to this problem, a method was devised to quickly remove the adsorbed moisture by heating the sensor to keep it higher than the ambient temperature (Japanese Patent Application Laid-open No. 54-94090). The humidity near the sensor will differ from the humidity that is originally the target of measurement. According to the apparatus of the present invention, measurements can be made quickly and reliably even in high humidity areas, as described above, and there is no such problem. Furthermore, since the device of the present invention uses gold electrodes and corrosion-resistant metal lead wires, it can be used continuously in an atmosphere surrounded by corrosive gases, and even if dust or the like adheres to it, it can be washed and will not deteriorate over time. Very few. Practical measurement is also possible within the operating temperature range of -80 to +250°C.

In summary, the water vapor amount sensor of the present invention has sufficient characteristics such as responsiveness, reproducibility, reliability, ease of maintenance, and changes over time to be applied to measurements aimed at continuous control of humidity. It has satisfactory performance.

In addition, as a special application of the present invention, the device of the present invention may be used as a sensor for estimating the moisture content in an area with a relative humidity of 100% or more, that is, in an ambient atmosphere where steam is present, and the device of the present invention may be used with a special semipermeable membrane. It can also be used as a sensor that is sealed and submerged in a solution to be measured to measure the amount of water in the solution.

In addition, in the above, an example in which a crystal piece is used as the piezoelectric body has been described, but instead of the crystal piece, a piezoelectric ceramic piece, a piezoelectric tuning fork, an ultrasonic resonator, etc. can be applied as the piezoelectric body. is also possible. The selection of these piezoelectric bodies may be made arbitrarily, taking into consideration the price, resonance frequency, etc., in accordance with the purpose of measurement, the specifications of the display circuit section, etc.

[Brief explanation of drawings]

FIG. 1 is a front view showing an embodiment of the water vapor amount sensor of the present invention, FIG. 2 is a plan view of the water vapor amount sensor of FIG.
The figure is a graph showing the results obtained by simulation of the characteristics of the water vapor amount sensor of the present invention. 1...Piezoelectric body (crystal piece) 2.2'...Gold electrode 8.8'...Lead wire 4...Base agent Patent attorney Ryuji Higashijima

Claims (1)

[Scope of Claims] (A piezoelectric body, a pair of foil-shaped gold electrodes each disposed at a predetermined position so as to apply an oscillating voltage to the piezoelectric body, one surface of which is in direct contact with the outside air, and A water vapor amount sensor having lead wires respectively led out from each of the gold electrodes. (2) The water vapor amount sensor according to claim 1, wherein the piezoelectric body is a crystal piece. (3) The piezoelectric body is a piezoelectric ceramic. The water vapor amount sensor according to claim 1, which is a piezoelectric member.
Water vapor amount sensor as described in section. (6) The water vapor amount sensor according to claim 1, wherein the piezoelectric body is an ultrasonic resonator. (6) The water vapor amount sensor according to any one of claims 1 to 5, wherein the lead wire is made of a corrosion-resistant metal.