JPS6138819B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a humidity measuring device using a metal oxide semiconductor as a humidity sensing element.

The moisture-sensitive element used in this type of measurement is formed by sintering a metal oxide as a main material, and its electrical characteristic is such that its resistance value decreases in inverse proportion to moisture absorption.

FIG. 2 is a diagram illustrating typical humidity-resistance characteristics of such a humidity sensing element. As can be seen from the diagram, as the relative humidity increases,
% or more, the change in resistance value becomes extremely slow, and it is said that the measurement accuracy at high humidity is reduced for this type of device that displays humidity based on changes in resistance value. There were flaws.

In order to improve the above-mentioned drawbacks when measuring high humidity, the present invention uses a new means to move the operating point of the humidity sensing element, so that even in a high humidity atmosphere, the resistance value changes within a large range. The aim is to improve measurement accuracy in high humidity areas.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 illustrates a schematic diagram of a moisture-sensitive element used in the examples, in which 1 indicates a resistance wire, for example, a metal with a large resistance temperature coefficient such as platinum is used, and the wire diameter is For example, the diameter is about 20 to 50 μm. 2 is a heat-resistant insulating material that covers the resistance wire 1. 4 is a porous wet body of metal oxide formed by sintering the resistance wire 1 and its coating 2 as a core, and terminals 3a and 3b are embedded at both ends thereof, for example. 1a and 1b indicate terminals of the resistance wire 1.

It is assumed that the moisture sensitive element 4 is thermally coupled to the resistance wire 1 and always maintains a predetermined temperature, which will be described later.

In the moisture sensing element having such a structure, the resistance value between the terminals 3a and 3b decreases as the moisture sensing element 4 absorbs moisture.

FIG. 3 illustrates a specific circuit of the embodiment, in which the moisture sensing element described in detail is a portion 4' delineated by a dotted line, the resistance wire 1 is connected to the resistor R1, and the humidity sensing element is 4 are shown replaced with signals corresponding to resistors r.

The resistor R1 and the fixed resistors R2, R3,
A bridge circuit is constructed by R4, and points (a) and
The potential change at point (b) is input to the amplifier circuit 5, and from its output, a current ( ) is fed back to the bridge circuit so that the bridge circuit maintains balance.

On the other hand, both terminals of the resistor r, which changes depending on the humidity, are connected to a resistance value detection circuit 6, and the resistor r is configured so that the value thereof is displayed by an indicator 7 as a humidity display.

The resistance values on each side of the bridge are approximately R2 = 224Ω, R3 = 200Ω, and R4 = 20Ω, and the value of R1 is 22.4Ω from equation (2) described later. Note that the resistance temperature coefficient α of the resistor R1 is approximately 0.004.

In such a configuration, the resistance R at 0°C
If the resistance value of 1 is R0, its temperature coefficient of resistance is α, and its temperature is T, then R1=R0(1+αT)) (1) holds true.

Also, the equilibrium condition for the bridge composed of resistors R1, R2, R3, and R4 is R1・R3・=R2・R4 (2) Currently, resistor R1 is not heated and its resistance value is low R1・R3<R2・R4 (3) If so, the potential at point (a) in Figure 3 will be lower than at point (b).

Since the inverting input terminal of the amplifier 5 is connected to point (a) and the non-inverting input terminal to point (b),
When the potential at point (a) becomes lower, the output current () of the amplifier 5 increases, and the resistor R1 is heated.

Conversely, if the resistor R1 is heated too much, R1.R3>R2.R4 (4), and in this case, the potential at point (a) becomes higher than at point (b), and the current () decreases.

Through this operation, the bridge equilibrium condition, equation (2), is automatically satisfied and the resistance value of R1 becomes constant.
As a result, as shown by equation (1), the resistance R1
temperature is constant. Note that R0 = 20Ω.

Assuming that the resistance values on each side of the bridge are the values described above, the temperature of the resistor R1 at this time will be 30° C. from equation (1).

That is, in the embodiment, the resistor R1 and the humidity sensing element 4'
Since the elements 4' are thermally coupled, the temperature of the element 4' is always maintained at 30°C.

Next, regarding humidity, the saturated vapor pressure of water is 17.53 mmHg at an ambient temperature of 20°C, and becomes 31.83mmHg at 30°C.

Relative humidity is the ratio of the water vapor pressure in a gas to the saturated water vapor pressure at the same temperature as the gas, expressed as a percentage. Therefore, in the case of the example, the ambient temperature was 20°C.
Even if the relative humidity is 100%, the temperature of the humidity sensing element 4' is maintained at 30°C as described above, so the element itself is equivalently 17.53/31.83 x 100 = 55.0738... ... (5) In other words, this corresponds to being in an atmosphere with a humidity of approximately 55%.

If we look at this in the relationship between resistance value and relative humidity shown in Figure 2, we can see that the relative humidity at the time of measurement is, for example,
Even if it is 100%, the resistance value indicates the value when the relative humidity is approximately 55%.

As is clear from the above explanation, in the present invention, even when the humidity at the time of measurement is in a high range of 80 to 100%, equivalently, when it is in the range of 40 to 55%, It shows a corresponding resistance change, and the second
As is clear from the resistance change curve shown in the figure,
Detection sensitivity is increased, and measurement accuracy can be improved especially at high humidity.

Furthermore, in the present invention, since the moisture-sensitive element is always heated to a predetermined temperature (for example, about 30°C in the embodiment), the moisture absorption of the element itself is significantly lower than that of conventional ones. Therefore, the drying operation of the humidity sensing element, which had to be carried out frequently when measuring high humidity, is reduced, and it also has the characteristics that it can be used extremely stably and with high precision even during continuous measurement over a long period of time. It also has excellent practical effects.

[Brief explanation of the drawing]

FIG. 1 is a partially cutaway sectional view showing an example of a humidity sensing element used in a humidity measuring device according to an embodiment of the present invention;
FIG. 2 is a graph showing the change in resistance value of this humidity sensing element with respect to humidity, and FIG. 3 is a circuit diagram showing the configuration of a humidity measuring device according to an embodiment of the present invention. 1...Resistance wire, 1a, 1b...Terminal, 2...Heat-resistant insulating material, 3a, 3b...Terminal, 4...Porous moisture material, R1, R2, R3, R4, r...Resistance,
4'...Moisture sensing element, 5...Amplifier, 6...Resistance detection circuit, 7...Indicator.

Claims (1)

[Scope of Claims] 1. A moisture sensing element configured by wrapping and thermally bonding a coated resistance wire having a resistance value depending on temperature with a moisture sensing element, and providing terminals at both ends of the humidity sensing element; a bridge circuit having a resistance wire in the moisture sensing element on one side; a feedback amplifier having a pair of terminals of the bridge circuit connected to an input terminal and keeping the bridge in balance by supplying a feedback current to the bridge circuit; A humidity measuring device comprising: a humidity detection circuit that detects humidity based on a change in the resistance value of a humidity sensing element of the humidity sensing element, which is maintained at a predetermined temperature by bridge equilibrium. 2. The humidity measuring device according to claim 1, wherein the humidity sensing element is formed by sintering a porous moisture material around a coated resistance wire as a core. 3. The humidity measuring device according to claim 1, wherein the resistance wire of the humidity sensing element is a metal wire having a large temperature resistance coefficient.