JPS6138820B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a device that uses a metal oxide semiconductor as a humidity sensing element and measures relative humidity by heating the element to a constant temperature state. On the other hand, the present invention relates to a humidity measuring device having a feature of a compensation circuit that compensates for this.

An example of an outline of a humidity sensing element used in this type of measurement is shown in Fig. 1, where 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, 20. It is about 50 μmφ. 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 maintained at a predetermined temperature, which will be described later.

A typical example of the humidity-resistance characteristics of such a moisture sensing element is shown in FIG. 2, and as the moisture sensing element 4 absorbs moisture, the resistance value between its terminals 3a and 3b decreases. It is something.

FIG. 3 shows a circuit example of this type of measuring device, in which the moisture sensing element described in detail is a portion 4' marked by a dotted line, the resistance wire 1 is connected to a resistor R1, and the moisture sensing element 4 is connected to a resistor. They are each shown replaced by the symbol r.

Therefore, the resistor R1 and the fixed resistors R2', R
3 and R4 constitute a bridge circuit, and the potential changes at points (a) and (b) are input to the amplifier circuit 5, and from its output, a current is applied to the bridge circuit so that the bridge circuit maintains balance. I will be returned.

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

Further, the equilibrium condition of the bridge composed of resistors R1, R2', R3, and R4 is R1.R3=R2'.R4 (2).

If the resistor R1 is not heated and its resistance value is low such that R1.R3<R2'.R4 (3), the potential at point a in FIG. 3 will be lower than 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 I 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 I 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 temperature of the resistor R1 is kept constant.

The device having the configuration illustrated in FIG. 3 operates as described above, and the humidity sensing element is heated to a predetermined temperature, thereby making it possible to correct defects in the characteristics of the relationship between humidity and resistance value. It is.

However, it does not have a function to compensate for changes in ambient temperature during measurement, and in this respect it cannot be said to be sufficient.

Figure 5 shows an example of how the characteristics of a moisture-sensitive element change when the ambient temperature changes.
and humidity-resistance characteristics at 60°C, and as is clear from the figure, the characteristics change greatly with changes in temperature.

In order to improve the above-mentioned disadvantage that the indicated value of relative humidity changes due to changes in ambient temperature, the present invention automatically adjusts the operating point of a humidity sensing element according to changes in ambient temperature using a novel means. move it to
This aims to obtain accurate measurement results that are less affected by temperature changes.

An embodiment of the present invention will be described below. In FIG. 4, R2 is a temperature compensation resistor, and the ambient temperature is detected by this resistor, and based on the change in resistance value that is the detection result, This changes the equilibrium conditions of the bridge.

As a result, the heating temperature of R1 is changed, and the temperature of the humidity sensitive element r, which is thermally coupled to the resistor, is changed together, resulting in an accurate relative humidity indication regardless of changes in ambient temperature, as explained below. It is something that can be done.

In the embodiment illustrated in FIG. R0=R4=20Ω R3=200Ω α=0.003 shall be.

R2 is a temperature compensation resistor whose resistance value changes as the ambient temperature changes, and can be expressed as R2=R02 (1+βTa) (5).

Here, R02 is the resistance value of R2 at 0℃, and Ta
is the ambient temperature, and β is the resistance temperature coefficient of R2.

As an example, a case where R02=200.5Ω β=0.00389 and the ambient temperature changes from 7° C. to 45.5° C. will be described.

Table 1 shows R2 when the ambient temperature Ta changes,
The values of R1 and T are shown. The value of R2 was determined in the same manner as in equation (5) above, and the values of R1 and T were obtained in accordance with equations (2) and (1), respectively. By determining each value as described above, when the ambient temperature Ta changes from 7℃ to 30℃ to 45.5℃,
The circuit operates so that the temperature of the humidity sensing element becomes 10°C, 40°C, and 60°C, respectively.

Next, the change characteristics of the resistance value of the humidity sensing element will be explained with reference to FIG. 5 and Table 2.

FIG. 5 shows the characteristics of the humidity sensing element, and shows the relationship between relative humidity and resistance value when the temperature changes.

Table 2 lists the relative humidity equivalently felt by the humidity sensing element and its resistance value when the ambient temperature changes from 7°C to 45.5°C when the atmospheric humidity is 98%, 70%, and 40%. are doing. For example, considering the case where the ambient temperature is 30°C, the temperature of the humidity sensing element is maintained at 40°C as explained above. The saturated water vapor pressure at 30°C is 31.83mmHg, so if the relative humidity is 98%, the vapor pressure will be 31.19mmHg as 31.83×0.98=31.19 (6).

The saturated water vapor pressure at 40℃ is 55.34mmHg, and since the humidity sensing element is kept at 40℃, the equivalent relative humidity felt by the element is 31.19/55.34×100=56.36… (7 ), and its resistance value is 8.3×10 ⁴ Ω. Table 2 shows the resistance values of the humidity-sensitive element obtained based on similar calculations for relative humidity of 98%, 70%, and 40%, and for atmospheric temperatures of 7°C, 30°C, and 45.5°C.

Figures 6a, b, and c are graphs showing the resistance values in Table 2, respectively. Even if the ambient temperature changes, the temperature can be detected and the heating temperature of the humidity sensing element can be automatically changed. This shows that relative humidity can be measured without being affected by changes in ambient temperature.

6A shows the atmospheric humidity when it is 98%, FIG. 6B when it is 70%, and FIG. 6C when it is 40%.

As described in detail above, the present invention is extremely effective in that the compensation operation works extremely effectively against changes in ambient temperature during measurement, resulting in high measurement accuracy.

[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, Fig. 3 is a circuit diagram showing the configuration of the humidity measuring device which is the premise of the present invention, and Fig. 4 is an embodiment of the present invention. A circuit diagram showing a humidity measuring device by
FIG. 5 is a graph showing changes in relative humidity and resistance value with respect to different temperatures of the humidity sensing element according to the present example;
Figure 6a is a graph showing changes in relative humidity and resistance value for different atmospheric temperatures when the atmospheric humidity is 98%, Figure 6b is 70%, and Figure 6c is 40%. It is. 1...Resistance wire, 1a, 1b...Terminal, 2...Heat-resistant insulating material, 3a, 3b...Terminal, 4...Porous moisture material, R1, R2', R3, R4, r...Resistance,
R2...Temperature compensation resistor, 4'...Moisture sensing element, 5
...Amplifier, 6...Resistance detection circuit, 7...Indicator.

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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 in which a resistance wire in the humidity sensing element is connected to one side and a temperature compensation resistor having a resistance value depending on the temperature is connected to the other side; and a pair of terminals of the bridge circuit are connected to an input end. a feedback amplifier that balances the bridge by supplying a feedback current to the bridge circuit; and a feedback amplifier that balances the bridge by supplying a feedback current to the bridge circuit; A humidity detection circuit to detect,
A humidity measuring device comprising: 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.