JPS585220Y2 - Ekimen Kenchiki - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a liquid level detector that utilizes a change in resistance value due to self-heating of a thermistor.

As shown in Figure 1, the conventional liquid level detector used in this type of oil level cage, etc. has a small heat dissipation coefficient.
A second thermistor RA having an equal resistance change rate with respect to temperature and a sufficiently large heat radiation coefficient is connected in series to the thermistor R8 for temperature compensation, and the first thermistor RB is connected to a predetermined position in the container. If the thermistor RB is placed so that it is always submerged in the liquid at a certain position, when the liquid level drops below the predetermined position and the thermistor RB is exposed above the liquid level, due to the difference in heat dissipation coefficient between the liquid and the air, The self-heating amount is accumulated, the temperature rises, and the resistance value decreases.

Therefore, a voltage change occurs at the connection point P between RA and 5RB, and the signal detected by the voltage comparator S operates the alarm indicator (lamp, buzzer, etc.) L.

By the way, in this conventional liquid level detector, when the temperature of the liquid and the air around it rises, the difference in heat radiation coefficient between the liquid and the air becomes smaller, and the resistance value when RB comes out from the liquid into the air becomes smaller. The change in voltage becomes small, making it difficult to detect the change in voltage at the point P.

Since RB itself already has a low resistance, a large current is required to detect a voltage change, which shortens the life of the thermistors RA and RB.

The object of the present invention is to provide a liquid level detector that does not have the above-mentioned drawbacks.

Examples thereof will be described below with reference to FIGS. 2 to 4.

Figure 2 is a diagram showing the basic structure of the composite thermistor for liquid level detection adopted in this invention. 2 is a resistor with constant resistance temperature characteristics, 3 is a thermistor with a large B constant, and these are electrically connected. are connected in parallel to maintain thermal equilibrium.

In Fig. 2A, a semi-cylindrical resistor 2 and a thermistor 3 are joined together to form a cylindrical shape, and terminal caps 1 and 4 are placed over this.
This shows an embodiment in which terminal caps 1 and 4 are arranged in a concentric cylindrical shape and covered with terminal caps 1 and 4.

FIG. 3 is a diagram showing the resistance-temperature characteristics of the composite thermistor R6 of the present invention. By combining the resistor 2, which has a constant resistance-temperature characteristic, and the thermistor 13, which has a large B constant, as described above, a certain temperature can be maintained. As a boundary, it is shown that the composite thermistor R6, which has the characteristic that the resistance value rapidly decreases at a temperature higher than that, is replaced.

In addition, the B constant referred to here is B=E/k ('K) However,
E is the inherent energy of the substance, k is Boltzmann's constant, and there is the following relationship with the temperature coefficient α of the thermistor.

However, T is the temperature (°K), and R is the resistance at T'K.As mentioned above, the composite thermistor Rc of the present invention has a characteristic that the resistance value decreases rapidly at a certain temperature and above that temperature. The inflection point temperature of the liquid is selected to be several tens of degrees ('C) higher than the maximum temperature of the liquid used.
In addition, in order to ensure that there is a sufficient difference in calorific value between when this composite thermistor Rc is in air and when it is in liquid, its shape is a structure with a small heat dissipation coefficient, for example, a cylindrical shape as shown in Fig. 2. Make it.

Next, its operation will be explained with reference to FIG.

Ro is a fixed resistor connected in series with the composite thermistor R6 of the present invention, and its connection point P is connected to the base of the first transistor TRI of the voltage comparator S.

The voltage comparator S constitutes a Schmitt trigger circuit by the transistor TRI of the preceding symbol 1 and the second transistor TR2, and the collector of the second transistor TR2 is connected in series with the alarm indicator (lamp, buzzer, etc.) L. It is connected to the base of the third transistor TR3.

Then, the high voltage at point P when R6 is in the liquid is the reference voltage E
(Fig. 5) The setting is made higher so that TRI is turned on and TR2 is turned off.

Therefore, since the emitter and base of TR3 are at the same potential as the power supply voltage V, TR3 is turned off, and L does not operate.

When the liquid level in which R6 is currently immersed falls and R6 is exposed to the air, the heat radiation coefficient in air is smaller than in liquid, so R6 accumulates self-heating and reaches the inflection point temperature shown in Figure 3. When the resistance value decreases rapidly.

In this way, when the voltage at point P decreases to below the reference voltage E, T
When RI is turned off and TR2 is turned on, TR3 is also turned on, current flows to the alarm display, and an alarm is displayed.
Notifies that the liquid level has fallen below the specified value.

Since the composite thermistor R6 of the present invention has the above-mentioned configuration and characteristics, when exposed from the liquid to the air, even if the ambient temperature is high, it will not flow as large a current as the conventional thermistor R8 shown in Figure 1. Even if it is not, the resistance value is immediately lowered by slightly exceeding the inflection point temperature, giving a predetermined voltage drop at point P.

Moreover, since the heat generated by the resistor forming one of the composite thermistors R6 heats the other thermistor, the above-mentioned effect is further ensured.

Also, since this composite thermistor R6 always has a constant resistance value when it is in the liquid, when the change in resistance value is detected by the voltage comparator S as a change in voltage at point P, as shown in FIG. In addition, the reference voltage E can be easily set.

Figure 6 shows another embodiment of the present invention, in which the composite thermistor R of the present invention
6 and fixed resistors R2, R3, and R4, and if Rc is balanced when it is in the liquid so that no current flows to the ammeter A, the resistance change of Rc, that is, the change in the liquid level. The resistance can be determined by ammeter A.

As described above, the liquid level detector of the present invention has a first sensing element installed in the liquid so as to be located at the liquid level to be detected, and a second sensing element installed separately in the air. In a liquid level detector that detects a liquid level by comparing a change in resistance value of the first sensing element with a resistance value of the second sensing element, the first sensing element has a resistance temperature characteristic. By using a composite thermistor that has an inflection point temperature where the temperature suddenly bends, it is possible to change the resistance value over a wide range without using a large current. It has the advantage of being easy to detect even when it is difficult to detect with a detector.

As mentioned above, since it does not require a large current, it has a long life, and has excellent features as a liquid level detector for oil gauges and other liquids.

[Brief explanation of the drawing]

Figure 1: Circuit diagram of a conventional liquid level detector, Figure 2: Diagram showing the structure of the composite thermistor of the present invention, Figure 3: Characteristic diagram of the composite thermistor of the present invention, Figure 4: Liquid level detector of the present invention. Circuit diagram of the surface detector, Figure 5: Diagram showing the voltage characteristics at point P in Figure 4, Figure 6
Figure: Another circuit diagram of the liquid level detector of the present invention. Symbol R6・・・Composite thermistor of the present invention, S・
...Voltage comparator, TRI, TR2, TR3...
...Transistor, L...Alarm indicator, R1
-R4...Fixed resistance, A...Ammeter,
■・・・・・・Power supply.

Claims (1)

[Scope of utility model registration request] It consists of a first sensing element disposed in the liquid so as to be located at the liquid surface to be detected, and a second sensing element disposed separately (in the air), and the resistance value of the first sensing element is In a liquid level detector that detects the liquid level by comparing a change in the resistance value of the second sensing element with the resistance value of the second sensing element, the first sensing element is
A resistor with constant resistance-temperature characteristics and a sanimister with a large B constant (activation energy specific to the substance/Boltzmann's constant) are closely bonded together, and terminals are provided at both ends.The resistance value increases rapidly above a predetermined temperature. 1. A liquid level detector comprising a composite thermistor having an inflection characteristic that decreases in temperature and a small heat dissipation coefficient.