Liquid level detecting apparatus

A level detecting resistor (Rd) is inserted into a liquid (5) and has a length immersed in the liquid. The level detecting resistor (Rd) is supplied a constant current (Id) from a constant current source (1). The voltage drop (Vd) developed across the level detecting resistor (Rd) is supplied to a non-inverting input terminal of a differential input amplifier (OP1, R1-R4). The output (Vo) of the differential input amplifier is then divided by a series-circuit of a temperature compensating resistor (Rc) and a resistor (R5). The divided voltage (Va) across the resistor (R5) is fed back via a voltage follower (OP2) to the non-inverting input terminal of the differential input amplifier.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a liquid level detecting apparatus for 
detecting the level or height of a liquid, and more particularly to a 
liquid level detecting apparatus in which a liquid level detecting 
resistor is placed in the liquid and the resistance of the detecting 
resistor varies in accordance with its length of immersed portion in the 
liquid. 
2. Prior Art 
As shown in FIG. 3, both a level detecting resistor Rd and a temperature 
compensating resistor of a conventional liquid level detecting apparatus 
take the form of a wire wound resistor that is wound around an elongated 
supporting member 3 or 8. The level detecting resistor Rd is placed 
vertically in the liquid as shown in FIG. 4 and is supplied a constant 
current Id from a constant current source 1. Likewise, the temperature 
compensating resistor Rc is inserted into the liquid as shown in FIG. 4. 
FIG. 2 shows a conventional liquid level detecting apparatus of this type. 
The voltage drop developed across the level detecting resistor Rd is 
supplied to an inverting input terminal (-) of an OP amplifier OP3 via the 
temperature compensating resistor Rc. The output of the OP amplifier OP3 
is fed back to the inverting input terminal (-) thereof via a feedback 
resistor R6. In other words, the resistors Rc, R6, and the amplifier OP3 
form a negative feedback amplifier whose gain is given by R6/Rc. 
The resistors Rd and Rc have the following temperature characteristics. 
EQU Rd=Rdt(1+.alpha.d.DELTA.T)L (1) 
EQU Rc=Rct(1+.alpha.c.DELTA.T)L (2) 
where Rdt and Rct are resistances per unit length of the level detecting 
resistor and temperature compensating resistor, respectively, at a certain 
ambient air temperature, for example, t=20.degree. C. Rd and Rc are the 
overall resistances of the level detecting resistor and temperature 
compensating resistor, respectively, when the ambient air temperature 
increases by an increment .DELTA.T from t.degree. C. .alpha.d and .alpha.c 
are the temperature coefficients of resistances of the level detecting 
resistor Rd and temperature compensating resistor Rc, respectively, at the 
ambient air temperature t.degree. C. L is the overall length of the 
respective resistors. 
When the constant current Id flows through the detecting resistor Rd, the 
current Id generates an amount of heat to further increase the resistance 
of Rd at the temperature t.degree. C. This heat is radiated to the liquid 
from a portion immersed in the liquid, causing the overall resistance to 
decrease somewhat. Thus, it is understood that the overall resistance of 
the level detecting resistor Rd at the ambient temperature t.degree. C. 
varies with the length immersed in the liquid. In other words, the voltage 
developed across the level detecting resistor Rd represents the liquid 
level. 
If .alpha.d=.alpha.c=.alpha. and both the level detecting resistor and 
temperature compensating resistor are inserted into the liquid, Eq. (1) 
and Eq. (2) are rewritten as follows: 
##EQU1## 
where X is a length of each resistor immersed in the liquid. a is a 
constant specific to the liquid. K is a quantity given by K=I.sup.2 
R.alpha.o.theta., that is determined by the temperature coefficient 
.alpha.o (.alpha.d or .alpha.c), the current I (Id or Ic) through the 
resistor R (Rd or Rc), and the thermal resistance .theta. in radiating 
heat generated therein to atmosphere . . . LS1 Therefore, 
##EQU2## 
Thus, the output voltage Vout of the OP amplifier OP3 is given as follows: 
##EQU3## 
If Id&gt;&gt;Ic, then the overall resistance of the temperature compensating 
resistor Rc does not vary with the length immersed in the liquid, thus 
##EQU4## 
Rdt and Rct are the resistances per unit length of the level detecting 
resistor Rd and the temperature compensating resistor Rc at a specific 
ambient air temperature, for example, t=20.degree. C., and therefore are 
fixed values. Now, Vout changes with the length X immersed in the liquid, 
not being affected by the change .DELTA.T in the ambient temperature. 
With this prior art apparatus, since a relatively large current Id is run 
through the level detecting resistor Rd for higher sensitivity, the 
voltage developed thereacross is relatively high. As a result, the high 
voltage across the level detecting resistor Rd tends to cause a current to 
flow through the temperature compensating resistor Rc, the current through 
Rc generates an amount of heat which in turn causes the resistance of Rc 
to vary. The change in resistance of Rc due to self-generated heat will 
result in deviation from the designed temperature versus resistance curve 
of the level detecting apparatus. To prevent this kind of measurement 
error, generally the temperature compensating resistor Rc is selected to 
be a large value so to as to maintain as low a current through Rc as 
possible. A large value of Rc requires more turns of wire and/or smaller 
diameters of the wire, which causes more complicated manufacture process. 
If Rd and Rc differ in diameter of the wire, they differ in temperture 
coefficient .alpha.. This causes a problem in temperature compensation of 
the level detecting resistor. Excess turns of wire that are wound around 
the supporting member tend to increase heat capacity of the resistor Rc, 
which takes a longer time for the resistance to settle when the ambient 
temperature changes, causing error in temperature compensation. In 
addition, the level detecting apparatus cannot follow a rapid change in 
liquid level. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a liquid level detecting apparatus 
in which the voltage developed across the liquid level detecting resistor 
is not affected by succeeding circuits or circuit elements to indicate 
correct liquid level. 
Another object of the invention is to provide a liquid level detecting 
apparatus in which the correct liquid level is indicated without being 
affected by ambient temperatures. 
A level detecting resistor is inserted into a liquid and has a length 
immersed in the liquid. The level detecting resistor is supplied a 
constant current from a constant current source. The voltage drop 
developed across the level detecting resistor is supplied to a 
differential type non-inverting amplifying circuit. The output of the 
non-inverting amplifying circuit is then divided by a series-circuit of a 
temperature compensating resistor and a resistor. The divided voltage is 
fed back via a voltage follower circuit to the non-inverting input 
terminal of the differential input amplifying circuit. The output voltage 
of the voltage follower circuit is free from the change of ambient 
temperature, thus the apparatus is temperature compensated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described in detail with reference to the 
drawings. 
FIG. 3 is a perspective view of assemblies 3 and 8 of a level detecting 
resistor Rd and a temperature compensating resistor Rd, respectively, of 
the present invention and FIG. 4 shows the level detecting resistor Rd and 
the temperature compensating resistor Rc immersed in the liquid 5 
contained in a tank 2 so as to measure a liquid level H. FIG. 1 is a 
circuit diagram showing an embodiment of a liquid level detecting 
apparatus according to the present invention. A constant current source 1 
supplies a constant current Id to the level detecting resistor Rd. The 
current Id causes a voltage drop Vd across the resistor Rd. The voltage Vd 
is applied to a non-inverting input terminal (+) of an OP amplifier OP1 
via a resistor R3. Between the inverting input terminal (-) of the OP 
amplifier OP1 and the output terminal thereof is inserted a feedback 
resistor R2. The inverting input terminal (-) is grounded through a 
resistor R1. The output voltage Vout of the OP amplifier OP1 is divided by 
a series circuit of a resistor R5 and a temperature compensating resistor 
Rc. The voltage drop Va developed across the resistor R5 is supplied to 
the non-inverting input terminal (+) of an OP amplifier OP2. The OP 
amplifier OP2 is a voltage-follower where the signal is directly fed back 
from output to input and the output voltage thereof is of the same 
amplitude as the input voltage thereto. The output of the voltage follower 
is fed to the non-inverting input terminal (+) of the OP amplifier OP1 via 
a resistor R4. The output voltage of the voltage follower or OP ampifier 
OP2 represents the level of the liquid 5. It should be noted that the 
resistors R1-R4 and the OP amplifier OP1 form a differential input 
amplifier as a non-inverting amplifier. 
When the constant current Id flows through the level detecting resistor Rd, 
the resistance of Rd varies due to the heat generated by the current Id. 
The heat is given off to the liquid to decrease somewhat. Thus, the 
resistance of the Rd varies with the length immersed in the liquid. The 
level detecting resistor Rd and the temperature compensating resistor Rc 
have the same value of temperature coefficient. The values of the various 
resistors in the circuit in FIG. 1 are selected to satisfy the following 
relations. 
##EQU5## 
The output voltage Vo of the OP amplifier OP1 is 
##EQU6## 
putting Eqs. (8)a and (8)b into Eq. (9), 
##EQU7## 
thus, the voltage across the temperature compensating resistor Rc is 
##EQU8## 
thus, the current Ic through the resistor Rc is 
##EQU9## 
The output voltage Vout of the OP amplifier OP2 is 
##EQU10## 
From Eqs. (2) and (5), Eq. (12) can be rewritten as follows: 
##EQU11## 
It should be noted that if R1&gt;&gt;R2 (=R4) and Rdt is nearly equal to Rct, 
then Ic&lt;&lt;Id. For example, R1 may be about 50 times larger than R2. This 
indicates that the resistor Rc generates much less amount of heat as 
compared to the resistor Rd. 
The resistors R1, R4, and R5 are selected such that their temperature 
coefficients are much smaller, e.g., 1/100 or so, than those of the level 
detecting resistor Rd and the temperature compensating resistor Rc. It 
should be noted that RctL is the total resistance of the temperature 
compensating resistor Rc at a specific ambient temperature t, e.g., 
t=20.degree. C. Therefore, RctL is a known fixed value. putting Eq. (14) 
into Eq. (13)a, 
##EQU12## 
and we known R5=R1 Rc from Eq. (8), thus 
##EQU13## 
It should be noted that the Vout is a function of the length X immersed in 
the liquid and is not affected by the change .DELTA.T in the ambient 
temperature. 
According to the present invention, since it is possible to prevent the 
current from the constant current source 1 from flowing through other 
succeeding circuits or circuit elements, the accurate measurement of the 
liquid level can be made. 
The temperature compensating resistor Rc will not contribute to error in 
measuring the liquid level. This is advantageous in that the temperature 
compensating resistor Rc can be of the same resistance as the level 
detecting resistor, which is convenient in production as well as inventory 
supervision. The resistors Rd and Rc of the same design permits to use a 
resistance wire of the same lot of wire production line, which in turn 
allows the temperature coefficients .alpha.d and .alpha.c of the resistors 
to be very close to each other. Since it is not necessary to increase the 
resistance of the temperature compensating resistor, the temperature 
compensating resistor Rc can be of a small heat capacity, being 
advantageous in quickly responding to the rapid change of ambient 
temperatures.