A salimeter based on the electric conductivity measurement of an objective liquid solution and devised so as to give the salinity of the solution instantly without any correction or adjustment with respect to a temperature of the solution. The apparatus is divided into a main electronic circuit portion and a measuring probe or probes to be detachably connected to the main circuit portion through a connecting cable. Each of the probes is provided with a pair of electrodes and a temperature sensor, both to be immersed directly in an objective solution, while the main circuit portion comprises a first means for measuring the conductivity between the electrodes, a second means for generating a temperature effect compensation signal according to an output from the temperature sensor and an arithmetic divider for outputting a ratio of the measured conductivity to the temperature effect compensation signal. The ratio, which gives the conductivity of the solution at a predetermined standard temperature, is converted to a corresponding value of salinity and then displayed on a display unit.

BACKGROUND OF THE INVENTION 
The present invention relates to a salimeter, and more particularly to a 
salimeter for determining the salinity of saline solution through the 
measurement of the electric conductivity of the solution. 
Based on the fact that the salinity of a saline solution is inseparably 
related to the electric (ionic) conductivity of the solution, one 
well-known type of salimeter is by means of measuring the electric 
conductivity of an objective solution. Salimeters of this type 
fundamentally consist of an exciting AC voltage source and a pair of 
electrodes. The pair of electrodes is either constituted in the form of a 
probe capable of being immersed directly in an objective solution or 
provided in a measuring cell into which the objective solution is to be 
sampled. The exciting AC voltage source supplies an AC voltage between the 
electrodes with the probe immersed in an objective solution or with the 
solution sampled into the cell. The conductivity of the solution is 
obtained, in principle, from the data of the voltage and current between 
the electrodes and of the geometry of the pair of electrodes. In practice, 
the apparatus is devised so as to give a resultant value of the 
conductivity and/or salinity of the solution according to a probe or cell 
constant reflecting the geometry of the pair of electrodes. However, the 
exciting AC voltage, which should be kept constant in principle, must be 
corrected against possible small variations in the probe/cell constants of 
individual probes or cells in use. 
Such an inconvenience is improved, for example, with the conductivity 
measuring apparatus disclosed in the Japanese Utility Model Application 
No. 56-65478. This apparatus is characterized by being provided with a 
reference resistor for use in calibrating the apparatus, in advance, 
against the errors resulting from the possible cell-constant variations 
with respect to the cells to be used. The reference resistor is 
incorporated so that it may be substituted for the cell through a switch 
operation. The incorporation of the reference resistor in the apparatus 
makes an easy and precise calibration possible. Furthermore, the circuit 
constitution of the apparatus provides another advantage that, once the 
calibration is made with respect to a specific cell, the conductivity 
measurements with the cell are secured from the possible voltage 
variations expected to the exciting AC voltage source, since the exciting 
voltage can easily be corrected. 
However, there are left some problems in applying the above-mentioned 
improved conductivity measuring apparatus to a salimeter under 
consideration, though it is also based on the same principle of 
conductivity measurement, since the above apparatus has been improved only 
to make it easy to correct the excitation voltage against the variation in 
the cell constant. Because the conductivity of a solution depends on 
temperature, a solution with a constant salinity shows different values of 
conductivity according to the temperature of the solution. In order to 
obtain a proper salinity at a predetermined standard temperature (of 
25.degree. C.), therefore, it is necessary to provide a temperature sensor 
and an arithmetic means for correcting the measured conductivity in 
accordance with the temperature and conductivity temperature-coefficient 
of the solution. However, the addition of such correcting means to the 
above apparatus has effect also on the excitation voltage correction to be 
made by the adjustment of the reference resistor, causing the reference 
resistor to be useless at temperatures other than the standard temperature 
(25.degree. C.). A further disadvantage is that the standard solution must 
always be reserved. 
OBJECTS AND SUMMARY OF THE INVENTION 
The present invention aims at eliminating the above mentioned disadvantages 
accompanying the conventional salimeters, and makes it an object to 
provide an improved salimeter capable of measuring a precise salinity of 
an objective solution without keeping the same at a standard temperature 
and, in addition, without necessitating a standard solution to be always 
reserved after the initial calibration procedures have been completed. 
Another object of the present invention is to constitute such an improved 
salimeter with a main electronic circuit portion and a measuring probe 
portion detachably connected to the main portion through a prolonged 
connecting cable for the purpose of making a common salimeter applicable 
to widely diverging kinds of objective solutions, for which the use of a 
common probe is to be avoided. By providing a plurality of probes carrying 
their respective pairs of electrodes to be immersed in their respective 
specific kinds of solutions, a common salimeter can be used for various 
solutions ranging from a soup to be kept at a well sanitary condition to 
the liquid wastes from human or animal bodies. 
A further object of the present invention is to assemble such an improved 
salimeter substantially in analog circuits to avoid the possible errors 
expected in digitalizing analog signals related to the conductivity and 
temperature data. 
According to the present invention, a salimeter consists of a main 
electronic circuit portion and at least one detachable probe in which a 
pair of electrodes and a temperature sensor are provided to be immersed 
directly in an objective liquid solution. The main electronic circuit 
portion comprises an exciting AC voltage generator whose output is made 
adjustable, an amplifier for amplifying the output of the exciting AC 
voltage generator in proportion to the conductance (of the solution) 
between the electrodes provided in said probe, a rectifier for rectifying 
the output of the amplifier, a compensation signal generator for 
generating a temperature effect compensating signal in accordance with an 
output of said temperature sensor incorporated in the probe, an arithmetic 
divider for dividing the output of the rectifier by the output of the 
compensation signal generator, a display unit for displaying an output of 
the arithmetic divider, a reference resistor made capable of being 
substituted for the pair of electrodes, and a voltage source for providing 
a standard voltage capable of being inputted to the compensation signal 
generator substitutionally for the output from the temperature sensor.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, which shows the circuit constitution of an embodiment 
of the present invention, an exciting AC voltage source 1 consisting of an 
oscillator 1a and an output attenuator 1b supplies an exciting voltage 
e.sub.i to an electrode 3 through a buffer amplifier 2 with a gain of 
unity. The electrode 3, together with another electrode 4, constitutes a 
pair of electrodes to be immersed in an objective solution whose salinity 
is to be measured. A resistance R.sub.c shown between the electrodes 3 and 
4 with a dotted line represents resistance expected to appear there when 
the pair of electrodes 3, 4 is immersed in a solution. The electrode 4 is 
connected to the inverting input terminal of an operational amplifier 5 
through a switch S1. The operational amplifier 5 constitutes an inverting 
linear amplification circuit together with the resistance R.sub.c and a 
feed-back resistance R.sub.f connecting between the output and inverting 
input terminals of the operational amplifier 5. The output from the 
amplifier 5 is led to a synchronous rectifier circuit 6 consisting of two 
capacitors C.sub.2, C.sub.3 and two switching means S3 and S4. With the 
switching means S3 and S4 alternately operated in synchronous with the 
output frequency of the oscillator 1a, the rectifier circuit 6 rectifies 
an AC output voltage e.sub.c from the amplifier 5 and outputs a DC voltage 
E.sub.c equal to the peak-to-peak value of e.sub.c. On the other hand the 
electrode 4 is provided with a temperature sensor 7 connected to a 
temperature detecting circuit 8, which outputs a temperature signal 
voltage E.sub.t proportional to the temperature t of the solution whose 
salinity is to be measured. The output E.sub.t of the temperature 
detecting circuit 8 is inputted through a switch S2 to a temperature 
effect compensation signal generating circuit 9, which outputs a 
compensation signal E.sub.r proportional to 1+.alpha.(t-t.sub.s), where 
.alpha. is a temperature coefficient of the conductance (and therefore, of 
the conductivity) of the saline solution and t.sub.s is a predetermined 
standard temperature value. Both the compensation signal E.sub.r and the 
output E.sub.c from the synchronous rectifier circuit 6 are inputted to an 
arithmetic divider circuit 10, which outputs a ratio E.sub.c /E.sub.r. The 
ratio E.sub.c /E.sub.r, which, as is described later, is proportional to 
the salinity of the objective solution, is displayed by a display unit 11. 
The salimeter according to the present invention further comprises a 
reference resistance R.sub.r and means for providing a standard voltage 
E.sub.s which corresponds to a predetermined standard temperature value. 
The means consists of a series connection of resistances R.sub.2 and 
R.sub.3, which divide a constant voltage E.sub.o to give the standard 
voltage E.sub.s. This standard voltage E.sub.s is to be inputted, in 
substitution for the temperature signal E.sub.t, to the temperature effect 
compensation signal generating circuit 9 through the switch S2, while the 
reference resistance R.sub.r is to be substituted for the pair of 
electrodes 3 and 4, namely, for the resistance R.sub.c. In the above 
circuit constitution the exciting AC voltage source 1 outputs an AC 
voltage e.sub.i as shown in FIG. 5. Furthermore, the pair of electrodes 3 
and 4 is constituted in the form of a probe 13 as shown in FIG. 4, in 
which equipotentially connected electrodes 14, 16 and single electrode 15 
correspond to the electrodes 3 and 4 shown in FIG. 1. The probe is to be 
immersed in an objective solution. 
Next, the use and function of the embodiment are described in the 
following. 
In the first place the pair of electrodes 3 and 4 (formed into a single 
probe as shown in FIG. 4 as is mentioned above) is immersed in a standard 
solution with the switches S1 and S2 turned to contacts A and a, 
respectively. The standard solution is, for instance, a pure NaCl solution 
having a concentration of 5%. Thereupon the linear amplification circuit 
(consisting of the resistance R.sub.c of the standard solution, the 
feed-back resistance R.sub.f and the operational amplifier 5 outputs an AC 
voltage given by: 
##EQU1## 
where R.sub.cs (=R.sub.c) is the resistance of the standard solution at 
the predetermined standard temperature t.sub.s, which is often chosen to 
be 25.degree. C. Then, the synchronous rectifier circuit 6 rectifies the 
AC voltage given by Eq. (1) to output a DC voltage E.sub.c which is equal 
to the peak-to-peak value of e.sub.c : 
##EQU2## 
where E.sub.i is the peak-to-peak value of e.sub.i (refer to FIG. 5), and 
R.sub.cs is the value of R.sub.c which the standard solution shows at the 
standard temperature. On the other hand the temperature detecting circuit 
8 outputs a temperature signal voltage E.sub.t corresponding to the 
temperature t of the standard solution. The signal voltage E.sub.t is 
inputted to the temperature effect compensation signal generating circuit 
9 to make it output a temperature effect compensation signal voltage 
E.sub.r given by the following Eq. (3) having the same temperature 
coefficient as that included in E.sub.c given by Eq. (2): 
EQU E.sub.r =E.sub.k [1+.alpha.(t-t.sub.s)], (3) 
where a proportionality factor E.sub.k is chosen to be equal to the value 
of E.sub.r at the standard temperature value. Eqs. (2) and (3) are 
graphically shown in FIG. 6. Their respective constant gradients give a 
constant ratio E.sub.c /E.sub.r in the entire range of temperature. 
The above two voltages E.sub.c and E.sub.r are inputted to the arithmetic 
divider circuit 10, which performs an arithmetic operation of dividing 
E.sub.c by E.sub.r giving a ratio: 
##EQU3## 
As is shown by Eq. (4), the ratio E.sub.c /E.sub.r does not contain a 
temperature term and is proportional to the peak-to-peak value E.sub.i of 
the exciting voltage e.sub.i and the conductance 1/R.sub.cs of the 
standard solution at the predetermined standard temperature. The ratio 
E.sub.c /E.sub.r is to be inputted to the display unit 11 to display the 
salinity. In this case the ratio E.sub.c /E.sub.r is made equal to the 
salinity value itself (5% in the present case) of the standard solution by 
adjusting E.sub.i through the operation of the attenuator 1b provided in 
the exciting AC voltage source 1. Once E.sub.i is adjusted so as to make 
the display unit 11 display the salinity value of the standard solution, 
the salimeter is calibrated so as to show the value of salinity also with 
respect to any other objective solution. Incidentally, an A-D converter 
can be used as the above arithmetic divider 10 by inputting E.sub.c 
thereto as a signal to be converted with E.sub.r used as a reference 
signal. 
Further, the present invention is devised so that, even if the E.sub.i 
-adjustment is put out of order for some reason or other, the calibration 
may easily be recovered without using the standard solution again. Just 
after the E.sub.i adjustment is made for calibration, turn the switches S1 
and S2 respectively to contacts B and b, and then read and take notes of 
the reading on the display unit 11. This ratio E.sub.c /E.sub.r displayed 
at this time is the value obtained with E.sub.s and R.sub.r substituted 
respectively for E.sub.k and R.sub.cs in Eq. (4), resulting in: 
##EQU4## 
Because R.sub.f, E.sub.s and R.sub.r are constant, the ratio is determined 
only by a set value of the excitation voltage E.sub.i, giving a value 
proportional to a probe constant with the variation corrected. In case a 
plurality of probes are provided for various kinds of objective solutions, 
the above procedure of calibration should be taken in advance with respect 
to each of the probes. 
Further, in case the temperature sensor is a resistance thermometer, the 
present invention can be modified as shown in FIG. 2. According to this 
embodiment the resistance thermometer 7a is supplied with an exciting 
current from the constant voltage E.sub.o (which is also the voltage 
source of E.sub.s) through a variable resistance R.sub.4 and a constant 
resistance R.sub.5, outputting a voltage given by E.sub.o R(t)/(R.sub.4 
+R.sub.5 R(t)) as the temperature signal voltage E.sub.t, where R(t) is 
the temperature-dependent resistance of the resistance thermometer 7a at 
a temperature t. According to such a circuit constitution of the 
thermometer exciting current supply system, possible E.sub.t -variations 
due to characteristic variations of the individual resistance thermometers 
belonging the different probes (provided for various kinds of objective 
solutions) can be eliminated by correcting the thermometer characteristics 
through the adjustment of the variable resistance R.sub.4. 
The present invention can be further embodied as shown in FIG. 3. In this 
embodiment the reference resistance R.sub.r is made up of a parallel 
connection of two resistances, namely, a constant resistance R.sub.r1 and 
a semi-variable one R.sub.r2 :R.sub.r1 is provided in the main circuit 
portion of the apparatus and R.sub.r2 is placed in each of the probes. 
Thus, the reference resistance corresponding to R.sub.r in the embodiments 
shown in FIGS. 1 and 2 is given by R.sub.r1 R.sub.r2 /(R.sub.r1 
+R.sub.r2). Therefore, the calibration values displayed on the display 
unit 11 with the switches S1 and S2 turned respectively to the contacts B 
and b can be made equal irrespective of the variation in the probe 
constant of the individual probes by adjusting the variable resistance 
R.sub.r2 in the calibration process with respect to each of the probes. 
Further, the embodiments shown in FIGS. 1 and 2 can be modified by moving 
the reference resistance R.sub.r to each of the probes. In this case the 
moved resistance R.sub.r is made variable to be adjusted in advance in 
accordance with the probe constant of each probe.