JPS62207968A - Conductivity meter circuit - Google Patents

Abstract

PURPOSE:To eliminate the influence of polarization without using a costly material such as platinum black for electrodes by sampling a measuring voltage near the time right after the impression of a square wave AC voltage when the influence of the current occurring in the capacity of an electrode cable is not received. CONSTITUTION:The square wave AC voltage V1 is impressed to one 1' of the electrode immersed in a liquid to be measured via a buffer amplifier 6 and is applied to a control signal generating circuit 9. The signal generating circuit 9 supplies sample pulses PA, PB, PC deviated to the time right after the rise timing of the square wave AC voltage V1 to sample holding circuits 10A, 10B, 10C and causes electrodes 1, 1', to discharge by a discharge circuit 11 in the period when the V1 is of a zero level. The measuring voltage proportional only to the current flowing between the electrodes is calculated by an arithmetic circuit 12 in accordance with the voltage sampled near the time right after the impression of the square wave AC voltage where the influence of the electric current occurring in the capacity of the electrode cable is not received. The conductivity of the liquid to be measured is thus determined.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an improvement of a two-electrode type liquid conductivity thickness meter circuit.

<Prior Art> The conventional configuration of a two-electrode liquid conductivity meter will be explained based on FIG. 1.1' is a pair of opposing electrodes immersed in the liquid to be measured 2, and 3.3' is a connection terminal of the cables 4, 4' to the electrodes. 5 is a generator circuit for rectangular wave AC voltage v1;
Reference numeral 6 denotes a current/voltage conversion circuit that converts the current to the amplifier.The non-inverting input terminal is at the reference potential, and the feedback resistor R is connected to the inverting input terminal. AC current flowing from cable 4! is V2”
--Io' Tt (The voltage is converted according to the relationship. 8 is a synchronous rectifier circuit driven by the rectangular wave AC voltage
2 is converted into a DC voltage signal EK.

FIG. 4 shows an equivalent circuit of the liquid to be measured via the electrode 1.1' and the cables 4, 4'.

, Ra is the parallel resistance of Ca, capacitor C6 and resistance Ra
The series circuit represents the capacitance and resistance of the cables 4, 4'.

<Problems to be solved by the invention> 21! When measuring the liquid resistance Rc based on the current flowing when a square wave AC voltage is applied in a polar method configuration, a method of increasing the frequency of the AC voltage is used to avoid the influence of Ca. However, when the frequency is increased, the influence of the cable's capacitance C6 appears.

In order to make C as large as possible, methods such as using platinum black for the electrodes 1 and 1' are taken, but platinum black is expensive and has the disadvantage of increasing costs.

The present invention uses electrodes made of materials such as SUS without using expensive materials such as platinum black for the electrodes, and improves conductivity without being affected by so-called polarization due to the capacitance between the electrode and the liquid. The aim is to realize a circuit that can accurately measure up to a high range.

Means for Solving the Problems> The structural feature of the present invention is that a square wave AC voltage is applied between a pair of measurement electrodes immersed in a liquid to be measured, and the measurement is proportional to the current flowing between the electrodes. In a circuit that measures the electrical conductivity of a liquid to be measured based on voltage, samples are taken at three points with the measurement voltage shifted in time immediately after the rectangular wave alternating voltage is applied and in the vicinity where the current does not affect the current caused by the electrode cable capacitance. means for calculating a measurement voltage that is proportional only to the current flowing between the electrodes at the timing of applying the rectangular wave alternating voltage based on the sample voltage of this means; The present invention also includes means for calculating conductivity.

<Operation> Three points are sampled at different intervals, and a measured voltage proportional to the inter-electrode current at the timing of applying the rectangular wave alternating voltage is calculated based on the sample voltage.

<Embodiment> An embodiment of the conductivity meter circuit of the present invention will be described based on FIG. 4 explained in Figure 3! Elements that are the same as the element are given the same reference numerals and explanations are omitted.

The rectangular wave AC voltage v1 is a ternary signal having a zero level for a certain period of time between the positive and negative half cycles, and is applied to one of the electrodes 1' via the buffer amplifier 6, and is also applied to the control signal generation circuit. given to 9.

The control signal generation circuit 9 includes a sample hold circuit 10A for sample pulses PA, PB, and PC whose time is shifted immediately after the timing of the rise of the rectangular wave AC voltage v1;
10B and 10o, and during the period when Kv1 is at zero level, electrode 1. A discharge control signal PD is supplied to the discharge circuit 11 which short-circuits the discharge control signal PD.

The sample voltages (2), VB, and Vo of the sample and hold circuits 10A, 1OB, and 1Oc are led to the arithmetic circuit 12.

The calculation result is transmitted as an output signal E.

jlE2 Figure η) is a waveform diagram of the rectangular wave AC voltage v1, (B)
is the waveform diagram of the measurement voltage v2, and the sample pulse pA is
pB.

A waveform diagram of P and discharge control signal PD is shown.

Timing t of the rise of the rectangular wave AC voltage v1. In a very short period of time, a spike occurs due to the capacitance and resistance of the cable, and after that, the liquid capacitance C9 liquid resistance R, the parallel resistance Rc of Ca
This results in a voltage waveform that gradually decreases.

Using the equivalent circuit in Fig. 4, and considering the measured voltage v2 when ■ is applied to the electrode, V2 = R(' J
10R(” 12 (1) where C
6・R, (Ca, the sample voltage ■A at timing t1, which is immediately after to and has a small influence of 12, is (
1) Ignoring the second term in formula, vA ” v2 (t=t) # Rf '1
(t=t1). Therefore, from equation (2), general K.

V2#A (1+ B e-”)
(Represented as 41.

The value we want to find now is the value of v2 at the timing of 1=1°, and with this value, Rc is expressed as E, R(
Since is a known value, if V2(tyo) is found by calculation, the value of Rc will also be found.

Here, sample voltages vA, VB, and vo at timing 11+tze t3, which are close to each other and have a small influence on 12 immediately after t=to, are as follows. Now, as an example, t=tt=2・IK y j3
If you decide '11 '21 t3 like ``I K# 2 3tK, A, B,
C will be as follows. Rc of the FJ formula (5) is given by these constants A, B, and C.

This indicates the value of interelectrode resistance that is not affected by interelectrode capacitance C1 and cable capacitance C6, and from this interelectrode resistance R, the conduction 1! The L rate J is J==-! It can be easily calculated using L (K: a constant determined by the shape of the electrode) Rc.

What is the means for calculating A, B, and C from the above equations (6) to (8)?

High-speed calculation is possible using a microprocessor.
The continuity of the output signal E is almost the same as that of the conventional analog system, and the calculation accuracy is also t. Since sample voltages at three points very close to are used, highly accurate approximate calculation can be realized.

In the above embodiment, based on equation (2), the form of equation (4) is v
2, but instead of this, V2#A't2+B't+C'
06 and a quadratic formula, the card can be found in the same way. Furthermore, by simplifying this and focusing on the fact that the time t and v2 have a relatively close linear relationship in the vicinity of t''to, Vz: A' t + B'
(It is also possible to similarly obtain Rc by regarding it as a linear equation of II.

Effects> As explained above, according to the present invention, the following effects can be expected.

(1) Conductivity can be measured with high precision without being affected by cable capacity.

(2) Even if you do not use expensive platinum black for the electrodes and use electrodes with relatively low C (capacity between electrode and liquid) such as 5US (stainless alloy) or titanium, the frequency of AC rectangular wave AC voltage can be controlled. Measurements can be made without raising the temperature.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing one embodiment of the present invention, Fig. 2 is a waveform diagram for explaining operation, Fig. 3 is a block diagram showing an example of the conventional technology, and Fig. 4 is equivalent between electrodes and cables. Shows the circuit. 1.1'...Counter electrode, 4.4'...Cable, 5
. . . Rectangular wave alternating current voltage generation circuit, 6 . . Buffer amplifier, 7 . . . Current/voltage conversion circuit, 9 . ...Discharge circuit, 12...
Arithmetic circuit.

Claims (1)

[Claims] In a circuit that measures the electrical conductivity of a liquid to be measured based on a measurement voltage proportional to a current flowing between the electrodes by a rectangular wave AC voltage applied between a pair of measurement electrodes immersed in the liquid to be measured, the rectangular wave means for sampling the measured voltage at three points at different times in the vicinity immediately after application of the alternating current voltage and not affected by the current caused by the electrode cable capacitance, and the timing of applying the rectangular wave alternating voltage based on the sample voltage of this means. A conductivity meter circuit comprising means for calculating a measurement voltage proportional only to the current flowing between the electrodes, and means for calculating the conductivity of the liquid to be measured based on the calculated voltage.