DE1179295B - Bridge circuit for measuring the electrical conductivity of an electrolyte - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Boarding school Kl .: GOIr

German class: 21 e - 29/02

Number: 1 179 295

File number: N18653IX d / 21 e

Registration date: Wed 19, 1960

Opening day: October 8, 1964

The invention relates to an alternating current operated bridge circuit for measuring the electrical conductivity of an electrolyte, in which a conductivity measuring cell intended for the electrolyte and one which determines the bridge measuring range, in the sense of achieving a linear bridge balancing characteristic measured, having a very high conductivity in relation to the test object Fixed resistor each form a bridge arm and are in series with the bridge operating power source as well as parallel to the other two bridge branches, which are connected by the series connection of one with the measuring cell to be kept at the same temperature, a negative temperature coefficient having resistance with one serving to take into account different temperature coefficients of different test objects Resistance network are formed and in which the resistor network has one connection end Contains bridge adjustment potentiometer located at one pole of the operating power source, the tap of which forms one of the output terminals of the bridge.

Such a bridge circuit is shown in FIG. 1 of the drawing. The bridge is with alternating voltage generated by a generator O of z. B. 1000 Hz fed. A bridge branch consists of the resistance element R _x ; in series therewith is a constant resistance R _m . The resistance element R _x represents the resistance of the conductivity measuring cell, which has two, generally platinum-plated electrodes which are immersed in the liquid whose specific conductivity is to be measured.

Another branch of the bridge consists of a resistor R ₂ and another branch of the parallel connection of two resistors R ₃ and R _p . The resistor R _v is designed as a potentiometer. A measuring instrument V is connected between the sliding contact of the potentiometer and the connection point of the resistors of the first branch of the bridge. The bridge is designed to be set to zero using the sliding contact of the potentiometer. This can be done automatically in a known manner by replacing the measuring instrument with an electronic amplifier, at the output of which an electric motor is connected which acts on the sliding contact so that the latter is always returned to the position in which the bridge is in equilibrium .

Electrolytes have a relatively high temperature coefficient of specific conductivity. This temperature coefficient is between 1.5 and 3% at 2O ⁰ C and it is necessary to reduce the measured temperature coefficient 2O ⁰ C. This bridge circuit for measuring the electrical
Conductivity of an electrolyte

Applicant:

N. V. Philips' Gloeilampenfabrieken,

Eindhoven (Netherlands)

Representative:

Dr. rer. nat. P. Roßbach, patent attorney,
Hamburg 1, Mönckebergstr. 7th

Named as inventor:

Pieter Geert Kuipers, Eindhoven (Netherlands)

Claimed priority:

Netherlands 23 July 1959 (241 559)

can be done in a manner known _per se in that a resistor with a likewise negative temperature coefficient is selected for the resistance R%. This resistance can then be used for a specific temperature interval

= RJl

- 20)

to be written.

R _{20 represents} the resistance at 2O ⁰ C, α the temperature coefficient of i? ₂ ^ur * d / represents the temperature in ⁰ C. The conductivity Gt of the electrolyte can be written in a similar way

Gt = G ₂₀ (1 + β [t - 20]),

where β is the temperature coefficient of the conductivity of the electrolyte.

If the two temperature coefficients are equal, it is always possible to precisely compensate if the resistance R _z and the cell R _x are exposed to the same external conditions. In practice, however, it turns out that between different resistors with a negative temperature coefficient there is always a greater deviation with respect to the temperature response, and it is necessary to use series resistors and sometimes parallel resistors in combination with R ₂ in order to also be used for two different temperatures to compensate exactly.

In general, a linearly divided scale is desired for the potentiometer R _p. R _m represents the

409 690/141

range resistance, which can be exchanged to change the measuring range. Sufficient linearity is then achieved in that a maximum resistance ratio of 1: 100 is selected for the two bridge branches R _n and R _x. Then, for each measuring range, R _1n is matched to one another equal to or greater than coefficients. This can be achieved by only changing the temperature during a test measurement while the concentration of the electrolyte remains the same. The bridge is set to zero.

With a correct compensation should

0.01

where C is the constant of the relevant conductivity cell and Gmax is the maximum conductivity (minimum resistance) occurring in the relevant measuring range.

In this way, the potential in point 1 of the circuit is proportional to the conductivity, because the voltage V ₁ at R _{x is} related to the bridge supply voltage as the resistance R _m is related to the total resistance of the first bridge branch. If R _{m is} negligible compared to the resistance of the conductivity cell, then

where G _{x represents} the measured conductivity.

The same relationship applies to the ratio of the resistances in the other bridge branch. In general, i? 3 is chosen to be very small compared to R ₂ , and Rj, is given a value almost equal to the maximum value of R ₂ . The resistance R ₃ is then about 10 times 3 <> smaller than R _p .

The invention is based on the object of creating a device of the type described which bridges the temperature coefficient of conductivity while maintaining the same scale division of the electrolyte can be easily adjusted.

Based on the bridge circuit described above, the invention consists in that the other Connection end of the adjustment potentiometer to the connection point of a series circuit of two Connected in parallel to an adjustable resistor, in series between the resistor with resistors negative temperature coefficient and a fixed resistor at the bridge operating current source Resistor is connected.

A circuit according to the invention is shown in Figure 2, in which R _{4 represents} the variable resistance and R _{5 represents} a constant resistance. Same parts as in Fig. 1 have the same reference numerals.

A resistor R _e + R ₇ lying parallel to the variable resistor R ₄ preferably has a high value compared to i? ₄ , and its tap is chosen such that the ratio between the resistors R ₅ + R ₇ and the resistor R ₂ with a negative temperature coefficient is small, e.g. B. of the order of 0.01, so that the parts R _e and R ₇ behave as 99: 1.

The potentiometer is again labeled R _p. The resistor R ₅ has a value of 10 Ω.

The actual measurement takes place according to FIG. 2 as well as according to FIG. 1 by moving the tap 2, which can be done automatically by hand or in a known manner. The measuring element R _x and the NTC resistor R ₂ should always have the same temperature.

If a different type of electrolyte is measured, the temperature coefficient of which is also different, the temperature (I --60 / J) 1000 -fÄ » is achieved by setting the resistance R _t.
C ₂₀ "300 + Rs

where C _{20 is} the conductivity of the electrolyte at 20 ° C and β is the temperature coefficient of conductivity. This is based on a temperature increase of 20 to 80 ° C, and the final resistance values of A ₂ are set at 1000 and 300 Ω. R _s is the required series resistance to R ₂ .
One then finds:

R _s = ™ - 300 Ω
6 -β

or if β is expressed as a percentage:

1166

-300Ω.

Taking into account the linearity of the scale, the lower resistance of the right bridge arm should be around 1% of the upper resistance A ₂ . The latter is at 20 ° C: R ₂₀ -f R ₈ . (R ₂₀ is the resistance of the NTC resistor R ₂ , thus 1000 Ω.) It is therefore the mentioned lower resistance:

R '= 0.01 (A ₂₀ -r Rs) - IO
The remaining part is:

11.66
ß

R '

R '

30010.99 ^ 10

-3 -10.

R ' thus consists of a resistance of 10 Ω with a correction for the influence of ß. Since β depends on the electrolyte to be measured, it is necessary to make the correction. This is done with the circuit according to the invention. This is because it is possible to change R ' _s , as a result of which the correct value of β can be set without having to correct the sensitivity of the bridge. At 20 ° C the output voltage of the bridge branch does not change when R ' _{s is} changed.

Claims (2)

Patent claims: 1. Bridge circuit operated with alternating current for measuring electrical conductivity an electrolyte, in which a conductivity measuring cell intended for the electrolyte and a determining the bridge measuring range, in the sense of achieving a linear bridge balancing characteristic measured, having a very high conductivity in relation to the test object Fixed resistor each form a bridge branch and are connected in series to the bridge operating power source as well as parallel to the other two bridge branches, which are connected by the series connection of one with of the measuring cell to be kept at the same temperature, have a negative temperature coefficient send resistor are formed with a resistor network serving to take into account different temperature coefficients of different test objects and in which the resistor network contains a bridge adjustment potentiometer which has one connection end at a pole of the operating power source and whose tap forms one of the output connections of the bridge, characterized in that the other connection end of the Adjustment potentiometer (Rp) is connected to the connection point of a series circuit of two resistors CR ₆ , R ₇ ) , which are connected in parallel to an adjustable, in series between the resistor CR ₂ ) with negative temperature coefficients and a fixed resistor (R ₆ ) at the bridge operating current source (O) lying resistor CR ₄ ) is connected. 2. Apparatus according to claim 1, characterized in that the tapping ratio of the resistor (R ₆ , R ₇ ) , which is preferably high-resistance _{in relation to the adjustable resistor CR 4} ), is selected such that the one on the side of the resistor CR ₂ ) is negative Temperature coefficient lying part CR ₆ ) is about a hundred times the remaining part (R ₇ ) . Considered publications:
German Patent No. 1,007,880;
British Patent Nos. 686 681, 690 701;
"Instruments", 24 (1951), pp. 710 to 715. 1 sheet of drawings 409 690/141 9.64 © Bundesdruckerei Berlin