APPLIANCES

To perform this measurement method can be used specially constructed devices or constituted by individual items conveniently assembled. The block diagram shown in Figure 2 and elements described in the following sections illustrate appropriate equipment.

Block diagram of measuring apparatus

Keys

1. Test cell
2. Heating System
3. Square wave generator
4. Measurement chain
5. Meter
6. Recorder

Test Cell
The test cells of three terminals, designed according to the recommendations of IEC 60247, are generally appropriate for these measures.

You can use an additional type of cell in which there is no bridge, made by any solid insulating material, between the measuring electrodes, as shown in Figure 3. This cell type is often more accurate with highly insulating liquids.

Example of a cell test designed for highly insulating liquids

Keys

1. Cover
2. Internal electrode
3. External electrode
4. Stainless steel container
5. Thermocouple or thermometer to measure temperature
6. Electrical connections for BNC terminals

The typical distance between the inner and outer electrodes is 4 mm, the minimum distance should not be less than 1 mm.

The recommended material for the electrodes is stainless steel. As an example, the diameter of the outer electrode is 43 mm, the outer electrode is 51 mm, the length of the electrodes is 60 mm, and the diameter of the container Stainless steel is 65 mm.

This type of test cell is designed to minimize the effects of contamination of surfaces

Contact: although the contact surface is large, the relationship ÷ = "electrode surface" / "fluid volume" is relatively small (÷ = 2.6 cm-1) due to the large volume of liquid (v = 200 cm3).NOTE - It is recommended restricting the use of a given cell to a particular type of liquid.

Heating System
The heating system must be adequate to maintain the temperature of the measuring cell to within ±1 ºC of set point. This can be a forced draft oven or in an oil bath controlled thermostatically controlled and equipped with a bracket to hold the cell.The heating system must be shielded electrical connections on the cell.

Square wave generator

The square wave generator must provide a quasi-rectangular voltage highly stable. The following features are appropriate:

- Amplitude: 10 V to 100 V;

- Frequency: 0.1 Hz to 1 Hz;

- Ripple: <1%;

- Rise time of tension: 1 ms to 100 ms.

Measurement Chain

The IR conduction current through the measuring cell, is measured in the second part of each half-wave and averages for a number of periods depends on the range of measures. The chain of measures gives the conductance G test cell.

As an example, the range of measured values ​​of conductance is 2 x 10-6 S a 2 x 10-14 S with a margin of error less than 2%.

The capacity C of the test cell is deduced from the measured current during the voltage rise. The values measurable capacity is between 10 pF and 1000 pF with an uncertainty of less than 1%.

As an example, for a liquid relative permittivity r = 2, a conductance value of 2 x 10-14 S gives

as ä = 0,8 x 10-6 a 50 Hz.

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Posted in: square wave generator,values ​​of conductance

PRINCIPLE OF OPERATION

The operating principle is the measurement of the capacitive current and the conduction current by applying a voltage square wave AC test cell. The capacitive current is measured during the rise time of tension and the conduction current is measured during the stable period of stress, but before any disturbance electric field due to the accumulation of ions. The currents can be measured both positive and half-waves in the negative square wave current for several cycles to increase the accuracy of the measurement (see Figure).

Principle of operation using the square wave method

The square wave voltage V (t) of amplitude ±V, is changed periodically with a slope dV / dt. The current I during the rise and fall of tension is the sum of the capacitive current (displacement current) and current driving:

The capacitive current IC is measured during periods of rising and falling of V (t).

The driving current IR is measured in periods of stable V (t) as V / R given angular frequency can be determined by the following relationships:

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Posted in: conduction,tension,voltage