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Features at a glance
- Absolute Accuracy to ± 2%
- High and medium sensitivity ranges
- Fast and easy to use
- Compact size
- Rugged, reliable and easy to clean
A Special Instrument
With the new BI-870 measuring of dielectric constants has never been so easy. Simply insert the probe in the liquid to be measured, adjust the two controls on the front panel and read the dielectric constant from the display. The BI-870 can
accurately measure in low and high dielectric solvents, including mixed liquids and solutions.
Good zeta potential determinations of suspended colloidal particles depend on an accurate knowledge of the dielectric constant of the liquid. For a range of particles of interest a number of different liquids may be used, including mixtures of various proportions creating the need for an accurate measurement of dielectric constant. While literature values of dielectric constants are suitable when using pure liquids values are not readily available for mixtures of liquids. There are no generally acceptable methods of calculating values for mixtures from pure component values. For example, weighting by mole fraction leads to errors of 17% and 6% for the two mixtures given in the table below. Weighting by volume leads to errors of -8% for Methanol/Water and 2.4% for Dichloro Methane/Benzene respectively.
The BI-870 has two selectable sensitivity ranges: 1-20 and 1-200. Absolute accuracy is ± 2% while repeatability and linearity are better than 0.2%. The measurement signal applied to the outer cylinder of the probe is a low-distortion sine wave at a frequency of 10 kHz. The amplitude is approximately 7 volts rms on the 1-20 range and 0.7 volts on the 1-200 range. The frequency is crystal-controlled and is, therefore, stable to approximately 1 part in 105. The dielectric constant of the liquid sample is determined by measuring the current between the outer and inner cylinders of the probe. With a stable voltage source and precisely known probe parameters, it is possible to display the dielectric constant directly. Calibration is simple using the back panel adjustment with a liquid of known dielectric constant.
The probe has an open structure and is easy to clean. It is constructed from two precision cylinders, machined from type 316 stainless steel. Six Teflon balls maintain the cylinder spacing. If the probe is used primarily with low-dielectric constant hydrocarbon fluids, we recommend cleaning by agitation in acetone or ethyl alcohol, followed by gentle drying with clean compressed air. It is important that the probe be cleaned before any residue dries on the cylinders. Since the probe is made of stainless steel and Teflon®, it may be cleaned in almost any solvent.
BI-870 Dielectric Constant Measurements for Common Liquids Material Temperature Measured
% Difference2 Cyclohexane 250C 2.03 2.015 0.7 Ethyl acetate 250C 6.05 6.02 0.5 Dichloro methane 250C 8.92 9.073 -1.6 Methanol 250C 33.2 32.63 1.7 De-ionized Water 250C
Dichloro methane +
250C 5.35 N/A - Methanol +
250C 61.2 N/A -
1. CRC Handbook of Chemistry and Physics.
2. %Difference = [(Measured - Published)/Published] x 100.
3. Corrected for temperature to match 25 °C measurements.
4. Liquids were mixed 1:1 by volume at 25 °C.
Full Scale Sensitivity Dielectric Constant of 1-20 and 1-200 Maximum Conductivity of Sample Range 1-20: 1 µS/cm; Range 1-200: 10 µS/cm,( Accuracy 2% Absolute; Repeatability and Linearity 0.2% Probe Materials Stainless Steel and Teflon® Minimum Liquid Required ~23 mL, standard probe. Power Requirements 100/115/220/240 VAC, 50/60 Hz, 10 watts Dimensions 190mm x 240mm x 70mm Operating Temperature 22-58 °C Measurement Signal Low-distortion 10 kHz sine wave Display Backlit LCD Output Analog Recorder (optional or customer supplied), Full Scale Reading 1.999 V Calibration Back Panel Adjustment with Reference Liquid
A policy of continual improvement may lead to specification changes