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What is conductivity? Electromagnetic flowmeter for measuring conductivity of liquid

Date: 2019-03-19
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Conductivity is the ability of an object to conduct electric current.  The measuring principle of conductivity meter is to put two parallel polar plates into the solution to be measured, add a certain potential (usually sine wave voltage) at both ends of the polar plates, and then measure the current flowing between the polar plates.  According to ohm's law, the reciprocal of conductivity (g)-resistance (r) is determined by voltage and current.

The basic unit of electrical conductivity is Siemens (S), originally called ohm.  Because the geometry of the conductivity cell affects the conductivity value, the unit conductivity S/cm is used in standard measurement to compensate for the difference caused by various electrode sizes.

Unit conductivity (C) is simply the product of measured conductivity (G) and conductivity cell constant (L/A). Here, L is the length of the liquid column between two polar plates, and A is the area of the polar plates.

  =ρl=l/σ

(1) Define or explain that the reciprocal of resistivity is conductivity.  σ=1/ρ

(2) Unit: In the International System of Units, the unit of electrical conductivity is Siemens/meter.

(3) Explain that the physical meaning of conductivity is to indicate the conductivity of a substance.  The higher the conductivity, the stronger the conductivity, and vice versa.


The conductivity of industrial water and its aqueous solution is greater than 10 -4 s/cm, and the conductivity of acid, alkali and salt solution is 10- 4 ~10 -1 s/cm, so there is no problem in using, and there is no problem in using low-degree distilled water 10 -5 s/cm.  Petroleum products and organic solvents cannot be used if their electrical rates are too low.  From the data, it is found that some pure liquid or liquid has low conductivity and cannot be used. However, in actual work, there will be some examples that can be used due to impurities. Impurities are beneficial to increase conductivity.  For aqueous solution, the conductivity in the data is measured in the laboratory with pure water ratio. The actual aqueous solution may be mixed with industrial water, and the conductivity will be higher than that found, which is also conducive to flow measurement.


The premise of using electromagnetic flowmeter is that the liquid to be measured must be conductive and cannot be lower than the threshold value (i.e. lower limit value).  If the conductivity is lower than the threshold value, the measurement error will be directly caused and cannot be used. if the conductivity exceeds the threshold value, the measurement can be carried out even if it changes. the indication error changes little. the threshold value of the universal electromagnetic flowmeter is between 10-4 ~ (5× 10-6) s/cm, depending on the model.  When in use, it also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance. The manufacturer's instruction book usually specifies the length of the signal line corresponding to the conductivity.  Instruments with non-contact capacitors coupled with large area electrodes can measure liquids with conductivity as low as (5 × 10 -8 )S/CM.


According to the use experience, the actual applied liquid conductivity is preferably at least one order of magnitude larger than the threshold specified by the instrument manufacturer.  Because the lower limit value specified in the instrument specification of the manufacturer is the lowest measurable value under various conditions of use, it is limited by some conditions of use, such as conductivity uniformity, connection signal line, external noise, etc., otherwise, output sloshing and other phenomena will occur.  We have encountered many times the measurement of low-grade distilled water or deionized water, the conductivity of which is close to the threshold value of 5 × 10 -6 s/cm, and the output shakes when in use.


There is also a kind of liquid that cannot be measured by electromagnetic flowmeter installed on the pipeline in the absence of available conductivity data, but it is considered useful to take samples from the field pipeline and go off-line to the laboratory to measure its conductivity with conductivity meter.  This is because the measured liquid in the off-line sampling process is different from that in the pipeline. For example, the liquid has absorbed CO2 or NO x in the atmosphere to generate trace amounts of carbonic acid or nitric acid, which changes the conductivity.


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