|Place of Origin:||CHINA|
|Minimum Order Quantity:||0-100|
|Packaging Details:||Common package or custom package|
|Payment Terms:||L/C, D/A, D/P, T/T, Western Union, MoneyGram|
|Range And Resolution:||0-5000μs/cm 1||Precision:||± 1.5% F.S.|
|Operating Temperature:||0 ~ 65 °C||Working Pressure:||< 0.6 Mpa|
|Power Supply:||12 ~ 24 VDC ± 10%||Signal Output:||RS-485(Modbus/RTU)|
Modbus Online Conductivity Sensor,
Ip68 Online Conductivity Meter
Drinking water/surface water/various water supplies/industrial water treatment
|Range and resolution||0-5000μs/cm||1|
|Precision||± 1.5% F.S.|
|Operating temperature||0 ~ 65 °C|
|Working pressure||< 0.6 mpa|
|Power supply||12 ~ 24 VDC ± 10%|
|Installation mode||Immersion mounting|
|Cable length||5 meters, other length can be customized|
|Temperature compensation||Automatic temperature compensation (PT1000)|
|Calibration mode||Two-point calibration|
|Power consumption||< 0.05 W|
Note: At least 2 cm away from the bottom and side walls of the container during installation and testing.
6.Electrolytic Conductivity and Its Measuring
Our body fluids — blood, lymph, and interstitial fluid — all have a high concentration of sodium chloride and other minerals; they are all electrolytes; the conductivity of blood is approximately 0.54 S/m at 37°C
The conductivity of aqueous solutions, in which the electric current is carried by charged ions, is determined by the number of charge carriers (the concentration), the speed of their moving (the ion mobility depends on the solution temperature) and the charge they carry (valence of ions). Therefore, in most aqueous solutions, the higher concentration will lead to more ions and hence to higher conductivity. However, after reaching some maximum concentration, the conductivity may start decreasing with increasing concentration. Therefore, two different concentrations of the same salt may have the same conductivity.
The temperature also affects conductivity because at higher temperatures ions move faster, increasing the conductivity. Pure water does not conduct electricity well. The ordinary distilled water in equilibrium with carbon dioxide containing in the air and total dissolved solids of less than 10 mg/L has a conductivity of about 20 µS/cm. The conductivity of various solutions is given in the table below.
The conductivity of distilled water is approximately 0.055 μS/cm
|Conductivity of various water solutions at 25°C|
|Pure water||0.055 μS/cm|
|Deionized water||1.0 μS/cm|
|Drinking water||50 to 500 μS/cm|
|Domestic wastewater||0.05 to 1.5 mS/cm|
|Industrial wastewater||0.05 to 10 mS/cm|
|Seawater||35 to 50 mS/cm|
|Sodium chloride, 1mol/L||85 mS/cm|
|Hydrochloric acid, 1 mol/L||332 mS/cm|
Two electrodes of a conductivity sensor (left) and the temperature sensor (right) used for automatic temperature compensation (ATC) in a TDS meter
To determine the conductivity of a solution, a conductance or resistance meter (they are technically the same) is usually used and the measured value is then manually or automatically recalculated to conductivity. This is done by considering the physical characteristics of the measuring device or sensor. This includes the area of electrodes and the separation distance between the two electrodes. The sensors are quite simple: they comprise a pair of electrodes immersed in the electrolyte solution. The sensors for measuring conductivity are characterised by a cell constant, which is given by the ratio of the distance between electrodes D to the area normal to the current flow A:
K = D/A
This formula works well when the area of electrodes is much greater than the separation between them because in this case most of the electric current flows directly between the electrodes. Example: for 1 cubic centimeter of liquid K = D/A = 1 cm/1 cm² = 1 cm⁻¹. Note that cells with small widely-spaced electrodes have cell constants of 1.0 cm⁻¹ or more while cells with larger and closely-spaced electrodes have constants of 0.1 cm⁻¹ or less. The cell constant of various devices for measuring conductivity varies from 0.01 to 100 cm⁻¹.
Theoretical cell constant: left — K = 0.01 cm⁻¹ , right — K = 1 cm⁻¹
To obtain the conductivity from the measured conductance, the following formula is used:
σ = K ∙ G
σ is the solution conductivity in S/cm,
K is the cell constant in cm⁻¹,
G is the cell conductance in siemens.
The cell constant is usually not calculated, but measured for a particular measuring device or setup using a solution of known conductivity. This measured value is entered into the meter, which automatically calculates the conductivity from measured conductance or resistance. Because the conductivity depends on the solution temperature, devices for measuring conductivity often contain a temperature sensor that allows measuring the temperature and providing the automatic temperature compensation (ATC) to the standard temperature of 25°C.
The simplest method of measuring the conductance is applying a voltage to two flat electrodes immersed in the solution and measuring the resulting current. This is called a potentiometric method. According to Ohm’s law, the conductance G is the ratio of current I to voltage V:
G = I/V
However, things are not as simple as they seem. There are many difficulties. When DC voltage is used, ions can accumulate near the electrode surfaces and chemical reactions can occur at the surfaces. This will lead to increasing polarization resistance on the electrode surfaces, which, in turn, may lead to erroneous results. If we try to measure the resistance of, for example, sodium chloride solution using a multimeter, we will clearly see that the reading on the display is increasing rather quickly. To mitigate this problem, often four electrodes are used instead of two.
Electrode polarization can be prevented or reduced by applying an alternating current and adjusting the measuring frequency. Low frequencies are used to measure low conductivity, where the polarization resistance is comparatively small. Higher frequencies are used to measure high conductivity values. Frequency is usually automatically adjusted considering the measured conductivity of a solution. Modern digital 2-electrode conductivity meters usually use complex alternating current waveforms and temperature compensation. They are calibrated at the factory and often recalibration is required in the field because of the cell constant changes with time. It can be changed due to contamination or the physical-chemical modification of electrodes.
In a traditional 2-electrode conductivity meter, an alternating voltage is applied between the two electrodes, and the resulting current is measured. This meter, though simple, has one disadvantage — it measures not only the solution resistance but also the resistance caused by the polarization of the electrodes. To minimize the effect of polarization, 4-electrode cells, as well as platinized cells covered with platinum black, are often used.
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