Conductivity unit conversion and determination

1. Conductivity:
Conductivity (total dissolved solids) is abbreviated as TDS. Conductivity is the ability of an object to conduct electric current. The measuring principle of the conductivity meter is to place two parallel plates in the solution to be measured, add a certain potential (usually a sine wave voltage) at both ends of the plates, and then measure the current flowing between the plates. According to Ohm's law, conductivity (G) - the reciprocal of resistance (R) is determined by voltage and current.
The basic unit of conductivity is Siemens (S), which was originally called mho, which means the reciprocal of the resistance unit ohm. Because the geometry of the conductivity cell affects the conductivity value, the unit conductivity S/cm is used in standard measurements to compensate for the differences caused by various electrode sizes. The unit conductivity (C) is simply the product of the measured conductivity (G) and the conductivity cell constant (L/A). Here L is the length of the liquid column between the two plates, and A is the area of ​​the 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 conductivity is Siemens/meter.
(3) Explain that the physical meaning of conductivity is to indicate the conductivity of a material.
The greater the conductivity, the stronger the conductivity, and vice versa.

Conductivity meter

1. Unit conversion between conductivity meter and resistivity meter
1. Conductivity meter is the reciprocal of resistivity, which is conductivity, and the unit is Siemens/m, 1 Siemens = 1/Ω.
The unit of conductivity is mho, also known as Siemens. It is expressed by S, because the S unit is too large. Millisiemens
1uS/cm = 0.001mS/cm; 1000uS/cm = 1mS/cm is often used
2. The unit of resistivity meter is Ω.cm, that is, ohm centimeter.
Measurement method between conductivity and resistivity of water
1. The resistivity of water refers to the resistance between the opposite sides of a cube with a side length of 1cm at a certain temperature, and the unit is Ω.cm or MΩ.cm. Conductivity is the reciprocal of resistivity, and the unit is S/cm (or μs/cm). The resistivity (or conductivity) of water reflects the amount of salt in the water. It is an important indicator of water purity. The higher the purity of water, the lower the salt content, and the greater the resistivity of water (the lower the conductivity).
2. The resistivity (or conductivity) of water is affected by the purity, temperature and various factors in the measurement. The measurement of pure water resistivity (or conductivity) is to select a dynamic measurement method, and use the temperature compensation method to convert the measured value into a resistivity at 25°C for easy measurement and comparison.
3. Water hardness
Water hardness refers to the concentration of calcium and magnesium ions in water. The hardness unit is ppm, and 1ppm represents 1 mg/L of calcium carbonate in water.
Hardness unit conversion:
Hardness unit ppm German hardness French hardness British hardness
1ppm = 1.000ppm 0.0560 0.1 0.0702
1 German hardness = 17.847ppm 1 1.7847 1.2521
1 French hardness = 10.000ppm 0.5603 1 0.7015
1 British hardness = 14.286ppm 0.7987 1.4285 1

TDS and conductivity
1. TDS is the abbreviation of tatal dissolved solids in English. Its Chinese translation is total dissolved solids (total solid content dissolved in water), and its unit is mg/L (ppm). It indicates how many mg of total dissolved solids are dissolved in 1 liter of water.
2. What does TDS specifically include?
TDS is the total content of dissolved substances in water. Due to the strong solubility of water, water includes calcium and magnesium ions, colloids, suspended particles, proteins, viruses, bacteria, microorganisms and corpses, as well as even smaller heavy metal ions. We all know that pure water contains very few dissolved solids, generally only zero to tens of mg/L. If the water is polluted or has dissolved many soluble substances, its total solid content will increase with the increase of soluble substances.
3. What is the TDS value? What is the purpose of the TDS pen?
The TDS pen is an instrument specifically used to measure the content of soluble substances in water. Its measurement principle is actually to indirectly reflect the TDS value by measuring the conductivity of water.
In a physical sense, the more dissolved matter there is in water, the greater the TDS value of the water, the better the conductivity of the water, and the greater its conductivity value. [page]
For example: There are two types of water quality, A and B, where the TDS value of water A is 200; the TDS value of water B is 260, then the conductivity of water B is higher than that of water A, indicating that the degree of pollution of water B is more serious than that of water A.
4. What is TDS value?
The TDS value displayed by a TDS pen or other similar instruments refers to conductivity, which represents the meaning of TDS and indirectly reflects the content of dissolved solids in a certain water. It is very important for human drinking.
The World Health Organization stipulates that the TDS value standard for drinking water suitable for human body is within 50. The
Hong Kong standard is within 80, and below 100 is acceptable.
The national standard GB5749-2006 "Sanitary Standard for Drinking Water" requires a limited amount of total dissolved solids (TDS) for drinking tap water: total dissolved solids ≤1000 mg/L. Obviously, the gap with the international standard is too large. With the development of industry, the types and quantities of pollutants in water have increased, and this value may no longer meet the standard for human drinking.
When the TDS value exceeds 200, it is the industrial water standard, and above 300 is industrially polluted water.
What are the TDS values ​​of various waters?
The TDS values ​​of tap water in different regions are different, and are higher in the northern region. Generally, the TDS value of water from a water plant is around 180-260. If it is deep well groundwater, it will be higher, around 300. Some places are in industrial areas, and the groundwater may be higher after being polluted.
Generally, water treated by reverse osmosis (RO) technology is within 50, which is suitable for human drinking. The conductivity can only be roughly estimated by TDS. TDS only refers to the total dissolved solids in the water body, that is, the sum of ions. However, the conductivity of different ions is different, so even at the same TDS, the conductivity is different due to different scaling of the water body.
Empirical formula: TDS equals conductivity multiplied by 0.5-0.7

Conductivity
measurement The physical quantity that measures the conductivity of an object is called conductivity G, which is the reciprocal of resistance R G=1/R=I/E (where I: current passing through the conductor; E: potential difference between two electrodes).
R = ρ*L/A, ρ is called resistivity, which represents the resistance of a cubic conductor with A=1m2 and L=1m.
G= 1/ρ*A/L=κ*A/L, where κ is called conductivity, which represents the conductivity of a cubic liquid column with A=1m2 and L=1m. The unit of G is West (symbol S, i.e. Ω-1), and the unit of κ is West•m-1. (symbol S•m-1).
The conductivity of an electrolyte solution is not only related to the nature of the electrolyte, the properties of the solvent and the temperature, but also to the concentration of the solution. Therefore, conductivity is often used as an important indicator of the salt content of the solution and the quality of pure water and steam; the analysis method based on the determination of the conductivity of an electrolyte solution is called conductivity analysis. The measurement of conductivity is affected by factors such as temperature, cell constant and AC power frequency.

1. AC conductivity measurement method
In AC conductivity measurement, in order to prevent the polarization of the conductivity cell from causing serious measurement errors, an AC power supply is required as the measurement power supply.
The error caused by electrode polarization is: , ΔR is the measured resistance error 2 / PRR ≈ ω Δ , P is the polarization electromotive force, ω is the angular frequency of the AC measurement power supply, and RSOL is the test solution resistance. Increasing ω can reduce the measurement error. After using an AC power supply, the inter-electrode capacitance of the conductivity electrode causes an error in the measurement result: −ΔR≈R3ω2Cp, where Cp is the inter-electrode capacitance. When the measurement power supply is AC, the conductivity cell is no longer a pure resistor, but an impedance including capacitive reactance. Its total conductivity value is the vector sum of the two conductivity values, causing errors in the actual measurement of the conductivity meter.


In the figure, RL1 and RL2 represent the lead resistance of the conductivity electrode, which can often be ignored; CDL1 and CDL2 are two electrode sheet double-layer capacitors, which are formed by the double electric layers on the surfaces of the two electrodes; Cp is the inter-electrode capacitance; RSOL is the solution resistance between the electrodes; the components represented by Z1 and Z2 represent the impedance of the induced electricity on the two electrode sheets, that is, the polarization resistance.
When an alternating current is applied across the conductivity cell, it will flow through CDL and RSOL, and also through Cp. Since CDL provides a low-resistance path for AC, Z1 and Z2 are short-circuited by CDL1 and CDL2 respectively. Therefore, if Cp is very small and CDL is very large, the impedance of the conductivity cell is approximately RSOL. In fact, the capacitance of the double-layer capacitor depends on the platinum electrode to be coated with a layer of sponge-like platinum black to greatly increase the surface area. Therefore, only Cp is effective when measuring high-resistance solutions, and the equivalent circuit can be simplified to the parallel connection of Cp and RSOL.
When ωCpRSOL<<1, Z=RSOL; when Cp and RSOL are constant, the measurement error increases as ω increases. Therefore, when the measured resistance is constant, the larger the inter-electrode capacitance of the conductivity electrode, the lower the frequency of the measurement power supply, and vice versa.
When the inter-electrode capacitance of the conductivity electrode is constant, the larger the measured resistance, the lower the frequency of the measurement power supply, and vice versa. This requires the correct selection of the frequency of the power supply when conducting conductivity measurements. Otherwise, for solutions with high conductivity, a high-frequency AC power supply should be used, and a platinum black electrode with a large conductivity cell constant (L/A) should be used to reduce the influence of polarization; For solutions with low conductivity, a low-frequency AC power supply should be used, and a bright electrode with a small conductivity cell constant (L/A) should be used to reduce the influence of capacitive reactance.

2. Measurement of pure water conductivity
When measuring water samples with high conductivity (low resistivity), the influence of electrode surface polarization resistance is greater. Therefore, a higher frequency measurement power supply is used to eliminate the influence of electrode surface polarization resistance through the short-circuit effect of differential capacitance; using a high-frequency AC power supply to reduce the capacitive reactance XC1, XC2 and XCp of CDL1, CDL2 and Cp will increase the measurement error; using a low-frequency AC power supply to increase the capacitive reactance XC1, XC2 and XCp of CDL1, CDL2 and Cp, at the same time, when measuring water samples with very low conductivity (high resistivity), the influence of electrode surface polarization resistance is small and can be ignored, so the values ​​of XC1 in parallel with Z1 and XC2 in parallel with Z2 are very small. Using an electrode with a small conductivity cell constant can increase the RSOL value to reduce the measurement error.
Recommended electrode constant:

From the above, we can see that the following points are generally true:
(1) The selection of electrode constant and measurement frequency are crucial to the accurate measurement of pure water conductivity. For low conductivity, choose low frequency and small electrode constant; for high conductivity, choose high frequency and large electrode constant.
(2) When measuring hydrogen conductivity of condensed water, feed water and steam, its temperature coefficient changes with temperature and conductivity. Strictly controlling the water sample temperature or selecting a conductivity meter with nonlinear automatic temperature compensation function can effectively reduce the measurement error caused by temperature change.
(3) Using the online method to check and calibrate the conductivity cell constant of the hydrogen conductivity measurement electrode can reduce the error caused by inaccurate electrode constant.
(4) When loading the resin in the hydrogen exchange column and using it, try to avoid bubbles in the resin layer.
(5) Care should be taken to avoid cracks in the cation exchange resin used in the hydrogen exchange column.
(6) Using hydrogen-type color-changing cation exchange resin is an effective measure to solve the error information caused by the failure of hydrogen exchange resin?
(7) Low regeneration of hydrogen cation exchange resin will cause low measurement results.