1. The resistivity of water When measuring the conductivity of water, it is related to the resistance value of water. The resistance value is large, the conductivity is poor, and the resistance value is small, the conductivity is good. According to Ohm's law, at a constant water temperature, the resistance value R of water is inversely proportional to the vertical cross-sectional area F of the electrode and proportional to the distance L between the electrodes, as follows: R = Ï Â· L / F where Ï -Resistivity, or specific negative. The unit of the negative electrode is ohm (Europe, code-named Ω), or micro-ohm (μΩ), 1Ω is equal to 106μΩ; the international unit of resistance (SI) is ohm (Ω · m). If the cross-sectional area F of the electrode is made 1 cm2, and the distance L between the two electrodes is 1 cm, then the resistance value is equal to the resistivity. The resistivity of water is related to the amount of salt in the water, the concentration of ions in the water, the number of charges of ions, and the speed of movement of ions. Therefore, the resistivity of pure water is large, and the resistivity of ultrapure water is even greater. The purer the water, the greater the resistivity. 2. The reciprocal of the electrical resistivity is called the electrical conductivity L. In liquids, the reciprocal of resistance is often used to measure the electrical conductivity. The calculation formula of conductance L is as follows: L = l / R = S / l, the unit of conductance is also called Siemens by Mho. Said by S, because the S unit is too large. Milli-Siemens is often used, and micro-Siemens unit 1S = 103mS = 106μS.
Effect of temperature on conductivity The resistance of a solution decreases with increasing temperature, that is, when the concentration of the solution is constant, its conductivity increases with increasing temperature, and the increase is about 2% ℃ -1. In addition, the electrolytes of the same type have different temperature coefficients when their concentrations are different. At low concentrations, the relationship between the temperature of the conductivity is expressed by the following formula: L1 = L0 [1 + α (t-t0) + β (t-t0) 2] due to the second term β (t-t0) 2 The value is small and can be ignored. The relationship between conductivity and temperature at low temperature can be expressed by the following approximation L1 = L0 [1 + α (t-t0)], so temperature compensation must be added during actual measurement.
Temperature coefficient of conductivity For most ions, the temperature coefficient of conductivity is approximately + 1.4% ℃ -1 ～ 3% ℃ -1. For H + and OH- ions, the temperature coefficient of conductivity is 1.5% ℃ -1 and 1.8% ℃, respectively. -1, this value is generally 1% or better than 1% relative to the accuracy requirement of the conductivity measurement, which cannot be ignored. 3. The conductivity of pure water even exists in pure water H + and OH- two ions, it is often said that pure water is a poor conductor of electricity, but strictly speaking water is still a very weak electrolyte, it exists The following ionization balance:
H2O ↠→ H ++ OH or 2H2O ↠→ H3 + O + OH-
Its equilibrium constant:
KW = [H +]. [OH-] / H2O = 10-14
Where KW is called the ion product of water
[H +] 2 = [OH-] 2 = 10-14
∴ [H +] 2 = [OH-] 2 = 10-7
lH2O, 0 = λOH-, 0 = 349.82 + 198.6 = 548.42S ​​/ cm.mol2
Known water density d25 ℃ / H2O = 0.9970781cm3
Therefore, the original assumption is that the water ion concentration can only reach 0.99707. In fact, only 0.99707 shares of water dissociate into [H +] and [OH-] of 0.99707.10-7, then the sum of the dissociated [H +] and [OH-] conductivity KH2O can be obtained by
KH2O = CM / 1000λH2O
= (0.99707.10-7 / 1000) .548.42
= 0.05468μS.cm-1≈0.054μS.cm-1
∴ÏH2O = 1 / KH2O = 1 / 0.05468 × 10-9
= 18.29 (MΩ.cm) ≈18.3 (MΩ.cm)
From the ion product of water is 10-14, it can be calculated that the theoretical ultimate conductivity of high-purity water is 0.0547 μS.cm-1, and the resistance is 18.3 MΩ.cm (25 ° C).
The temperature coefficient of water conductivity has different temperature coefficients in different conductivity ranges. For the commonly used distilled water of 1 μS.cm-1, it is about + 2.5% -1. 4.TDS
TDS is the abbreviation of total dissolved solids in English. The Chinese translation is called dissolved total solids. The unit of measurement is milligrams per liter (mg / L). It indicates how many milligrams of total dissolved solids are dissolved in 1 liter of water.
The TDS concept is a foreign product, widely used in the water treatment field in the United States and Taiwan. The TDS value measurement tool is generally a TDS pen. Its measurement principle is actually to indirectly reflect the TDS value by measuring the conductivity of the water. In a physical sense, the more dissolved matter in water, the greater the TDS value of water, the better the conductivity of water, and the greater the conductivity value.
In layman's terms: the TDS value represents the content of impurities in the dissolved matter in water. The larger the TDS value, the higher the content of impurities in the water. On the contrary, the content of impurities is small.
How to use the TDS pen: Open the probe cover of the TDS pen, press the button labeled ON OFF, and after the LCD screen displays, insert the TDS pen into the water under test. After the value stabilizes, press the button labeled HOLD to take out the TDS pen Only read the value. After the test, wipe the TDS pen probe with dry paper.
Factors affecting TDS value test:
Water temperature: TDS pen can not be used to measure high temperature water (for example: hot boiling water)
Water flow rate: TDS pen can not be used to measure water quality pollution of large sloshing water body: TDS pen cannot be used to measure water body with high pollution concentration 5. Hardness of water Some metal cations in water are combined with some anions in water. During the heating process, scale is easily formed due to evaporation and concentration, which adheres to the heated surface and affects heat conduction. We express the total concentration of these metal ions in water as the hardness of water. For example, the most common metal ions in natural water are calcium ions (Ca2 +) and magnesium ions (Mg2 +), which interact with anions in water such as carbonate ions (CO32-), bicarbonate ions (HCO3-), and sulfate ions (SO42 -), Chloride ions (CL-), and nitrate ions (NO3-) are combined together to form calcium, magnesium carbonate, bicarbonate, sulfate, chloride, and nitrate hardness. Metal ions such as iron, manganese, and silver in water can also form hardness, but because they have very little content in natural water, they can be ignored. Therefore, the total concentration of Ca2 + and Mg2 + is usually regarded as the hardness of water. The hardness of the water has a great influence on the boiler water. Therefore, the water should be softened or desalted according to the requirements of the boiler for the water quality of various parameters. 6. The relationship between TDS and conductivity For most water sources, the conductivity / TDS ratio is between 1.2-1.7, seawater uses 1.4 ratio and brackish water uses 1.3 ratio for conversion, usually can get better approximate conversion rate. 7. How does temperature affect water production
Effect of temperature on conductivity The resistance of a solution decreases with increasing temperature, that is, when the concentration of the solution is constant, its conductivity increases with increasing temperature, and the increase is about 2% ℃ -1. In addition, the electrolytes of the same type have different temperature coefficients when their concentrations are different. At low concentrations, the relationship between the temperature of the conductivity is expressed by the following formula: L1 = L0 [1 + α (t-t0) + β (t-t0) 2] due to the second term β (t-t0) 2 The value is small and can be ignored. The relationship between conductivity and temperature at low temperature can be expressed by the following approximation L1 = L0 [1 + α (t-t0)], so temperature compensation must be added during actual measurement.
Temperature coefficient of conductivity For most ions, the temperature coefficient of conductivity is approximately + 1.4% ℃ -1 ～ 3% ℃ -1. For H + and OH- ions, the temperature coefficient of conductivity is 1.5% ℃ -1 and 1.8% ℃, respectively. -1, this value is generally 1% or better than 1% relative to the accuracy requirement of the conductivity measurement, which cannot be ignored. 3. The conductivity of pure water even exists in pure water H + and OH- two ions, it is often said that pure water is a poor conductor of electricity, but strictly speaking water is still a very weak electrolyte, it exists The following ionization balance:
H2O ↠→ H ++ OH or 2H2O ↠→ H3 + O + OH-
Its equilibrium constant:
KW = [H +]. [OH-] / H2O = 10-14
Where KW is called the ion product of water
[H +] 2 = [OH-] 2 = 10-14
∴ [H +] 2 = [OH-] 2 = 10-7
lH2O, 0 = λOH-, 0 = 349.82 + 198.6 = 548.42S ​​/ cm.mol2
Known water density d25 ℃ / H2O = 0.9970781cm3
Therefore, the original assumption is that the water ion concentration can only reach 0.99707. In fact, only 0.99707 shares of water dissociate into [H +] and [OH-] of 0.99707.10-7, then the sum of the dissociated [H +] and [OH-] conductivity KH2O can be obtained by
KH2O = CM / 1000λH2O
= (0.99707.10-7 / 1000) .548.42
= 0.05468μS.cm-1≈0.054μS.cm-1
∴ÏH2O = 1 / KH2O = 1 / 0.05468 × 10-9
= 18.29 (MΩ.cm) ≈18.3 (MΩ.cm)
From the ion product of water is 10-14, it can be calculated that the theoretical ultimate conductivity of high-purity water is 0.0547 μS.cm-1, and the resistance is 18.3 MΩ.cm (25 ° C).
The temperature coefficient of water conductivity has different temperature coefficients in different conductivity ranges. For the commonly used distilled water of 1 μS.cm-1, it is about + 2.5% -1. 4.TDS
TDS is the abbreviation of total dissolved solids in English. The Chinese translation is called dissolved total solids. The unit of measurement is milligrams per liter (mg / L). It indicates how many milligrams of total dissolved solids are dissolved in 1 liter of water.
The TDS concept is a foreign product, widely used in the water treatment field in the United States and Taiwan. The TDS value measurement tool is generally a TDS pen. Its measurement principle is actually to indirectly reflect the TDS value by measuring the conductivity of the water. In a physical sense, the more dissolved matter in water, the greater the TDS value of water, the better the conductivity of water, and the greater the conductivity value.
In layman's terms: the TDS value represents the content of impurities in the dissolved matter in water. The larger the TDS value, the higher the content of impurities in the water. On the contrary, the content of impurities is small.
How to use the TDS pen: Open the probe cover of the TDS pen, press the button labeled ON OFF, and after the LCD screen displays, insert the TDS pen into the water under test. After the value stabilizes, press the button labeled HOLD to take out the TDS pen Only read the value. After the test, wipe the TDS pen probe with dry paper.
Factors affecting TDS value test:
Water temperature: TDS pen can not be used to measure high temperature water (for example: hot boiling water)
Water flow rate: TDS pen can not be used to measure water quality pollution of large sloshing water body: TDS pen cannot be used to measure water body with high pollution concentration 5. Hardness of water Some metal cations in water are combined with some anions in water. During the heating process, scale is easily formed due to evaporation and concentration, which adheres to the heated surface and affects heat conduction. We express the total concentration of these metal ions in water as the hardness of water. For example, the most common metal ions in natural water are calcium ions (Ca2 +) and magnesium ions (Mg2 +), which interact with anions in water such as carbonate ions (CO32-), bicarbonate ions (HCO3-), and sulfate ions (SO42 -), Chloride ions (CL-), and nitrate ions (NO3-) are combined together to form calcium, magnesium carbonate, bicarbonate, sulfate, chloride, and nitrate hardness. Metal ions such as iron, manganese, and silver in water can also form hardness, but because they have very little content in natural water, they can be ignored. Therefore, the total concentration of Ca2 + and Mg2 + is usually regarded as the hardness of water. The hardness of the water has a great influence on the boiler water. Therefore, the water should be softened or desalted according to the requirements of the boiler for the water quality of various parameters. 6. The relationship between TDS and conductivity For most water sources, the conductivity / TDS ratio is between 1.2-1.7, seawater uses 1.4 ratio and brackish water uses 1.3 ratio for conversion, usually can get better approximate conversion rate. 7. How does temperature affect water production
The higher the temperature, the higher the water production, and vice versa. When operating at a higher temperature, the operating pressure should be lowered to keep the water production unchanged, and vice versa.
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