Commonly Used Units of Measurement in Ultrapure Water Treatment

The process of preparing pure water using an ultrapure water system is a precise process. We often see various parameters of ultrapure water equipment, which involve many units of measurement. Today, we've compiled explanations of commonly used units of measurement in ultrapure water treatment:

1. Units of Volume

1 cubic meter (m³) = 10³ liters; 1 liter (L) = 10³ milliliters; 1 US gallon (G) = 3.785 liters.

2. Weight-to-Volume Ratio (Solution Concentration)

1 ppm = 1 mg/L (parts per million); 1 ppb = 1 μg/L (parts per billion); 1 ppt = 1 ounce/L (parts per trillion).

3. Units of Pressure

The force acting uniformly and perpendicularly on the surface of an object is called pressure. The pressure acting uniformly and perpendicularly on a unit area of the surface of an object is called pressure intensity. In engineering and daily life, pressure is often referred to as force, and the symbol "P" represents the unit of pressure, which is Newton-meter (N/m²). This represents the pressure produced when a force of 1 N is applied uniformly and perpendicularly to a surface of 1 m², also known as Pascal (Pa), or simply Pascal. 1 Pa = 1 N/m².

1 MPa = 1 x 10⁶ Pa = 1,000,000 Pa; 1 kgf/cm² (kilogram-force) = 9.80665 x 10⁻² MPa (megapascal) ≈ 0.1 MPa ≈ 14.5 PSI (pounds per square inch). 1 PSI ≈ 0.06895 kgf/cm².

4. Conductivity (μs/cm) & Resistivity (MΩ·cm)

These values are reciprocals of each other and are one of the units characterizing the purity of water. Water contains cations and anions. Ions are conductive media, and by measuring the conductivity (resistance) of water, we can indicate the amount of ions present, thus indirectly reflecting the purity of the water. Generally, we use conductivity to reflect the purity of pure water and resistivity to reflect the purity of ultrapure water. The international unit for conductivity is Siemens, abbreviated as S. Pure water has very low conductivity and is commonly expressed in μS/cm, although some water quality standards also use mS/m. The international unit for resistance is ohm, abbreviated as Ω. Ultrapure water has very high resistance and is commonly expressed in MΩ. 1 mS/m = 10 μS/cm; 5 μS/cm = 0.2 MΩ·cm; 1 μS/cm = 1 MΩ·cm; 0.5 μS/cm = 2 MΩ·cm; 0.1 μS/cm = 10 MΩ·cm; 18.2 MΩ·cm·cm = 0.055 μS/cm.

5. TDS Value

TDS refers to the total dissolved solids (TDS) content in water. It is commonly used to measure the quality of source water, and the measuring tool is a TDS meter. The measurement principle is actually to indirectly reflect the TDS value by measuring the conductivity of water. The more dissolved solids in water, the higher its TDS value, and the higher the conductivity of the water. Typically, the TDS value is about half of the conductivity test value. In China, the TDS value of tap water in cities along the Yangtze River is usually 100-200 ppm, while in the Yellow River basin it is usually 300-600 ppm.

Note: TDS value and water salinity (also known as mineralization) are two different concepts. TDS value is usually greater than water salinity because TDS includes not only dissolved salts but also dissolved organic matter. TDS (Total Dissolved Solids) can only be used to approximate the salinity (or mineralization) of water when it is very clean and has a very low organic matter content. Generally speaking, TDS, mineralization, and salinity are directly proportional to the concentration of pollutants in water.

6. Turbidity

Turbidity occurs in water due to the presence of suspended and colloidal particles. The degree of turbidity is called turbidity (measured in "degrees"). One degree of turbidity is defined as 1 mg of SiO2 (silicon dioxide) in 1 liter of water. The national standard for drinking water turbidity is less than 5 degrees.

7. Hardness

Water hardness refers to the content of dissolved calcium and magnesium salts in water (calcium salts account for approximately 85%, magnesium salts 10-15%, and other metal salts 1-3%). It mainly refers to the content of Ca²⁺ and Mg²⁺ ions in the water. The Ca²⁺/Mg²⁺ in water primarily originates from the dissolution of calcium and magnesium salts (CaCO₃/MgCO₃) from rocks. The dissolution rate is greatly accelerated when the CO₂ content in the water is high. Hardness is divided into temporary hardness (carbonate hardness, which can be removed by heating and precipitation) and permanent hardness (non-carbonate hardness, which cannot be removed by heating and boiling). The sum of both is called total hardness. The unit of water hardness is German hardness. 1 liter of water containing 10 mg of CaO (calcium oxide) is 1 German hardness. Water with a hardness greater than 8 degrees (equivalent to 200 ppm) is considered hard water. The Chinese national standard for drinking water stipulates that the total hardness index should not exceed 450 ppm.

8. PH Value

pH value represents the negative logarithm of the H+ concentration in water, also known as the hydrogen ion index. pH value indicates the order of magnitude of the variation in the acidity or alkalinity of a solution.

Strongly acidic aqueous solution: pH < 5.0; Weakly acidic aqueous solution: pH = 5.0~6.4; Neutral aqueous solution: pH = 6.5~8.0; Weakly alkaline aqueous solution: pH -8.1~10; Strongly alkaline aqueous solution: pH > 10.