Definition
A total dissolved solids analyzer, commonly known as a TDS analyzer, is a laboratory and process testing instrument used to determine the total dissolved solids content in a solution. Total dissolved solids refers to the total amount of inorganic salts and a small amount of organic matter dissolved in water, and its value is generally expressed in mass concentration, commonly used in milligrams per liter. This instrument has a wide range of application values in environmental monitoring, industrial process control, water quality assessment, and other fields.
Principle
The core working principle of the TDS analyzer is based on the conductivity measurement of the solution. Ionic substances dissolved in water increase the conductivity of the solution, and the conductivity is positively correlated with the ion concentration within a certain range. The instrument works by measuring the conductivity of the solution and applying an empirical formula to convert it to the total dissolved solids concentration. The common conversion formula is: TDS = k × EC, where EC is the conductivity measurement and k is the conversion coefficient, and its value is usually between 0.5 and 0.8, depending on the main ionic component of the dissolved solid. Some advanced models incorporate temperature compensation algorithms to eliminate the effect of temperature on conductivity measurements.
Measurement method
Conventional measurements are made using the direct immersion electrode method. When measuring, the conductive electrode of the instrument is immersed in the solution to be tested, and the instrument automatically applies AC voltage and measures the current response, thus calculating the conductivity value. Based on a preset conversion factor or calibration curve, the instrument processor converts the conductivity value to the total dissolved solids concentration and displays the result. To ensure measurement accuracy, calibration is carried out using a standard conductivity solution before operation. For complex matrix samples, measurements may need to be taken after filtration removal of suspended solids.
Influencing factors
Measurement results are influenced by a variety of factors. Changes in solution temperature can significantly alter ion mobility, which in turn affects conductivity, so most instruments are equipped with automatic temperature compensation. The electrode constant changes slightly over time and needs to be calibrated regularly. Air bubbles or suspensions in the sample may interfere with the electrode's contact with the solution. Even if the total dissolved solids concentration of solutions with different ionic compositions is the same, the conductivity may be different due to the difference in ion mobility, so the selection of conversion coefficient should consider the characteristics of the sample. Contaminated or fouling of the electrodes can also reduce measurement accuracy.
Applications
In the field of water quality monitoring, this instrument is used to assess the mineral content of drinking water, surface water, groundwater. In industrial processes, it is commonly used to monitor the water quality of boiler feedwater, cooling circulating water, and semiconductor ultrapure water systems. Water quality assessment of agricultural irrigation water also often relies on such instruments. The food and beverage industry uses it for production water monitoring. In addition, in water treatment facilities, it is used to evaluate the treatment efficiency of processes such as reverse osmosis, ion exchange, etc.
Instrument selection considerations
The measurement range should be considered when selecting an instrument, and the expected concentration of the daily sample should be covered. Resolution and accuracy need to meet the data quality requirements of the specific application. The temperature compensation range and mode of the instrument affect the measurement stability in different environments. The electrode material and structure should be adapted to the sample characteristics, such as corrosion resistance or anti-fouling design. Convenient calibration functions and data storage and output interfaces should also be taken into account. For field applications, instrument portability, battery life, and protection level are important factors. The user-friendly design of the operator interface helps to reduce operating errors.