Definition
Conductivity TDS meter is a laboratory and field testing instrument used to measure the conductivity and total dissolved solids content of a solution. Conductivity reflects the ability of a solution to conduct current, measured in Siemens per centimeter. Total dissolved solids refers to the total mass concentration of inorganic salts and small amounts of organic matter dissolved in water, usually expressed in milligrams per liter. There is a correlation between the two, and the instrument can indirectly estimate the TDS value from the conductivity data.
Principle
Conductivity measurement is based on Ohm's law, where an alternating current is applied to a solution through an electrode, measuring the current generated to calculate the conductivity value. Electrode constant is a core parameter defined as the ratio of electrode spacing to area. The formula for calculating conductivity is:
G = k × (I/V)
where G is the conductivity, k is the electrode constant, I is the measured current, and V is the applied voltage. TDS calculation usually uses the empirical coefficient method:
TDS = kc × σ
where σ is the measured value of conductivity, kcAs a conversion factor, its value is related to the ionic composition of the solution, with a common range of 0.5 to 0.7.
Measurement method
The measurement process is subject to standardized operating procedures. Instrument calibration is performed first, using a standard potassium chloride solution to calibrate the electrode constants at specific temperatures. Completely immerse the electrode in the solution to be tested during measurement to ensure that there are no bubbles attached to the measuring cell. For non-homogeneous samples, it is necessary to stir moderately and then stand still for measurement. High-precision measurements require automatic temperature compensation with temperature sensors, usually set at two hundredths per degrees Celsius. The measurement results need to record the ambient temperature and compensation mode.
Influencing factors
Solution temperature is the main influencing factor, and ion mobility increases with increasing temperature. The type and concentration of ions affect the polarization effect of the electrode, and it is recommended to use the high-frequency measurement mode for highly conductive solutions. Electrode contamination can cause constant drift and require regular cleaning and maintenance. The flow state of the solution can cause measurement fluctuations, and a flow cell is recommended under turbulent conditions. In addition, factors such as electromagnetic interference, electrode aging, and cable impedance can introduce measurement deviations.
Applications
It is used in environmental monitoring for surface water and groundwater salinity assessment. Monitoring of boiler feedwater, cooling circulating water and wastewater treatment in industrial process control. It is used in the agricultural field for irrigation water quality and salinity analysis of soil extracts. The food processing industry monitors the purity of production water. Maintain an appropriate water ion concentration in aquaculture. Laboratories are used for chemical reagent preparation and reaction process monitoring. Each application scenario needs to determine the measurement range and accuracy requirements according to the corresponding industry standards.
Key points of selection
The selection should take into account the measurement range, with conventional instruments covering 0 to 200 millimeters per centimeter, and special models up to 1000 millimeters per centimeter. The accuracy level should meet the requirements of the testing standard, and the basic error limit of laboratory-grade instruments is usually within one hundredth of the full scale range. The electrode material needs to be adapted to the measuring medium, the platinum black electrode is suitable for low conductivity measurement, and the stainless steel electrode has good corrosion resistance. The instrument should have temperature compensation function and data storage capability. On-site use needs to pay attention to the protection level and power supply method. Supporting calibration solutions and maintenance tools should be taken into account for procurement.