How does the instrument calculate total dissolved solids in water from conductivity?

This article explains how TDS instruments estimate total dissolved solids by measuring the electrical conductivity of water. Since most dissolved solids exist in ionic form and affect conductivity, there is a correlation between the two. The instrument first measures conductivity and temperature, then corrects the conductivity to 25°C using a formula, and finally multiplies it by an empirical coefficient (typically between 0.5 and 0.8) to obtain an estimated TDS value. This method is fast and convenient, but the coefficient can vary depending on the ionic composition of the water, so the result is an estimate and may be inaccurate in water with complex ion compositions. For critical applications, laboratory methods should be used for calibration.

The basic relationship between conductivity and total dissolved solids

When analyzing water quality, conductivity is a key physical parameter that measures the ability of aqueous solutions to conduct current. Its value mainly depends on the type, concentration and temperature of ions in the solution. Total dissolved solids refer to the total mass concentration of dissolved inorganic salts and small amounts of organic matter in water, usually measured in milligrams per liter. Since most of the dissolved solids exist in ionic form, they directly contribute to the conductivity of the solution, so there is a significant correlation between the two. This correlation provides a theoretical basis for indirectly estimating the total dissolved solids concentration by conductivity measurement.

Principles and formulas

The core of the instrument's calculation of total dissolved solids is based on conductivity measurements, which are estimated by an empirical conversion factor. The conversion coefficient is not a universal constant, and it will vary within a certain range due to the specific composition ratio of dissolved ions in the water body. For most natural freshwater and common aqueous solutions, this factor is generally considered to be between 0.5 and 0.8. A widely used basic calculation formula is as follows:

TDS ≈ k * EC₂₅

where TDS stands for total dissolved solids estimate in milligrams per liter; k is the empirical conversion coefficient; EC₂₅Represents the conductivity value calibrated to a standard temperature of 25 degrees Celsius in microsiemens per centimeter.

Temperature calibration is an integral step in the calculation. Because the conductivity is significantly affected by temperature, the instrument has a built-in temperature sensor, and the measured conductivity is automatically compensated to the reference value of 25 degrees Celsius using the following approximate formula:

EC₂₅ = EC_t / [1 + α(t - 25)]

In the formula, EC_tThe conductivity measured at temperature t is α the temperature compensation coefficient, usually about 0.02 degrees Celsius.

Workflow

The workflow of modern TDS instruments is an automated process that integrates measurement, compensation and calculation. First, the instrument measures the conductivity and real-time temperature of the water sample through electrodes. The microprocessor then normalizes the conductivity value to 25 degrees Celsius based on a preset temperature compensation algorithm. Finally, the system multiplies the normalized conductivity value by a preset conversion factor to directly display the total dissolved solids reading.

To be clear, this method yields an "estimated total dissolved solids". Its accuracy is highly dependent on how well the preset conversion factor k matches the actual ionic composition of the water sample. For example, a water body dominated by sodium chloride and a water body dominated by calcium sulfate may differ in the actual total dissolved solids mass, even if the conductivity is the same. Therefore, some advanced instruments allow users to adjust the k value within a specific range based on the known characteristics of the water sample being tested to obtain more targeted readings.

Reference for conversion coefficients for different water bodies

Examples of water body types	Commonly used conversion factor (k) range
Generally natural fresh water	0.55 - 0.75
Drinking water, mineral water	0.60 - 0.75
Sodium chloride-based solution	About 0.5
Industrial recycled water	It needs to be determined experimentally

Scope of application

The conductivity-based method for measuring total dissolved solids has the advantages of being fast, continuous, and easy to monitor online. It is ideal for trend monitoring and process control in scenarios where water quality is relatively stable. However, this method also has limitations in that it cannot distinguish between separator species and can be biased in estimates for water samples with complex or variable ionic compositions. In the case of strict requirements for results, this method should be compared and calibrated with standard gravimetric analysis methods such as laboratory drying and weighing method.

Summary

The total dissolved solids instrument realizes the rapid estimation of the total dissolved solids concentration in water by measuring the conductivity supplemented by temperature compensation and coefficient conversion. Understanding the calculation principles behind it, the impact of coefficient selection, and the scope of application of the method are crucial for the correct use of instruments and the interpretation of data appropriately. In practical applications, users should evaluate the applicability of this indirect method based on specific water quality conditions and use standard methods for verification if necessary.