How Do You Find TDS?

You can find Total Dissolved Solids (TDS) using a conductivity meter and a conversion formula, or by using a filtration and weighing method.

Here's a breakdown of both approaches:

1. Using a Conductivity Meter

This is the most common and convenient method.

Measure Electrical Conductivity (EC): Use a TDS meter or a conductivity meter to measure the electrical conductivity of the water sample. The meter will typically display the EC value in microsiemens per centimeter (µS/cm).
Apply the Conversion Formula: Calculate TDS using the following formula:

TDS = KE × EC

Where:
- TDS is the Total Dissolved Solids in milligrams per liter (mg/L) or parts per million (ppm).
- KE is the correlation factor, which varies depending on the water source and composition. It typically ranges from 0.55 to 0.8. A common value is 0.67 or 0.71. Check the specific instructions for your meter or the typical value for your water type.
- EC is the electrical conductivity in µS/cm.
Example: If your EC reading is 300 µS/cm and you are using a KE of 0.67, then:

TDS = 0.67 × 300 = 201 mg/L (or ppm).

This method is more accurate but also more time-consuming.

Filter the Water Sample: Pass a known volume of the water sample through a pre-weighed filter paper with a pore size small enough to retain dissolved solids (typically 0.45 micrometers).
Dry the Filter Paper: Dry the filter paper containing the residue in an oven at a specific temperature (usually 105°C) until a constant weight is achieved. This ensures all the water has evaporated.
Weigh the Filter Paper: Weigh the filter paper again after drying.
Calculate the TDS: Subtract the initial weight of the filter paper from the final weight to determine the weight of the dissolved solids. Then, calculate the TDS using the following formula:

TDS (mg/L) = [(Weight of filter paper with residue (mg) - Weight of clean filter paper (mg)) / Volume of water filtered (L)]
Example:
- Weight of clean filter paper: 100 mg
- Weight of filter paper with residue: 105 mg
- Volume of water filtered: 0.1 L (100 mL)
TDS = [(105 mg - 100 mg) / 0.1 L] = 50 mg/L (or ppm)

Temperature: Conductivity (and therefore TDS readings derived from conductivity) is temperature-dependent. Many meters automatically compensate for temperature.
Water Source: The mineral composition of the water significantly affects the correlation factor (KE) needed for accurate TDS calculation when using a conductivity meter.
Accuracy of Equipment: Ensure your meter and scale are calibrated for accurate readings.