Measuring Total Dissolved Solids (TDS) is an accessible way to gain initial insight into the composition of your water supply. Understanding the level of dissolved substances helps assess factors like taste, the efficiency of filtration systems, and potential maintenance issues within plumbing. This article is a practical guide to understanding what TDS represents and how to accurately test your water using a handheld meter.
Understanding Total Dissolved Solids
Total Dissolved Solids (TDS) measures the combined content of all inorganic and organic substances present in water in molecular, ionized, or micro-granular form. These dissolved materials are measured in parts per million (ppm), which is equivalent to milligrams per liter (mg/L). A TDS reading quantifies everything dissolved in the water that is not the pure water molecule ($\text{H}_2\text{O}$) itself.
The primary constituents of TDS are typically inorganic salts, including positively charged ions (cations) like calcium, magnesium, sodium, and potassium, and negatively charged ions (anions) such as carbonates, chlorides, and sulfates. These substances originate from natural sources, such as the dissolution of rocks and soil, or from human activities like agricultural runoff and water treatment chemicals. Because these dissolved materials are electrically charged ions, they increase the water’s ability to conduct an electrical current.
Step-by-Step Guide to Using a TDS Meter
The most common method for measuring TDS at home uses a handheld digital TDS meter. This device measures the electrical conductivity of the water sample and applies a fixed conversion factor to translate the value into the final TDS reading, expressed in ppm. Before testing, prepare the meter by removing the protective cap, turning the device on, and confirming the display reads zero.
Calibration is the first step in using a meter, ensuring accuracy, as performance can drift over time even if the meter is factory-calibrated. To calibrate, immerse the probe into a commercial standard calibration solution with a known concentration. If the meter’s reading does not match the known ppm value, gently turn the adjustment screw until the reading is accurate. Calibrating the meter at a temperature close to $77^\circ \text{F}$ ($25^\circ \text{C}$) yields the most precise results.
To collect a sample, pour the water into a clean, non-metallic container, such as a glass or plastic cup. Carefully immerse the meter’s electrodes into the water, ensuring they are submerged up to the maximum immersion level marked on the device. Gently swirl or tap the meter to dislodge any air bubbles clinging to the electrodes, as these can interfere with the electrical current and skew the result.
Allow the reading to stabilize, which usually takes between ten and twenty seconds, or slightly longer if the water is very hot or cold. Once the number stops fluctuating, record the final stabilized reading in ppm. Many meters feature a “HOLD” button that freezes the display, allowing you to lift the meter out of the sample to record the result easily. After measurement, the probe should be rinsed immediately using distilled or deionized water to prevent contaminants from coating the electrodes.
Interpreting Your TDS Reading
Once a TDS reading is obtained, it must be put into context to determine its significance for water quality. A TDS level below 50 ppm is often found in water treated by reverse osmosis (RO) or distillation, but this water may taste flat due to the lack of dissolved minerals. The optimal range for drinking water is generally considered to be between 50 and 150 ppm, balancing purity with beneficial minerals like calcium and magnesium that contribute to a pleasant taste.
The U.S. Environmental Protection Agency (EPA) established a secondary maximum contaminant level guideline of 500 ppm for TDS, based on aesthetic factors. Water exceeding 500 ppm may possess a noticeable bitter, salty, or metallic taste, and can lead to excessive scaling in appliances and pipes. A high TDS reading does not automatically indicate the water is unsafe to drink, as many beneficial minerals contribute to the total number.
A consistently high TDS reading, especially over 1,000 ppm, signals the need for further testing, as it can indicate high concentrations of potentially harmful substances. For those with water filtration systems, a TDS meter is an effective tool for monitoring performance. A sudden increase in the TDS of filtered water is a reliable sign that the filter, such as an RO membrane, needs replacement. The TDS number is best used as a general indicator of overall water composition and as a metric for taste and appliance longevity.