Total Dissolved Solids (TDS) is one of the most vital indicators of water quality, especially when it comes to drinking water. Understanding how TDS is measured in water helps individuals, households, and industries ensure their water is safe, palatable, and suitable for their specific needs. TDS levels influence everything from the taste and clarity of water to its impact on plumbing, appliances, and health.
In this guide, we will dive deep into the meaning of TDS, why it matters, and how it can be accurately measured using various methods both at home and in laboratory settings.
What is TDS in Water?
TDS refers to the combined total of all organic and inorganic substances—primarily minerals, salts, and metals—that are dissolved in a given volume of water. These solids are so small that they cannot be seen with the naked eye and pass through a filter with pores sized around 2 microns or smaller.
Common constituents of TDS include:
- Calcium
- Magnesium
- Sodium
- Potassium
- Bicarbonates
- Chlorides
- Sulfates
- Nitrates
These minerals usually come from natural sources like soil and rocks, but they can also be introduced through industrial waste, agricultural runoff, sewage, and plumbing.
Why Measure TDS in Drinking Water?
TDS levels directly affect the following:
- Taste: Higher TDS may make water taste salty, bitter, or metallic. Low TDS may result in flat or bland taste.
- Health: While some minerals like calcium and magnesium are beneficial in small quantities, extremely high or low TDS levels can impact health.
- Plumbing and Appliances: High TDS leads to scale build-up in pipes, water heaters, and appliances, reducing efficiency and lifespan.
- Water Purification Monitoring: TDS testing helps evaluate the performance of water filtration systems like RO (Reverse Osmosis).
Standard TDS Levels for Drinking Water
Different organizations around the world provide guidelines for acceptable TDS levels:
| Organization | Recommended TDS Level |
|---|---|
| WHO (World Health Organization) | < 300 mg/L (Excellent), up to 600 mg/L (Good), 900 mg/L (Fair), 1200 mg/L (Poor) |
| BIS (Bureau of Indian Standards) | Desirable: 500 mg/L, Maximum Permissible: 2000 mg/L |
| US EPA (Environmental Protection Agency) | Secondary Standard: 500 mg/L |
A TDS level below 500 mg/L is generally considered ideal for drinking water.
Methods to Measure TDS in Water
1. Gravimetric Method (Laboratory Analysis)
This is the most accurate method and is commonly used in certified labs.
Steps:
- Filtration – Water is passed through a filter to remove undissolved particles.
- Evaporation – The filtered sample is placed in a pre-weighed container and evaporated at ~103°C.
- Drying – It is further dried at ~180°C to remove any residual moisture.
- Weighing – The weight of remaining residue is calculated.
- Calculation – TDS (mg/L) = (Final weight – Initial weight) / Volume of sample (L) * 1000
Pros:
- Highly accurate
- Suitable for research and compliance
Cons:
- Time-consuming
- Requires laboratory setup
2. Conductivity-Based Method (Most Common for Field Use)
TDS meters estimate TDS using the water’s electrical conductivity. Dissolved salts conduct electricity; hence, the more ions in water, the higher the conductivity.
Key Components:
- Electrodes: Detect conductivity
- Conversion Factor: Converts conductivity (µS/cm) to TDS (mg/L) using a factor (usually between 0.5 to 0.7)
Example: If conductivity = 700 µS/cm and conversion factor = 0.67, Then TDS = 700 x 0.67 = 469 mg/L
Pros:
- Fast and convenient
- Portable
- Good for domestic and industrial monitoring
Cons:
- Approximate value
- Affected by water temperature and composition
3. Digital TDS Meters / TDS Pens
These handheld devices are user-friendly and ideal for home users. Simply dip the pen into a glass of water and wait for the digital display.
Advantages:
- Inexpensive
- Easy to use
- Ideal for RO water testing, aquariums, and borewells
Limitations:
- Not 100% accurate compared to lab tests
- Needs regular calibration for consistent results
Key Factors for Accurate TDS Measurement
a. Calibration with Standard Solutions
Calibration is crucial for accurate TDS readings. Standard calibration solutions are used (e.g., 342 ppm NaCl) to verify and adjust device accuracy.
b. Temperature Compensation
Water temperature significantly affects conductivity. Most modern meters include automatic temperature compensation (ATC). For best results, let the meter adjust to water temperature before taking a reading.
c. Proper Sampling Techniques
- Use clean containers
- Avoid touching electrodes with hands
- Remove air bubbles by gentle tapping
- Rinse the meter after every use
Comparing TDS Testing Methods
| Method | Accuracy | Time Required | Skill Needed | Ideal For |
| Gravimetric | Very High | Several Hours | Trained Lab Technicians | Research, Compliance, Reports |
| Conductivity Meter | Medium | Instant | Low | Routine Checks, Domestic Use |
| Digital TDS Pen | Medium-Low | Instant | Very Low | RO Checks, Field Use, Aquariums |
What to Do If TDS is Too High or Too Low?
High TDS (>500 mg/L)
- Install RO System: Best for removing dissolved salts
- Use Water Softeners: Removes hardness due to calcium and magnesium
- Blending: Mix high TDS water with low TDS sources to reduce concentration
Low TDS (<50 mg/L)
- Re-mineralization Filters: Add essential minerals like calcium and magnesium
- Avoid for Long-Term Consumption: Extremely low TDS can leach metals from pipes and lack necessary minerals
Frequently Asked Questions
Q1. What is the ideal TDS level in drinking water?
A: Between 100 to 300 mg/L is ideal for taste and safety.
Q2. Can boiling water reduce TDS?
A: No. Boiling can only kill microbes; it cannot remove dissolved solids.
Q3. Is bottled water always low in TDS?
A: Not necessarily. Some bottled mineral waters have TDS above 300 mg/L for enhanced taste and mineral content.
Q4. Does RO water have zero TDS?
A: RO water can reduce TDS below 50 mg/L, but not zero. Often, it is remineralized before consumption.
Final Thoughts
Understanding how TDS is measured in water empowers you to take control of your drinking water quality. Whether you rely on digital TDS meters at home or conduct precise lab tests, regular monitoring helps ensure your water is both safe and pleasant to drink. With the right tools and knowledge, you can make informed decisions about filtration systems, maintenance schedules, and even your water source.
For any TDS testing kits, lab water analysis services, or expert consultation, reach out to ROSP. We’re here to help ensure your water is clean, balanced, and healthy.
Contact ROSP:
📞 Phone: +91-9560654995
📧 Email: info@roservicepoint.com
