Guide to TDS Meters
When dealing with conductivity, the Total Dissolved Solid (TDS) in a liquid is useful to know but often hard to identify. TDS Meters effectively help determine the total dissolved solid and, therefore, the liquid’s conductivity. Check out PE Facts to find out how to get the most reliable results from your TDS Meter!
TDS is defined as the Total Dissolved Solid in a liquid solution. The Total Dissolved Solid is considered the total weight of all solids that are dissolved in a given volume of water, expressed in units of milligram (mg) per unit volume of water (mg/l). It is also referred to as parts per million and is directly related to the cleanliness of water and water purification systems.
Dissolved solids are typically any salts, minerals, metals and even dissolved gases present in water. It also includes anything else present in water other than the pure water (H2O) molecule and suspended solids. Dissolved solids also consistently contribute to the conductivity of a solution.
Different dissolved solids give various conductivity readings. This allows us to use conductivity to determine what materials are dissolved in a solution. There are instruments available to measure conductivity and, therefore, they can also identify the dissolved solid. Dissolved solids are measured in parts per million (ppm) or parts per thousand (ppt) and are used in various chemical and industrial procedures.
Dissolved solids can produce a variety of problems such as causing water to taste bitter or salty, causing corrosion of plumbing fixtures and piping, causing the effectiveness of water filters to be reduced, along with poor performance in the processes of the manufacturing industries.
Conductivity can be defined simply as the measure of how well the electricity, or current, flows through a solution. It is measured in siemans, micro (µS), or milli (mS) per cubic Centimeter (cm). In general, the more ions in a liquid, the easier it is for a current to flow through it. For instance, it is much easier for a current to flow through salt water than through distilled water, or a similarly pure liquid. The following are conductivity levels of some common liquids.
| Deionized Water | 055µS/cm |
| Drinking Water | 100µS/cm |
| Ocean Water | 53 mS/cm |
There are numerous manufacturers of high quality TDS/Conductivity Meters that provide reliable water testing an analysis. These instruments are reasonably inexpensive and are calibrated and certified according to NIST standards. Some meters have features such as automatic temperature compensation and push-button calibration functions.
Any of the available instruments will provide excellent performance given that you read, understand and adhere to the manufacturer's guidelines. It is particularly important to follow the manufacturer's instructions when calibrating your instrument in order to guarantee that it is operating to its fullest potential.
Manufacturers cannot stress the importance of regularly maintaining their instruments enough. Proper maintenance, outlined in the instruction manual, is the only way to keep instruments functioning properly, accurately and reliably for a long lifespan. In order to help users do so, manufacturers provide calibration, buffer, and storage solutions as well as a technical service support team who can be called anytime for assistance.
Manufacturers usually give a certificate of analysis with their maintenance solutions showing the date of manufacture, lot number, accuracy and expiration dates.
Calibration (Typical):
The tester is factory calibrated. However, to guarantee accuracy, calibrate regularly according to your specific instrument's manufacturer instructions -example below. Select a calibration standard appropriate for your tester:
| TDSTESTR LOW: | From 200 to 1990 ppm |
| TDSTESTR HIGH: | From 2.00 to 10.00 ppt |
| ECTESTR LOW: | From 200 to 1990 µS |
| ECTESTR HIGH: | From 1.0 to 19.90 mS |
It is best to select a Standard close to the test solution value.
- Open the battery compartment lid (end with lanyard loop). The Increment (INC) and Decrement (DEC) calibration keys are the two white buttons.
- Rinse the electrode in deionized water, then rinse it in the calibration standard, then dip it into a container of the calibration standard.
- Switch the unit on (ON/OFF key). Wait several minutes for the display to stabilize.
- Press the INC or DEC keys to adjust the reading to match the calibration standard value.
- After 3 seconds without a key press, the display flashes 3 times, and then shows "ENT". The tester accepts the calibration value and then returns to measurement mode.
- Replace the battery cap.
Tester Maintenance:
To enhance performance, rinse the electrodes in alcohol for 10-15 minutes.
If the low battery indicator appears, or if readings are faint or unstable, replace all batteries.
If you experience drift, occasionally let the electrode dry fully.
If you need a new electrode, refer to your manufacturer's instruction manual concerning replacements.
Commonly Asked Questions
Q. Is there a difference between micromhos and microsiemens?
A.No, there is no difference. Micromhos are more common in the US, and microsiemens are more common in Europe.
Q. How and when do I need to Calibrate?
A. Calibrate using a standard solution within the range of what you expect the samples you are testing to be at. Follow the manufacturer’s instructions for your specific instrument. Recalibrate when you change ranges or if readings seem to be inaccurate.
Q. Are probes interchangeable?
A. No, you cannot put one manufacturer’s probe on another manufacturer’s meter even if they seem to fit together. There are many differences between manufacturers and “universal” is not the norm if you want the best performance.
Q. Will any calibration solution work?
A. Total Dissolved Solids (TDS) standard calibration solution should contain the same types of solids known to be in the test solution. Failure to do so will result in significant discrepancies between the ppm or ppt reading and the actual ppm or ppt in the test solution. Most TDS meter manufacturers offer convenient and accurate ready-made solutions.
Please Note: The information contained in this publication is intended for general information purposes only. This publication is not a substitute for review of the applicable government regulations and standards, and should not be construed as legal advice or opinion. Readers with specific questions should refer to the cited regulation or consult with an attorney.
