What is the difference between a conductivity meter and a TDS Meter?
Conductivity and TDS what are they? Why do we measure them? What is the difference? Why is it important to know?
Whether you work in agriculture, aquaculture, perform environmental or wastewater testing, you’re likely to have come across these terms. You may have used the measurements these meters produce.
This article provides (relatively) simple answers to these questions, addresses the differences between a conductivity meter and a TDS meter, the variances between the readings they produce, as well as what it all means for you.
What is Conductivity and what is TDS?
Let’s start with a definition of both these terms.
Conductivity – i.e. electrical conductivity (EC) is the measure of the concentration of ions present within a sample. This is calculated by the ability of the substance to transmit an electrical current over a defined area. The measurement unit for electrical conductivity is called Siemens(S). The units are typically displayed as Milli Siemens per centimetre mS/cm or Micro Siemens per cm μS/cm.
Total dissolved solids (TDS) - Put simply, TDS refers to anything present in the water that is not either:
- pure water; or
- a suspended solid.
Examples of dissolved solids include organic compounds, inorganic compounds, salt and other minerals. The measurement of TDS is presented in parts per million or ‘PPM’.
So, what is the difference between conductivity and TDS meters?
In pure scientific terms, conductivity tells you how well electricity is passing through a substance. On the other hand, TDS indicates the amount of dissolved solids within a liquid, accounting for both conductible and non-conductible particles.
In practice, when measuring conductivity and TDS both conductivity meters and TDS meters pass electricity through a liquid. In fact, conductivity meters are used to estimate the TDS in a substance.
How is this done? Simply, TDS meters measure conductivity, then apply a conversion factor. Even if your meter is a TDS-only meter and does not display conductivity it still uses electrical conductivity with a conversion factor to calculate the TDS results.
Why? Well, for a true TDS measurement you have to evaporate all the moisture from your sample, then weigh the solid residue. While this is the most accurate method, in practice it is often impractical.
In summary, there is a correlation between conductivity and TDS. They are not the same. TDS meters that use conductivity to generate measurements only estimate TDS.
What are the common TDS conversion factors?
Conversion factors for TDS can be a source of confusion. Don’t panic! TDS meters generally use one of two main conversion factors (although there are many, many more). These are:
- TDS NaCI – This is a conversion factor based on Sodium Chloride. This is approximately 500 x EC (in Milli Siemens); and
- Natural Water Conversion (also known as 442 Conversion) – This is common in hydroponics. Measurements are calibrated from a base of 40% sodium sulphate, 40% sodium bicarbonate and 20% sodium chloride. This equates to approximately 700 x EC (in Milli Siemens).
Now here’s where you need to pay attention. There is a signicant difference in the results generated by TDS meters, depending what conversion factor is used - as you can clearly see in the table below.
| Electrical Conductivity (In milliSiemens) | NaCl Conversion TDS (500 x EC) | Natural Water Conversion TDS (700 x EC) |
| 0.2 mS | 100ppm | 140ppm |
| 0.5 mS | 350ppm | 350ppm |
| 1.0 mS | 500ppm | 700ppm |
| 2.0 mS | 1000ppm | 1400ppm |
| 3.0 mS | 1500ppm | 2100ppm |
Table 1 Demonstration of electrical conductivity converted to TDS using the NaCI and Natural Water Conversion
For this reason, you must know the conversion factor your Conductivity and TDS meter are using to have an idea of your TDS estimation. Most Conductivity and TDS meters have adjustable TDS factors to suit individual needs.
Other factors affecting accuracy
Having seen how the conversion factor that a TDS deploys can affect results, there are other environmental factors that impact accuracy. For example, conductivity readings can vary significantly depending on the concentration of ions in, and the temperature of your environment or sample.
How to choose and operate the best meter for your application?
The solution is not as hard as it seems. Firstly, the manufacturers of conductivity and TDS meters build their meters with particular applications in mind. Each design will incorporate a degree of accuracy, a conversation factor (in the case of TDS meters), methods of calibration etc. Secondly, good online stores will make it clear what meters suit what applications, even provide set-up and how-to-use instructions or videos, along with the support of expert customer service teams. All you have to do is know your application.
Examples of conductivity and TDS meters
To get an idea of how this works in practice, check out some of the conductivity and TDS meters listed below that you can find in the Instrument Choice online store.
Product Code: EC-33
Adjustable TDS: 0.40 to 1.00
Waterproof Multi-Purpose Meter.jpg?Action=thumbnail&algorithm=fill_proportional&width=104)
Product Code: EC-PCSTestr35
Complete with five sensors in one and measures conductivity, TDS, pH, salinity and temperature with ease.
Adjustable TDS factor from 0.4 to 1.00.
Ideal for testing swimming pools, spas, aquariums, hydroponic operations, boilers and food processing facilities.
PC60 Premium Multiparameter Pocket Tester Kit 
Product Code: IC-PC60
Simultaneously tests conductivity, TDS, pH, salinity and temperature in a quick, accurate, and reliable manner.
Adjustable TDS factor from 0.4 to 1.00.
Designed for Agriculture, Aquaculture, hydroponics, soil testing and boiler testing.
Waterproof PC 450-meter kit 
Product Code: EC-PCWP4150-04K
Highly durable and suited to professional use this meter measures conductivity, TDS, pH, mV salinity and temperature.
Adjustable TDS factor from 0.4 to 1.00.
Excellent for use in laboratories, agriculture, aquaculture, hydroponics and more.
Conclusion
While conductivity and TDS meters are complex instruments they are designed to be tough and easy to use. They are essential tools for determining the nutrient content and the total dissolved solids within a solution.
Understanding how they work is the key to selecting the right device for your application, as well as how to use your device correctly. (For example, it is always important to check the TDS conversion factor you are using to ensure accuracy and consistency of measurements).
Choosing a good manufacturer and retailer also helps. Look for businesses that offer a comprehensive range of meters, so you can find the right device for your application, ask about how-to instructions, demand expert customer support and after sales service. That way you can’t go wrong.
Want to browse more examples of conductivity and TDS meters? Search the Instrument Choice online store. You’ll find an extensive range of quality, cost-effective units that have been personally selected by our scientists.
Do you have questions about conductivity or TDS? Need a bit more information on any of the meters listed above? Ask one of the Instrument Choice Scientists, we’re here to help! Call 1300 737 871 or email [email protected]
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