Conductivity, Resistivity, TDS and Salinity Measurements

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Conductivity, Resistivity, TDS and Salinity Measurements

What is Conductivity?

Conductivity is a measure of the ability of a substance to conduct an electrical flow through it. In water, many compounds and molecules (such as dissolved salts and inorganic materials) dissociate into constitutive ions. Ions are small sub-atomic particles and carry with them an electrical charge; either positive (cations) or negative (anions). These particles help conduct an electrical charge through a substance. The conductivity of water is directly related to the concentration of ions present, therefore the more ions present means a higher conductivity and conversely, in more pure water, there is a lower conductivity. It is worth noting that even when the amount of ions increases (increasing the conductivity) in a water sample, the overall electrical charge would stay the same as the cations and anions would cancel each other out.

Conductivity can be used as a measure of dissolved salts and inorganic materials in a sample


A conductivity reading is used in reporting water quality in many industries, but can also be measured directly in soil for example. It is good for giving an overall indication of quality. Readings are taken using a specialist electrochemistry meter – a conductivity meter. Often, these meters will also measure a number of related parameters – resistivity, TDS and salinity, using pre-programmed conversion settings.


Conductivity measuring units

The SI units for conductivity is S (siemens)/m however the unit μS/cm is also very commonly used. Other measurements units include micromhos or millimhos/cm. One siemen (S) = one mho.


Relationship with other measurements

Resistivity is the reciprocal of conductivity (resistivity = 1/ conductivity) and is a measure of a substances ability to not conduct (i.e. to resist) an electrical current. The unit of measurement is ohms/m (Ω m). Pure water quality is often measured using resistivity.

TDS is a measurement of the dissolved solid content of the water sample – a measure of all particles with a size greater than 2 microns, which includes dissolved salts and also dissolved organic compounds (such as hydrocarbons). This measurement can be derived from a conductivity reading using a conversion factor that can be found pre-programmed into some conductivity meters. This relationship between conductivity and TDS is not constant however is often estimated as a constant conversion factor of around 0.65 –  this may need to be increased for water with high ionic content and decreased for pure water. Measuring units include g/L and ppt. The TDS of water affects the osmotic balance of organisms and therefore its monitoring has importance in wastewater and environmental monitoring. Want to know more? Click here.

Salinity is a measurement of all dissolved salts in water. It is usually measured indirectly and is derived from a conductivity reading using a conversion factor that would often be pre-programmed into your conductivity meter. Typical measuring units are PSU, % and ppt. Salinity can affect levels of dissolved oxygen in water. Industries that use salinity measurements include agriculture, hydroponics, aquaculture and pool and spa.

The HI98192 conductivity meter by Hanna Instruments


Conductivity Meters + Probes

We stock a wide range of conductivity meters, ranging from small pocket meters, robust portable meters for field use all the way up to benchtop meters ideal for in the lab. If a meter is also capable of displaying the resistivity, TDS and/or salinity based on the conductivity then this will be detailed in the product description.

Types of conductivity probe:

Care is needed when choosing a suitable conductivity meter and probe for your application. Below we list the two main types of conductivity probe available and the types of application/sample they are most suited for. Factors to take into consideration when making your selection are: your expected conductivity range, sample volume and sample make-up (acidic, viscous, dirty etc). Conductivity probe bodies can be  made of a number of materials, so you will need to ensure the body type is suitable for your sample type.

Amperometric Probes work by applying a known voltage between two internal electrodes. The current is then measured. The electrodes themselves can be made of graphite or steel. This type of probe is best used in clean water applications due to the likelihood of a build up of mineral deposits and polarisation at high conductivities. This probe tends to be more affordable than the potentiometric probes and only a small sample volume is required. 

Potentiometric Probes contain four rings. They work by applying a voltage to the two outer rings to induce a current. This drives a current in the solution and this is measured between the two inner rings. The voltage drop would be dependent on the conductivity of the sample.  These probes have high accuracy, high precision and a larger application range due to their ability to be used at a higher conductivity values. They are suitable for most applications. These probes however require a larger sample as vent holes need to be covered. 


If you are looking for a meter/probe suitable for measuring ultra pure water, please check out our other blog post for advice.

We have a large range of conductivity meters and probes suitable for wide ranging applications. If you have any questions about selecting the correct product to suit your needs feel free to contact us using the form below. Alternatively, you can talk directly to our technical team on 01954 233 120.

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