Soil Moisture / EC sensor

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Measuring salinity

A quick and convenient way of measuring the salinity of a solution is to use a portable conductivity meter (EC meter). This enables instantaneous salinity readings to be obtained. The ‘electro-conductivity’ of the solution increases with the concentration of the salts.

There are different measuring units of electrical conductivity:

  • microSiemens per centimeter (µS/cm). Commonly called: ‘EC Units’
  • deciSiemens per meter (dS/m)
  • milliSiemens per centimetre (mS/cm)

Salinity levels are also measured in terms of the total dissolved salts (TDS) concentrated in the solution and may be expressed as:

  • milligrams per liter (mg/L) of TDS or
  • parts per million (ppm) of TDS

General conversions between the different measurements

  • ppm is numerically the same as mg/L
  • 1000 EC = 1000 µS/cm (that is 1:1)
  • 1000 µS/cm = 640 ppm (or mg/L) TDS
  • 1000 µS/cm = 1dS/m
  • mS/cm = dS/m (that is 1:1)
  • 1000 EC = 640 ppm = 1 dS/m
  • To convert ppm (or mg/L) to dS/m, divide by 640
  • To convert dS/m to ppm (mg/L), multiply by 640

Frequency of calibrating a salinity meter

The salinity meter should be calibrated regularly, depending on how frequently the meter is used.

For most irrigation farm purposes, recalibration should occur once or twice per irrigation season. Generally, this is at the start of the irrigation season and again mid-way or towards the end of the irrigation season.

More frequent calibration of the salinity meter would give you more confidence in the accuracy of the readings. This might be advisable where the water salinity was of concern, such as increasing salinity levels with each reading, fluctuating salinity levels, or if the water source is at a threshold level for preferred use.

Handling and storing calibration solution

It is important that standard solutions are kept stored in an airtight bottle and unused portions are not returned to stock solution. The stock solution should be replaced once a year.

Cleaning stainless steel electrodes

The stainless steel electrodes need to be kept clean and free from dirt etc., to ensure accurate readings are obtained.

  • To a beaker add enough 50 percent methylated spirits to cover the electrodes only. Allow standing for 15-20 minutes.
  • After the elapsed time, pour into the waste bottle and gently wipe the electrodes with a tissue then rinse with distilled water.