Choosing a meter

There are basically two types of meter commonly available - small 'stick' types and larger, more complex and more accurate bench-top and portable models.

Stick types are less expensive and fit in your pocket, making them highly portable. It should be remembered that these are not as accurate as the larger models and tend to irretrievably break down after a season or two depending on use. There are also more expensive waterproof versions that can last much longer. The digital reading on these is usually in mS/cm, thus a reading of 1.4 is 1.4 mS/cm or 1400 µ S/cm. Because they only have one decimal place, it means that each step is 0.1 mS/cm (100 µ S/cm) which makes for reasonably coarse readings and less than perfect calibrations. There is usually a small screw on the meter that is used to calibrate it.

Larger models often have a number of functions (often combined with a pH meter). They usually give readings to 0.001 dS/m (1 µ S/cm) and have a more convenient knob to calibrate them and are more accurate. The electrode is separate and plugs into the instrument.

Electrodes can be the classic two separated platinum plates (these are black as the plates are given a layer of platinum black) with a separate reference electrode or other arrangements that are often round and encased in a protective PVC sheath. These latter models are meant for environmental monitoring and are built to withstand field conditions.

Electrodes come with various cell constants (written as 'K'). Different cell constants are suitable for measuring different strength solutions. For agricultural measurements K=1 is suitable. This will be written on the electrode or the cable.

Calibrating the instrument

It is important to calibrate your instrument every time you use it otherwise you will get inaccurate readings. Calibration means reading a solution of known conductivity and adjusting your meter to read the same. The adjustment is done with a screw or knob.

The commonly used standard solution is potassium chloride (KCl). A solution of 0.01 M gives a conductivity of 1.413 dS/m and a 0.02 M solution gives 2.76 dS/m. It is good practice to calibrate using a solution that approximates the readings you expect. Agricultural applications are usually in the 0.5 to 4.0 dS/m range and both 1.413 and 2.76 dS/m calibration solutions would be suitable. After calibration, never return the used standard solution to the bottle as it will contaminate the unused standard. Discard it.

Calibration solutions can be bought from scientific supply companies, but tend to be quite expensive. A cheaper method, if you have access to an accurate balance and volumetric glassware, is to make up a stock solution of 1M KCl and then use that to make up your calibration standard a litre at a time using a 1:100 dilution (e.g. use a 10 mL pipette and a 1 L volumetric flask). It is important to ensure that the solutions are kept airtight to avoid evaporation, as this will concentrate your salts and so change the conductivity. Provided the solutions are tightly sealed, kept from extremes of temperature and out of the light (e.g. in a cupboard) then they will last for years.
To make up a stock solution of 1M KCl use 74.555 g of dry reagent grade KCl and make up to 1 litre with distilled water. Keep this as your stock and do 1:100 (0.01 M KCl) for 1.413 dS/m or 1:50 (0.02 M KCl) dilutions for 2.76 dS/m standard solutions.

Measuring the conductivity

Taking a conductivity reading is easy:

• immerse the electrode in the solution
• jiggle up and down a few times to remove any bubbles without bumping it
• wait 30 seconds or so for the sample and electrode to come to the same temperature and then read the measurement.

The instrument will do the temperature correction for you. The electrode should be immersed over the plates and reference electrode or up to the hole in the protective PVC sheath.

With the stick types, immerse the pins to the level indicated in the instructions. It is most important that the level of the liquid is not above the waterproofed section of the electrode or meter.

If you are making a series of measurements, you should rinse the electrode with distilled water and dab dry with a tissue between samples (without rubbing the electrode itself).

Ensure that the battery is in good condition for portable types. Most modern instruments will give a low battery warning. If you suspect a low battery problem, replace it.

Storing the electrode

After making your last measurement, rinse with distilled water and dab dry and then store it according to the manufacturers instructions. This may be storing in air or in a solution. Ensure that it is out of strong light and away from temperature extremes and dust. Remember that fertilisers are very corrosive, so keep all instrumentation protected from them. The stick types usually have a protective cover for the electrodes.

Maintaining the electrodes

Maintenance is only possible on the separated plate type electrodes. As electrodes age or if they are not rinsed thoroughly after each reading, they will eventually get a build up of dirt and scale on them and this will result in inaccurate readings, readings that drift or very low readings.

Never attempt to touch, scrub or rub electrodes, as this will remove the platinum coatings. If you suspect that you may have contaminated your electrode with fats or oils, then these can be removed by soaking in methylated spirits for 10 to 15 minutes, rinsing in distilled water and recalibrating the electrode.

Scales (accumulated salts) can be removed by dipping the electrode briefly in a solution of one part concentrated hydrochloric acid and 10 parts distilled water and then rinsing it thoroughly with distilled water and recalibrating it. If neither of these procedures rectify your problem, then your electrode may need replatinisation. This is done at the factory and is far cheaper than the replacement cost of the electrode.