Selecting the right meter for testing soil and water.

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Here are a few notes on choosing and using test meters.

Conductivity meters. Meters (and testers) which display in ppm are using a TDS (Total Dissolved Solids) conversion factor. Most aquarium meters use 0.5 to convert between microS/cm and ppm. If you are into horticulture, soils, irrigation water (not too salty) and hydroponics then choose a meter which uses closer to 0.7. Better still, a lot of meters have user adjustable conversion ratios.

For fresh water aquariums a conductivity meter which reads to 1990 microS/cm is satisfactory. For general horticulture choose a meter which goes to at least 4000 microS/cm. For hydroponics choose a meter which goes to at least 3000 microS/cm. For monitoring saline water go to 10000 microS/cm or above.

Waterproof meters are best for outdoors or harsh conditions. Also think about whether you will need a probe on a long or longer cable to get to the water you are measuring.

All meters should be calibrated so that you can trust the measurements. This is not too hard once you've done it the first time. Make sure you read the instructions and choose a calibration solution to suit the intended or anticipated range of values. ORP (Oxidation Reduction Potential or REDOX) and dissolved oxygen meters generally use a single solution. Some meters need you to twist a dial or adjust a screw to set the meter. This is not hard - just requires patience. There are some meters which recognize the strength of a calibration solution and adjust automatically.

Some meters come with a minimum set of calibration solutions but with most meters these are purchased separately so you may need to stock enough to carry you through. Use distilled water to wash electrodes but use proper storage solution to store them if required.

Here is Tim using a pH meter in the lab.

You will find more details by reading the notes and catalogue. Remember to click on the pictures to see specifications.


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