Which TDS/EC-meter to choose

TDS-3, EC-3, AP-1, AP-2 or COM-100, all from HM Digital, USA.

You can choose between 2 diff. types:

  1. an EC-meter, that measures electric conductivity in the unit MicroSiemens (µS).
  2. a TDS-meter, that measures total dissolved solids (TDS) in ppm (parts per million or mg/L).

A TDS meter and an EC-meter is the same device, they just show different units. TDS meter and an EC meter both measures the water's electrical conductivity. Total Dissolved Solids (TDS) is the number of charged ions (minerals, salts and metals) that is dissolved in a given quantity of water expressed in mg/l, also referred to as parts per million (ppm).

HM Digital uses in TDS-meters a NaCL calibration, that converts from µS to the unit for a TDS-meter: ppm with factor 2, that is:

1ppm = 2µS (maybe not true for other manufacturers)

It is recommended to buy a meter to check the water in different situations. You can see how well the filter and membrane are working, and thereby evaluate, when it is time to change or membrane filters.

EC-3 (µS) and

TDS-3 (ppm)

AP-2 (µS) og

AP-1 (ppm)

COM-100 (ppm & µS) 
In relation to the newer model: easier to read, slightly bigger.

To see temp. you have to press a button.

Display at higher resolution. Shows at the same time temperature.

advanced combo EC-TDS meter. Several americans are prasing it for accuracy.
Not water proof and not magnetic Water resistant and magnetic Water proof and it floats.
Automatic calibration with a calibration fluid: no, mini screwdriver is needed Yes Yes

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