According to HM Digital's website, although there is a close relationship between TDS and electrical conductivity (EC), they are not the same. The company writes that TDS, in layman's terms, is the combined total solids dissolved in water, while electrical conductivity is the ability of something to conduct electricity – in this case, water's ability to conduct electricity.
HM Digital, Culver City, Calif., says that one of the only true methods of measuring TDS is to weigh residue found in water after the water has evaporated. The spots left on a glass after it has been washed and air dried are an easily understood example of total dissolved solids. The residue has mass, and it's possible to weigh it in a lab, but if you're not in a lab, according to the website, TDS levels can be estimated based on the conductivity of the water because the hydrogen and oxygen molecules of the H2O carry almost no electrical charge. However, there is an electrical charge carried by most metals, minerals and salts.
A TDS meter measures the electrical conductivity level and converts it to a TDS measurement. Since different metals, minerals and salts will be more or less conductive than others, there are different conversion factors that can be used: ppm or mg/L are the most commonly used scales to measure TDS, and µS (micro-Siemens, abbreviated from µS/cm) is the most commonly used scale to measure electrical conductivity.
The FAQ page on HM Digital's site asks the following questions and provides the answers listed below.
Q.Why do I experience different readings in the same water with the same meter?
A.The reasons for varied readings include:
Ions. The nature of charged
positive ions, which is what the TDS meters are measuring, is that they always are
moving. Therefore, there may always be variances in the conductivity and, thus,
Temperature. Even with automatic temperature
compensation, temperature changes by a 10th of a degree may increase or
decrease conductivity. Additionally, the temperature coefficient (what the
reading is multiplied by to adjust for temperature differences) changes
slightly depending on the range of ppm. HM Digital's meters and virtually every
meter under $500 has a single temperature coefficient, regardless of the range,
the site notes. HM's COM-100 meter offers three temperature coefficient
options, but each is linear once selected.
Air Bubbles. Even a tiny air bubble adhering
to one of the probes might affect conductivity, and thus the reading.
Lingering Electrical Charges. Electrical
charges coming off fingers and static electricity from clothing can affect the
meter reading, and lingering electrical charges in the water will affect the
Beaker/Cup Material. Plastic cups retain
lingering electrical charges more than glass. If the meter touches the side of
the glass or plastic, it could pick up a slight charge. If the plastic is
retaining a charge, it could also affect the water.
Volume Changes. The amount of water in the
sample may affect conductivity. Different volumes of the same water may have
different levels of conductivity. Displacement may affect the conductivity as
- Probe Positioning. The depth and position of the probe in the water sample may also affect conductivity. For example, if a meter is dipped into the water, removed and then dipped into the water again, but in a different spot, the reading may change.
A.Here are several suggestions.
Shake. Always make sure to shake
excess water off the meter before dipping it into a water sample, even if it's
the same water.
Tap and Stir. After dipping the meter in the
water, always lightly tap it against the water container's side and stir the
meter to remove any lingering air bubbles or electrical charges.
Positioning. When taking the reading, always
hold the meter straight up without it touching the sides or bottom of the
glass, beaker or cup. The probes should be suspended as close to the center of
the water sample as possible.
Time. The longer the meter is in the water,
the more accurate the reading will be.
Temperature. The ideal temperature for
conductivity readings is 77°F (25°C) even if the meter has automatic
- Rinse. If switching between very low and very high ppm water, always rinse the probes with distilled water first to avoid any buildup on the probe.