Why are my EC/TDS readings inaccurate?

There are multiple factors that can influence your EC/TDS readings.

Polarization Effect (Two Electrode Probes)

The polarization effect happens when a charge builds up between the two electrodes. This extra charge can cause your EC readings to be lower than they are expected to be. This can be minimized by two electrode probes that are made out of graphite instead of stainless steel.

Fringe Field Effect (Four Ring Probes)

The fringe field effect is when the measurement field, the constant electrical current, extends outside of the probe. You only need to worry about the fringe field effect if your probe is too close to the sides of the container or pipe where you are taking an EC measurement. A good rule of thumb is to keep the probe at least an inch away from all surfaces. The distance you need to keep the probe varies, so just check the manual.

Is the rubber ring removed? (Four Ring Probes)

If your probe is a new four ring probe, take a look inside the probe sleeve near the bottom. There is a small clear rubber circle around the internal section of the probe. This is to help preserve the probe during shipping, but it can interfere with your readings. Carefully remove the rubber circle, then re-calibrate the probe.

Did you calibrate?

If you are testing for USP standards, or in aggressive chemicals, you will need to calibrate more often. If the probe is used daily, calibrate daily. If not, calibrate the probe prior to use.

Did you calibrate correctly?

Proper and frequent calibration is key. If the probe is used daily, calibrate daily. If not, calibrate the probe prior to use. Certain calibration steps are vital as EC calibration standards have no buffering capacity and are easily contaminated.

Did you use the right calibration standard?

When doing a single point calibration, calibrate in the air first (zero point) and then the calibration standard used should be as close to the actual concentration of your sample as possible. If your sample concentration fluctuates regularly in a wide range, you may need a probe and meter that can calibrate to multiple standards to increase your accuracy.

Thermal equilibrium

Even with temperature compensation, it takes time for the probe to reach a stabilized temperature state. Make sure to allow a few minutes for the probe to reach this equilibrium.

Air Bubbles

Air bubbles can wreak havoc on readings as the sensing parts of the probes may not be completely submerged in the sample. Gently swirl the probe and/or tap the probe on the bottom of the beaker. This should be enough to dislodge any trapped bubbles.

Maybe what’s in your analyte is non-ionic.

Not all things that cause TDS break apart into ions while in solution. Double check that your analyte breaks apart into ions when in solution. If it does not become ionic, there could be other means of testing available.