What Do You Do When Your EC Readings Aren't Quite Right?
Electrical conductivity is a useful measurement in a variety of industries, applications, and situations. Knowing exactly what electrical conductivity (EC/TDS) is, probe options, proper measurement techniques, and how to give probes a bit of TLC when they need it can jump-start your testing.
Top 3 Problems In EC Testing
Problem #1: My readings keep drifting or are erratic.
Our Question for You: Is your probe dirty?
Cleaning your probe regularly can keep debris from building up on your probe. Two electrode probes can get buildup between the two electrodes, and the vent hole on a four ring probe can get clogged. Simply rinse the probe in between readings, before storage, and after storage. Even if you are testing in water, you can still get residue.
Our Question for You: Is the probe set up correctly?
Check the connections between the probe to the meter. 3.5 mm connectors can sometimes fool you into thinking it is connected, but push on it a bit to ensure that the pin is inserted all the way into the port. If you have a meter with a pin connector (DIN/Quick DIN) check that the pins aren't bent, and that they are lined up when you insert the connector.
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.
Our Question for You: What is your sample, and what probe is right for it?
Just like with pH probes, you need to make sure that your EC probe is submerged correctly. Smaller sample sizes are better suited for two electrode probes due to the plates only needing to be slightly submerged. Four ring probes require the sample to cover the vent hole as well.
Make sure that there is at least one inch of clearance between the probe and all sides of the beaker.
Check the usable measurement range for your two electrode probe. If your sample is out of range you need a four ring probe, or a different two electrode probe.
Our Question for You: Did you calibrate?
Proper and frequent calibration is key. If the probe is used daily, calibrate daily. If not, calibrate the probe prior to use.
Problem #2: My readings are inaccurate.
Our Question for You: Are you working with a two electrode probe, and do you know about the Polarization Effect?
Two electrode probes can have something occur called the Polarization Effect. Due to residue buildup, an electrical charge can build up between the electrodes. The increase in charge can cause you to have lower EC readings. Look for two electrode probes that have graphite pins instead of the traditional stainless steel.
Our Question for You: Are you working with a four ring probe, and do you know about the Fringe Field Effect?
The Fringe Field Effect is when the measurement field, the constant electrical current, extends outside of the probe. If you keep your probe at least 1 inch away from the sides of the container or pipe where you are taking a measurement, you should be in the clear.
Our Question for You: 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.
Our Question for You: Did you calibrate correctly?
Check out our Maintenance Guide to verify that you followed the proper calibration steps.
Our Question for You: 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.
|Code||Electrical Conductivity Value @ 25°C||Code||Electrical Conductivity Value @ 25°C|
|HI7031L||1413 μS/cm||HI7034L||80000 μS/cm|
|HI7039L||5000 μS/cm||HI7035L||111800 μS/cm|
Our Question for You: Did you reach Thermal Equilibrium?
Patience is key! We aren't saying that you need to wait 10 minutes for your result; you just need to let the temperature equalize. 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.
Our Question for You: Did you get rid of air bubbles?
Who cares about little air bubbles? Well, you should when measuring EC. 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.
Question 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.
Problem #3: My probe will not calibrate.
Question: Are you using a contaminated standard?
Remember that the calibration standards have no buffering capacity. Try taking some new/clean beakers and pouring fresh standard into each. Then, use one beaker as an additional “rinse” to help keep the standard for calibrating as free of contamination as possible.
Question: Is your rinse water contaminated?
EC probes are very sensitive, and not all water is created equal. As mentioned above, the calibration standards are easily contaminated. Old rinse water, or water from the tap, can introduce contaminants. Distilled water can be used, with deionized water being the best option.
Question: Did you store your probe properly?
EC probes should be stored dry unless they are a combination EC/pH probe. If you have a combination probe, the probe should be stored in HI70300 storage solution in order to keep the pH glass probe hydrated and in working order.
Remember to: Clean Regularly, Calibrate Often, Care Always
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Allison graduated from Bryant University with a Master’s Degree in Global Environmental Studies. She is passionate about nature, and how science is connected to the world around us. At Hanna, she provides an array of content and support to customers through the Hanna Blog, SOPs, and Data Sets.
Allison may be reached at email@example.com.