Question:
What could be the cause of suddenly fluctuating or negative readings from a chlorine dioxide (ClO2) sensor?
Answer:
Background: One of our distributors had just completed testing a different, (not GfG), brand of single-sensor ClO2 instrument with 10 ppm of chlorine dioxide. He confirmed a problem that his customer had brought to his attention. As our distributor put it: “When continuously exposed to 10 ppm ClO2 the meter runs accurately for about 30 minutes, then begins a nose dive that drives the reading negative (with associated negative drift alarm) over the next 20 minutes. Is it possible that the self-protection is kicking in? This is a very serious issue with use of this meter.”
Answer: The exposure limit for ClO2 is 0.1 ppm (TWA) and 0.3 ppm (STEL). GfG uses Sensoric 3E 1 O chlorine dioxide sensors in our instruments. The sensors are optimized for performance at exposure limit concentrations. The resolution is normally set to 0.01 ppm, over a full linear range of 0 – 2.0 ppm. The sensor goes into over-range alarm when the concentration exceeds the linear range limit by more than 30%, (for ClO2 this would be about 2.6 ppm). We normally use a chlorine generator set to 1.0 ppm to calibrate ClO2 sensors.
Electrochemical sensors used to measure oxidizing gases like ClO2 utilize water molecules in the detection reaction. That makes oxidizing gas sensors more susceptible to humidity related fluctuation than the sensors used to measure reducing gases such as CO and H2S. Our distributor stated that when his customer’s instruments were exposed to 10 ppm ClO2 the readings were initially stable, but then, after 30 minutes, the readings suddenly started counting downwards, and eventually wound up in negative alarm. This would be consistent with the reaction having consumed all of the available water in the sensor electrolyte. The sensor takes on water as necessary from the surrounding atmosphere. But if the available water in the electrolyte is consumed faster than it can be replaced, eventually you run out of water, and the detection reaction comes to a halt until the sensor has a chance to replenish itself.
An additional factor is the dryness of the gas and air used to test the sensor. Gas from a cylinder contains no moisture whatsoever. We normally recommend using either a chlorine generator, or a chlorine dioxide generator to test and calibrate ClO2 sensors. Cl2 and ClO2 generators mix the test gas into a stream of ambient air. The humidity of the ambient air in which the generator is located may vary from one day to the next, but even on a very dry winter day, it is never zero. However, the drier the air, the harder the sensor will have to work to replenish the water used up in the detection reaction. It doesn’t surprise me that using a very high concentration of test gas, mixing the gas into dry winter air, and prolonging the exposure over a 30 minute period, eventually led to fluctuating or declining readings.
Both the GfG Micro IV single-sensor and G460 Multi-sensor gas detector can be equipped with ClO2 sensors. GfG instruments perform very well when equipped with ClO2 sensors. Prolonged exposure to ClO2 at concentrations within the linear detection range should not cause the GfG sensor to lose sensitivity, or begin to display negative readings. Deliberate exposure to concentrations that exceed the upper range limit are not recommended.
The only other time you are likely to see negative readings is when the ClO2 sensor is exposed to a high concentration of a reducing gas (such as H2S).
The ClO2 sensors we use in GfG products are pretty resistant to most interfering gases. However, H2S has a negative interfering effect on ClO2 sensor readings. Exposure to 10 ppm H2S will produce a reading of about – 2.5 ppm. On the other hand, high concentrations of ClO2 or chlorine have very little effect on the H2S sensors we use in GfG products. GfG technical note TN2019 lists relative response values for the electrochemical toxic gas sensors used in GfG products.
I normally suggest using a multi-sensor instrument equipped with both ClO2 and H2S sensors when both hazards are potentially present. If you see a high reading on the H2S sensor at the same time you see a negative reading on the ClO2 sensor, you can be reasonably certain of the cause.
Our distributor also asked if using a colorimetric measurement tube might be the best solution for high-range ClO2 measurement. If the customer needs to take action at 10 ppm ClO2 (or higher), using a tube would probably make sense.
An alternative approach for this customer would be to continue to his current instruments, but use them for periodic rather than continuous sampling. As long as you operate the instrument only for brief intervals, and give the sensor time to recover between readings, you will probably not deplete the electrolyte sufficiently to run into any problems. The customer should be careful to discontinue use the instrument if the readings suddenly begin to fluctuate or decline.