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Electrochemical sensors vs. solid state sensors

Summary: in Argentina thousands of workers are exposed to the presence of hydrogen sulfide, H2S, a highly toxic and flammable gas. Industries such as oil and gas exploration, mining, petrochemicals, water treatment, among others, use detectors to prevent exposure of personnel.

The selection of the detector is crucial for safety, the criteria for its selection is often misunderstood, here we will deal with two very different sensor technologies, underlying the market (electrochemical and solid state), this document aims to facilitate the choice of a detector by highlighting the pros and cons of each technology.

1. Motivation

For any H2S detector, which is acquired in the world market, it would be a utopia to think of it as a perfect detector, ignorance of the type of sensor or work environment can cause an instrument, in apparent perfect operation, to fail drastically, the following aspects should be of consideration:

• Cross sensitivity: all H2S detectors, and gases in general, suffer, to a greater or lesser extent, from cross-sensitvity, that is, an H2S detector not only responds to the presence of H2S, it also responds to the presence of other gases, a particular case would be that the sensor does not respond to the presence of H2S if it is combined with another gas.

• Saving resources, the difference in costs for instruments of this type can be 10 to 1, a correct interpretation of the specifications of the detectors, added to the knowledge of the environment, can provide an early warning system with a cost of a fraction to Another proposed one, on the other hand, a false alarm can cause huge operating costs in lost man-hours.

Other characteristics to take into account are: measurement drift, repeatability, resistance to contamination (poisoning), overload, sampling system and filters.

2. OperationPrinciples, solid state sensors

In solid-state sensors, a semiconductor material is applied to a non-conductive substrate and two electrodes are placed on it. The substrate is heated to a temperature, which may or may not be controlled, facilitating the change in conductivity of the semiconductor in the presence of H2S gas. In the zero gas condition, the O2 molecules keep the electrons of the semiconductor fixed, preventing the circulation of electric charges, in other words, the semiconductor is in its high impedance state. When H2S molecules come into contact with the surface they replace those of O2, this results in the release of electrons and a decrease in electrical resistance, this variation is related in a logarithmic way with the concentration of H2S in the environment. An unwanted effect is that the displacement of the O2 molecules, by those of H2S, is not the only mechanism by which the impedance of the sensor changes, this is one of the weak points of solid state sensors, producing a long list of gases that cause this same effect.

3. Operation Principles, electrochemical sensors

Electrochemical sensors are classified as combustion cells, where a process of conversion of chemical energy to electrical energy takes place, there are two or three electrodes, but the basic principle of operation is the same. There is a membrane that separates the outside environment from the inside of the cell, inside there is an electrolyte, the membrane works as a water-repellent barrier, when the gas molecules from the outside enter, an oxidation-reduction takes place, closing the circuit between the anode and cathode the free electrons produced can be measured with a potentiostatic circuit, the current signal is in linear proportion with the amount of gas that enters the cell and consequently with the concertation of the external gas, although it has a selectivity greater than in the case of solid state sensors, as in these, H2S gas is not the only one that produces a signal, even the presence of certain gases can reverse the oxidation-reduction processes obtaining negative measurements, for obscure reasons the manufacturers of Solid-state sensors generally do not make public documents showing a cross-sensitivity table, differently, manufacturers of electronic sensors Rochemicals normally include cross-sensitivity tables in their data sheets, although this is not the whole story, in my experience in the field, and then making measurements, I have found that a long list of gases, as common as those that appear in the tables provided by the manufacturer, they have cross sensitivity with the gases of interest, in several cases the manufacturers have sent me complementary tables, as a conclusion tests should be done if it is suspected that a certain type of gas will be in contact with the detector, even if it is not specified by the manufacturer.

4. Calibrations and lifespan of solid state sensors

 The resistance value for solid state sensors is approximately 106 Ohms, in the case of clean air, the resistance decreases to approximately 105 Ohms for 1ppm and 104 Ohms for 100ppm of H2S in air, which is a typical full scale for this type of sensors. This logarithmic behavior would imply that the calibration must be carried out with at least two different concentrations of H2S. It is notable that a change in the baseline, small in percentage, will be appreciable at low concentrations (but much more for high concentrations), this is evident in figure 1 (response of a solid state sensor at different concentrations) , the change from the baseline is the so-called "long time drift". Exposure for prolonged periods of these sensors to free air of H2S, causes the known state called "go to sleep", once in this state, the sensor is insensitive to the presence of low concentrations of H2S, for this reason the calibration has a double effect, on the one hand to carry out the calibration itself, necessary due to the great influence of the long time drift for this type of sensors, and on the other hand, to prevent the sensor from entering the “go to sleep” state, new technologies in the Solid state sensor construction have eliminated this effect. The useful life of these sensors is 3-5 years depending on the manufacturer and model.

5. Electrochemical sensors calibrations and lifespan

 Generally, electrochemical cells have a duration of 6 months to 3 years depending on the cell, the duration is related to the resolution of the cell, when you have membranes with a larger surface, the depletion of electrolyte is faster, but you have a greater sensitivity. (high sensitivities 0.1 ppm are associated with environmental measurements rather than alarm systems). The periodic calibration responds to a decrease in the gain, it is interesting that the zero shift of an H2S detector can be null, with respect to its resolution, in the total useful life of the cell, if for some reason at a electrochemical cell based detector is overlooked by a routine calibration the measured value would be somewhat lower than the actual, say 9ppm instead of 10.0ppm, in many applications an irrelevant error (suppose an early warning system where the alarm triggering is set to a value well below dangerous concentrations), electrochemical cells have absolute zero, do not produce current if there is no chemical reaction caused by an external gas, detectors based on electrochemical cells can reach the 0 drift condition , ideal for measuring low concentrations.

6. Humidity and temperature, electrochemical sensors

Humidity is a factor that has an effect on different aspects than for each sensor, conditioning the range in some and repeatability in others.

Electrochemical sensors: measurements are not affected by changes in humidity, they work in conditions of 15-95% relative humidity. A change in temperature implies a change in the transformation ratio of H2S-current, in instruments that measure the working temperature this value is compensated, saving the inconvenience, otherwise errors of less than 10% are obtained.

7. Humidity and temperature, solid state sensors

Solid state sensors: changes in humidity affect the repeatability of their values, they work in conditions of 0-100% relative humidity, this characteristic is what makes detectors based on solid state sensors more suitable for very hostile environments. to take into account that the contact of the sensor surface with water de-sensitizes the sensor. With regard to temperature, all solid state sensors have a heater, the detector may or may not control the temperature, if the detector does not control the temperature, temperature errors are introduced, affecting the measurement of low and high concentrations.

8. Maintenance and conclusions

Generally, the cost of an electrochemical sensor at the time of replacement is less than 50% of that of a solid state sensor. In general terms, solid state sensors need more maintenance but can be used in extreme environments, their selectivity is low and their drift is high, electrochemical sensors are more reliable in the detection of small concentrations, with a higher selectivity and a low drift, a disadvantage of electrochemical cells is the over-load, generally if the concentration exceeds 10 times the range of its maximum scale the cell could be damaged. In general, the manufacturers of both sensors recommend calibrations once a month or every three months, although the electrochemical cells have an absolute zero, they do not produce current without a reactive gas, every company with a good safety scheme makes weekly checks, for cases In extremes, as in the exploration industry, up to two daily checks are carried out, one per shift. The consumption in electrochemical sensors allows developers to implement 4-20ma 2-wire transmitters, something that is not possible for solid-state sensors due to their consumption.