Why do electrochemical cells require periodical ventilation?
Every electrochemical sensor under the extended influence of measured gas deteriorates.
Measurements, particularly of the low concentrations, are then burdened with a higher error.
Periodic ventilation allows the sensor to recover its full accuracy.
If the concentration of provided gas is high then the ventilation should be performed more frequently.
During ventilation process all of the toxic gas sensors are assigning their gaseous zeros and the
O2 sensor designates its measuring range (calibrates itself on the atmospheric air).
What is the “forming of electrochemical sensors”?
Over many years of production, we have observed that the parameters of electrochemical sensors
change most rapidly at a rather initial stage of their use.
In order to widely avoid this event, during production of the devices, we apply a method to
stabilise the parameters of the sensors.
This is what we name forming of the sensors, which generally bases on reviving them periodically
through administration of concrete doses of certain gases.
A sensor that underwent this process of forming before the first calibration is effectively
more stable over time and this way ensures the durability of the calibration impact.
What does the „cross-sensitivity” term mean?
A common disadvantage of practically all electrochemical sensors is their limited selectivity.
This means that the sensor does not react only to the gas it should detect (e.g. H2S),
but also to other components being part of the measured gas (e.g. SO2).
In practise, this means that sensor H2S will give an electrical signal just because of the presence
of SO2, even in case of total absence of H2S. Cross-sensitivity concerns all electrochemical
gases and can be accumulative (the foreign gas increases the signal of the sensor)
or diminutive (the foreign gas decreases the signal of the sensor).
How can I avoid measurement errors caused by cross-sensitivity of electrochemical sensors?
The leading producers of sensors are attempted to construct them in a way that they obtain the lowest
possible sensitivity towards foreign gases. In practise, this helps to reduce, but not avoid the effect
of cross-sensitivity. Another method is to equip the sensors with built-in filters that eliminate the foreign gases.
The main disadvantage is the limited durability of the filters and in further consequence lower effectiveness of this method.
The most sophisticated method used by advanced producers of gas analysers is the calculated elimination of the cross-sensitivity.
What method does madur use to avoid errors caused by electrochemical sensor’s cross-sensitivity?
We use all available methods:
a. we use solely high-quality sensors with minimised cross-sensitivity from the leading producers,
b. we use sensors that are equipped with filters for foreign gases,
c. our analysers are calibrated with many single-component gases and the cross-sensitivity is eliminated during measurements by calculation.
What is the calculated elimination of cross-sensitivity applied in madur analysers?
Suppose the analyser is equipped, besides sensor O2, with 5 more following electrochemical gas sensors:
CO, NO, SO2, NO2 and H2S. In this case, sensor H2S will react sensitive not
only to his own component, but also the remaining gases.
This reaction pattern applies to other sensors as well.
During the process of calibration the analyser with its sensors is being fed with corresponding single-component gases.
The reaction of each sensor to each gas is then noted and saved in the long-term memory of the
analyser in a form of a two-dimensional calibration table. During the measurements, the analyser uses the calibration
table and electrical signals from all electrochemical cells to solve the series of equations to calculate the
results for all sensors, that are deprived of cross-sensitivity error.
Why are madur analysers calibrated with several single-component gases while other manufacturers calibrate their devices with gas mixtures?
Calibration with gas mixtures is used to facilitate the process of calibration, mainly in terms of saving time of the calibration process.
This, however, implies that there is no cross-sensitivity between the sensors, which is not really true. Even if instantly
after production of a device, thanks to new filters in the sensors, the cross-sensitivity is almost zero,
this value will get worse with increasing operating life of the filters implemented.
In such a situation, a gas mixture calibrated device will measure the gas mixture in a correct way,
but will show faulty results in case of other proportions of gases to be measured.
Thanks to our calibration method and calculated elimination of cross-sensitivity,
our devices will show correct results even with exhausted filters.
In our practice, we also use gas mixtures, but only to control the operation of the devices and never for calibration purposes.
Are electrochemical sensors in madur devices compensated thermally?
Yes. Despite the efforts of producers, all electrochemical sensors show a significant and mostly non-linear dependency of the signal on the temperature.
All madur analysers have a built-in thermal compensation of the sensors, basing on a non-linear
algorithm and constant temperature measurement of the sensors with an accuracy of 0.1°C (sometimes even 0.01°C).
However, this method is not always sufficient, as the thermal characteristics of sensors change during their life-times and the compensation becomes less efficient over time.
In most advance solutions we also use active thermal stabilisation that allows us to stay almost completely free the from the influence of temperature.
Some manufacturers use pre-calibrated electrochemical sensors with built-in electronics, what are the pros and cons of this method?
Producers, that use these kinds of sensors emphasise the simplicity of replacement and absence of calibration necessity.
On the other hand, due to the built-in electronics, the price of such a sensor rises significantly.
Moreover, it is not true that such a sophisticated sensor does not need any calibration anymore, because it lasts for only six months,
whereas the life-time of the sensor itself may be rather longer. So, in order to receive correct measurement results,
the user would have to buy a new pre-calibrated sensor every six months, which constitutes a quite cost-intensive issue.
Therefore, we have rather quickly discarded the idea of using such sensors, especially as our clients appreciate the independence in the purchase of sensors.
The algorithms for calibration are accessible in all our analysers, so that the user can perform calibration by oneself or any other facility that offers such service.
Using pre-calibrated sensors, in our opinion, resembles the politics of manufacturers of ink-jet printers. Users of these printers will definitely know what we mean.
What determines a lifetime of an electrochemical sensor?
Electrochemical sensors are based on electrolyte that wears out slowly while the sensor is being used.
The expected operating lifetime of sensor is determined by concentration of a gas sample and the time that the sensor is exposed to it.
What is the main difference between classic electrochemical sensor and sensor that measures partial pressure?
Classic electrochemical sensor measures concentration of a gas, independently from air pressure
(if air pressure changes measurement stays the same). Sensor that measures partial pressure works in a different way.
It measures the hypothetical pressure of that gas if it alone occupied the volume of the mixture at the same temperature.
So if air is measured in 1000hPa pressure, oxygen partial pressure sensor should measure about 209,5ppm of oxygen (as amount of oxygen in atmosphere equals 20,95%).
Measurement can change either because the concentration changes or because the pressure does.