Ever since MOS H2S sensors came into existence, their historical reputation of 'going to sleep' has persisted. The fact is that certain solid state sensors tend to degrade in sensitivity at such a rapid rate, that from one calibration interval to the next, it seems as though the sensor has totally lost its sensitivity to scale.
This means that the sensor would need a much higher concentration of H2S before even attaining the on-scale level and is therefore an unsafe device. In a number of these compromised cases, it was discovered that repeated high doses of H2S could temporarily recover some of the lost H2S sensitivity, suggesting that it had “gone to sleep” because it appeared that “it could partially be woken up”.
This was very true in the 1980s and 1990s for Manufacturers of H2S MOS sensors, and thus at that time it was commonly suggested that calibration intervals do not exceed one month.
The typical feature of an MOS sensor losing sensitivity (“going to sleep”) is the result of a number of factors that may vary over time. A correctly designed MOS H2S sensor film can be made to provide steady H2S sensitivity over several years irrespective of how frequently it is exposed to H2S. Conversely, a weakly designed MOS H2S sensor can display a total on-scale loss of sensitivity in less than three months. It all depends upon the sensor design, manufacturing consistency and operating condition.
Factors that Influence Loss in H2S Sensitivity
A Metal Oxide Sensor film which ages over time in the direction of the sensor film becoming greater in resistance will unavoidably lead to lower “as-found” H2S signal when gas tested.
Higher film resistances can occur because of negative changes in the grain boundary structure of the sensor film over accumulated time and temperature. Different metal oxide based films that are deposited with different fabrication methods can differ widely on this point.
Higher resistances could be the result of inter-migration between the sensor film and the substrate it rests on.
Higher film resistance can occur because of environmental gas exposures of oxidizing gases such as ozone, which sometimes permanently shift the sensor film in the direction of higher resistance. In certain other cases sustained exposure to either low or high moisture conditions can also add to higher sensor film resistance.
Generally, MOS sensor films that function at higher temperatures are under more pressure to age in the direction of increasing sensor film resistance that results in reduced H2S sensitivity. Hence, sensor designs that can function effectively at lower temperatures tend to age better; and sensor designs that are forced to function at higher temperatures are more susceptible to age-based sensitivity losses.
A massive differentiating factor in sensor design is the more recent calculated use of mixed metal oxide catalyst additives which are used to accelerate the H2S reaction and assist in allowing the effective operation of the sensor at lower operating temperatures. However, metal oxide catalysts can also vary over time (including being potentially poisoned) so that is another potential failure mode for H2S sensitivity loss.
MOS H2S Sensors
Certain MOS H2S Sensors have experienced dramatic improvement in long-term stability including those from 3M Gas & Flame Detection.
Manufacturers that thoroughly control the technology of their MOS sensor have successfully optimized improvements in the sensor technology to handle long-term aging. These best-in-class sensor designs have shifted the MOS sensors to an operational safety level where for a majority of applications three month calibration periods are standard.
These sensors will sufficiently respond to H2S within a broadened accuracy window, even if the calibration frequency was prolonged further to six months, although we do not suggest longer than three months.
The graph below shows the response of two market-leading H2S MOS detectors to 50 ppm H2S test gas over a period of two years without a span calibration performed. These sensors were placed outdoors in Houston, Texas for the length of this test period.
Although a possible gradual loss of sensitivity may still take place in competitive MOS H2S sensors, the slow rate of degradation in the best designed sensors is such that a service life of up to 10 years can be attained in a majority of applications.
It is only the MOS H2S sensors of poor quality design and manufacture that has caused the negative myth of “going to sleep”. But regardless of what expression is used to label the problem, end-users of H2S sensors should assess and choose sensors with great care to avoid possible failures with the related dangers and higher costs of ownership that can result.
This information has been sourced, reviewed and adapted from materials provided by 3M Gas & Flame Detection.
For more information on this source, please visit 3M Gas & Flame Detection.