Sensors are vital to a gas detector but can stop working properly for a number of reasons. One might be simply that someone dropped the detector and the sensor was damaged. Another might be exposure to toxic gases which poison the sensor. A third cause might be inaccuracy of reading because of excessive atmospheric humidity. And of course, an oxygen sensor may just stop giving accurate readings because it has become too old to work.
Traditional Oxygen Sensors
Like other electrochemical sensors, oxygen sensors become less sensitive with time, and this period is about two years, whereas with other classes of similar sensors such as hydrogen sulfide and carbon monoxide sensors, this period is usually longer than four years. Thus this may be considered a case of falling below performance standards.
Oxygen sensors which are too old are a primary cause of gas detection failures, and the expenditure of money on replacing sensors as well as waste of time because the detector is not working are annoying and frustrating. However, long-life oxygen sensors have now been developed which is a long-desired boon for gas detector users.
Long-life Oxygen Sensors
Long-life oxygen sensors are the advanced way to go with gas detectors, and alleviates much of the cost and pain associated with the traditional sensors. For one, these long-life sensors have a lifetime of up to five years if maintained properly, and if calibration and bump testing are performed as recommended, which is far greater than the earlier two-year span.
This increase in life is associated with less sensor (and hence gas detection) failures, less servicing time, and saving of money on repairs and replacements. In addition, the long-life oxygen sensors are lead-free, which prevents much environmental pollution. And finally, there is not much of a difference as far as the cost goes, between the conventional and long-life sensors.
With all these benefits, there’s bound to be some things on the negative side. These include the fact that they are designed differently, and therefore operate on a different plan. For instance, even after the power is switched off the instrument continues to draw on the battery, and these instruments must be allowed time to warm up in some situations. These are only superficially disturbing, but pose very little problem to the typical gas detector user.
The long-life oxygen sensor is designed to perform in the field for three to five years, solving the common gas detection industry problem of traditional oxygen sensor failure after eighteen to thirty months of use.
Biased Sensors – The Reason Why
The long-life sensor uses up some current even if the device is powered off, because of its biased design. This simply means that the reference electrode and the sensing electrode are at different potentials, with the former being at about 600 mV above the latter. Biased sensors are necessary to keep them stable and accurate.
Once the device is switched on, and begins to give readings, the sensor has a current sufficient for its bias, but when it is turned off, it requires battery power to remain stable. However, once the detector is at full charge, it can be stored without power for 15 days and still run for one hour, which proves that the battery power is not affected significantly, nor is the instrument run time adversely affected.
Warm up is required for the sensors because of the constant need for current if they are to maintain their charge. One example of this is when the battery is exhausted and requires to be changed or recharged. Once this is done, and the device is powered on, it will take a while for the sensor to become stable, the period varying with the length of dormancy. The table below gives typical periods for sensor stabilization:
||CHARGING TIME TO STABILIZE
Another situation in which sensors must be stabilized is when the long-life sensor needs to be replaced, as when the old one is almost worn out. While the new one which has been ordered comes with its coin cell battery to maintain sensor bias until the time of installation, it is seen that there will still be a delay before the instrument starts to function. During this time the instrument is applying bias to optimize the sensor readings, which requires under 15 minutes before completion.
Instability is seen to occur with readings from biased oxygen sensors if:
- The battery is removed for some time
- The instrument is not powered off as soon as the warning of critical battery level appears
However, provided the users maintain proper charging and testing protocols, these differences should not make much of an impact upon the use or functions of these sensors.
In conclusion, while long-life sensors are different from conventional oxygen sensors, they have many plus points, such as reducing the chance of sensor failure, less time expended on servicing the instruments, and lessened pollution. By giving due attention to calibrating, bump testing and maintenance schedules the long-life oxygen sensors may be expected to take gas detection a step ahead with respect to convenience and especially safety.
This information has been sourced, reviewed and adapted from materials provided by Industrial Scientific.
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