Huge importance is always placed on the size and shape of the primary pores of the sensor, which is critical and determines the sensitivity and speed of response of the sensor, however very little attention is given to the construction of Base Layer of the sensor.
The construction of this Base Layer determines the base capacitance of a sensor and uniformity of this layer is critical for a reliable, stable and drift free sensor.
Pores or cracks in the Base Layer act as capillary pores and they absorb and desorb water molecules in the same way as the primary pores, although at a slower rate, therefore sensor with this construction, when used in ‘dry’ conditions will shift calibrations as these capillary pores dry out.
Other manufacturers do not have the capability to manufacture the sensor’s Base Layer with such precision or this layer is porous or cracked, which allows the base capacitance to change when the sensor is permanently installed in a continuous ‘dry’ condition - e.g. when at the outlet of an air dryer. This results in large sensor drift and in this case the sensor will indicate dryer Dewpoint readings than actual - eventually reaching the ‘bottom stop’ of the instrument display.
In addition to being relatively slow to dehydrate (drying down), this layer can also be slow to rehydrate (wetting up), giving the impression of a flat spot, i.e. where the sensor is very slow or does not respond to increase in moisture levels at low dewpoints.
Aluminium oxide sensors operating on an impedance circuit will show an exaggerated drift caused by this effect, especially if the capillary pores in the Base Layer extend to the aluminium core, as this would increase the resistive effect of the sensor.
Many years of development have resulted in a sensor with a very evenly constructed Base Layer, which eliminates dry end drift and does not demonstrate a flat spot response.
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