The 8800 series uses field proven aluminum oxide (Al2O3) sensing technology to accurately detect trace moisture on either a continuous or spot checking basis. All Al2O3 sensors share the same basic operating principle: the capacitance measured between the sensor’s aluminum core and gold film deposited on the oxide layer varies with the water content. The 8800 series moisture sensor employs unique Hyper Thin Film (HTF) technology, which offers three major structural improvements in Al2O3 sensor design. These structural changes, noted below, provide the user with increased sensitivity, greater stability and a quicker response time when compared to other conventional aluminum oxide sensors on the market today.
In HTF sensors, the transition between the aluminum oxide and the aluminum core is sharp and clearly defined. This inner barrier layer produces a capacitor with its electrodes very close together, which, in turn, leads to the sensor’s high wet to dry capacitance ratio. The benefit of this high ratio is that drift in capacitance, due to undesirable factors, is much less significant. This is clearly a benefit when comparing HTF versus conventional sensors where temperature sensitivity and aging drift are concerned. An added benefit associated with this sharp transition in the barrier layer is a reduction in metal migration, one of the major causes of aging drift in conventional sensors.
Hyper Thin Film (HTF) Layer
The thinner oxide layer of the HTF sensor results in higher capacitance changes (stronger signal generated than conventional sensors) because capacitance is inversely proportional to the distance of the capacitors’ plates from each other (the distance between the aluminum core and the gold film deposited on the oxide layer). The thinner layer also means water molecules will travel faster in and out of the sensor pores, therefore responding several times faster than conventional sensors.
The most significant difference between HTF and conventional sensors in their pore geometry. While conventional sensors rely on hygroscopic Al2O3 structures to attract water, the HTF sensors instead rely on a pore geometry that slows the Brownian motion of the water molecules entering the pores. The HTF sensor design results in more dielectric in the pores and consequently a higher capacitance. An added benefit is derived from the fact that HTF pore geometry does not significantly change over time. Conventional Al2O3 structures, however, are not stable and collapse slowly into non-hygroscopic structures. As a result, conventional sensors are subject to higher drift rates and require frequent re-calibration.
Pressure Correction Function
The moisture readings are typically displayed at sensor pressure (atmospheric). Alternatively, the operator can display the readings at the line pressure by selecting and setting the Pressure Correct Mode key. Readings are typically read on a dew point basis in °C or °F. For readout on a ppm basis, the operator must control/know the line pressure and program this value into the controller via the pressure correct function.
Common Electronics Platform
Each unit uses a state-of-the-art microprocessor offering advanced intelligent features. In genera, the MODE button navigates through different user options; the UP and DOWN buttons modify the units, values or choices in the selected mode; and Pressure Correct button is used to either abort out of a mode or to activate the pressure correction function of the instrument. Most versions offer the common options of two adjustable alarms, an isolated 0-24 mA or 4-20 mA output or RS-232 serial interface. The instrument also has an indication for sensor open, short or electronic system failure to quickly alert the operator to a possible malfunction situation. In addition, most versions offer extensive self-diagnostic features that are performed at both start-up as well as once every two minutes.
Sensor Installation Considerations
Proper sample handling is crucial to maintaining the tight moisture control demanded by the industry today. Operators can assist in achieving a successful dew point measurement by:
Mounting the sensor as close to the measurement point as possible
Mounting the instrument as close to the measurement point as practical
Leakage, pressure/temperature gradients and moisture absorption/desorption characteristics also need to be taken into careful consideration when designing the appropriate sample system for dew point analysis. Ideally a minimum number of joints/fittings and other plumbing upstream of the Al2O3 sensor should be maintained to reduce the number of locations where moisture could collect or a leak could form.