Application
Aluminum oxide moisture sensor technology is one of the most commonly used equipment in industrial and process applications for
moisture analysis, due to the rugged nature of the sensor technology in terms of immunity to contaminants and its ability to be used directly in process lines or sample systems. These devices require periodic recalibration, (6-12 month cycle depending on manufacturer) which requires that they be removed
from service and brought to a laboratory. In the lab, the moisture probe is exposed to a calibration gas produced by a moisture generator and the output signal from the sensor is recorded. The moisture generator / calibration system often includesa chilled mirror hygrometer as the primary standard against which the aluminium oxide sensors are calibrated. (GE Sensing / Michell Instruments).

The accuracy of a chilled mirror hygrometer is ±0.5˚C dew frostpoint whereas an aluminium probe is ±2˚C dew frostpoint making it ideally suited for calibrating aluminium oxide probes
 

Background
Drax Power PLC use aluminium oxide hygrometers from GE Panametrics for field measurement of moisture in Hydrogen within the generators spot testing is done with a MBW chilled mirror hygrometer. Drax Power validates its field sensors at it’s central lab.

Drax uses bottled nitrogen and a GE Panametrics MG-101 generator to flow gas at specific frost points through flow cells containing the sensors being tested. Before testing, each sensor is cleaned in toluene and dried in an oven as per the GE Sensing recommended cleaning procedure for aluminium oxide sensors. The chilled mirror hygrometer is placed in series with the sensors and used as the standard. The majority of probes tested remain within tolerance, but any that are out of tolerance get returned to the manufacturer for factory recalibration. Several GE Panametrics probes continue in service—even after 10years. This process has and continues to serve Drax well.