What is the rehydration procedure to recover the sensor from exposure to organic solvents or high humidity environments?
Typically, initial accuracy can be recovered by baking the sensor at 125 °C for 12 hours followed by ~2 days storage period in normal ambient conditions with ~50%RH. High RH exposure (i.e., 75% RH for 12 hours) will accelerate the post-bake recovery, but, after high RH exposure, approximately two days at normal RH is still recommended for the device to fully recover its accuracy. The bake/hydrate procedure can be found in AN607 along with further details.
What would be causing the sensor’s relative humidity readings to be much less than an accurate reference’s readings?
The most common source for a low RH reading is excess heat occurring near the sensor, such as PCB heat. The sensor and surrounding air must track the ambient temperature in order to accurately measure humidity. In general, every 1C of heat decreases the RH reading by 5%. A more detailed relationship between temperature changes and RH readings can be found in Appendix B of AN607. Please reference AN607 for design considerations such as sensor placement and PCB layout.
What would be causing the sensor’s relative humidity readings to be much greater than an accurate reference’s readings?
There are three common sources for a high RH reading. The first is the presence of conductive dust particles or liquid which can get lodged in the humidity sensing film. To protect against this, it is recommended to use the factory-installed ePTFE cover. The second and third sources of high RH readings is exposure to organic solvents and high temperature, high humidity environments. The sensor can recover from this through a rehydration cycle, please reference AN607 for details on the procedure.
What distance can I measure using the Wake on Prox feature of the TouchXpress CPT devices?
Wake on proximity is very dependent on the layout and the configuration of the capacitive sensing electrodes with regard to pad design, drive strength, etc. It is recommended to characterize different electrodes and dielectric materials to determine the correct distance for a reliable proximity measurement. An example for proximity is to use an antenna design with minimal ground planes in proximity to the electrode. Either route the sensor with NO ground surrounding it or anywhere nearby, or add a wire to the design that routes inside the product case and connects to the PCB+CPT at a single point.
The TouchXpress tools (Capacitive Sense Profiler and TouchXpress Configurator) provide the ability to monitor the capacitive sensing parameters to dynamically characterize electrodes in the end system.