Does high concentration helium degrade absolute pressure sensor performance? We use MPXA4115AC6U. 3 units returned from the field use were way out of calibration (reading low). Voltage supplies are fine. Customer then told us they use almost pure helium. Could this be the cause of low readings (by helium leaking into the vacuum side of the pressure sensor)?
Hi Andrew Lintz,
Yes, it is probably that the helium has leaked into the chamber since Helium molecules are smaller than other air molecules.
Leakage is an unspecified and therefore not guaranteed performance characteristic of our devices. However, Freescale does perform a 100% leakage test on all ported devices after assembly, with a maximum allowable leakage rate of 0.133kPa/sec. This maximum rate can be considered "worst case" (prior to any handling/processing by the customer which may induce package stresses which affect port-to-package sealing) and the typical production distribution for leakage rate is much better.
Now, it’s important to mention that all the Freescale Pressure sensors are designed to work in a clean dry air so we do not recommend direct contact with corrosive gases and liquid substances since this could damage the protective gel and this could lead to out of spec device we can only guarantee direct contact with "Clean Dry Air".
However, I recommend you to take a look of the Application Note AN3728 about the Media Compatibility for the pressure sensors. URL: http://cache.freescale.com/files/sensors/doc/app_note/AN3728.pdf
The gel coat protects the die and wire bonds that are encapsulated. When a corrosive (or a different media than “Clean Dry Air”) is introduced to the sensor that is not compatible, it has the potential to dissolve, or be absorbed by the gel and corrupt the die surface or lead to wire-bond breakage. Some types of media will take longer to corrupt the die than others, so a disposable application is possible. Unfortunately, applications using Helium has not been tested, so we recommend you test and validate your application with our sensor before the final application.
Our media compatibility disclaimer suggests that our customers are responsible for determining media compatibility in other environments. Customers are urged to conduct their own in-house evaluations and characterizations to determine if our devices are suitable for use in their non-clean dry air environment.
Media Compatibility Disclaimer:
"Freescale has tested media tolerant sensor devices in selected solutions or environments and test results are based on particular conditions and procedures selected by Freescale. Customers are advised that the results may vary for actual service conditions. Customers are cautioned that they are responsible to determine the media compatibility of sensor devices in their applications and the foreseeable use and misuses of their applications."