Need to check the central heating medium pressure drop
We do not recommend all our sensors to become in direct contact with any kind of liquid substance or corrosive gases which would damage the gel and cause the sensor to become out of specification.
The strain gauge and the electronic circuitry for calibration and compensation are protected by a nitride layer but the aluminum bonding pads which provide electrical connections between the leadframe and the gauge are not protected, in order to make the bonding feasible. The complete die is also protected with a silicone gel. This gel is not fully hermetic, although we use much better gel for our newer types of pressure sensors, water or any other fluid can penetrate the gel and can reach the die. When the sensor die is in contact with water e.g., oxydoreduction reactions between Al/Al3+ and water start as soon as the sensor is biased. After some working hours, or maybe days, the aluminum pad of the supply pin is definitely destroyed, so that an open circuit on the Vcc pin occurs. This is an aluminum corrosion phenomena. But the corrosion phenomena is stopped when the sensor supply voltage is switched off. There are also some other failure causes like galvanic corrosion, but the Mean Time To Failure (MTTF) due to these other causes much longer than the MTTF caused by electrocorrosion of the Vcc pad. Therefore electrocorrosion is the major failure cause, and they are permanent.
However, as you may know, our products are used in washing machine applications, which would expose the gel to a humid environment. In these applications, the device is located at the top of a measurement tube - so water level is indirectly measured through the water pushing a trapped air column up to the sensor. As you can see in the following application notes, the sensor does not have a direct contact with water, but is separated by the air column.
AN1950: Water Level Monitoring
AN1516: Liquid Level Control Using a Pressure Sensor
We have also an application note about media compatibility.
AN3728: Media Compatibility for IPS PRT Pressure Sensors
It seems that the indirect measuring is the best choice for water-level measuring as shown in Fig 2 of AN3728. But the air trapped in the pipe will expand when increasing the temperature (and vice versa) and consequently the applying pressure on the diaphragm will depend on the temperature. The measuring system will depend on the temperature although the MEMS pressure sensor is temperature compensation.
Could you please give me your comments about this? Any suggestion to overcome this problem? Thank you very much.
MEMSiTech, Saigon Silicon City, Vietnam
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