30% of MPXM2202GS failed within 2 months after installation

[mikedrebby]

Hi All,

I am having too many sensors failing, its MPXM2202GS, about 30% within 2 months after installation.

They are used to measure water level with air column between water and the sensor, the sensors were installed upside down.

1) Using 5vdc as power supply

2)Using pulsing, so the sensor is only On for 10% of the operating time as recommended in the document AN3728

3)I used/tried to insert silicone oil by putting the sensors in vacuum to remove the air and replace it with silicone oil

4)Sensors were installed up-side-down as recommended in the document AN3728

5)There is an air column between sensors and water, so basically we followed the advice in "AN3728, Media Compatibility for IPS PRT Pressure Sensors"

6)Temperature is between 10 and 30 Celsius and all devices installed inside, so no harsh environment

7)The applied pressure is within the range of the sensor (0 to 200Kpa)

8)I noted that the silicone oil (very thick) got out of the sensor port when installing the sensor up side down. It took three or two days before all the silicone oil comes out

When I tested the failed sensors the resistance between Out+ and Out- where infinity while its about 2K2 ohms on a good/new sensor.

So what have we done wrong here?

Is it possible that putting the sensor in vacuum has affected the gel layer of the sensor?

Was it better if we did not try to put silicone oil at all?

any idea how to improve life of the sensor or solve the problem of the failing sensors?

Best regards,

Mike.

[Joshevelle]

Hello Mike,

Thanks for using our community.

What was the process you followed to add silicon oil to the sensor? did you exposed the entire sensor to vacuum or just the inlet pressure port?

After the silicon oil got out of the sensor, is there any chance that high humidity could reached the inlet pressure port?

It would help us to better understand the problem if you could share some pictures of your hardware.

Thanks in advance.

Josh

[mikedrebby]

Hi Josh,

The sensor package was put in a vacuum container during the process of inserting the silicone oil.

The device will be installed vertically , so there is guarantee always will be a trapped air in the pipe(s). The trapped air will eventually get very humid anyway. the distance between the sensor port and the water is between 3 to 12 cm depending on the applied pressure.

I have cut one of the devices to make it clear how it was installed.

I hope this will help a bit making clear how the sensor was installed inside the device.

Best regards,

Mike

[Joshevelle]

Hello Mike,

Thanks for the detailed information.

The document you followed (AN3728) has something I would change. Take a look at page two, figure 2 "Media Separated by a column of Clean Dry Air"

http://cache.freescale.com/files/sensors/doc/app_note/AN3728.pdf

The air column should go from the _BOTTOM_ of the tank, up to the pressure sensor, not at the same level of the water or higher as shown in the image. This air column inside the tube would be naturally created, just connect the sensor to the tube while the tank is empty (or before inserting the tube into the water), then when the water goes up, the level of the water will _INCREASE_ the pressure of the air inside the tube.

I would modify the image as follows:

Otherwise, if you insert the tube into the water before connecting the sensor, the water level inside the tube will be the same than tank level, hence, if the water level goes down, the pressure of the air inside the tube would become vacuum.

All of our pressure sensors are designed to be used under P1>P2 condition at any given moment. The MPXM2202GS pressure port #1 (P1) is the only pressure port available, however pressure port #2 (P2) is underneath the device (small hole), hence P2 is always exposed to atmospheric pressure. Therefore, while you apply pressure into P1, you're actually measuring the differential pressure between P1 and atmospheric pressure (gauge pressure type).

If the pressure inside of the tube, goes below atmospheric pressure, P2 would become greater than P1, which would permanently damage the sensor.

Would you please take a look at this and confirm that the "air column" is being created from the bottom of the tank up to the sensor and not from the water level up to the sensor?

Hope it helps.

Josh

[mikedrebby]

Hi Josh,

Thank you for taking so much effort in replying.

The note of p1 > p2 is noted. I dont think can happen in our configuration/device but its good to keep in mind though.

I can confirm that there is always "air column" is being created. There is no way otherway, and if there is a leak in air, then water will start leaking too after some time but we did not encounter any leakage with our devices.

My question is ... is it ok, or mostly ok, to use the sensor just like in the picture above (with air column) without adding extra silicone grease or silicone oil? I mean...Is the gel layer alone good enough if the sensor was used with pulsing duty cycle, air column, p1 >= p2, no harsh environment ? can we expect from the MPXM2202GS to survive for 2 to 3 years in these circumstances?

Thank you

[Joshevelle]

Mike,

In your specific application, the air column is very small, hence high humidity will reach the sensor and eventually the humidity will condensate into the pressure port of the sensor. I would highly recommend you to protect the devices with silicone grease _AND_  change the blue "flexible silicone tube" for a smaller diameter tube and make some loops with it, that way, the high humidity would condensate inside the loops and not in the sensor. Also the silicone grease would be trapped in the loops, protecting the sensor from the high humidity.

Hope it helps.

Josh

[mikedrebby]

Thank you Josh for your replies.

It seems that the sensors in our application suffer from large amount of drift, probably 5% or more with in weeks after first installation.

Do you have some data/documents regarding drifting?

Best regards

[Joshevelle]

Mike,

Are you using a calibration routine in your application?

We do not have a specific document for what you're looking, but we do have a document to auto calibrate our pressure sensors. The software technique name's "Auto Zero" and you can find all the information in the following document:

http://cache.freescale.com/files/sensors/doc/app_note/AN1636.pdf

-Josh

[mikedrebby]

Hi Josh,

I am aware of that document. unfortunately the job of the device is to prevent the water pressure to become zero(that is when the tank and pipe are both empty).

So if the pressure is zero, then  the device has failed to do its job. I was thinking if the drift not so much, then one might be able to ignore it but if its much then there is no way around.

Mike

[Joshevelle]

Mike,

You could use the autozero function just before plugging the pressure sensor to the pipe (as a calibration routine), so you can eliminate the offset of that specific sensor. Remember that the offset is different from sensor to sensor even if they share the same part number.

I did a test of a similar pressure sensor (MPX5700), which I connected to a 100PSI tank, directly exposed to car coolant at 25°C for over a month and I didn't have any problems related to the offset of the sensor.

-Josh

[mikedrebby]

tanks Josh,

The initial offset of any given sensor is not an issue for out application. What I am afraid of is a constant drifting over time.

Mike