MPX5700 sensor, water compatibility

[jrgranola]

Hi

I'm intending to use the MPX5700 sensor to sense cold water pressure (50-75F). The data sheet says the sensor has only been tested for dry air, but has anyone used the sensor in a water sensing application longer than 1 year?

[david_diaz]

Hello John,

Indeed, Freescale's standard pressure sensor products are qualified to be used at dry air, only.

For the use of pressure sensors in any other environments than dry air will require additional precautions and protection of the sensing element inside the pressure sensor:

Usually in a water measurement application, a head tube is used. This is simply a piece of tubing between the sensor and the container in which the water level or pressure needs to be measured. Air is trapped in the tube and this isolates the sensor from the liquid water.

I would like refer to the following application note that would be worth to be considered in addition to the provided information.

Media Compatibility for IPS PRT Pressure Sensors

Besides, your will find a couple of application notes that give some ideas for using the devices in liquid environments or you can download the Application Notes from the following links:

1 - Liquid level control using a Pressure Sensor

2 - Water level monitoring

I hope this information will be useful for you.

If I misunderstood your question, feel free to let me know.  I will be glad to help.

Have a great day.

David Diaz.

[jrgranola]

Thank you David

I read the application sheets and they where helpful.

Even when using the air tube between the sensor and water measured, it seems that the air trapped in the tube would eventually achieve near 100% relative humidity if exposed to water constantly, and condensation in the sensor may occur, which conflicts with the sensor being only compatible with "dry air".

Does the length of tube with trapped air have any bearing on pressure measured? Or can it be assumed that a sensor attached to a tube of air of infinite length will accurately measure the pressure of the water?

Thanks David!

[david_diaz]

Hello John,

I am glad to know that the documentation provided was useful for you.

Indeed, some vaporization of the media (in this case water) will occur in the air column, but for most applications Media Resistant gel will provide adequate protection here.

The length of a tube, may become an issue if you would need to measure differential pressure when the application calls for measuring the difference between two pressure points. One side of the sensor is hooked up to one pressure point, and the other side is hooked up to a different pressure point.

Please let me know if this information answer your questions.

If I misunderstood your question, feel free to let me know.  I will be glad to help.

Have a great day.

David

[jrgranola]

Hi David

Do the MPX5700 or other compatible freescale sensors come with Media Resistan Gel?

[david_diaz · Accepted solution]

Hello John,

In this case, the MPX5700 contains fluorosilicone gel which protects the die from harsh media.

The Media resistance Gel material is specified in the MCD (Material Composition Declaration) of the MPX5700. Please find more information at:

Material Composition Declaration - MPX5700

I hope this information will be useful for you.

If I misunderstood your question, feel free to let me know.  I will be glad to help.

Have a great day.

David