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LMT70: Sensor gets faulty in field post 2-3 months of deployment.

Bhumika Jain
Prodigy 20 points
Part Number: LMT70

I have integrated LMT70 into my battery-powered node. My node is powered through 2 AAA batteries.

These temperature sensor nodes are deployed in the field and after running for 2-3 months, I get incorrect readings from my sensor. By incorrect reading, I mean any random value. High random values for temperature like 122, 121, -139, etc in degree Celcius.

I have attached my circuit diagram for LMT 70 sensor in my battery-powered node.

In the above circuit, LMT70 is placed on a small PCB having J8 and J9 connectors. A 3-meter wire is drawn from the J8-J9 connector and gets attached to my main node on the J7 connector where my SoC is present.

Kindly suggest something which can resolve this field issue where my sensor turns faulty post 2-3 months in the field. 

  • 0
    TI__Genius 10070 points

    Hi Bhumika,

    Thank you for posting to the Sensing forum.

    Can you please provide the voltage at the TAO pin when T_ON is high? Additionally, are there any signs of condensation or other possible damages to the system? If you can provide further information on the field environment that may also be helpful in debugging further.

    Nicole

  • 0 in reply to Nicole Khoury
    Prodigy 20 points

    Hello Nicole,

    I have to measure TAO voltage, give me some time for it.

    As far as the field environment is concerned, we are putting these sensors across Freezers, cold rooms, chillers, makelines, and normal ambient environments. 

    In some sensors, we can see small white deposits around it, but that is not the case across all faulty sensors.

  • 0 in reply to Nicole Khoury
    Prodigy 20 points

    Hi Nicole,

    Following is some additional information with respect to our application.

    1.  We have a separate PCB for the sensor which goes inside a metallic probe and a wire comes out of that probe. The probe is then closed with a heat shrink (it is not a waterproof seal). Refer attached image:

    With reference to the above setup of sensors, kindly guide me if it is the right way to package the sensor. Also, is there anything we can do to improve our packaging of sensor, any filling inside the probe to make it waterproof so that we can monitor temperature easily inside refrigeration units?

    2. Most of these sensors are failing in refrigeration units.

    Also, I have the following additional queries:

    1. Do we need this capacitor c17 in our circuit or can it be skipped?

    2. Should we power the sensor via GPIO rather than power it via battery voltage?

  • 0 in reply to Bhumika Jain
    TI__Genius 10070 points

    Hi Bhumika,

    Thank you for providing these details.

    Since most of the failing sensors are in refrigeration units, I would expect that the issue is with moisture entering the probe. Is it possible that the small white deposits you mentioned are frost formations? If so, this would indicate that the moisture levels may allow for condensation to form on the sensors. For your application I would recommend waterproofing the probes by sealing them with an epoxy.

    To address your additional questions:

    Bhumika Jain said:
    1. Do we need this capacitor c17 in our circuit or can it be skipped?

    A bypass capacitor may be required if the supply line is noisy, but this is optional. For noisy environments, we recommend a 100 nF capacitor placed across the VDD and GND pins of the device.

    Bhumika Jain said:
    2. Should we power the sensor via GPIO rather than power it via battery voltage?

    There shouldn’t be a problem with the way you are powering the LMT70, and the current schematic does not seem to have any issues.

    Best regards,

    Nicole

  • 0 in reply to Nicole Khoury
    Prodigy 20 points

    Hi Nicole,

    Thank you for your response.

    I have one more observation in my sensor application.

    This observation is that some sensors give out of range values for some time and after that, they start giving correct values again. This is on cyclic manner.

    Is it acceptable behavior, that sensors try to heal themselves repeatedly, or once the sensor has got bad due to moisture or humidity it won't work fine?

    Regards,

    Bhumika

  • 0 in reply to Bhumika Jain
    TI__Guru* 91146 points

    Bhumika - 

    Your issue sounds like corrosion, due to flux contamination and is not uncommon if the board was not cleaned completely before the probe was assembled and if the connections on the board are not sufficiently protected from moisture. 

    If you have or can share additional images which are closer to the parts on the board we may be able to help discern if this indeed might be the case.