BME280 temperature offset 4 Degree after 3 months

[RainerS]

Hi,

Problem Statement:
I am using in total 6 BME280 sensors In my smart Home setup.

Initially the sensors worked fine with accurate temperature and Humidity readings.
However after ~3 Months one of the sensors suddenly shows a much too high temperature. The temperature is about 4 Degrees higher than the other sensors (e.g. 28 degree instead of real temperature of 24 degree).
This also causes invalid/too low humidity readouts (as temperature is used to calculate relative humitidy)
The other 5 sensors work fine with the same hardware setup and software.

Technical Details:
The affected sensor is positioned in the Bathroom (which will sense a humidity change from 30% to 85% to 30% each day during showering) and controls the Fan to blow humid air out of the room.
I sample sensor values every 5 seconds.
Regarding PCB layout, I am using the Breakout boards with integrated 5V regulator and level shifter commonly found on Amazon or Ebay.
The sensor is mounted Head-down (metal casing pointing towards the floor) inside of a Plastic casing with slots to allow free airflow.
I am using the Bosch BME driver software v3.5.0 on AVR controller.

This is how I initialize the device (Weather monitoring mode):

....
struct bme280_dev m_dev;
m_dev.intf_ptr = &m_i2c;
m_dev.intf = BME280_I2C_INTF;
m_dev.read = IicCWrapper::smRead_c_wrapper;
m_dev.write = IicCWrapper::smWrite_c_wrapper;
m_dev.delay_us = Bme280::delay_us;
checkResult(bme280_init(&m_dev));

/* Recommended mode of operation: Weather monitoring */
m_dev.settings.osr_h = BME280_OVERSAMPLING_1X;
m_dev.settings.osr_p = BME280_OVERSAMPLING_1X;
m_dev.settings.osr_t = BME280_OVERSAMPLING_1X;
m_dev.settings.filter = BME280_FILTER_COEFF_OFF;
m_dev.settings.standby_time = BME280_STANDBY_TIME_1000_MS;

uint8_t settings_sel = BME280_OSR_PRESS_SEL;
settings_sel |= BME280_OSR_TEMP_SEL;
settings_sel |= BME280_OSR_HUM_SEL;
settings_sel |= BME280_STANDBY_SEL;
settings_sel |= BME280_FILTER_SEL;
...

 
Questions:
It shis a known issue?
Did I do something wrong, e.g. use the sensor outside of its spec?
Does the sensors intended use cover usage inside a Bathroom to control the room ventilation?
Might this be related to the affected sensor being exposed to numerous humidity changes as ist is situated in the bathroom?
Is this a defective sensor?
If I replace the sensor, will it show the same problem after 3 month?

Thanks for looking into this 🙂

[RainerS]

Short update:
I made some pictures, on how I physically positioned the Sensor in the Casing and on the wall.

I also took the opportunity when I had the case disassembled, to swap the Sensor with a new one.
After the swap the new sensor as expected reads the correct temperature again (as did the old sensor for 3 months).

From this I conclude the issue is not from any ambient heat source, but sensor degradation over time.
Now let's see if the new sensor also degrades over time.

I would be happy on any feedback on this. Is the way I mounted the sensor  OK for a humid Bathroom environment (I let the sensor face downwards to avoid any condensation forming on and dripping into the sensor)?

[BSTRobin]

Hi RainerS,

According your feedback, we would like to get more information from you:
1. Where are the other five BME280 installed? And what about the surrounding environment?
2. The BME280 in the bathroom has a temperature drift of 4 °. Is the humidity value normal at that time?
3. Is the temperature and humidity normal when the BME280 in the bathroom is taken out of the bathroom?

[RainerS]

Hi, thanks for the reply.

Regarding question 1:

• All of the sensors are installed indoors.
• Three of those in directly adjacent rooms to the bathroom.
• I usually have all doors completely open to the adjacent rooms (except when showering) which provides free air circulation.
• All sensors are at same height of 1.4m mounted in a slotted casing 1cm from the wall (internal, wall, not perimeter wall). Casing and mounting method see photos in previous post.
• I checked the correct temperature / offset as follows (All methods showed 3.5....4.5 Degree offset to the affected sensor, affected sensor always reading a too high temperature):
  • Using an IR thermometer measuring the surface temperature of the casing
  • Using an analog thermometer
  • Replacing the BME280 module with a brand new one (which then showed the correct temperature)
  • Comparing to Sensor readouts of the other three BME sensors in adjacent rooms with free air circulation

Regarding question 2:

• The Humidity value also reads a way too low value
• If I remember correctly (I did not make protocol for humidity): When showering: I read about 65% RH and 35 Degree Celsius, When in reality it should have been >80% RH and 30 Degree Celsius

Regarding question 3:

• A week ago, I replaced the affected sensor module in the bathroom with a new one. This now shows the correct temperature (4 degrees lower) again (as did the affected sensor for ~3 month).
• I just now swapped the sensor module in the living room with the affected sensor module (Which I stored in a plastic box for 1 week now), to see if outside of bathroom environment the temperature offset is still there:
  • Original living room sensor: 22.5 Degree Celsius, 62% RH
  • Living room sensor with BME280 module from bathroom (after 3 minutes wait time to acclimatize): 25.4 Degree Celsius, 50% RH

I will still keep the affected sensor module. If you are interested, I can send it to you.

[BSTRobin]

Hi RainerS,

Thank for your feedback.

Could we know which region you located?