BME688 in shower room - 100% humidity, condensing

[Jon1]

Hi,

I'd like to use a gas sensor in a bathroom with a shower, where it will be exposed to 100% humidity and condensation.  I'm considering the BME688.

Can the BME688 survive exposure to those conditions?  Will it give me good readings afterwards?  What do I need to do to make that happen?

From the datasheet, I've understood that:

1. The BME688 can survive condensation if no power is applied. (Source: Datasheet Table 11)
2. The humidity readings are valid up to 100% humidity, non-condensing. (Source: Datasheet Table 8 )
3. The gas sensor is "electrically operable" up to 95% humidity, non-condensing.  (Source: Datasheet Table 2)
4. The temperature readings don't have any humidity limits. (Source: Datasheet Table 10)

So I don't expect the BME688 to give me VOC/gas readings at >95% humidity, or while it has condensation in it.  That's fine.

Can I run the BME688 even if there is condensation?  Or do I need to turn the power off with an external MOSFET?

If water does condense inside the sensor, how do I get that water out and return the sensor to normal operation?  How do I automatically detect that there's water in the sensor, so my system can automatically return to normal operation?

Is there a preferred orientation for the sensor?  E.g. should the PCB be horizontal, with the BME688 on the bottom, so condensed water can drip out of the hole in the BME688 ?

(Obviously I will need to coat my PCB, and the coating will need to seal to the metal can of the sensor, and I'll need to leave the sense hole open.  I'm not worried about that, I'm worried about what's inside the BME688).

I believe I could stay strictly within the datasheet limits by:

• putting a small external heater under the BME688 (possibly just a PCB trace heater, or perhaps a PTC heat pad on the opposite side of the PCB)
• having a separate room temperature and humidity sensor that are rated for 100% humidity, condensing.
• when I detect conditions approaching condensing (e.g. >80% humidity reported by the BME688), use the external heater to heat the BME688 to 5-10 degrees C above room air temperature.  This should prevent condensation on the BME688.  (Although this may not work in the case where the room's humidity rises very quickly, if the heater cannot heat the BME688 quickly enough - see next bullet point for how that could be handled).  And obviously, heating the BME688 makes the BME688's built-in temperature and humidity sensors useless for measuring the room, hence the need for a separate room temperature and humidity sensor to use during this time.  But the BME688 gas sensor could presumably still be used.  When the separate humidity sensor reports the room humidity has dropped, the external heater can be switched off again, and the BME688 can return to room temperature.
• if I detect condensing conditions (e.g. >90% humidity reported by the BME688), turn off the BME688, use the external heater to heat the BME688 to perhaps 45C to discourage further condensation, wait for the room's humidity to drop, then wait for an hour to allow any condensation inside the BME688 to evaporate, then turn the external heater off and turn the BME688 back on.  Then wait for the BME688 to return to room temperature.

Would that work?

However, that seems massively complicated, massively inconvenient, and is probably massively overkill.  Surely there must be a much better, simpler way?  "I want to sense bad smells in a bathroom" seems like an obvious application of this sensor.  I can't be the first person to want to do this?

[BSTRobin · Accepted solution]

Hi Jon1,

Please refer to the following reply from us.

1, Can the BME688 survive exposure to those conditions?  Will it give me good readings afterwards? 

• The sensor passed the THB(temperature humidity bias) test which followed JESD22 spec
• Based on the test the sensor will be ok at the environment 85℃/85% rH with bias

2, What do I need to do to make that happen?

• Suggest to add waterproof film on top of the sensor

From the datasheet, I've understood that:

1. The BME688 can survive condensation if no power is applied. (Source: Datasheet Table 11)
2. The humidity readings are valid up to 100% humidity, non-condensing. (Source: Datasheet Table 8 )
3. The gas sensor is "electrically operable" up to 95% humidity, non-condensing.  (Source: Datasheet Table 2)
4. The temperature readings don't have any humidity limits. (Source: Datasheet Table 10)

So I don't expect the BME688 to give me VOC/gas readings at >95% humidity, or while it has condensation in it.  That's fine.

3, Can I run the BME688 even if there is condensation?  Or do I need to turn the power off with an external MOSFET?

• As the mention from datasheet, doesn't suggest to run the sensor at condensation status

4, If water does condense inside the sensor, how do I get that water out and return the sensor to normal operation?  How do I automatically detect that there's water in the sensor, so my system can automatically return to normal operation?

• When the humidity output come back to normal, the condensation on the top of the sensor will dismiss

5, Is there a preferred orientation for the sensor?  E.g. should the PCB be horizontal, with the BME688 on the bottom, so condensed water can drip out of the hole in the BME688 ?

• Customer can follow the document https://www.bosch-sensortec.com/media/boschsensortec/downloads/handling_soldering_mounting_instructi...
• At same time, suggest to use waterproof film

6, (Obviously I will need to coat my PCB, and the coating will need to seal to the metal can of the sensor, and I'll need to leave the sense hole open.  I'm not worried about that, I'm worried about what's inside the BME688).

• suggest cover the hole when coating material on the sensor

I believe I could stay strictly within the datasheet limits by:

• putting a small external heater under the BME688 (possibly just a PCB trace heater, or perhaps a PTC heat pad on the opposite side of the PCB)
• having a separate room temperature and humidity sensor that are rated for 100% humidity, condensing.
• when I detect conditions approaching condensing (e.g. >80% humidity reported by the BME688), use the external heater to heat the BME688 to 5-10 degrees C above room air temperature.  This should prevent condensation on the BME688.  (Although this may not work in the case where the room's humidity rises very quickly, if the heater cannot heat the BME688 quickly enough - see next bullet point for how that could be handled).  And obviously, heating the BME688 makes the BME688's built-in temperature and humidity sensors useless for measuring the room, hence the need for a separate room temperature and humidity sensor to use during this time.  But the BME688 gas sensor could presumably still be used.  When the separate humidity sensor reports the room humidity has dropped, the external heater can be switched off again, and the BME688 can return to room temperature.
• if I detect condensing conditions (e.g. >90% humidity reported by the BME688), turn off the BME688, use the external heater to heat the BME688 to perhaps 45C to discourage further condensation, wait for the room's humidity to drop, then wait for an hour to allow any condensation inside the BME688 to evaporate, then turn the external heater off and turn the BME688 back on.  Then wait for the BME688 to return to room temperature.

Would that work?

• Doesn't suggest put the external heater under the sensor,   https://www.bosch-sensortec.com/media/boschsensortec/downloads/handling_soldering_mounting_instructi...

However, that seems massively complicated, massively inconvenient, and is probably massively overkill.  Surely there must be a much better, simpler way?  "I want to sense bad smells in a bathroom" seems like an obvious application of this sensor.  I can't be the first person to want to do this?

• Suggest to add waterproof film on top of the sensor