Experience with a humidity sensor?

You may benefit from potting the components in epoxy wherever possible.

Do you have any pressure/temperature sensors in mind?

I also need to have the air pressure sensor exposed and that has no filter or protection.

You could house the air pressure sensor inside a sealed pressurised container with a 'bladder'.

Any changes in the air density will exert force onto the bladder and raise the pressure inside which the sensor can measure.

It should be relatively easy to calibrate, and keeps it from the toxic environment.

For the electronics, I suggest housing them inside a rubber or latex balloon (used for RC What Kind of Balloons Do You Use to Protect RC Electronics?) or similar then inside a sealed container, with only the connections external.

If possible apply positive pressure into the sealed container, avoid using double insulated wire at the external junction, as the air will leak down the outer sheath.

I'm afraid your humidity sensor needs to be exposed, so pick something designed for chemical environments, or throw it away after 30 days.

Let us know what you designed and how it copes.

Mark

Hi,

'Conformal Coating' (best and costly, but the cost is going down somewhat) or try encapsulating (soft potting) insides of jiffy box or specific components, depending on your budget. Keep in mind the addition of capacitance/impedance, so recalibration maybe necessary after post potting/coating of board (check the material's permittivity, E). Pretest and mask whatever you do not want to be protected (i.e. adjustable pot screws, mounting holes, etc) with conformal coating or potting before sending it out. If you are doing it yourself, very messy and wear a good breathing mask.

When I was in HS, they made humidity sensors by attaching a strain gauge to a curly-headed person's hair.

Silicone RTV can be your friend.  SiO2 being pretty stable stuff.  I don't think a modern gauge has to be too sensitive to the environment you describe.  IIRC they are made of a porous membrane of chloro-fluronated polymer and gold electrodes to make it into a capacitor.

Another way to measure humidity is to put a conductivity gauge on the cold side of a peltier cooler in order to determine dew point.

"Do you have any pressure/temperature sensors in mind?"

Not yet.  My prototype is using a 10 DOF (GY-87) only because I had it lying around.  I'll probably end up buying a dedicated pressure sensor.  For temperature I'm currently using thermistors.

Mark,

I was considering placing the board & batteries in zip-lock sandwich bags, but had never thought of balloons.  Interesting idea which I'll have to explore; which material survives the environment - plastic or rubber?  I know cable ties "corrode" in the environment, becoming brittle and eventually breaking.

Positive pressure would be a good idea, but pressurizing the box for a month adds many complications.  The unit will be inserted down a man hole and placed on the sewer wall.  Because of the threat of vandalism/ theft/ the public, the unit can't have any components above the man hole.  Ideally I'd have a unit on the surface receiving an RF signal from the sensors and a solar panel powering it.

Ernesto hufana,

Thanks for that idea.  I had considered that option, but didn't know the technical name for doing it.  The only problem I see is that the sensor's surface is protected, but the inside of the sensor isn't.  This may lead to it corroding from the inside out .  The other worry is that the epoxy may cover the tiny pin hole sensor and stop it working.

Don,

I was considering using lots of HOT GLUE in it's various chemical variants.  I think APE may be the winning chemical, but the glue sticks never say what they are made from , so it could be trial and error!

Andrew

which material survives the environment

I'm picking the latex.

The zip lock bags are fine but I have found they can sometimes have issues with the zip part.

You also need to get the wires out, which means the seal is difficult, hence the rubber balloon.

Silicon and hot glue are fine as long as they remain bonded to the wire/box.

If there is any movement the seal is broken, so it's important to consider that when using it.

You can purchase electrical boxes that are IP67 rated and are cheap enough.

They should be able to be pressurised if you pick the right one.

Some are chemical resistant as well.

Positive pressure is just a valve on the side, but ensuring the container is sealed is key.

Electrical compression glands are available, and are worth looking at.

Mark