Humidity sensitivity and how to keep humidity low

[jmaja]

I'm using a MEMS sensor, which is according to manufacturer sensitive to humidity so that over 50%RH may affect accuracy. I'm using an IP67 enclosure, which may be enough (is it?) for keeping constant absolute humidity inside, but how can I get <50%RH in cold temperatures?

E.g. using this: http://www.humidity-calculator.com/index.php you can calculate that 50%RH at -20C is 512 PPMv, which is 2%RH at 20C or 0.2%RH at 70C.

How can I get such a low humidity say at 20C when sealing the enclosure? It also may be necessary to open the enclosure at site, which can be at any outdoor temperature/humidity. Is there an easy way to ensure low humidity while closing the enclosure at site?

Are MEMS sensors really sensitive to RH even at low temperatures or are they actually sensitive to absolute humidity?

The obvious solution to heat up the enclosure is not an option, since it is attached to heavy metal object and needs to be low power.

[dfmischler]

Dessicant?

[kxenos]

Why don't you enclose your sensor in it's own small sealed enclosure that will not open on site? Like a OCXO case or something like that?

[jmaja]

Silica gel looks like a good option. It was quite hard to find any data for it, except for museum use for keeping constant ~50% RH, but what I have found it seems to have good properties for keeping RH below 50% at wide temperature range.

I have seen silica gel bags in many places and they are also recommended for many application. However, I haven't found much for using them inside electronics enclosures. Is there a good standard practice for this? Are these small bags (e.g. sorb-it: http://www.agmcontainer.com/desiccantcity/desiccant_sorb-it.htm) used? How are they attached inside the enclosure for not flying around during handling/vibrations and still doing their job and being easily replaceable? How are they (or should be) packed so that they don't absorb moisture out of air before installation? If installation is done in humid environment, how long is the window for opening up the package to sealing the enclosure?

[Thor-Arne]

These might be interesting since they are panel mount and have a indicator showing the humidity.

RS stock those just search for "Desiccator".

[Nerull]

I've worked with scientific astrophotography cameras which have a desiccant plug - based on silica gel I believe - which threads into the sealed portion of the camera body containing the CCD chip. The sensor is cooled via a pelter to reduce thermal noise and must be protected from moisture to prevent frosting. The plug can be removed and baked in an oven for a couple hours to reactivate if it starts to lose effectiveness.

[jmaja]

Those panel mount desiccators are interesting, but they are quite big and a bit expensive. Also you need to make a big hole in the enclosure (adding leak?) and still you need to open the enclosure for replacing the desiccator. Also I don't like to spoil the good EMC properties of the aluminum enclosure with a big plastic item.

Now I also found desiccator sticks, which enter through a M5 hole, are stainless steel and can be replaced without opening the enclosure. Haven't found any seller/price yet.