Fan humidity sensor type

Usually capacitance change. For example, see here:

It might be worth cleaning the sensor, perhaps with a little IPA. Easy to forget to turn off the power first!

Another example of a MEMS type device

Probably not, unless the air is blocked from entering the device

My 15 year old dehumidifier uses a synthetic equivalent, which is looped many times to fit a long length into a small-ish switch. Looks like a length of sellotape without sticky on it.

However, nowadays, an electronic device will be used. They measure the conductance of some organic material which absorbs moisture. One really important thing is there must never be any DC voltage across the sensor, as it will electrolise the sense electrodes and destroy it. Even a test meter on a resistance scale for a second will wreck the accuracy.

You could make sure it's not blocked with dust. They can be damaged by the use of volatiles nearby (aftersave, airosols?). Some of the high precision ones I use can be baked in a low oven to reset them if this happens, or if they got wet (although they will recover by themselves in time from water).

e.g.

Andrew Gabriel used his keyboard to write :

No obvious signs of dust, that I noticed and it is 7 feet up from the floor, so unlikely to be volatiles. I will pull its PCB out and take a proper look at it on the bench. Will the IPA mentioned by Jeff, be OK for giving it a clean?

The electronics in some fans only respond to a rapid change in humidity such as happens when you take a shower or bath and not to a slow change in humidity as would occur naturally with a weather change. This is meant prevent the fan triggering for long periods on a humid day.

In this case I'm not sure how setting the lower humidity would work:

A rapid change from 40% to a threshold level turns it on and it doesn't go back off until 40% is registered again (??????)

A rapid change from a RH below 40% to 40% turns the fan on (?????)

The timer turns it off (?????)

Perhaps its seaweed :-) Brian

Relative - it cannot be minus can it?

Brian Gaff submitted this idea :

No, there was no seaweed in it, just a surprising amount of dust. Its a two terminal device, with a small proportion of the air being drawn past it. I have washed it down with IPA, reinstalled it and it seems to run continuously now - probably the sensor needs to dry out, before normal operation resumes.

Just fitted a Vent Axia Lo Carbon Trickle fan in Laundry/Shower Rm. Has Humidity sensor to switch to higher extract rate. The adjustment available, which is fiddly, results in either "off" or "on" all the time , I want a switch point somewhere between 55% and 70% RH ! Not impressed.