Replace an electrolytic humidity sensor from a dehumidifier

[Frank_38]

Hi all,

I've got a DeLonghi DDSX220 dehumidifier that I've bought a few years ago. It works quite well except for one thing : the humidity sensor. Overtime the humidity measurement become less and less reliable until a point where the dehumidifier refuses to operate (humidity too low, <30% RH). I've already changed it twice. It's not that this is a complex task, it's just that the damn board, that is the size of my thumb nail with two components on it, costs >20€ on eBay. And of course today I've got the same problem. Again. And I've got to say that I'm kind of pissed off to buy another 20€ sensor board that I know for sure will turn bad in a year or so.

The sensor is an electrolytic humidity sensor that looks like this one. It is operated using AC voltage and you measure the impedance to get the relative humidity (based on the temperature as well, there's what looks like a glass thermistor close to the humidity sensor).

Now I want to find a reliable solution but I'm not quite sure what is possible. I'd still want to have a real humidity measurement but what could be an alternative ? Could it be "simulated" with something else (like a board with another more standard humidity sensor, a mcu, etc.) ? Or could it be easily and safely replaced (i.e. without frying the main circuit board) with a potentiometer ?

I could also just replace the humidity sensor itself on the board but I'm not sure if the one that I found on bb-sensors.com has the same characteristics than the original one. And this kind of component seems to be really sensitive to heating so I might fry it when trying to solder it. To be fair I'm not fond of this solution.

I'm open to all suggestions and remarks. Thanks in advance for your help.

[TheMG]

Perhaps the sensor is just getting contaminated with something? Is it located directly in the air stream of the dehumidifier or just sits in still air?

Any chemicals or anything else used or stored near this dehumidifier that might be getting into the air and contaminating the sensor?

[Frank_38]

Perhaps the sensor is just getting contaminated with something? Is it located directly in the air stream of the dehumidifier or just sits in still air?

Any chemicals or anything else used or stored near this dehumidifier that might be getting into the air and contaminating the sensor?

The dehumidifier is used in my bathroom and in my laundry room (because it has a "laundry" function where it blows warm air). The sensor is located on the right side of the dehumidifier and is not directly exposed. It is also not in the way of the air stream.

I have two hypothesis. The first one is that it is a bad conception (maybe the sensor is not driven correctly). The second one is that this specific kind of humidity sensor goes bad after a while. I even unplug the dehumidifier from the main when I'm not using it just to be sure everything is off and not in some kind of sleep mode where the humidity sensor is still powered on.

[fzabkar]

Does the dehumidifier report the relative humidity and temperature? If so, then you could replace the RH sensor with a potentiometer and determine the expected RH-versus-resistance characteristic.

[Frank_38]

The dehumidifier only shows the RH, not the temperature. But it uses the temperature to compute the correct RH. Here's what I found in the datasheet of the electrolytic humidity sensor that I've found on bb-sensors.com :



I'll try to post a picture of the board with the sensor and the thermistor.