humidity

I have a de humidifier which is costing rather a lot to run.

I have another in my shed that works perfectly with a heater which keeps to about 8 deg c.
The heat comes from the dehumidifier itself which is oversized deliberately, shed is well insulated.

Expensive one is in a conservatory with large glass and has a stat but never shuts off in cold weather.
It keeps the air dry as expected but will not keep dry enough to dry out existing condensation from the glass.

In the conservatory i would like to save money, i expect the humistat is a cheap sipmle one .

I suspect i do not understand enough about humidity and think i need to keep the conservatory heated to a degree.

Internal to conservatory door house is opened regularly
Never had the problem until winter came.

I'm hoping an arduino could make a better job of this than the onboard (simple ? sensor ).

Any ideas please ?

I wonder if a wet/dry bulb using a couple of thermistors and a fan would give a better result than a bit of wet plastic string although i may be fighting basic physcs here

If the glass is getting sufficiently cold so that the temperature of the glass falls below the dew point of the air in the room, the glass will fog up.
There are only 3 solutions and thats heat the glass or heat the air or dry the air.
An Air conditioner in dehumidify mode would help to dry the air .

the dehumidifier works but runs continuously.
The stuff on the glass can be wiped off.
only a problem when it gets really cold,the glass is double glazed.
Air conditioner too expensive option i only wish to prevent condensation.

If you're looking for a humidity sensor, Bosch make some great ones. The BME280 is kind of top-of-the-line and relatively expensive but I reckon it's amazingly good for humidity, temperature and pressure.

Sparkfun BME280 breakout

If you're only going to buy one, you might as well make it a good one.

Seriously, the datasheet says it's good enough to use for indoor navigation, by remembering the humidity of each room and detecting the pressure change of opening and closing doors.

The parameter you need to calculate is "dew point", which is the temperature when dew starts to form. It's one way of measuring absolute humidity, instead of relative humidity. If you can keep the dew point of the room below the temperature of the inside of the glass, then condensation won't form on the glass.

i am not sure what i am needing.
According to the dehumidifier specs it should turn off at some undefined point.

I do not think it does as it runs continuously. and costs lots to run.

It could have a faulty sensor but cannot say. I have an independent humidity stick from maplins but am not really sure from the manual what it means.
It says RH but not convinced as i cannot see a temp sensor.

I think I have seen advertised some sort of liquids that you can spray or rub on glass making them much more resistant to condensation.

MorganS:
The parameter you need to calculate is "dew point", which is the temperature when dew starts to form. It's one way of measuring absolute humidity, instead of relative humidity. If you can keep the dew point of the room below the temperature of the inside of the glass, then condensation won't form on the glass.

Dew point should do for me i think, i am not sure how the sensor on my dehumidifier works however.
I guess its designed for room temp use.

jrdoner:
I think I have seen advertised some sort of liquids that you can spray or rub on glass making them much more resistant to condensation.

Whetting agents i think.
Room is much better now but machine continues to run.

About 250 watts.