How to control a dehumidifier?

Humidity sensors do take account of temperature, but perhaps that isn't what you want.

1. The amount of water that the air can hold depends on temperature. When its cooled to the dew point condensation occurs.
2. For a particular air mass the quantity of water in the air does not change with temperature.
3. A humidity sensor measures the amount of water in the air as a percentage of the amount it holds. So "Relative Humidity" or RH...
4. So as you cool a volume of air it can hold less water and the RH% rises, until at the dew point it is 100% and condensation occurs.

Dave

What matters is RH. A no-controller dh is generally a terrible performer, with either a very coarse heatsink or a peltier.

Are you saying air at 30°C saturated with water vapour will retain the same amount of water vapour when cooled to -10°C? I don't think so!

The dehumidifier we have is actually very good at what it does, it easily pulls over a litre of water out of the room every 24 hours in the deepest depths of winter.

In previous years I used to run it non stop once conld weather started and then turned off when cold weather ended

Given teh cost of electricity, I thought a dew point controller was a sensible thing to do.

You are right, he should have added 'above the dew point'. For example, air at -10°C will still hold ~4% water vapour (g.m^3). The absolute water vapour content (g.m^3) of air containing ~4% water vapour at 30°C (i.e. pretty dry air by any standards), won't change as the air is cooled until it reaches -10°C. However, the _relative_humidity_ (RH) will rise as the temperature falls to -10°C, at which point the RH hits 100%.

Something like this would do you

The issue is often where it occurs, often you need to take a lot more moisture from the air in cold weather if the problem is condensation on walls or window frames,since your device is not located in those areas to measure them locally. Bathrooms even those with fans etc, seem almost impossible to stop getting condensation. Brian

I think I also missed this...

So when the Relative Humidity is at 100% the temperature is at, or below, the dew-point. No difference between an RH sensor and a dew point sensor...

So turn it on when the temperature reaches dew point, to stop condensation?

That is equivalent to saying you will turn it on when RH reaches 100%. The dew-point is the temperature at which condensation occurs because the air is saturated with water.

Saying "the air is saturated with water" is another way of saying "the relative humidity is 100%"

Dave

I want this done automatically.... :-) at just before the dew point

Then a RH sensor alone should be fine.

Exactly ! Walls and windows especially in poorly insulated rooms will be significantly colder than the air the dehumidifier or sensor is monitoring. One is trying to reduce condensation on these cooler surfaces. RH% is not useful for this, setting a Dewpoint temperature of say 12C may be.

Or it may not. If the ambient is 22 and the dew point is 12 you are unlikely to get condensation. Why because the RH is around 53%

On the other hand if the dew point is 20 and the ambient is 22 you will get condensation on any cool surface, because there is a lot of water in the air, the RH is 88%.

The dew point tells you zilch about the probability of condensation. You need to know the likely hood the air will be cooled to that temperature.

what is important is the difference between ambient and dew point which tells you the relative humidity.

Dave

Agree, but if the aim is to economically reduce the risk or amount of condensation on cold surfaces then monitoring RH isnt an efficient way of doing it My poorly insulated Laundry is running at 17C 77% RH so a dew point around 13C .

True.

No, you would have to monitor the temperature of the coldest surface and turn the dehumidifier on when it is getting close to the dew point temperature.

It would certainly be possible with your own system but not really feasible for a dehumidifier to do.

Perhaps with a Pi using a thermocouple attached to the coldest part of the wall (for me thats the bottom corners) or even the window corners.

alogn with a humidity meter and them some mathematical code in teh Pi and then interface to a relay to control said dehumidifier?

Yeah, that's what I meant.

So we are back to RH% again because that is what a humidity meter reports. Most of the meters I have seen have a probe on a wire that can be placed near the coldest part of the wall or window, but they are expensive...

The air near the coldest part of the wall must be closer to dew point? Will it be good enough to place the sensor close to the window.

PI's are getting expensive so how about an Arduino? In fact some one has a project to do that sort of thing :-

Or would it be enough to simply detect condensation is occurring with something like too bits of alarm tape stuck to the window..

.. and now we have it sorted when to switch on, what about switching off...

Dave

David Wade snipped-for-privacy@dave.invalid wrote

When the coldest surfaces aren't near the dew point.