Our dehumidifier is a simple manual one so there is no humidistat or
thermostat.

I know that condensation only occurs when dew point occurs which is a
function of temperature and humidity, is there a dew-point controller I
can buy rather than a humidistat as the humidistat will only switch on
humdity levels and does not take into account temperature?

On 29/09/2022 15:12, SH wrote:
> Our dehumidifier is a simple manual one so there is no humidistat or
> thermostat.
>
> I know that condensation only occurs when dew point occurs which is a
> function of temperature and humidity, is there a dew-point controller I
> can buy rather than a humidistat as the humidistat will only switch on
> humdity levels and does not take into account temperature?

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

On Thursday, 29 September 2022 at 15:12:52 UTC+1, SH wrote:
> Our dehumidifier is a simple manual one so there is no humidistat or
> thermostat.
>
> I know that condensation only occurs when dew point occurs which is a
> function of temperature and humidity, is there a dew-point controller I
> can buy rather than a humidistat as the humidistat will only switch on
> humdity levels and does not take into account temperature?

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

On 29/09/2022 17:28, David Wade wrote:

> 2. For a particular air mass the quantity of water in the air does not
> change with temperature.

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!

--

Jeff

On 29/09/2022 23:01, Animal wrote:
> On Thursday, 29 September 2022 at 15:12:52 UTC+1, SH wrote:
>> Our dehumidifier is a simple manual one so there is no humidistat or
>> thermostat.
>>
>> I know that condensation only occurs when dew point occurs which is a
>> function of temperature and humidity, is there a dew-point controller I
>> can buy rather than a humidistat as the humidistat will only switch on
>> humdity levels and does not take into account temperature?
>
> What matters is RH. A no-controller dh is generally a terrible performer, with either a very coarse heatsink or a peltier.

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.

On Fri, 30 Sep 2022 07:52:19 +0100, Jeff Layman <Jeff@invalid.invalid>
wrote:

>On 29/09/2022 17:28, David Wade wrote:
>
>> 2. For a particular air mass the quantity of water in the air does not
>> change with temperature.
>
>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!

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%.

--
Chris

On Fri, 30 Sep 2022 08:42:18 +0100, SH <i.love.spam@spam.com> wrote:

>On 29/09/2022 23:01, Animal wrote:
>> On Thursday, 29 September 2022 at 15:12:52 UTC+1, SH wrote:
>>> Our dehumidifier is a simple manual one so there is no humidistat or
>>> thermostat.
>>>
>>> I know that condensation only occurs when dew point occurs which is a
>>> function of temperature and humidity, is there a dew-point controller I
>>> can buy rather than a humidistat as the humidistat will only switch on
>>> humdity levels and does not take into account temperature?
>>
>> What matters is RH. A no-controller dh is generally a terrible performer, with either a very coarse heatsink or a peltier.
>
>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.

Something like this would do you https://tinyurl.com/mrxdc5eu

--
Chris

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

--

--:
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...
briang1@blueyonder.co.uk
Blind user, so no pictures please
Note this Signature is meaningless.!
"David Wade" <g4ugm@dave.invalid> wrote in message
news:th4h3b$mob0$1@dont-email.me...
> On 29/09/2022 15:12, SH wrote:
>> Our dehumidifier is a simple manual one so there is no humidistat or
>> thermostat.
>>
>> I know that condensation only occurs when dew point occurs which is a
>> function of temperature and humidity, is there a dew-point controller I
>> can buy rather than a humidistat as the humidistat will only switch on
>> humdity levels and does not take into account temperature?
>
> 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
>

On 30/09/2022 09:18, Chris Hogg wrote:
> On Fri, 30 Sep 2022 07:52:19 +0100, Jeff Layman <Jeff@invalid.invalid>
> wrote:
>
>> On 29/09/2022 17:28, David Wade wrote:
>>
>>> 2. For a particular air mass the quantity of water in the air does not
>>> change with temperature.
>>
>> 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!
>
> 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%.
>
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...

On 30/09/2022 08:42, SH wrote:
> On 29/09/2022 23:01, Animal wrote:
>> On Thursday, 29 September 2022 at 15:12:52 UTC+1, SH wrote:
>>> Our dehumidifier is a simple manual one so there is no humidistat or
>>> thermostat.
>>>
>>> I know that condensation only occurs when dew point occurs which is a
>>> function of temperature and humidity, is there a dew-point controller I
>>> can buy rather than a humidistat as the humidistat will only switch on
>>> humdity levels and does not take into account temperature?
>>
>> What matters is RH. A no-controller dh is generally a terrible
>> performer, with either a very coarse heatsink or a peltier.
>
> 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.

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

On 30/09/2022 12:42, David Wade wrote:
> On 30/09/2022 08:42, SH wrote:
>> On 29/09/2022 23:01, Animal wrote:
>>> On Thursday, 29 September 2022 at 15:12:52 UTC+1, SH wrote:
>>>> Our dehumidifier is a simple manual one so there is no humidistat or
>>>> thermostat.
>>>>
>>>> I know that condensation only occurs when dew point occurs which is a
>>>> function of temperature and humidity, is there a dew-point controller I
>>>> can buy rather than a humidistat as the humidistat will only switch on
>>>> humdity levels and does not take into account temperature?
>>>
>>> What matters is RH. A no-controller dh is generally a terrible
>>> performer, with either a very coarse heatsink or a peltier.
>>
>> 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.
>
> So turn it on when the temperature reaches dew point, to stop condensation?

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

> 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
>

On Fri, 30 Sep 2022 22:27:05 +1000, SH <i.love.spam@spam.com> wrote:

> On 30/09/2022 12:42, David Wade wrote:
>> On 30/09/2022 08:42, SH wrote:
>>> On 29/09/2022 23:01, Animal wrote:
>>>> On Thursday, 29 September 2022 at 15:12:52 UTC+1, SH wrote:
>>>>> Our dehumidifier is a simple manual one so there is no humidistat or
>>>>> thermostat.
>>>>>
>>>>> I know that condensation only occurs when dew point occurs which is a
>>>>> function of temperature and humidity, is there a dew-point
>>>>> controller I
>>>>> can buy rather than a humidistat as the humidistat will only switch
>>>>> on
>>>>> humdity levels and does not take into account temperature?
>>>>
>>>> What matters is RH. A no-controller dh is generally a terrible
>>>> performer, with either a very coarse heatsink or a peltier.
>>>
>>> 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.
>> So turn it on when the temperature reaches dew point, to stop
>> condensation?

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

Then a RH sensor alone should be fine.

>
>> 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
>>

On 30/09/2022 09:36, Brian Gaff wrote:
> 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
>
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.

On 01/10/2022 18:34, Robert wrote:
> On 30/09/2022 09:36, Brian Gaff wrote:
>> 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
>>
> 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

On 01/10/2022 20:42, David Wade wrote:
> On 01/10/2022 18:34, Robert wrote:
>> On 30/09/2022 09:36, Brian Gaff wrote:
>>> 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
>>>
>> 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 .

On Sun, 02 Oct 2022 04:34:48 +1100, Robert <robert@invalid.invalid> wrote:

> On 30/09/2022 09:36, Brian Gaff wrote:
>> 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
>>
> 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,

True.

> setting a Dewpoint temperature of say 12C may be.

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.

On 01/10/2022 21:58, Rod Speed wrote:
> On Sun, 02 Oct 2022 04:34:48 +1100, Robert <robert@invalid.invalid> wrote:
>
>> On 30/09/2022 09:36, Brian Gaff wrote:
>>> 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
>>>
>> 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,
>
> True.
>
>> setting a Dewpoint temperature of say 12C may be.
>
> 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?

On Sun, 02 Oct 2022 19:11:37 +1100, SH <i.love.spam@spam.com> wrote:

> On 01/10/2022 21:58, Rod Speed wrote:
>> On Sun, 02 Oct 2022 04:34:48 +1100, Robert <robert@invalid.invalid>
>> wrote:
>>
>>> On 30/09/2022 09:36, Brian Gaff wrote:
>>>> 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
>>>>
>>> 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,
>> True.
>>
>>> setting a Dewpoint temperature of say 12C may be.
>> 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.

> 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?

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 :-

https://create.arduino.cc/projecthub/sachendra003/arduino-humidity-sensor-288146

https://tinyurl.com/5n6ncejy

and the sensor is available on E-Bay for less that a fiver

https://www.ebay.co.uk/itm/203578893519

and mains relay boards are cheap

https://www.ebay.co.uk/itm/264435776787

as the sensor report ambient temp and rh% you can calculate the dew-point...

https://iridl.ldeo.columbia.edu/dochelp/QA/Basic/dewpoint.html

so you could stick the sensor on the window, or even have multiple
sensors on multiple windows....

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 <g4ugm@dave.invalid> wrote

>
>> 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?
>
> 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 :-
>
> https://create.arduino.cc/projecthub/sachendra003/arduino-humidity-sensor-288146
>
> https://tinyurl.com/5n6ncejy
>
> and the sensor is available on E-Bay for less that a fiver
>
> https://www.ebay.co.uk/itm/203578893519
>
> and mains relay boards are cheap
>
> https://www.ebay.co.uk/itm/264435776787
>
> as the sensor report ambient temp and rh% you can calculate the
> dew-point...
>
> https://iridl.ldeo.columbia.edu/dochelp/QA/Basic/dewpoint.html
>
> so you could stick the sensor on the window, or even have multiple
> sensors on multiple windows....
>
> 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...

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

On Sat, 1 Oct 2022 21:05:13 +0100, Robert <robert@invalid.invalid>
wrote:

>On 01/10/2022 20:42, David Wade wrote:
>> On 01/10/2022 18:34, Robert wrote:
>>> On 30/09/2022 09:36, Brian Gaff wrote:
>>>> 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
>>>>
>>> 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 .

But can you control dew point directly? I think not.

Dew point is a function of absolute humidity (the amount of water
vapour per unit mass or unit volume of air, sometimes quoted as the
partial pressure of water vapour in the air). Only one parameter
influences the dew point - the humidity. At any given humidity, when
the temperature falls low enough for the air to become saturated,
condensation will occur. That is the dew point (it's a temperature)
for that degree of absolute humidity. The only way to control the dew
point is to control the humidity. To control the humidity you need a
dehumidifier. Whether that dehumidifier uses relative humidity or
absolute humidity for its control, is neither here nor there. AIUI
most, if not all, dehumidifiers control on relative humidity and do a
perfectly acceptable job.

Of course, you can vary whether or nor condensation occurs by varying
the temperature, keeping it above the dew point for that degree of
humidity, but that is not controlling the dew point.

--
Chris

On Sun, 02 Oct 2022 21:11:28 +1100, Chris Hogg <me@privacy.net> wrote:

> On Sat, 1 Oct 2022 21:05:13 +0100, Robert <robert@invalid.invalid>
> wrote:
>
>> On 01/10/2022 20:42, David Wade wrote:
>>> On 01/10/2022 18:34, Robert wrote:
>>>> On 30/09/2022 09:36, Brian Gaff wrote:
>>>>> 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
>>>>>
>>>> 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 .

> But can you control dew point directly? I think not.

Corse you can by dehumidifying the room.

> Dew point is a function of absolute humidity (the amount of water
> vapour per unit mass or unit volume of air, sometimes quoted as the
> partial pressure of water vapour in the air). Only one parameter
> influences the dew point - the humidity.

But you can obviously change the humidity with a dehumidifier.

> At any given humidity, when
> the temperature falls low enough for the air to become saturated,
> condensation will occur. That is the dew point (it's a temperature)
> for that degree of absolute humidity. The only way to control the dew
> point is to control the humidity. To control the humidity you need a
> dehumidifier.

And that is what he is using.

> Whether that dehumidifier uses relative humidity or
> absolute humidity for its control, is neither here nor there.

It is when you want it to do what it needs to do automatically
rather than having to be turned on and off manually as required.

> AIUI
> most, if not all, dehumidifiers control on relative humidity and do a
> perfectly acceptable job.

His does too, but he wants to avoid having to turn it on and off as
required.

> Of course, you can vary whether or nor condensation occurs by varying
> the temperature, keeping it above the dew point for that degree of
> humidity,

Not possible with his outside walls.

> but that is not controlling the dew point.

But turning the dehumidifier on does.

On Friday, 30 September 2022 at 13:27:16 UTC+1, SH wrote:
> On 30/09/2022 12:42, David Wade wrote:
> > On 30/09/2022 08:42, SH wrote:
> >> On 29/09/2022 23:01, Animal wrote:
> >>> On Thursday, 29 September 2022 at 15:12:52 UTC+1, SH wrote:
> >>>> Our dehumidifier is a simple manual one so there is no humidistat or
> >>>> thermostat.
> >>>>
> >>>> I know that condensation only occurs when dew point occurs which is a
> >>>> function of temperature and humidity, is there a dew-point controller I
> >>>> can buy rather than a humidistat as the humidistat will only switch on
> >>>> humdity levels and does not take into account temperature?
> >>>
> >>> What matters is RH. A no-controller dh is generally a terrible
> >>> performer, with either a very coarse heatsink or a peltier.
> >>
> >> 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.
> >
> > So turn it on when the temperature reaches dew point, to stop condensation?
> I want this done automatically.... :-) at just before the dew point

Then you'll get mould. Keep it down to 65% to avoid mould.

On Sunday, 2 October 2022 at 09:11:41 UTC+1, SH wrote:
> On 01/10/2022 21:58, Rod Speed wrote:
> > On Sun, 02 Oct 2022 04:34:48 +1100, Robert <rob...@invalid.invalid> wrote:
> >
> >> On 30/09/2022 09:36, Brian Gaff wrote:
> >>> 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
> >>>
> >> 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,
> >
> > True.
> >
> >> setting a Dewpoint temperature of say 12C may be.
> >
> > 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?

Funny how people like to use computers where 2 contacts would work. Put contacts on the window glass to detect condensation directly.

On Sunday, 2 October 2022 at 09:52:01 UTC+1, David Wade wrote:
> > 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?
> 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...

under £2 last time I got them. Ebay.