UKH

I have a little Wi-Fi enabled device in my living room which logs the room temperature. It also claims to measure relative humidity and dew point. I’ve noticed that the dew point seems to follow room temperature. For example, when we were away from home with the central heating off the room temperature slowly dropped over a few days from about 18 to 12.4. Over the same period the dew point dropped from about 13 to 6.8. It does similar when the heating is off during the night or the middle of the day. 
I can’t understand this; assuming that the absolute humidity is staying roughly constant why should the dew point change? What am I failing to understand, or is my device talking rubbish?
I think I had to retake my thermodynamics exam at uni. 

In reply to Thugitty Jugitty:

As a guess,

If you have a falling temperature and the humidity is constant then you have less and less water in your house.

This happens because your house won't be airtight so humidity will average out with the outdoors.

With less water in the house, the dew point will drop.

In reply to montyjohn:

This is exactly the kind of thing I struggled with at uni. If the absolute humidity remains constant (i.e. g/m3 of water in air) then surely the amount of water in my house remains constant? But are you saying that relative humidity (i.e. %) will remain roughly constant instead, which I think would explain things. 
And yes my house is old and draughty. 

In reply to Thugitty Jugitty:

Humidity is a percentage of the air's capacity to hold water vs how much water is i the air.

The air's capacity to hold water reduces with temperature.

So there's more water in air at 20 degrees at 60% humidity than there is in air at 10 degrees at 60% humidity.

In reply to montyjohn:

What has not been mentioned is the moisture generated by breath sweat and drying laundry.

None when the house is unoccupied.

Anybody who has used a plastic survival bag will concur!

In reply to Thugitty Jugitty:

as I understand it the humidity and therefore the dew point will not be staying constant as the temperature is not constant 

In reply to Rick Graham:

Sorry, should have been clearer. We’re generally also in the house in the middle of the day when heating is off. We just wear more layers. Obviously this wasn’t the case when we were away with the heating off, but the same thing happens in both cases.

In reply to Inhambane:

Do you mean relative humidity or absolute?

In reply to montyjohn:

I think the data supports your idea.

In reply to Thugitty Jugitty:

Even if your house was perfectly sealed from the outdoors, it will still contain sinks and sources of water vapour. Therefore your activity and room temperature will influence the water content of the air. The fact that the dewpoint drops when you leave suggests that you and your activities increase the water content of the air. You might be generating water vapour by showering, washing up, drying laundry, breathing and sweating. 

Because your house isn't perfectly sealed, there will also be exchange of air with the outside, so while you're not there the dewpoint inside your house should gradually approach the dewpoint of (drier) outside air.

Also dewpoint is not solely determined by air water content but also by pressure, so it would vary even in a perfectly sealed box of air with no other sinks/sources of water vapour in it.

In reply to Myr:

The same thing happens to dewpoint when the central heating is off during the daytime but we're still in the house.

In reply to anyone:

I've just put the sensor into a tightly sealed tupperware container and left it in its usual position in the living room. What should happen to the reported dewpoint over the next 24 hours, assuming there's no dramatic changes in air pressure and that I can afford good tupperware?

In reply to Thugitty Jugitty:

Fun game.

As house cools the humidity will rise.

Dew point will stay the same.

Assuming you can afford decent tupperware and/or humidity/temperature sensors

In reply to Thugitty Jugitty:

Electronic moisture/humidity isn't particularly easy to measure accurately and repeatably long term, 'measurements' from cheap kit should be treated with a decent pinch of salt.

That said, as the temperature in your house and it's airmass changes the moisture available will move around between materials to maintain an equilibrium. Also, you're not in it breathing and cooking while you're away.

Jk

In reply to jkarran:

The same thing happens to dewpoint (i.e. it falls as temperature falls) when the central heating is off during the daytime but we're still in the house.

In reply to Thugitty Jugitty:

Dew point is a function of relative humidity (RH) and temperature.  See this chart.

In reply to jkarran:

Terrible suggestion. Salt absorbs water and will muck up the results.

In reply to montyjohn:

The sensor was only twenty-odd quid but it seems reasonably accurate on temperature. The rest I'm not so sure about yet. I bought it mainly to let me know if my pipes are about to freeze due to central heating malfunction whilst we're away on holiday.

In reply to NobleStone:

Yes, if relative humidity remains approximately constant as the house temperature falls then the dew point will fall. What I was struggling to understand is what the mechanism is which keeps the relative humidity constant as temperature falls - this would require a fall in absolute humidity. My tupperware experiment might prove something.

In reply to Thugitty Jugitty:

I think from your graphs, your humidity sensor is inaccurate. RH should be varying with temperature, but it isn't, and that's giving the false impression that dewpoint is varying.

I think your tupperware experiment will continue to show the dewpoint spuriously varying over time, purely driven by temperature changing and a calculation using invariant RH.

In reply to jkarran:

This all day, a good humidity sensor needs an air flow across it to read accurately. I was called out over Christmas to an Ozone generation plant because the humidity sensor on the compressed air line was reading - 15 but it needs to be - 60.. The sensor vent port was blocked, a tiny bit of dessicant from the dryer had blocked it, 1 quick clean and a almost undetectable flow of air over the port and the dew point was back to - 60 again in no time. 

In reply to Thugitty Jugitty:

My guess is that your little device contains the ubiquitous DHT11  sensor, which is ok-ish on temperature but a bit crap at humidity, because measuring humidity electronically is not a £1.50 job.

In reply to Thugitty Jugitty:

As others have said, dew point is the temperature at which the moisture in the air condenses/becomes saturated, which isn't constant as cold air can hold less moisture. 

In reply to purplemonkeyelephant:

If I take a cubic metre of air and cool it down by ten degrees, with the absolute humidity remaining constant, then I think that the relative humidity should increase but the dew point should remain constant. That is, the temperature which I’d have to cool it to in order to reach saturation would remain unchanged. Do you think this is incorrect?

In reply to Thugitty Jugitty:

Your understanding is the same as mine

In reply to Thugitty Jugitty:

Here’s the results of the Tupperware experiment. I messed up arranging the graphs a bit - the last one’s title is relative humidity. 

Relative humidity is remaining constant as the temperature changes, which is wrong according to most people’s understanding in this thread.

I don’t understand why relative humidity jumped and then settled down when I put the sensor in the box. 
 

In reply to Thugitty Jugitty:

I guess that means the relative humidity sensor is a bit of a finger in the air. 

You could try repeating the experiment with a fan in the box to mix the air up based on a comment above. But I wouldn't hold much hope.

In reply to Thugitty Jugitty:

This is correct, until you reach the dewpoint of course.