Inside humidity

_Sam_

tony3619 said:

My understanding is that if humidity outside is 80-90% at 8 degrees then opening windows will allow the colder air in. If this air is increased in temperature it can hold more moisture bring RH down. 

I think I'm getting it a bit now. So whenever you see the reading in percentages, this by definition should mean RH?

However in practical terms, if you define humidity as the "amount of water vapor in the air", isn't it true that that amount is going to be the same regardless of the temperature. In other words, if one wishes to have a dryer air inside the house, would they not want to reduce the amount of water vapor full stop. 

Basically I'm wondering why the advice is not given in absolute humidity terms, e.g. an air containing 10g of water vapor is healthier than the air containing 20g of water vapor.

  

subjecttocontract

How confident are you that the measuring device is accuate ?

Apodemus

_Sam_ said:

tony3619 said:

My understanding is that if humidity outside is 80-90% at 8 degrees then opening windows will allow the colder air in. If this air is increased in temperature it can hold more moisture bring RH down. 

I think I'm getting it a bit now. So whenever you see the reading in percentages, this by definition should mean RH?

However in practical terms, if you define humidity as the "amount of water vapor in the air", isn't it true that that amount is going to be the same regardless of the temperature. In other words, if one wishes to have a dryer air inside the house, would they not want to reduce the amount of water vapor full stop. 

Basically I'm wondering why the advice is not given in absolute humidity terms, e.g. an air containing 10g of water vapor is healthier than the air containing 20g of water vapor.

Relative humidity is the more useful figure, because virtually nothing in nature really experiences absolute humidity.  The 'feel' that you experience of humidity is the relative humidity.  The ability to dry your laundry at a given temperature varies with the relative humidity.  The ease with which you can breathe the air, is the relative humidity (both too low or too high being problematic).  The presence or absence of condensation in your home is the dew point of the walls, which varies with wall temperature and the relative humidity of the air.  In comparison absolute humidity tells you very little, unless you are undertaking chemical or physical engineering tasks in a tightly controlled environment.  

FreeBear

_Sam_ said:

tony3619 said:

My understanding is that if humidity outside is 80-90% at 8 degrees then opening windows will allow the colder air in. If this air is increased in temperature it can hold more moisture bring RH down. 

I think I'm getting it a bit now. So whenever you see the reading in percentages, this by definition should mean RH?  

In a nutshell, yes. But the %RH should also be qualified with a temperature.

e.g. My hallway is at 51%RH @ 17.5°C - Using one of those calculators, I can punch in different numbers to see how the RH changes in relation to an increase/decrease in temperature.

Note - RH has a +/-3% tolerance.

Apodemus

FreeBear said:

_Sam_ said:

tony3619 said:

My understanding is that if humidity outside is 80-90% at 8 degrees then opening windows will allow the colder air in. If this air is increased in temperature it can hold more moisture bring RH down. 

I think I'm getting it a bit now. So whenever you see the reading in percentages, this by definition should mean RH?  

In a nutshell, yes. But the %RH should also be qualified with a temperature.

e.g. My hallway is at 51%RH @ 17.5°C - Using one of those calculators, I can punch in different numbers to see how the RH changes in relation to an increase/decrease in temperature.

Note - RH has a +/-3% tolerance.

The calculators assume a sealed/isolated air system, though, with no input or output to wider ambient humidity.  In practice the desired RH change may not match the theory.

_Sam_

Apodemus said:

Relative humidity is the more useful figure, because virtually nothing in nature really experiences absolute humidity.  The 'feel' that you experience of humidity is the relative humidity.  The ability to dry your laundry at a given temperature varies with the relative humidity.  The ease with which you can breathe the air, is the relative humidity (both too low or too high being problematic).  The presence or absence of condensation in your home is the dew point of the walls, which varies with wall temperature and the relative humidity of the air.  In comparison absolute humidity tells you very little, unless you are undertaking chemical or physical engineering tasks in a tightly controlled environment. 

Is not the absolute humidity just a way to measure the moisture, just as relative humidity is? So that at any point in time you can get a measure of either, e.g. RH 60% at n temperature and AH 10g per m3 of air. (Appreciate it would quickly fluctuate, but this would appear to apply to both RH and AH.)

It does make sense to take temperature into account in many of the situations where moisture is involved, as you say laundry dries faster when the temperature is higher. But in terms of understanding how moist the air is, it seems to me that sometimes RH is almost like a red herring.

For example in terms of what we feel. It is said that in winter if the air in the house is humid, a person would feel colder than if the air in the house is dry. This comparison presupposes identical temperature in the house, but different amount of water vapor in the air. Hence what is at issue appears to be absolute humidity.

Now lets say a person in a cold humid house wants to feel warmer by reducing the amount of moisture. They turn on a dehumidifier which measures the moisture as RH. In 2 hours the person looks at the dehumidifier's display and sees that the RH reading dropped from 60% to 50%. However, because dehumidifiers produce a certain amount of heat as a by-product of removing water vapour from the air, this reading is a combination of two things - a reduced amount of moisture in the air AND a raised temperature. And so the person doesn't know how well did the dehumidifier do its job in terms of reducing the amount of moisture in the air (without doing further calculations, like weighing the water in the dehumidifier's tray )

Or in the OP's example, they wish to reduce humidity in the house and so they open the window to ventilate. But the outside air is very humid say 80% at 8C. Once that air comes into the house it warms up and its RH drops to 70%. But this drop feels like a red herring because by letting the outside air in, the OP let a certain amount of moisture in the house, and that amount of moisture remained the same regardless of the air being subsequently heated.

(I'm clearly overthinking all of this )

Apodemus

_Sam_ said:

Apodemus said:

Relative humidity is the more useful figure, because virtually nothing in nature really experiences absolute humidity.  The 'feel' that you experience of humidity is the relative humidity.  The ability to dry your laundry at a given temperature varies with the relative humidity.  The ease with which you can breathe the air, is the relative humidity (both too low or too high being problematic).  The presence or absence of condensation in your home is the dew point of the walls, which varies with wall temperature and the relative humidity of the air.  In comparison absolute humidity tells you very little, unless you are undertaking chemical or physical engineering tasks in a tightly controlled environment. 

(I'm clearly overthinking all of this )

I think we all are! 

I still see the RH as the more valuable info.  Probably best seen from the drying clothes scenario.  The ability of the air to absorb more moisture (from the wet clothes) varies with the RH, which is a function of the absolute humidity and the temperature.  You get to cite a single direct variable, rather than two indirect variables.  I guess it's a bit like we could talk about volt amps, but it is more normal to talk about Watts.

JGreenwood

I don't have anything to check humidity, however I do know the attic room in my house has been very humid over the past couple of months. I've had mould developing around the window. So I've placed a dehumidifier up there and it's helped lots.

It probably doesn't help that I virtually use it as a laundry room