Barometers reading impacted by dehumidifiers

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fmoran

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Anyone know what impact a dehumidifier will have when running in a boat on the barometer reading. Will it make it read higher due to the drier air - I thought it just reflected atmoshperic pressure. Anyway mine appears to be reading higher than it should be when the dehumidifier is on.
 
I think you've found the answer to the British weather. If we all run our dehumidifiers then this should promote a high pressure system with allied settled weather and sunshine.Saving the condensation from the heat exchanger would solve any water shortage from the good weather.
Can't see a downside.
 
A former geography teacher writes...

There are several corrections to be applied to a met. standard barometer (the Kew or Fortin types).

1 Index error
2 Latitude
3 Altitude
3 Temperature

Additional sources of error can be the effect of gusty wind, mercury fouling, local temperature.

Aneroid barometers usually have temp correction built in to the bellows mechanism.

Relative humidity within the range normally experienced on board (i.e. 0 - 100% :D) does not require adjustments to be made to the reading. All temperature corrections are based on the dry-bulb reading.
 
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it's important , I think to bear in mind that for us sailors it's the rate of change that is important rather than the actual reading on the barometer.
I just set mine up by looking at a the bbc and turning the adjusting screw to suit.
 
The barometer measures air pressure which wil remain unaffected by the humidity of the air.

Consider, in removing water vapour from the air inside the boat this may reduce the air pressure inside which would in turn allow air from outside, at a higher pressure, to force it's way inside restoring the pressure.
 
bobgoode said:
The barometer measures air pressure which wil remain unaffected by the humidity of the air.

Consider, in removing water vapour from the air inside the boat this may reduce the air pressure inside which would in turn allow air from outside, at a higher pressure, to force it's way inside restoring the pressure.
Click to expand...

... but have you considered the partial pressure of water vapour in this equation?
 
fmoran said:
Anyone know what impact a dehumidifier will have when running in a boat on the barometer reading. Will it make it read higher due to the drier air - I thought it just reflected atmoshperic pressure. Anyway mine appears to be reading higher than it should be when the dehumidifier is on.
Click to expand...

The vapour pressure of water at 20deg is 2.34 kpa, or 23mBar about half that at 10degC.
So it might be possible to observe a drop in pressure by removing the partial pressure of the water.

I wonder if it is possible that the barometer reacts to the sound pressure of the dehumidier's motor? Stranger things have happened!

Does the reading change the instant you switch it on? or is it a slow effect as the air is dried?
Is it a conventional mech barometer?
An electronic one could easily be affected by humidity, in either direction.
 
Not necessary as the boat is open to the outside which will allow air, and some water vapour, to flow inside raising the pressure to compensate. What you will have is the same pressure inside and outside but produced inside by dry air and outside by air and water vapour pressure.
 
I just notice its reading higher than I would expect - the dehumidifier is "on" all the time, though only running when the humidity is high. Its an oldfashioned barometer, in no way electronic / digital!. Thanks
 
If air and water vapour were ideal gases, the Law of partial pressures would suggest to me that:
The partial pressures of nitrogen and oxygen (etc) will be the same inside your boat as outside.
If your boat is more humid than the outside world, then the partial pressure of water will be higher inside than on dry land.
The total pressure indicated by the barometer will therefore be higher onboard by a factor of the vapour pressure of water at that temp times the difference in relative humidity between bateau and weather station.
I don't know how significant the non-ideal nature of the gases is.

If the above argument actually held true, then if you consider the water vapour pressure in my bathroom after a hot shower, 100% humidity vs the (say) 50% humidity outside the door, should be about 1% of a bar. That's 1% of about 15 psi, lot of square inches in a door, you could not open it!
I will ask a Physics teacher!
I have tried moving a barometer from room to room, but one barometer is **** and the other is screwed to the wall! FWIW, the variation seems to go the other way, I suspect a temperature error.
 
lw395 said:
The total pressure indicated by the barometer will therefore be higher onboard by a factor of the vapour pressure of water at that temp times the difference in relative humidity between bateau and weather station.
Click to expand...

stress-reduction.jpg
 
Air is a fluid. If you manage to drop the pressure in one place by taking the water vapour out, air will flow in from next door and equalise the pressure. You cannot possibly achieve a change in pressure, measurable by a normal barometer, between the inside of a non airtight boat and the surrounding air - which is what bobgoode told you in the 4th response to the OP.

lw395 - if there was a significant difference in water vapour pressure inside and outside the boat the partial pressures of nitrogen, oxygen etc would not be the same inside and outside the boat - the total pressure would be the same, so the partial pressures would be different.

Vince Hall Not Dalton's Law - you need the Ideal Gas Equation.
 
andygc said:
Air is a fluid. If you manage to drop the pressure in one place by taking the water vapour out, air will flow in from next door and equalise the pressure. You cannot possibly achieve a change in pressure, measurable by a normal barometer, between the inside of a non airtight boat and the surrounding air - which is what bobgoode told you in the 4th response to the OP.

lw395 - if there was a significant difference in water vapour pressure inside and outside the boat the partial pressures of nitrogen, oxygen etc would not be the same inside and outside the boat - the total pressure would be the same, so the partial pressures would be different.
Click to expand...

Really not convinced by this.
It implies nitrogen flowing from a lower partial pressure to a higher one.
I think what actually happens is the xs partial pressure of the water vapour causes it to diffuse outwards. Probably quite quickly. So the water vapour pressure either side of the door is almost equal.

The atmosphere is full of large and small scale pressure variations.
 
lw395 said:
Really not convinced by this.
It implies nitrogen flowing from a lower partial pressure to a higher one....
Click to expand...

Andygc has it. I think the term 'Partial Pressure' is confusing you - it's just a bit of shorthand to represent the proportion of each ingredient in a gaseous mixture. It isn't real pressure. The dehumidifier is taking water vapour out of the mix, so the proportion of the other gasses present is higher. (Their partial pressure is higher.) The water vapour is replaced by an equal volume of air drawn in from outside the boat (which also contains a water vapour).

None of this will affect an aneroid or mercury barometer.

Andy
 
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