Hydrometer; baffling

[Ricco1]

I moved in today, after 3 weeks of visiting the boat most days to get it ready.

 

My boat has a ship's wheel wooden thermometer and hydrometer. The hydrometer is baffling. It has a needle and scale. The scale ends at 100% but the needle can go beyond that. The first time I looked at it the reading was in the region of 120%. Doesn't this mean that the boat has sunk? When I opened the front doors the needle slowly moved back to 90%. Every time I've visited the boat the reading has been over 100% when I've got there.

 

I lit a fire this evening. It was a bit chilly but I only really lit it to celebrate my first night aboard. It was probably a mistake because I'm now sweltering in 82 degree heat. Anyway, the fire has prompted the hydrometer to dip alarmingly. It's now down to 57%, well below the previous record low.

 

Can anyone explain this? particularly how it can be more than 100%. I was brought up/ educated to believe that no single 'thing' could ever be more than 100%

[dmr]

I think it means there's something badly wrong with your batteries or do you mean hygrometer ?????

[howardang]

I moved in today, after 3 weeks of visiting the boat most days to get it ready.

 

My boat has a ship's wheel wooden thermometer and hydrometer. The hydrometer is baffling. It has a needle and scale. The scale ends at 100% but the needle can go beyond that. The first time I looked at it the reading was in the region of 120%. Doesn't this mean that the boat has sunk? When I opened the front doors the needle slowly moved back to 90%. Every time I've visited the boat the reading has been over 100% when I've got there.

 

I lit a fire this evening. It was a bit chilly but I only really lit it to celebrate my first night aboard. It was probably a mistake because I'm now sweltering in 82 degree heat. Anyway, the fire has prompted the hydrometer to dip alarmingly. It's now down to 57%, well below the previous record low.

 

Can anyone explain this? particularly how it can be more than 100%. I was brought up/ educated to believe that no single 'thing' could ever be more than 100%

 

Looks like you need to calibrate your hygrometer; there should be an adjustment screw on the back. Looking it up the suggested method is to wrap it in a wet cloth for around six hours and it should then read around 85per cent. If not adjust until it does.

 

Howard

[Phil Ambrose]

Hydrometer measures Specific Gravity of liquid

 

Hygrometer measres Humidity

I would guess that as the boat has been shut up it has allowed the humidity to climb and opening the boat up has allowed an exchange of air (as in airing the boat) Why you get readings of over 100% is not in my knowledge range.

Phil

[geoffwales]

When the air warms up, it expands. But the physical properties ie the moisture (RH) content cant expand as they are a given density so what happens is they disperse themselves more into the expanded air thus spreading themselves further and lowering the relative humidity. But whilst its worth noting that the humidity is lower the moisture content will stay the same, so once the air cools down again the rh will rise inline with dry bulb temp dropping.

100% rh is when they wet and the dry bulb are of equal temperature, normally chucking it down with rain then.

[BEngo]

A Hygrometer measures relative humidity. That is the amount of water in the air as a percentage of the maximum amount of water air of the same temperature could hold.

 

Anything above 100% is a false reading. Howard's suggestion for correcting your hygrometer is a good one.

 

Now Why does the reading fall when the boat warms up?

 

Warmer air can hold more moisture than cold air. So when your boat is cool inside there is some amount of water in the air Let's say 1 kg per cubic metre. The hygrometer sees this as some percentage of the maximum amount of water that air could hold. Lets say 1.2 Kg per cubic metre, so the hygrometer sees that the air is holding 10/12 of its maximum amount and reads 84%. When you warm the air up, without adding any more moisture the hygrometer notices that the air is warmer so it COULD hold more moisture. Lets say you warm it up so that it can hold 1.5 Kg per cubic metre, but don't allow any more water into the air, which is crudely what happens when you light a stove, or turn on an electric heater. The hygrometer then sees that there is 10/15 of the maximum amount and will now read 66%.

 

The numbers are not real, but show how it works. Actual numbers are nearer 30g per cubic metre at 30C

 

If you cool the air without changing its water content the opposite will happen and the hygrometer reading will rise.

 

N

[RLWP]

Some of those hygrometers are really only toys.

 

Richard

[bizzard]

As the atmospheric pressure falls (Barometer) the humidity rises (Hygrometer)= rain, storm.

[smileypete]

I've found the cheap digital ones aren't bad, have one like this with min/max but there's loads of others:

 

http://www.ebay.co.uk/itm/LCD-Digital-Thermometer-Hygrometer-Humidity-Temperature-Meter-Indoor-Home-H0-/281166286901

 

cheers, Pete.

~smpt~