Barometer calibration

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BlueSkyNick

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SWMBO has just bought me a new barometer for the house (silver wedding pressie!).

when I unwrapped it on Tuesday, it was showing 1030mb when it was blowing a hoolie outdoors.

Have just hung it on the wall reading 1018mb. Data from Lymington Met Here which is about an mile away as the seagull flies, is showing 1016mb.

There are no instructions, but there is a little adjustment screw on the back.

Is it a simple case of tweaking it until I think its about right, or is there a recommended method for calibration?
 
just tweak it on a day when the pressure is stable, alternatively take it to the obs post an tweak it there 7 then.

do a couple of taps just to settle the works after the adjustment.
 
Get a reading from the nearest air port to you if it is within 10mls it wont be far out. The screw on the back may well be for adjustment a very careful tweak so see what happens to the finger will tell you.
The most important thing is not so much the reading it shows but the rise or fall over an period of time ie a rise or fall of 10/12 mb in 50/60 Min's will indicate extremely violent weather in is eminent your immediate area. where as a rise or fall of 1 /3 mbs in an hr would mean very little or no change.
Thats what is its prime purpose is for.
You have to read and maintain a chart to predict for more than a few hours ahead.
My apologies if you already know all this.

You may be able to get a near reading
from :- here
or :- Or here.
 
Just had another look at your post 1016 - 1018 mbs,
I personally would not be concerned with a error that small it could differ by that much from one end of your house to the other and not be an effect of the true situation outside.
 
If you are able to set it up in a porch or covered area out side I think it would be possible to set up to the local mets reading.
I know this is a bit of thread drift, Some time ago 5/6 years now I set my plastimo battery clock up using the BT time signal it took me a 5/6 months to get right but I did in the end and now it is spot on to within 2 secs a month and right for the mid 6 months or so till the battery is done then it loses 1/2 secs so I know it's due a new battery.
So go for it the feeling of achievement is worth it.
 
SOUTHAMPTON/EASTLEIGH EGHI 071520Z 19019G30KT 180V250 9000 VCSH SCT016 BKN023 11/07 Q1008

The above is the aviaton METAR for 1520 today. The Q1008 is the QNH, this the air pressure corrected to sea level. so therfore air presure at Southampton at the mo is 1008mb. Dont forget that for every 27ft of alltitude the pressure will change by 1mb. Hope this helps.

Paul.
 
thanks Starboard, I forgot about the altitude effect, although we are probly not much more than 27ft up anyway.

Just leaving it to stablise on the wall for a few hours, with the occassional light tap, it has become to closer to the Lymington Met data.
 
Nice to have it right I know, but the main use for the beast at sea is its rate of change.

Calibrationis is best done in stable weather conditions, weather man usually says when. But do it near to sea level.

We used to use barometers to measure altitude, and they can be accurate to a several metres in the right conditions.
 
Not the best of days to be trying to set a barometer with such a steep pressure gradient.

If its within 2mb of local actuals I would be inclined to leave, you can always subtract the 2mb from your baro to arrive at correct pressure.

Other thing to bear in mind is that most weather station data ie XC weather/met office is at least 30mins to 1hr old so on a day like today very difficult to get a precise reading
 
"SOUTHAMPTON/EASTLEIGH EGHI 071520Z 19019G30KT 180V250 9000 VCSH SCT016 BKN023 11/07 Q1008

The above is the aviaton METAR for 1520 today. The Q1008 is the QNH, this the air pressure corrected to sea level. so therfore air presure at Southampton at the mo is 1008mb. Dont forget that for every 27ft of alltitude the pressure will change by 1mb."

Being a 'bit' thick, probably more than a bit today, can you pls clarify. /forums/images/graemlins/confused.gif

"The Q1008 is the QNH, this the air pressure corrected to sea level. therfore air presure at Southampton at the mo is 1008mb."

Since Soton Airport is obviously above sea level, why is it adjusted to sea level? Why not state actual? It must be higher than 1008 presumably.
 
[ QUOTE ]
I forgot about the altitude effect

[/ QUOTE ] You <u>should</u> forget about the altitude effect It is normal to set a barometer that is going to be used in one place eg at home to read the equivalent sea level pressure. Similarly the pressures quoted for metreological observation sites are eqivalent sea level pressures. ( All those quoted on this site are the sea level pressures despite the fact that many of the stations are well above sea level.)

So set your barometer, where it is going to be used, to the reading you get from Lymington. Ideally do this when the pressure is close to the average of 1013 mb. That way it will be at its most accurate in the centre and most often used part of its range. If it is as good as you deserve for a silver wedding prezzie then hopefully it will still be reading accurately at the extreme ends of its range as well.

Just for interest you could compare the reading with other nearby stations in the link I have given above but ignore readings from unofficial and amateur souces eg Yacht club met stations as there are often way out.

It is a different matter if you are setting a barometer at home for use on the boat . Then you must set it to read lower than the observation at your nearest met station by 1mb for every 27 ft you are above sea level (personally I use a correction figure of 11.4mb per 100m which is closer to 1mb per 28 ft but that is splitting hairs!) Then it will be reading correctly when you take it to the boat.
 
""Calibrationis is best done in stable weather conditions, weather man usually says when. But do it near to sea level.""

I agree. When RAF Mountbatten in Plymouth was in operation, I used to ring 'em up and they'd give me the exact barometric pressure AT SEA LEVEL. I used to jump in the car, shoot down to where my is moored, about 3 minutes, and adjust it there. Definitely the best way IMHO.
 
In aviation we use 3 types of pressure setting the SAS, QNH and QFE. As you realise aircraft altimeters are really barometers working on pressure changes. Whilst fling along an airway system aircraft will fly at flight levels and the subscale on the altimeter will be set to the SAS standard altimeter setting of 1013.2mb. Whilst general flying below the airways system we use the QNH value, this will be lowest forcast pressure corrected to sea level and therefore when setting set on subscale altimeter will read alltitude above sea level. Approaching the airport the subscale will be set to QFE, this is the local pressure at aerodrome elevation, this will give you the height above airfield elevation when set on the subscale.

Confussed??? well you should be, Altimetry is always a confusing subject in Air Traffic Control!!!!!! to us simple minded air traffikers!!!

Paul.
 
I may be getting rusty, but 10/12 mb fall in 1 hour? I think that would be extremely violent and would by that time have run out of clean t.t.t.t.trousers! AFAIR 3mb/hr over 6 hours would be quite bad in our latitudes. I appreciate that it depends on boatspeed relative to pressure gradient, latitude and speed of depression movement. It could be age creeping up on me...
 
[ QUOTE ]
10/12 mb fall in 1 hour? I think that would be extremely violent

[/ QUOTE ] You bet it would. As a rough guide gales can be expected if the pressure falls by more than 8mb in 3 hours.
 
WARNING WARNING

The site in the link tried to download some anti-spyware program as soon as I opened it. Told it to go away in short jerky movements.

(At least I think it was this site)
 
You did indeed spot the obvious point I was trying to make. /forums/images/graemlins/cool.gif
There is however a revision I should make the rise or fall of 1 to 3 mb in an hr should really be 5/6 hrs as pointed out by phlim.
It all to easy to get it wrong sometimes. /forums/images/graemlins/grin.gif
 
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