pmagowan
Well-Known Member
I have two, one in the Garmin wind vane and another plastimo clock style one. The Garmin shows 1026 while the plastimo shows 1050. How do I know which is right or do I need to buy a third and average the closest two?
Barometers
- Thread starter pmagowan
- Start date
maby
Well-Known Member
They should not read exactly the same - pressure drops surprisingly quickly with increased altitude - probably not 24 millibars though unless you have a very tall mast. The Met Office does sometimes publish accurate ground level pressure readings to help people calibrate their barometers - but obviously only in periods of very stable weather when it is not going to change too quickly to be useful. I remember hearing a BBC weather forecaster relatively recently saying words to the effect of "For anyone wanting to calibrate their barometer, the reading in xxxxx is currently yyyyy millibars".
Wait for a period of stable weather in your area, then check the Met Office web site for the latest charts...
GHA
Well-Known Member
Have a look here..
http://aprs.fi/
the WX icons are weather stations, many with live pressure readings, you could use local stations to calibrate your units. Assuming there's a button to do so. 1050 sounds a wee bit high...
http://aprs.fi/
the WX icons are weather stations, many with live pressure readings, you could use local stations to calibrate your units. Assuming there's a button to do so. 1050 sounds a wee bit high...
maby
Well-Known Member
Follow up: just checked with a very sensitive barometer in our three storey house and the pressure difference between the ground floor and top floor is about 0.6 millibar...
oldbilbo
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It used to be a requirement that every Harbourmaster had to keep a calibrated and corrected barometer ( altitude, temperature ) in the office so that ship's masters could check/calibrate their own. It's worth checking IMHO what your local office has to offer....
And if you find a harbour where the 'balls' are still dropped to signify the moment of noon, so that ships' chronometers can be checked, do let us know.
And if you find a harbour where the 'balls' are still dropped to signify the moment of noon, so that ships' chronometers can be checked, do let us know.
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Are you sure the Plastimo isn't telling the time ?
If you have Navtex then on the National Station, coastal station reports are published at least once a day. Alternatively air pressures are regularly published on the net.
You should be able to get fairly close ! Averaging won't work but what they will both show is the speed of rise and fall which should be consistent.
If you have Navtex then on the National Station, coastal station reports are published at least once a day. Alternatively air pressures are regularly published on the net.
You should be able to get fairly close ! Averaging won't work but what they will both show is the speed of rise and fall which should be consistent.
Stork_III
Well-Known Member
It used to be a requirement that every Harbourmaster had to keep a calibrated and corrected barometer ( altitude, temperature ) in the office so that ship's masters could check/calibrate their own. It's worth checking IMHO what your local office has to offer....
And if you find a harbour where the 'balls' are still dropped to signify the moment of noon, so that ships' chronometers can be checked, do let us know.
Deal in Kent does, at 1.00pm BST
Fantasie 19
Well-Known Member
Throw one of them away - the remaining one will then be "right"... 
prv
Well-Known Member
Pilots need to know air pressure to calibrate their altimeters before each flight. Assuming you can't listen in to a nearby airfield's radio, perhaps there are flyers' websites that can help?
Pete
Pete
VicS
Well-Known Member
I'd hazard a guess that 1050 is not right.
Calibrate your barometers aginst a reliable nearby pressure report. The reports on http://www.xcweather.co.uk/ are all from airfields etc so are reliable. They are "sea level" pressures.
Met Office reports also reliable.
Other "official" sources should also be OK but yacht club pressure readings can be miles out!
Do it when the pressure is relatively steady. There is not then going to be a big error due to time delay or a few miles discrepancy in location.
Best also done as close as possible to the average reading of 1013 mb.
If you are setting them at home reduce the setting by an amount equivalent to your altitude ( 11.4 mb per 100m above sea level).
At the end of the day it's pressure trend you are more interested in rather than an accurate reading
johnalison
Well-Known Member
My house is about 30m above sea level and pressure is about 3mb lower. Many years ago in Scientific American there were instructions on how to build a tsunami-detector, which was an ultra-sensitive barometer. All I remember is that they suggested that you test it by lifting it a metre from the table!
jbweston
Well-Known Member
I think you should calibrate them both against an accurate actual observation. Amongst other stations, airports give frequently updated actual pressure readings as these are vital for safe air navigation. They form part of the weather report knows as a METAR (Met Actual Report) updated frequently for all main airports - its buddy the TAF is a Terminal Area Forecast and isn't what you want. Pick an airport as close a possible to you geographically. Then look for the QNH as it's called. That's what the pressure is at the airport but at notional sea level - so for example if the airport is 200 feet above sea level, it's what the pressure would be at the bottom of a 200 foot well at the airport. This enables the aircraft to set its altimeter so it reads how high the aircraft is above sea level when near the airport. The other pressure is QFE or threshold pressure. That tells you what the pressure is when sitting at the start of the runway. It's not what you want (because your boat is at sea level, not at the height of the airport) but it's important to the fly boys when the aircraft is landing as obviously if the altimeter is zeroed to that pressure, it reads zero feet altitude as the aircraft's wheels touch the ground.
You need QNH - actual reported pressure at the airport adjusted to sea level - not QFE - pressure at the elevation of the airfield.
For example I've just googled 'METAR Southampton' and got 'METAR: EGHI 262050Z 29003KT 260V330 CAVOK 16/10 Q1029'. The only two bits you need are the date/time '262050Z' which is 26th of month 2050 UTC or Zulu as the fly boys call it, and the 'Q' which is QNH or sea level pressure at Southampton - namely 1029 hectopascals or millibars as old guys like me call them. EGHI is code for Southampton but we don't need to know that as we aren't reading it off a list on a teleprinter and Google gives us the right report.
Failing that method (which is the easiest) you could always use forecast pressure for example from GRIB file but that is just a forecast. Met actual observation is fact.
So If you're in Cowes or Hamble, your barometer should have been reading 1029 at 2050 UTC, that is about 40 minutes ago. If the pressure is changing really fast, you might need to know what your barometer was actually reading 40 minutes ago, but in most circumstances half an hour doesn't make much difference. So you simply twiddle the little adjustment screw at the back so the barometer reads 1029 like the METAR says it should.
Bingo.
Hope that helps.
You need QNH - actual reported pressure at the airport adjusted to sea level - not QFE - pressure at the elevation of the airfield.
For example I've just googled 'METAR Southampton' and got 'METAR: EGHI 262050Z 29003KT 260V330 CAVOK 16/10 Q1029'. The only two bits you need are the date/time '262050Z' which is 26th of month 2050 UTC or Zulu as the fly boys call it, and the 'Q' which is QNH or sea level pressure at Southampton - namely 1029 hectopascals or millibars as old guys like me call them. EGHI is code for Southampton but we don't need to know that as we aren't reading it off a list on a teleprinter and Google gives us the right report.
Failing that method (which is the easiest) you could always use forecast pressure for example from GRIB file but that is just a forecast. Met actual observation is fact.
So If you're in Cowes or Hamble, your barometer should have been reading 1029 at 2050 UTC, that is about 40 minutes ago. If the pressure is changing really fast, you might need to know what your barometer was actually reading 40 minutes ago, but in most circumstances half an hour doesn't make much difference. So you simply twiddle the little adjustment screw at the back so the barometer reads 1029 like the METAR says it should.
Bingo.
Hope that helps.
25931
Well-Known Member
Does it really matter ? I assume that they show similar rise and fall which is all that I need to know.
bitbaltic
Well-Known Member
Brilliant- just checked mine against Cardiff Airport and it's bang on at 1028 Mb, which will do for me!
Cheers
An Airband radio is very useful in this regard. The airports give these reports out in plain language on a repeating loop which is updated with any change in conditions. Windspeed and direction, cloud amounts and height, dewpoint etc are also included.
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I called the Coastguard station (which was nearby us) for a pressure reading on one very quiet and uneventful night, they were very willing to oblige.
AntarcticPilot
Well-Known Member
Well, plain language for some value of plain! Many years ago I took some flying lessons. The hardest part of the whole thing was understanding the heavily abbreviated and encoded weather forecast information. Mind, it didn't help that it was given in fairly strong SouthWest States American acents!
Slipstream 34
Well-Known Member
A change of 1mb equates to 30 feet. So if your mast is 45 feet expect the Barometer at deck level to read 1.5mb higher than the masthead unit.
oldbilbo
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"This is London Volmet...."
Aviation has been doing this, helpfully and unremarkably, for decades. Some years ago several of us - and the then-editor of PBO Magazine - tried to persuade the MCA and the Met Office to pilot such a provision for us yotties - and other small craft users. In trying to anticipate 'objections', we pointed out that spare frequencies were then available and we suggested a way it could be funded. They found a handful of other well-practiced excuses - er, reasons - for not bothering.
There's a 'Komitee' which meets at MCA Towers in So'ton every now and then - the Maritime Safety Information Group ( or, 'the lunch club', as MCA other ranks call it ) which is supposedly the channel for such communications. However, the last minutes available on the MCA website are dated 2009. That would be about when the renowned Jean Whitaker, the expert on all such stuff over many years, was 'retired'. Oddly, her MCA boss refused consent for a 'light touch' accolade-in-print, to recognise her fine service. There's something about middle-management civil servants that is often faintly odious....
Making minutes of meetings supposedly 'free for public access' out of reach by simply not publishing them where they should be is just another way of maintaining the culture of the Official Secrets Act ( the Met Office as part of the MoD, and the MCA being, well, simply Officious ) and frustrating the FoI Act.
The one thing that British bureaucracy is really good at is hiding and obscuring information, even innocuous stuff.
"It's Official, so we make it Secret, Bernard. And once it is An Official Secret, we keep it secret. on a Need To Know basis, of course..." "Yes, Sir Humphrey...."
PabloPicasso
Well-Known Member
Two chronometers are useless. It's better to have just one or at least three. Perhaps this applies to barometers as well