To calibrate a barometer

[choppy]

We have inherited basic barometer with new boat - Looks ok but is Danish possibly german so cant find any other info - It has 1 small screw /knob on back - is this likley to be a sensitivity adjustment or to set the hand to the current pressure as you would with a clock ?

Any info gratefully received.

 

[Salty John]

It's to set the barometer to the local pressure.

 

[VicS]

As salty john says to set the instrument to the correct pressure.

I'd advise setting it at a time when the reading is fairly steady as there is often a bit of a time delay between the time of the observation and the availability of the figures. Also do it when the pressure is reasonably close to the average which is 1013mb.

If you want to take it home to set it you can do so but you will need to take into account your altitude ie height above sea-level.
You will have to set it to read a lower pressure than the observations, which are usually if not always corrected to sea-level pressures, by 29mb for every 250m you are above sea-level. That's 0.58mb per 5m. 5m being the interval between contours on a 1:25000 OS map. It holds good up to 750m

You can confirm these figures if you wish. They have been deduced from a table in the Astronomy and Geophysics section of Kaye and Laby's Tables of Physical and Chemical Constants which you can find on the National Physical laboratory website at http://www.kayelaby.npl.co.uk/ You'll need to refer to that table if you live above 750m.

Use "Official" sources for your pressure observation. Any Met Office figures or http://www.xcweather.co.uk/ Avoid yacht club weather stations and other amateur sources as they are sometimes inaccurate.

Having said all that the most important thing is the rate of change of readings. The actual reading is a bit academic, but it is nice to have a barometer that does give the correct reading.

 

[Gunfleet]

Fitzcarraldo!

Very comprehensive instructions, Vic. Just the thing for a man keeping a boat on Lake Titicaca. /forums/images/graemlins/wink.gif

 

[choppy]

Great - thanks very much

 

[johnalison]

If you go to the Channel Isles, the forecasts on VHF will give you the latest pressure for each area. If only our other coastguard reports did the same.

 

[VicS]

Re: Fitzcarraldo!

[ QUOTE ]
Just the thing for a man keeping a boat on Lake Titicaca.

[/ QUOTE ]

Ah well. What you will have to do is determine a figure for the average pressure at your height above sea level where you live in Hull by interpolation of the figures in the table in K&L or simply calculate it by deducting 0.58mb for every 5m you live above sea-level from the mean of 1013mb. Then subtract from this figure 632mb, which is the mean pressure for Lake Whatsiticaca. Now add the result to the actual pressure at home when you carry out the calibration and set your barometer to that reading. Then when you take it to your boat it will read the pressure corrected to sea level.

However it is unlikely that any commonly available barometers will have the range of adjustment or the scale length to allow you to do this.

Therefore the best advice I can give you is to move your frigging boat to somewhere more sensible /forums/images/graemlins/smile.gif

 

[mortehoe]

Re: Fitzcarraldo!

Yavari's barometer (original) is calibrated in inches!

 

[VicS]

Re: Fitzcarraldo!

[ QUOTE ]
is calibrated in inches!

[/ QUOTE ] It'll cost too much to have it converted. Put it in an auction of scientific antiques and curios. Buy a new one with the proceeds

 

[mortehoe]

The pin is/was taken out (ie it turned into an altimeter)

There's a translation by the side which shows in inches that "This barometer was corrected to 'standard sea level' and is 12,561.6' above Newlyn Datum (corrected 2004)"

 

[VicS]

Re: The pin is/was taken out (ie it turned into an altimeter)

[ QUOTE ]
and is 12,561.6' above Newlyn Datum

[/ QUOTE ] That's interesting. Presumably it is a specially made instrument. If it dates from the 1860s it's not a modified aeronautical altimeter, because I don't think there were any aeroplanes that flew that high in those days, about 40 years later that they got off the ground at all wasn't it, but would there have been altimeters that explorers used?

Incredible as well that it has been set with such precision. to 0.6 ft., a bit of flannel perhaps. Ive got a pretty good barometer, a proper scientific one that lives in a plush lined leather case. It is just possible to see a small movement of the pointer in going from ground level up to the 2nd floor. I guess that was the latest and best measurement for the height of the lake.