Setting a barometer

[Chocolate]

I am not sure if this is the correct part of the Forum to post this message, but hope someone will be able to help me please. I have an old Edwardian aneroid barometer which we recently brought from England. It was bought in an antique shop some years ago, but 'set' by the local clockmender who knew what to set it at for the part of England we lived in. It has 'worked' very well for many, many years.  Clearly this setting is not applicable now we have moved to France. Can anyone tell me how to find someone who will know where to turn the pointer to please? 

 The circular dial, (think of the so-called 'banjo' barometers) has numbers around it ranging from 26 to 32 with incremental markings in between to measure in 10 small divisions each time. Outside these is the usual dry, wet, change etc. words. The pointer is changed via a small screw in the back, turned by a small screwdriver. This is a gold-coloured setting pointer which only moves when the screw is turned. There is another black one which moves according to the changing pressure and gives the reading.

If there is no-one such as a clock repairer who does this, can we sort it out ourselves? Is there a way of working out the setting as I believe it has something to do with how high we are above sea-level. That is shown on the local 'blue' map for walkers I think but we do not know how to use the information to set the barometer. All help/info welcomed. Thanks.

[Chezstevens]

I am not an expert.

However it looks like your barometer corrections are in inches of mercury. This would be 26.00 thru to 32.00 with 29.92 being the ''ICAO'' (DON'T ASK!) standard pressure setting. If you go to Meteo.fr they should give you the immediate pressure setting for your area, or town. You could then set this. If the pressure is given in millibars e.g. 1013 mbs or hpa then there is a relatively simple conversion you can apply. The ICAO standard atmosphere is 1013.25mbs or 29.92 in (inches of mercury) - you add or deduct .03 in for every mb increase or decrease e.g

1008 mb is 5.0 x .03 = 29.77in

then apply this to your barometer - I presume that barometers would accept QNH (pressure to display 0 feet at mean sea level) rather than QFE (pressure to display 0 feet at your altitude).

Hope this helps ..... unless I am going off at a tangent.

There are probably sufficient ex-aviator or sailors around to correct all errors in my synopsis.

[Jc]

Or see if someone in your area has a mercury barometer and set it to that.If it was set correctly in England it should not need adjustment.The pressure goes down with altitude but you do not adjust for it once set correctly.

[Jonzjob]

You will probably find a small screw on the back of the instrument for the adjustment. Almost certainly brass on one of that age..

[Gluestick]

Surely, going on from an earlier post, the only instrument that has to be zeroed, is an aneroid barometer altimeter?

It is set to true atmospheric ground at the time of take off and then as the air pressure changes it show the height from original zero? (Atmospheric changes and different airfields, relative to sea level - from which all such instrument are such instruments are referenced - would otherwise produce an inaccurate height reading).

Unless the red stuff has further addled my brain tonight![B]

My simple old barometer only needs the adjustable hand set to record any changes of the moving hand to indicate, roughly, a move from fair to stormy or visa versa.

 

[Chezstevens]

Ha, Ha VMT. To be fair there was no mention of zeroing I was hoping just setting the barometer once to an accurate QNH would do the job ... but I am no expert. What do you reckon are the 26 thru 32 graticules?

Being an aviator surely a barometer with a graticule giving 1/10th between 26.00 and 32.00 acts in a similar manner to an altimeter but, instead of feet in error from actual or 1013.25, has markings such as rain, stormy etc which relies upon the instrument showing a drop, or rise, in measured pressure. The increments allowing the instrument to be calibrated accurately.

[La vieille]

I remember standing behind some French people earlier this year who were buying a barometer as a wedding present for someone and the shopkeeper told them that they had to set the pressure for wherever the couple were, it would not be the same as the pressure where the shop was.   They were told to go to the church and it would be marked somewhere there (I think).   Didn't pay that much attention as I wasn't buying one, but remember thinking it was interesting that different places had different settings.

[Bob T]

Yes Vern, an altimeter is just a posh barometer. The reason that the altimeter needs to be adjusted is for low flying situations. For high level stuff the ISO standard of 1013 millibars is used as you are not close enough to the ground to make any difference. You are perfectly correct in htat a weather barometer should show change rather than pressure, as that is what affects weather. If the barometer were used up a 3000 meter mountain then the change may go off the scale due to the pressure being lower at that altitude, but in normal use at ground levels the actual readings, be they in inches of mercury or millibars,  are totally irrelevant.

[Chezstevens]

Anybody would think I was an ex-Halton brat! Failed miserable on electronic theory so went single trade - so don't ask me any wiring problems!! The graticules go from the equivalent of about 900 hPa to 1080 hPa - so that would be a fair blow then!

[Jonzjob]

[quote user="Chezstevens"][quote user="Gluestick"]

Surely, going on from an earlier post, the only instrument that has to be zeroed, is an aneroid barometer altimeter?

It is set to true atmospheric ground at the time of take off and then as the air pressure changes it show the height from original zero? (Atmospheric changes and different airfields, relative to sea level - from which all such instrument are such instruments are referenced - would otherwise produce an inaccurate height reading).

Unless the red stuff has further addled my brain tonight![B]

My simple old barometer only needs the adjustable hand set to record any changes of the moving hand to indicate, roughly, a move from fair to stormy or visa versa.

 

[/quote] Ha, Ha VMT. I was hoping just setting the barometer once to an accurate QNH would do the job ... but I am no expert. What do you reckon are the 26 thru 32 graticules? Being an aviator surely a barometer with a graticule giving 1/10th between 26.00 and 32.00 acts in a similar manner to an altimeter but, instead of feet in error from actual or 1013.25, has markings such as rain, stormy etc which relies upon the instrument showing a drop, or rise, in measured pressure. The increments allowing the instrument to be calibrated accurately.[/quote]

The problem is that when you move to a new home with a different altitude to where the barometer was set for the barometric pressure will be different. If the altitude is higher then the barometer will show a lower reading so that the little pointer will start to get nerer the 'Stormy' bit.

If you really want to find out what the pressure reading for your barometer is then have a look at this site http://www.digitaldutch.com/atmoscalc/ . Very interesting!!

This also gives the pressure variation as you go up.

To take it more simpely perhapse

Altitude           Pressure           Altitude          Pressure
    0,000                 29.92              20,000               13.75
    1,000                 28.86              25,000               11.10
    2,000                 27.82              30,000               8.886
    3,000                 26.82              35.000               7.041
    4,000                 25.84              40,000               5.538
    5,000                 24.89              45,000               4.355
  10,000                 20.58              50,000               3.425
  15,000                 16.88              60,000               2.118
  18,000 *               14.94            100,000               0.329
* This is almost exactly one-half the sea-level value.
      To convert in/Hg to psi, multiply by 0.491.

I hope this helps and if you can understand the web link, will you tell me how it works [8-)] ?

[Jonzjob]

By the way, I'm an ex St Athan boy..... AFE later turned EFA

[Chezstevens]

John,

With respect we all appreciate that they have moved house. The question is what datum to use. Your illustration uses ICAO std of 1013.25 or 29.92 (If you are North American) and the website is not really applicable, with respect, to barometers. Is this the std used on Barometers or do they set an accurate barometric pressure which then calibrates their barometer.

I don't know - I guess we need somebody that moved up from seaweed thru basic Barometers to the instrument we are now discussing.

Postnote: St Athans boys are Ok - my only time there was to do the MT driving course in 1980.

[Bob T]

Shall we start a new thread on Saints? I hated the place and vowed never to return after RSS moved there. The funny thing is that most readers won't know what we are on about.

[Jonzjob]

I was at St Athan from June 60 to December 61 and I swore blind that I would NEVER go back and I haven't! The camp was not too bad, just a shame as to where it is and who's around it!!!![+o(] At 16/17 years old that leaves a VERY strong impression!

Back on thread. I knew that there was a simple rough way to work our pressure drop with altitude and at last I've found it. Pressure drops approx 1% for every 80 metres up.

 Edit : If you have an active airfiel near by they will be able to tell you the pressure when you phone and their altitude. From that you should be able to get your correct (ish) reading. Near enough so it makes no difference. If you want to find the correct height of your house have a look on google Earth. France's Geoportail.com doesn't give the heights.

[Chezstevens]

www.meteo.fr has the pressure in hPa included with your village weather. What can be simplier than that? Call me old fashioned but RTFR.

[Alan Zoff]

Or you can try the following link:

http://fr.weather.com/common/home/localweather.html

The actual pressure will be changing of course but if you enter your location on the web page and then reset the moving needle on your barometer soon after the info has been updated, it should be close enough for domestic use. As I think someone has already said, it is the way the barometer shows movement in pressure that tends to be of most interest.

[bejay]

The accurate way to find air pressure at a particular place it to use a mercury barometer. This is a mercury column in  a Torricelli tube and the mercury settles at about 760 mm above the suface of the mercury reservoir. This height can be measured very accurately and in the trditional instrument a vernier scale was included so you could measure the height to an accuracy of  0.1mm. I imagine that this is now achieved electronically. This is an absolute measure of air pressure, Well, almost,  before anybody tells me, a correction has to be made for temperature variation in the mercury.

This figure can be converted mathematically to any set of units you desire. I can think of about half a dozen but I think the Pascal is the international standard.

If the actual numerical readings of an anereroid barometer are important then it will need to calibrated against a mercury barometer and because it is a sensitive mechanical instrument it will need recalibration from time to time. The only reason I can think of for recalibration in a different location is if the gravitational force is different. (Unless it has had a very rough journey of course). And anyway as Chezstevens says, its the change in pressure which is important.

Anyway to change the subject while I have the attention of some of the nation's finest. I have just been buzzed by a Chinook. I was driving along a main dual carriageway and it came at me at about 150feet. About 30 seconds later in came back along the the same road right over the car.  The conditions were very hazy almost foggy. I formed the distinct impression that it was lost and had dropped down to find its location. Please tell me I am wrong.

bj

[Chocolate]

I am absolutely staggered at the wealth of scientific information, speed and helpfulness of everyone who replied to my query. The result is that I have done it! The barometer is 'set'. A combination of the info posted on here and another google search resulted in a quick and speedy outcome. A chap called Dr. Giles Harrison from the Dept of Meteorology at Reading University wrote a paper which is on the 'net giving details which backed everyone else up on here. I simply looked on meteo.fr as advised (thanks), used the calculations given here and by Dr. Harrison and with a calculator (!) had set the barometer in less than 5 minutes. Thank you everyone for your time, help and wisdom.

[Chezstevens]

Pleasure. £1.5M well spent!!

Bit late now but another simple way of converting altitude to inches of mercury is 10' of altitude = 0.01 in. Others may be more familiar with 1mb (hPa) = 30'.

[Gluestick]

Perhaps that's why the UK has so many economic problems and no industry to speak of; all the scientists and engineers have buzzed off to France?