Barometer Dismantling

[thescullster]

Hi all

Back again with the old baro!

So I decided to take a peak last night to see if the chain was crudded up or the bellows unit shot. But fell at the (almost) first hurdle. Unscrewed three screws around the glass perimeter expecting the whole gubbins to lift out but no, just the glass lifts away and the dial etc remains housed in the wood casing. Unscrewed the screws around the dial, but that is held captive by the needle.

So, how do I remove the needle - it appears to be kind of riveted to the shaft end?

Before and after investigations, when the glass is tapped the needle moves 3-5mm. The needle seems fairly free to rotate, so presumably the chain gubbins is likely to be OK, but the bellows not? I have not seen the needle move more than an inch or so in either direction (from the 12 o'clock position).

I guess if I can get the needle off without damaging the internals, all will be revealed.

Thanks

Phil

[newshound]

Clock hands are normally "push fit" on the spindle but can get very tight. Like you, I think I'd expect the whole assembly to come out. Is it held in from the back?

[Nightjar]

It is probably a push fit, like a clock hand. There are special tools for taking clock hands off:

A pair of end cutters can be used, with care, instead.

Which could simply be an indication of a stable pressure system in the area.

Except where that little part that fell off and rolled away went. :-)

Colin Bignell

[The Natural Philosopher]

I went in from the back I think

yes. I am worried about taking the needle off..

[Nightjar]

On 28/06/2013 09:50, The Natural Philosopher wrote: ...

Also, how would you know where to align it when putting it back, unless you had another barometer to work from?

Colin Bignell

[thescullster]

Hi newshound

No there is no back access except for a small hole to an adjusting screw. The way the glass and dial are fixed the internals are clearly accessed from the front only.

Phil

[John]

Two quick ways to test the diaphragm (discovered by accident):

1. Put barometer in car with windows, vents and all doors except one closed. Slam remaining car door. Needle should jump 1/2" or so.
2. Find local big hill and drive up and down, checking change in barometer reading. Can't remember what the rate of change is for inches of mercury in this metric era, but it should be noticeable in 200 feet change of elevation. NB. Doesn't work in E. Anglia as there aren't any hills.

rusty

[news]

At sea level is is approximately 3mb/100 ft which if my calcs are correct its 0.1inHg per 100ft.

[The Natural Philosopher]

That's easy. go to nearest RAF weather station.

1018.85 mb at Lakenheath right now Cambridge showing 1020 give or take.

This is fairly useful also

[Brian Gaff]

If it has actually moved then surely the bellows must be working. Tapping is often needed on these things to make them move as I'm sure you are aware. The original touch sensitive devices! You need another one to see if the readings agree, as pressure has been not hugely different in the last few days or so. You can hear the pressures on forecasts or on the web. I have an air band radio so being near an airport I can tune in to the volme, and there you can get your elevation to set the offset of courset. Brian

[Brian Gaff]

You can get your ears to pop even going up and down the southa nd north downs so it has to be fairly substantial. Brian

[Steve Firth]

Err 100ft isn't sea level, HTH.

[Dave Liquorice]

I think that depends on how often you go up and down hills. My ears don't pop when going from near sea level up to 2,000' down to 1,000 and then back up to 1,400' at home.

My AWS barometer has 57 mb of correction, or 4 mb per 100' close enough to the previosly quoted 3 mb per 100' as it's non linear. 3 mb is 0.088" Hg, 4 mb 0.118" Hg.

[Dave Liquorice]

Not overly important provided you get it fairly close. The altitude adjustment ought to cope with a good 100 mb (3" Hg).

[polygonum]

I think it depends on health.

A few years ago I was finding somewhere around 200 feet could be uncomfortable - now back as I always was, several hundred feet and I don't notice at all most of the time.

Amazing what the right medicine does.

[hamiltons111]

[Brian Gaff (Sofa 2)]

2013? where did that come from? Brian

[Andrew]

Jet Stream ?, piggybacked on Storm Dennis ?