Fortin Barometer Restoration

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Barometer Restoration.

Being a lover of scientific instruments and all things old I was quite excited when I discovered an old mercury barometer stored at the back of the chemical store. Initial examination showed it to be complete, apart from the glass column which holds the mercury was missing, presumed broken.

Some internet research showed that it was a Fortin style barometer made by Darton of London, and commonly sold to schools and colleges throughout the land in the 1950s and 1960s.

I very quickly decided to recommision this lovely instrument, even more important as the powers that be have decided to outlaw the manufacture of mercury barometers in the near future. As a way of keeping a record of this project I decided to post an on-going thread on the technicians website TecHKnow where the ever helpful community there helped me with information and encouragement. For example, if it wasn't for chrisv and his impressive collection of old catalogues, we would never have known that in 1964 this model would have cost £65 0s 0d, or about £1000 in modern money.

Hopefully this article will help anybody else attempting a similar task, and this will show how I did it. Even though there are, doubtless, better ways of doing things, I was learning as I went along and working with limited resources.



Background

A barometer is an instrument for measuring atmospheric pressure.

A mercury barometer, at its simplest, is a glass tube (sealed at one end) filled with mercury and a reservoir of mercury open to the air at its base. High atmospheric pressure creates a force on the surface of the reservoir mercury, thus pushing the mercury up the tube, likewise lower pressure exerts less force and allows mercury to drop lower in the tube. By measuring the height of the column of mercury the air pressure can maintain, it is possible to calculate air pressure. In the past units have included inches of mercury(inHG), Millimetres of mercury(mmHg), millibars, and most recently kiloPascals.

Important

The techniques in this article involve contact with and exposure to mercury. At the very least mercury is toxic by inhalation, and can have cumulative effects on health. In some areas the use and supply of mercury is strictly regulated, so I would advise everybody to perform their own risk assement if attempting this and to be fully aware of any risks.

Some basic safety hints are:

  • always work in a well ventilated area.
  • never heat mercury, unless adequate ventilation is used.
  • always wear gloves.
  • remove all jewellery, mercury forms an amalgam with metals (combines with them), and gold and silver rings etc, will absorb mercury, even from its vapour.
  • mercury is a slippery customer, and it is hard to clean up spillages. Always work over a large tray which will catch any spills, and have to hand a mercury spill kit, and know how to use it.

The Project

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The offending item, a rather poor photo of the 'as found' state, standing at just over a metre tall the barometer was intact, all apart from the glass column being broken. The reservoir was still full of mercury, and this was probably why it had escaped the dustbin. Nobody had been bothered to empty it for disposal, so it spent years hidden behind a cupboard until my discovery.


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The scale at the top of the barometer reads in both mmHg and inHg, with a fine vernier scale for accurate measurement. Attatched to the body of the column is a working mercury thermometer with a hand etched scale.

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The resevoir still contained mercury, so I knew that part was salvageable.


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The main body of the barometer was removed from its backing board, and after removing 4 very small brass screws the protective casing around the column was removed. This revealed that the glass column was, indeed, broken and would need to be replaced.

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The reservoir is clamped together by 3 long screws passing through the base and the 'lid', sandwiching in between a short glass tube to view the mercury level. Dismantling meant that I now had access to the glass column, this passes through a wooden 'lid' to the reservoir. This wooden lid is very fine wood and acts as a porous plug. The glass column is secured in it by an interference fit then sealed in using a winding of thread and a varnish. No amount of soaking in various solvents would shift the varnish/glue, so there was only one option.

Take a deep breath and 'gently' have a go at chiseling it out.

Lid into vice, and ......STOP!! This is a detailed and fragile piece of wood with several steps machined into it. Out with the vernier calipers and make some detailed measurements and drawings, just in case. Lid back into vice and tap, tap, PHEW, the glass gently cracks and mostly comes out.

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Next was to accurately clean out the rest of the debris from the hole with the help of a lathe. Luckily at this point I double checked, and found out that the column was not 9.5mm (as thought), but was infact 3/8inch. Everything else was metric though.

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Several likely looking glass columns were unearthed from the back of the storeroom, they all had the correct dimensions, apart from the inside bore. The original had a 6.5mm bore, and these ranged from 5mm - 7.5mm. Some overnight head-scratching until I remembered that I was measuring the height of a column of mercury and not its volume, therefore the bore would make very little difference. In the end I used a 5mm bore column, that was a little longer than the original, but it could be cut to length at the final assembly stage.

I found it quite hard to find these glass columns in todays scientific catalogues, and the firms I found that were able/willing to make them seemed quite expensive. Luckily a good science technician never throws anything away, so these had probably sat in the back of the storeroom for 20+ years waiting to be used. It should be possible to manufacture one, with a bit of practise, by sealing the end of a suitable tube using heat, but remember that the wall thickness of the tube will have to be thick enough to support the considerable weight of the mercury, and also to hold the vacuum that is needed at the top of the column when in use.

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As the column had been sitting in a dusty store for ages it meant that I had to ensure it was spotlessly clean. To do this I filled it with Nitric acid and left to stand for a weekend, it was then rinsed several times, with a final rinse of acetone and left to air dry to ensure there was no moisture left. Any dirt inside the tube will cause small air bubbles in the mercury and lead to inacurate readings. The mercury must be able to move freely.

Filling a sealed end 5mm bore tube is not as easy as it looks, a funnel will simply not do the job. I found that tilting the column at an angle and using a small dropper allowed the fluid to run down the incline, and air to escape. Any air bubbles could be removed by gentle tapping and/or inverting the column gently a few times. The same technique was used for the final fill of mercury, though this meant using a very shallow angle, and being very careful when inverting (a column full of mercury is heavy).

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Now onto cosmetics. The metal surround for the column turns out to be brass painted black, and the black paint was somewhat chipped and scratched. I decided to remove it completely, with the option of later re-painting. In the end I decided that I prefer shiny things, so it has been polished by hand and remains in a shiny brass state. Other metal parts were stripped and re-painted in either satin or gloss black using car spray paint.

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The backing plate is a solid piece of hardwood(Teak??), this was cleaned up using white spirit, and thankfully came up quite well. I didn't fancy the prospect of having to strip it all the way down and re-varnish. The metal fixtures were removed and re-painted, and the bits of white paper stickers that had been applied behind the scales were removed. These will be replaced with something more pleasing when I find it.


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With everything now ready it was time for a dry rebuild. Luckily everything went back together with no bits missing, and none left over. Only minor disaster was three of the very small brass screws, that the whole thing hangs from, snapping. These were drilled out and re-tapped to be replaced with steel screws from the supply of dead microscope parts. Also the glass column was 60mm too long, a nerve wracking moment as I cut it, but success.

Re-filling the Barometer

The final step is re-filling the barometer with mercury. Remember that this will involve spillage of mercury, so all safety rules obeyed.

In theory this is simple, you fill the column with mercury, place a finger over the end, invert it, and place the open end in the reservoir of mercury then remove your finger.

Not a chance!!!

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Firstly the reservoir is clamped firmly, note the tray and bowl to catch any spills, also the large weight to counter-balance the weight of mercury.

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The glass observation tube and cork sealing washers are now added and filled with mercury, this is only held in place at this stage by gravity. This leads to the problem that, no matter how careful, you will knock it off when trying to introduce an inverted glass tube with your finger over the end.

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After many attempts the glass tube was secured to the reservoir using masking tape and with practice the column introduce, the lid replaced and the whole thing clamped back together with the three long screws. The masking tape was then removed.


Brilliant, it works!!

Oh no it doesn't.

The atmosphere should be able to support 760mm of mercury, but my barometer will only support 520mm at best. Lots of re-cleaning the column, checking for air bubbles, swearing etc. concluded that my mercury was not as pure as it claimed and had picked up impurities over the years. Clean(er) mercury used managed to produce a column very close to the expected height.

Now assemble the whole thing and hang it somewhere suitable.


Conclusion

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I now have a functional, attractive Fortin barometer hanging in my prep-room. It reads a consistent 12mm below the Met. Office readings. Any greater accuracy would probably need the services of a professional firm to calibrate it. For the use that it gets in school (up to age 16) it doesn't need to be any more accurate, but still demonstrates the use of a barometer, changing air pressure, and how to read a scientific instrument properly.

In a similar vein it also comes with tables to show how to compensate for altitude, temperature, and latitude. As well as conversions between the various units used. A good example of using tables to show pupils who are more used to simply entering numbers into a computer.

Would I do it again? Given the opportunity the answer would probably be yes. In all it took about 7 weeks of doing little bits of work, as and when, between my normal day job as a school technician, mainly lunchtimes and after school. Obviously having a science, and a technology department at hand helped as well. In the end the satisfaction of seeing a nice piece of equipment back in action is a good reward.

Useful Links

Without the aid of the internet I probably would never have taken on this project and found the following people very useful.

A loverly bunch on the forum, loads of advice and encouragement (and of course hosting this Wiki).

A firm specialising in restoration/building barometers in the UK. They have a passion for barometers, and are a good source of spares. They even gave advice and support, even though there was little chance of a sale. Interesting website.

Source of weather reports and statistics for the UK.


--Prep room boy 09:41, 1 May 2009 (UTC) Beaker.gif


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