Aneroid barometers that have not been properly serviced and calibrated may be prone to one or more of the many common faults that are found. Buying an instrument that has not been properly checked is therefore a lottery.
Many people buy an aneroid barometer with little assurance of its function or accuracy. The widely used Plastic Bag Test – placing the instrument in an inflated plastic bag, applying pressure to the bag and observing movement of the pointer – is a clue at least that the instrument might be working, but it is not conclusive and does not properly check the instrument for accuracy or full range of movement.
The vital component in all aneroid barometers is the capsule – a hollow structure manufactured in a number of metals and usually constructed in two halves soldered together, the air pressure inside reduced to a pre-determined and critical point depending upon the instrument design. Failed capsules are commonplace, the failure often at a soldered joint and the result of gradual deterioration of the joint to a point that it becomes porous. Since there may be no free flow of air across the failure point, the instrument may still register a deflection when submitted to the Plastic Bag Test, thus the test may be inconclusive.
With a view to setting the standard within the trade, Mark Jarrold designed, developed and built two pneumatic control systems, one to manage the test chambers with a high level of accuracy, the other to precisely set the pressure within repaired capsules that had failed.
All the instruments on the Vavasseur Antiques website will have passed through our rigorous series of tests and inspections, and many will be complemented by a graph indicating the level of function and accuracy demonstrated under test conditions. These attributes will vary from instrument to instrument as they are inherent in each barometer’s design, quality and date of manufacture.
Our descriptions, as far as possible, accurately reflect the important criteria of the instrument so described. Further, generally we only buy and sell instruments in fine original condition, and any departure from this will be noted in the descriptive material.