The Royal Observatory has announced that, at some point over the next two weeks, compasses at Greenwich will point true north for the first time in about 360 years. And for some parts of the UK, this may not happen for another 20 years. Either way, it is a once-in-a-lifetime event.
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 and the oft-cited Plastic Bag Text is fallible. Buying an instrument that has not been properly checked is therefore a lottery.