More than thirty years ago, at the time when hydrometric columns used mercury ampoules, a steam boiler suffered severe overheating which completely impaired the central tunnel and the back plate as a result of having been operating without water.
The strangest part of this case is that the boiler's water injection pump was in perfect operating conditions.
However, a detailed inspection of the equipment allowed technicians to observe that the top cap of the hydrometric column had been left open, allowing a few wasps to build a small clay nest! This subsequently prevented mercury from moving.
Consequently, false alarm signals were generated indicating that the equipment had water when it wasn't really the case. The steam boiler became a total loss.
This experience confirms that during protocols of preventive maintenance we must absolutely check all the equipment's components periodically. It also further clarifies why, in addition to the polluting nature of the element, mercury ampoules have been discontinued.