Fixed Survey Meter The Fixed Survey Meter was a specialist detection instrument used by the Royal Observer Corps during the Cold War between 1958 and 1982 to detect ionising radiation from nuclear fallout generated by a ground burst . Overview . The instrument was designed and built by the Atomic Weapons Establishment at Aldermaston as a replacement for the Radiac Survey Meter No 2 which could only be used above ground . The Royal Observer Corps ’ need was for an instrument that could be read from inside the protected environment on the underground post . The instrument had an analogue mechanical dial with a pivoted needle indicator on a scale that covered 0 . 1 roentgen s to 500 roentgens . Powered by three obsolete high voltage batteries ( 15 volt and 30 volt ) , that had to be specially manufactured , the meter was contained in a sturdy enamelled metal case . The controls featured an on - off switch combined with a calibration adjustment and a multi - position battery test switch . The batteries were contained within a clip - on cassette that took several minutes of careful preparation . A spare cassette was also pre - prepared to enable quick battery changes during operations . The meter was connected by a heavy duty cable to an ionisation chamber mounted onto a telescopic rod that was fed up a probe pipe in the ceiling of the monitoring post so that the probe was above ground . The top of the probe was protected by a polycarbinate dome that prevented fallout from entering the post . Operations . The delicate instruments were kept at the group controls in an air conditioned and de - humidified storage room and only issued to posts during Transition To War . Once at the posts the instrument was unpacked from its wooden transit case and prepared for use . If radiation readings approached the 400 roentgen level the telescopic rod was quickly collapsed and the probe reinserted to a distance below the surface that reduced the dial reading by a factor of ten . Thus the instrument became capable of producing accurate readings to a level of 5 , 000 roentgens per hour external reading . Shielded readings were prefixed with the word " Red " . When readings fell again to an indicated reading of 40 the probe was restored to full height . Once fallout had arrived anywhere in the group regular five minute readings were taken from all posts and displayed on the post boards in the operations room Codeword . The arrival of any radioactive fallout at the post was reported immediately with the codeword FIRST FALLOUT followed by post designation and the time e . g . " First Fallout - Shrewsbury 56 post - Oh three fifty two " . Training . Posts were provided with a dummy FSM trainer , known as the FSMT , that was identical in every way to the operational instrument except that it had a slot on its rear that permitted a roll of plastic trace , fed into the machine by a clockwork mechanism , to make the dial operate realistically over the 24 or 48 hours of a weekend exercise . Withdrawal . Between 1983 and 1984 the FSM and FSMT were withdrawn from use and scrapped . The instrument was replaced by the Plessey PDRM82 ( F ) , the fixed version of the new Civil Defence meters . The new polycarbonate cased meters were less delicate and this permitted them to be permanently stored at the monitoring posts instead of being retained at the group controls . References . Main reference " Royal Observer Corps Training Manual " published by HMSO