Fixed Survey Meter

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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. [1]

Royal Observer Corps

The Royal Observer Corps (ROC) was a civil defence organisation intended for the visual detection, identification, tracking and reporting of aircraft over Great Britain. It operated in the United Kingdom between 29 October 1925 and 31 December 1995, when the Corps' civilian volunteers were stood down. Composed mainly of civilian spare-time volunteers, ROC personnel wore a Royal Air Force (RAF) style uniform and latterly came under the administrative control of RAF Strike Command and the operational control of the Home Office. Civilian volunteers were trained and administered by a small cadre of professional full-time officers under the command of the Commandant Royal Observer Corps; latterly a serving RAF Air Commodore.

Cold War Geopolitical tension after World War II between the Eastern and Western Bloc

The Cold War was a period of geopolitical tension between the Soviet Union with its satellite states, and the United States with its allies after World War II. The historiography of the conflict began between 1946 and 1947. The Cold War began to de-escalate after the Revolutions of 1989. The collapse of the USSR in 1991 was the end of the Cold War. The term "cold" is used because there was no large-scale fighting directly between the two sides, but they each supported major regional conflicts known as proxy wars. The conflict split the temporary wartime alliance against Nazi Germany and its allies, leaving the USSR and the US as two superpowers with profound economic and political differences.

Nuclear fallout residual radioactive material following a nuclear blast

Nuclear fallout, or fallout, is the residual radioactive material propelled into the upper atmosphere following a nuclear blast, so called because it "falls out" of the sky after the explosion and the shock wave have passed. It commonly refers to the radioactive dust and ash created when a nuclear weapon explodes. The amount and spread of fallout is a product of the size of the weapon and the altitude at which it is detonated. Fallout may get entrained with the products of a pyrocumulus cloud and fall as black rain. This radioactive dust, usually consisting of fission products mixed with bystanding atoms that are neutron-activated by exposure, is a form of radioactive contamination.

Contents

Overview

Fixed Survey Meter Royal Observer Corps Fixed Survey Meter.jpeg
Fixed Survey Meter

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.

Atomic Weapons Establishment

The Atomic Weapons Establishment (AWE) is responsible for the design, manufacture and support of warheads for the United Kingdom's nuclear weapons. It is the successor to the Atomic Weapons Research Establishment (AWRE) with its main site on the former RAF Aldermaston and has major facilities at Burghfield, Blacknest and RNAD Coulport.

Aldermaston village in the United Kingdom

Aldermaston is a mostly rural, dispersed settlement, civil parish and electoral ward in Berkshire, England. In the United Kingdom Census 2011, the parish had a population of 1015. The village is in the mid-Kennet alluvial plain and bounds to Hampshire in the south. It is roughly equidistant from Newbury, Basingstoke and Reading, centred 46 miles (74 km) west-by-south-west of London.

The instrument had an analogue mechanical dial with a pivoted needle indicator on a scale that covered 0.1 roentgens 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.

Roentgen (unit) legacy unit of measurement for the kerma of X-rays and gamma rays up to 3 MeV; equals 0.258 mC/kg

The roentgen or röntgen is a legacy unit of measurement for the exposure of X-rays and gamma rays, and is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air. In 1928, it was adopted as the first international measurement quantity for ionising radiation to be defined for radiation protection, as it was then the most easily replicated method of measuring air ionization by using ion chambers. It is named after the German physicist Wilhelm Röntgen, who discovered X-rays.

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 polycarbonate 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

Plessey PDRM82F Fixed Survey Meter PDRM82F.jpeg
Plessey PDRM82F

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.

The Plessey Company plc was a British-based international electronics, defence and telecommunications company. It originated in 1917, growing and diversifying into electronics. It expanded after the Second World War by acquisition of companies and formed overseas companies.

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References

Main reference "Royal Observer Corps Training Manual" published by HMSO

See also

Operational instruments of the Royal Observer Corps

The Royal Observer Corps (ROC) was a civil defence organisation operating in the United Kingdom between October 1925 and 31 December 1995, when the Corps' civilian volunteers were stood down.. Composed mainly of civilian spare-time volunteers, ROC personnel wore a Royal Air Force (RAF) style uniform and latterly came under the administrative control of RAF Strike Command and the operational control of the Home Office. Civilian volunteers were trained and administered by a small cadre of professional full-time officers under the command of the Commandant Royal Observer Corps; a serving RAF Air Commodore.