Bomb Power Indicator, also known by the acronym BPI, was an instrument built to detect nuclear explosions and measure the peak

overpressure Overpressure (or blast overpressure) is the pressure caused by a shock wave over and above normal atmospheric pressure. The shock wave may be caused by sonic boom or by explosion An explosion is a rapid expansion in volume of a given amoun ...

of their blast waves. It was used at the twenty five

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Royal Observer Corps (ROC) controls and nearly 1,500 ROC underground monitoring posts, across the

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during the

Cold War The Cold War was a period of global Geopolitics, geopolitical rivalry between the United States (US) and the Soviet Union (USSR) and their respective allies, the capitalist Western Bloc and communist Eastern Bloc, which lasted from 1947 unt ...

Overview

The instruments, operated by volunteers, measured the level of peak-overpressure at the instrument's location. Once readings had been combined with information provided by the

Ground Zero Indicator The Ground Zero Indicator, known by the acronym GZI was a specially designed shadowgraph instrument used by the United Kingdom, British Royal Observer Corps during the Cold War to locate the Hypocenter, Ground Zero of any nuclear explosion. It co ...

s it would be possible to estimate the size of the nuclear explosion in megatons. Detailed BPI information was backed up by the automatic AWDREY readings. The BPI was designed and built by the

Atomic Weapons Establishment } The Atomic Weapons Establishment (AWE) is a United Kingdom Ministry of Defence research facility responsible for the design, manufacture and support of warheads for the UK's nuclear weapons. It is the successor to the Atomic Weapons Researc ...

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. The design was tested for performance and accuracy using real nuclear explosions at the 1957 Kiritimati (or Christmas Island) nuclear weapons tests, after being mounted on board a ship. A number of BPIs were also tested in

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during the Operation Buffalo series of nuclear tests, with units located at various distances from

ground zero A hypocenter or hypocentre (), also called ground zero or surface zero, is the point on the Earth's surface directly below a nuclear explosion, meteor air burst, or other mid-air explosion. In seismology, the hypocenter of an earthquake is its p ...

Operations

Above ground, a pair of circular plated baffles would be affected by the passing of the blast wave. The detector was connected by a steel pipe to the indicator dial below ground in the protected monitoring post. The dial was wall mounted and measured readings from 0.1 to 5 pounds per square inch peak overpressure. Readings below 0.3 pounds per square inch were noted but not reported. The baffles were normally stored below ground and only screwed onto the top of the pipe at the start of exercises or at Transition To War. Outside of operations the BPI pipe was protected by a screw on cap and there was a drain valve at the base of the instrument to remove any excess rainwater. If the BPI registered a reading of 0.3 or higher the operator would wait ten seconds before pressing the reset button and making a report to the group control. One minute after a BPI reading an observer is sent above ground to change the photographic papers in the

Codeword

Royal Observer Corps reports following a reading on any BPI were prefixed with the codeword "Tocsin" e.g. "Tocsin Shrewsbury 56 post - (time) oh five fifty five - pressure two point six".

References

{{reflist Royal Observer Corps Cold War military equipment of the United Kingdom

Overview

Operations

Codeword

See also

References