Page 18 - Royal Observer Corps

Radiac Survey Meter No 2

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 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. 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, 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.
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.