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Method (b) is suitable for vessels of small diameter ditions there will always be a signal fed to the ratemeter
(up to about 15 inches) and gives a reasonably linear which will keep the relay energised but in the event of
response over the middle portion of its range; the maxi- complete failure of the Geiger Muller tube (or any com-
mum range being about 10 inches. This type of ponent or connection to the ratemeter) leading to com-
installation has been highly successful on certain types plete signal failure, the relay will become de-energised
of pulp manufacturing plant in the Paper Industry; the and thus actuate an alarm or safety circuit. Such a
output being fed into a proportional control system.. system is built into many industrial level control equip-
Method (c) suffers from the disadvantage that the ments.
response curve is exponential and the maximum range RADIATION SAFETY
is about 30 inches for most liquids. The response can In designing installations for level control by means
be made more linear and the range extended by the of gamma-ray sources, care must be taken to ensure
modification shown in Method (d), a form of which has that there will be no radiation hazard to personnel
been used as a method of level indication for some bead responsible for operating the plant or maintaining the
catalyst towers. equipment. There are two types of hazard associated
One important limitation of these continuous indica- with radio-activity. Firstly there is a danger arising
ting systems is that the calibration is affected by source from ingestion or contamination should radio-active
decay. In the first instance the whole system must be materials escape from the source container. This risk
calibrated by making absolute determinations of the is virtually negligible if standard radio-graphic source
contents level over the whole range, and thereafter capsules are used. These are supplied by the Atomic
adjustments must be made to the ratemeter sensitivity Energy Research Establishment, Harwell (Cobalt 60)
at intervals to compensate for the decay of the source. or the Radiochemical Centre, Amersham (Caesium
A Cobalt 60 source decays by 1% in about 24 days, 137).
while a Caesium 137 source decays by 1% in about 6 Secondly, there is the hazard due to direct exposure
months. Since pulse ratemeters can be expected to of the body to external gamma-radiation. This has
operate with an accuracy of ± 1%, the overall been dealt with in the Preliminary Draft of Regulations
accuracy on a continuous level indicator may be about under the Factory (Ionising Radiations) Act, which lays
± 2% of the range. down maximum permissible levels for gamma-ray dose
If it is essential to cover a large vertical range of to be received in any working period by persons who
level indication by the gamma-ray method, the best may be exposed to radiation in the course of their
practical method is to employ an On/Off control system work.
and arrange for both source and detector to be traversed For certain categories of worker, the maximum per-
vertically together. The traversing gear can be driven missible level corresponds to a dose-rate of 2.5 mR/hr.
by a reversing motor controlled by the detector relay so and for other workers the maximum permissible level is
that source and detector follow any changes in level and .75 mR/hr. Obviously, all steps should be taken to The source activity required, depends of course on
are always positioned in line with the liquid surface. ensure that the actual dose received is below this the diameter of the vessel, the material and thickness of
For a large vessel, the main cost of such an installation maximum permissible level. A dose-rate of 2.5 mR/hr. the walls, and the signal strength required to give the
would be in the mechanical hoisting gear and electrical corresponds to about 250 counts/second in a typical desired accuracy and response speed.
drive. Geiger Muller Tube for gamma-rays from Cobalt 60 or Table 1 shows approximate source activities re-
RELIABILITY Caesium 137. A level detector or level indicator can, quired for various vessel diameters and wall thicknesses.
The Geiger Muller tubes most commonly used as therefore, operate satisfactorily with a dose-rate of Wall thickness is given in terms of steel, but conversion
detectors for level measurements are of the low voltage about one tenth tolerance at the detector site. for other materials can be made by reference to
Halogen-quenched type. These tubes, made almost Source holders are designed so as to surround the Table 2.
entirely of metal, are extremely robust and have a source pellet with a shielding material (usually lead) on
working life which is independent of the number of all sides except for a radiation aperture directed to- EXAMPLES
pulses conducted. In practice a large number of them wards the detector. Persons are not normally required A well-established application of gamma-ray level
have been in continuous use for 4 or 5 years without to work within the "beam" from this aperture, except detectors is the indication and control of molten glass
showing any signs of deterioration. Their temperature beyond the detector, where the intensity is well below level in the feeder channel of a furnace used for making
rating is from -55°C to +70°C so that they may be maximum permissible level. Should it be necessary for glass bottles. Fig. 6 shows a sectional view of such an
operated at the outside of many industrial vessels persons to work inside the vessel for maintenance pur- installation in which it can be seen that the vessel walls
without artificial cooling. Water-cooled detector units poses, the " beam '' can be attenuated drastically by consist of a considerable thickness (about 9 inches) of
can, however, be provided where necessary. The maxi- closing a shutter on the radiation aperture. The lead refractory materials. The contents of course are at
mum pulse rate of operation is about 1000 counts/sec- filling in the source holder ensures that the radiation about 1100°C. Under typical operating conditions it
ond but, in practice, the tubes are normally operated at outside the "beam," around the sides and back of the is desirable to hold the level steady to within ± inch
about 100 counts/second or less. Reliability and com- holder is below the maximum permissible level at all throughout a production run, and to have indication of
ponent life are thus comparable with the best types of points where operators may normally work or stand extreme deviations up to ± ½ inch. Control of level
equipment employing electronic valves. continuously. can only be achieved by regulating the supply of raw
There is always a small background level of radiation In practice it is found that source strengths ranging materials into the furnace tank.
which is detected by a Geiger Muller tube and this can from 0.5 to 250 mC are required, and the lead thickness For this type of application a single Geiger Muller
be used to operate a fail-safe system in many cases. necessary to shield these may range from 1 inch to 5 tube in a water-cooled jacket is mounted on one side of
For instance, if a ratemeter is used to indicate the level inches. The source holder, with its shutter closed, of the feeder, and a Cobalt 60 source of activity 150-250
of liquid in a tank, the pulse signal can also be used to forms a suitable container for transit or storage of the mC on the other. The source is housed in a lead shield
energise an alarm relay. Under normal working con- radio-active source up to the time of installation.

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