Page 40 - Spring Summer 2026

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(c) An Electronic Unit. This is usually mounted
remote from the point of measurement and its
purpose is to convert the detector output into a
useful electrical signal for indication or control.
In the case of Geiger Muller Tube detectors, the
Electronic Unit comprises a Pulse Ratemeter,
whose d.c. output can be displayed on a meter
or recorder, can be used to operate relays, or can
be fed into a proportional controller.
ON/OFF CONTROL

Fig. 2. Gamma Ray Source Holders for level
detection systems.
a range of source holders which can be used with this
type of detector. The detector can be built in weather-
proof or certified flameproof form and contains, in
addition to the Geiger Muller Tube, a simple Ratemeter
circuit and control relay. The trip level can be set for
operation at any intensity between 20 pulses/second
and 200 pulses/second. The response time constant of
the circuit is normally 8 seconds but can be adjusted
from about 1 second upwards.

Fig. 1 (a). A simple Level Detector employing a
Geiger-Müller Tube. (Weatherproof version).

Fig. 4. Continuous Level Indication Systems [S = Source, D = Detector, R = Range].

Fig. 3. On/Off Level Detector mounted on a Sand Hopper.
Detectors of this sort have been installed on hoppers
of sand, coke, lime, wood-chips, etc., storage tanks for
latex, acids, etc., catalyst vessels, plastics coating plant,
etc. Fig. 3 shows a typical high level alarm installation
on a hopper.
Fig. 1 (b). Flameproof Level Detector.
Owing to the physical size of the components, and CONTINUOUS LEVEL INDICATION
the way in which gamma-rays are absorbed, the method Owing to the physical size of the components and
is particularly suited to two-step or On/Off operation the way in which the gamma-rays are absorbed, con-
at a fixed level. For this purpose it is simply necessary tinuous level indication is generally limited to small
to mount a "point" source and detector on opposite ranges up to, say, 2 or 3 feet, and above this range the
sides of the vessel at the critical level. The change in system becomes cumbersome or completely impractic-
signal intensity as the contents fall below or rise above able, depending on the type of vessel. Fig. 4 shows
this level is usually more than sufficient for reliable schemes which can be used for continuous level indica-
operation of a control relay. Thus, the trip level of the tion over a moderate range.
relay can be set without further need for adjustment Method (a) uses a linear source and linear detector
over the period of one source half-life. Recalibration and this can be made to operate extremely well over
is not required and no access to the contents is neces- ranges of up to 2 feet, provided that the vessel dimen-
sary in order to determine the level at which control is sions are not too large. The response can be made
taking place. Generally, accuracy within ± ¼ inch is linear in most applications. This type of installation
easily achieved. Fig. 1 shows a typical self-contained has been provided for certain types of Catalyst Towers Fig. 5 (a). Linear Source and Fig. 5 (b). Linear Detector
detection system for On/Off control and Fig. 2 shows and Storage Tanks for corrosive liquids. Detector for continuous level showing Geiger-Muller Tubes
indication. and Head Amplifier.
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