Page 41 - Spring Summer 2026

Institution of Electronics News Institution of Electronics News

(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.
31 32

40 41
Become a member: https://membermojo.co.uk/ie Become a member: https://membermojo.co.uk/ie
Become a subscriber: https://membermojo.co.uk/subscriber Become a subscriber: https://membermojo.co.uk/subscriber
Become a sponsor: https://membermojo.co.uk/sponsor Become a sponsor: https://membermojo.co.uk/sponsor