Page 577 - Instrumentation Reference Book 3E

Detector = Mechanical measurements 559

portable box, have been made to detect the level
of liquid COz in high-pressure cylinders. This is a
Detector (high level)
much simpler method than that of weighing the
cylinder and subtracting this value from the
Control
weight when the cylinder was originally received.
(low
level)
23.3.1.2 Using neutrons
u Some industrial materials have a very low atomic
Figure 23.11 Dual detector level gauge. number (9, such as oils. water, plastics, etc., and
by using a beam of fast neutrons from a source
such as l4lAmlBe or 25ZCf, and a suitable thermal-
Chart
Servosystem recorder neutron detector such as cerium-activated lithium
glass in a scintillation counter, or "BF; or 3He gas-
filled counters, it is possible to measure the level of
such material (Figure 23.13). Fast neutrons from
the source are moderated or slowed down by the
low-Z material and some are scattered back into
the detector. By mounting both the source of fast
neutrons and the slow neutron detector at one side
of the vessel the combination may be used to fol-
low a varying level using a servocontrolled
mechanism, or in a fixed position to control the
Figurc 23.12 Level gauge (continuous) with automatic level in the container to a preset position. This
control.
device is also usable as a portable detector system
of a level gauge is to select a radioactive source to to find blockages in pipes. valves, etc., which often
give the optimum path difference signal when the occur in plastics-manufacturing plants.
material or liquid to be monitored just obscures
the beam from source to detector. The absorption
of the beam by the wall of the container must be 23.3.2 Measurement of flow
taken into account, as well as the absorption of
the material being measured. A single source and There are several methods of measuring flow
using radioactive sources, as follows.
single detector will provide a single response, but
by using two detectors with a single source (Fig-
ure 23.1 l), it may be able to provide three read- 23.3.2.1 Dilution nietlzod
ings: low (both detectors operating), normal (one This involves injection of a liquid containing
detector operating), and high (neither detector radioactivity into the flow line at a known con-
operating). This system is used in papermaking stant rate: samples are taken further down the
systems to measure the level of the hot pulp. line where it is known that lateral mixing has been
Another level gauge to give a continuous indi- completed. The ratio of the concentrations of the
cation of material or liquid level has the detector radioactive liquid injected into the line and
and the source mounted on a servocontrolled that of the samples allows the flow rate to be
platform which follows the level of the liquid or computed.
material in the container (Figure 23.12). This
provides a continuous readout of level, and it is
possible to use this signal to control the amount
of material or liquid entering or leaving the con- Fast
tainer in accordance with some preprogramed neutron
source
schedule.
Another level gauge uses a radioactive source Shield
inside the container but enclosed in a float which
rises and falls with the level of the liquid inside the
container. An external detector can then observe
the source inside the container, indicate the level
and initiate refilling procedure to keep the level at a
predetermined point. Electronics
Portabie level gauges consisting of radioactive and display
source and Geiger detector in a hand-held probe, Figure 23.13 Level measurement by moderation of fast
with ithe electronics and counting display in a neutrons.