Page 576 - Instrumentation Reference Book 3E

558 Measurements employing nuclear techniques
conditions with the maximum care taken to To find the position of large leaks in water
reduce sampling errors. This is best achieved by distribution pipes two methods have been devel-
calibrating while the instrument is operating in oped. The first uses an inflatable rubber ball with
closed loop with a series of homogeneous samples a diameter nearly equal to that of the pipe and
recirculated in turn. This gives a straight line containing 100 or so MBq of 24Na which is
relating percentage of calcium carbonate to the inserted into the pipe after the leaking section
total X-ray count. has been isolated from the rest of the system.
With slurry a line relating percentage calcium The only flow is then towards the leak and the
carbonate to X-ray count at each dilution is ball is carried as far as the leak, where it stops. As
obtained, producing a nomogram which enables 24Na emits a high-energy gamma ray its radiation
a simple special-purpose computer to be used to can be observed on the surface through a consid-
obtain the measured value indication or signal. erable thickness of soil. etc., by means of a sensi-
In normal operation the sample flows continu- tive portable detector. Alternatively, radioactive
ously through the instrument and the integrated tracer is introduced directly into the fluid in the
reading obtained at the end of 2-5 min, represent- pipe.
ing about 2 kg of dry sample or 30 liters of slurry, After a suitable period the line of the pipe can
is a measure of the composition of the sample. be monitored with a sensitive portable detector,
An indication of CaO to within &0.15 percent and the build-up of activity at the point of the
for the dry method and 410.20 percent for slurries leak can be determined. 24Na is a favored radio-
should be attainable by this method. active source for leak testing, especially of domes-
tic water supply or sewage leaks, since it has a
short half-life (15 h). emits a 2.7 MeV gamma ray
23.2.6 Wear and abrasion and is soluble in water as "Na C1. Thus, leak
The measurement of the wear experienced by tests can be rapidly carried out and the activity
mechanical bearings, pistons in the cylinder block will have decayed to safe limits in a very short
or valves in internal combustion engines is extreme- time.
ly tedious when performed by normal mechanical
methods. However. wear in a particular compo- 23.3 Mechanical measurements
nent may be easily measured by having the
component irradiated in a neutron flux in a 23.3.1 Level measurement
nuclear reactor to produce a small amount of
induced radioactivity. Thus the iron in, for exam- 23.3.1. I Using X- or gariinia rays
ple, piston rings, which have been activated and Level measurements are usually made with the
fitted to the pistons of a standard engine, will source and detector fixed in position on opposite
perform in an exactly similar way to normal pis- sides of the outer wall of the container (Figure
ton rings, but when wear takes place, the active 23.10). Because many containers in the chemical
particles will be carried around by the lubrication engineering and oil-refining industries, where
system, and a suitable radiation detector will most level gauges are installed, have large dimen-
allow the wear to be measured, as well as the sions, high-activity sources are required and these
distribution of the particles in the lubrication sys- have to be enclosed in thick lead shields with
tem and the efficiency of the oil filter for removing narrow collimators to reduce scattered radiation
such particles. which could be a hazard to people working in the
To measure the wear in bearings, one or other vicinity of such gauges. Because of cost, Geiger
of the bearing surfaces is made slightly radio- counters are the most usual detectors used,
active. and the amount of activity passed to the though they are not as efficient as scintillation
lubricating system is a measure of the wear counters. The important criterion in the design
experienced.

Detector
23.2.7 Leak detection
Leakage from pipes buried in the ground is a
constant problem with municipal authorities,
who may have to find leaks in water supplies, Colliiator
gas supplies, or sewage pipes very rapidly and (if required)
with the minimum of inconvenience to people -
living in the area, as essential supplies may have
to be cut off until the leak is found and the pipe
made safe again. Figure 23.10 Levelgauge (fixed)