Page 100 - Instrumentation Reference Book 3E

Methods providing full-range level measurement 85

Non Contact Nuclear

Sensdive scintillaton
detector plus
unit mounted in
weatherproof cast
aluminum housing
Microprocessor
controller simplifies
installation and
, mmmissioning ‘I

Figure 5.6 Nuclear gauging system needs mounting only on the outside of existing pipes or containers. Courtesy, Mineral
Control Instrumentation.

Table 5.1 Some guidelines for selecting a suitable level sensor. Adapted from Lazenby (1980)
Is remote control or indication desirable? A YES excludes mechanical float gauges. sight glasses,
dipsticks, and other devices designed specifically for
local control.
With level indicators. is time spent in taking a reading A YES excludes dipsticks, sight glasses, mechanical
important? float-gauges and certain balance systems.
Can the sensor contact the material being measured? A NO eliminates all but ultrasonics, radiation. radar,
and optical and load cells.
Must weight be measured rather than height‘! A YES means the choice is limited to load cells: but in
uniform-sided tanks, other devices such as capacitance
meters can be calibrated for weight. particularly if a
liquid is being measured.
Are there objections to mechanical moving parts? A YES gives one a choice of sight glasses and
capacitance, ultrasonic, radiation, conductivity. radar,
load-cell, optical, thermistor, and bubbler devices.
Is the application in a liquid? A YES eliminates vibrators and certain paddle types.
Is the application for a powdered or granular material? A YES eliminates dipsticks, sight glasses, floats,
themistors, conductivity devices, pressure (except
pressure switches) instruments, bubblers, displacers.
Do level indicators have to be accurate to about 2‘%? A YES eliminates thermistors, vibrators. paddles.
optical devices, suspended tilting switches, and
conductivity instruments, as those types only provide
control. All other types can be considered, but the
poorest accuracies probably come from float gauges
and radiation instruments.
Do level indicators need to have an accuracy that is a A YES reduces the list to dipsticks. some of the
lot better than 1%? displacers, and balance devices.

borosilicate) glass connected through unions and gives the basic structure of such a device. Where
valves into the tank wall. The diameter of the the contents are under pressure it will be neces-
tube must be large enough not to cause “climb” sary to use safety devices to control pressure
of the contents due to capillary action. The level release in case of tube breakage. In addition,
will follow that of the contents. Figure 5.7(a) when the contents are corrosive, other methods