Page 104 - Instrumentation Reference Book 3E

Methods providing full-range level measurement 89

city of the waves is known, the distance may then
be calculated.
Many variations exist upon this basic theme.
The choice of radiation, use from above or from
below, and of frequency depend much upon the
material and the accuracy sought.
Although pulses are sent, the repetition rate is
fast enough for the output to appear continuous.
The method can be made suitable for use in
hazardous regions.

5.4.7 Force or position balance
In these methods a short-range sensor, such as a
float resting in the surface or a surface sensor of
an electronic nature, is used to provide automatic
control signals that take in, or let out, cable or
wire so that the sensor is held at the same position
relative to the surface. Figure 5.13 gives the
arrangement of one such system that makes use
Figure 6.32 Assembly view of Fisher torque tube unit.
Courtesy, GEC Elliot ConrrolValves Ltd.
granular materials may not fulfill this require-
ment) then pressure acting upon a given area at
the bottom of a tank is proportional only to the
density of the fluid and the head of pressure. In
most cases density can be assumed to be uniform,
thereby allowing a pressure sensor, placed on the
bottom, to be used as a measure of tank level.
Pressure gauges are described in Chapter 9.
Lying in the same class are purge methods. The
pressure needed to discharge gas or liquid from a
nozzle placed at the bottom of the tank depends
upon the head of liquid above and its density.
Bubblers, as these are called, are simple to
arrange and give readout as a pressure-gauge
reading that can be read directly or transduced
into a more suitable form of signal.
Obviously bubblers do not work in granular
materials. The addition of small quantities of
liquid or gas must not significantly affect the
contents of the tank.

5.4.6 Microwave and ultrasonic, time-transit
methods
A source of coherent radiation, such as ultra-
sound or microwaves, can be used to measure
the distance from the surface to the top of the
tank or the depth of the contents. In essence a Figure 5.13 Schematic of self-balancing level gauge
pulse of radiation is transmitted down to the sur- using RF surface sensing. Courtesy, GEC-Elliot Process
Instruments Ltd.1. Sensing element. 2.Tape insulator. 3.Tape
face where some proportion is bounced back by counter-weight. 4. Flexible coupling used on fixed roof only.
the reflecting interface formed at the surface. The 5. Pulley box over tank. 6. Pulley box over tankside unit.
same concept can be used with the waves being 7,Temperature cable junction box. 8,Temperature bulb
sent upward through the material to be reflected mounting kit. 9. Averaging resistance thermometer bulb.
downward from the surface. With relatively 10. Cable counter-weight. 11. Stainless steel perforated
sophisticated electronic circuitry it is possible to measuring tape. 12. Radio frequency cable. 13.65 mm dia.
standpipe. 14. Servo-electronic box. 15. Level indication.
measure the flight time and, given that the velo- 16.Tape retrieval housing.