961-20A0-011 Datasheet, PDF (2/12 Pages) Magnetrol International, Inc. – Echotel® Model 961/962 Ultrasonic Level Switches

TECHNOLOGY
BASIC OPERATION
Model 961/962 switches utilize ultrasonic energy to detect
the presence or absence of liquid in a single or dual point
transducer. Ultrasonic contact level technology uses high-
frequency sound waves that are easily transmitted across
a transducer gap in the presence of a liquid media, but
are attenuated when the gap is dry. Model 961/962
switches use an ultrasonic frequency of 2 MHz to perform
this liquid level measurement in a wide variety of process
media and application conditions.
The transducer uses a pair of piezoelectric crystals that are
encapsulated in epoxy at the tip of the transducer. The
crystals are made of a ceramic material that vibrates at a
given frequency when subjected to an applied voltage.
The transmit crystal converts the applied voltage from the
electronics into an ultrasonic signal. When liquid is present
in the gap, the receive crystal senses the ultrasonic signal
from the transmit crystal and converts it back to an elec-
trical signal. This signal is sent to the electronics to indi-
cate the presence of liquid in the transducer gap. When
there is no liquid present, the ultrasonic signal is attenu-
ated and is not detected by the receive crystal.
Ultrasonic
transducer
Transmit
crystal
Receive
crystal
Ultrasonic signal transmission
across transducer gap
PULSED SIGNAL TECHNOLOGY
Challenging process conditions such as
aeration, suspended solids, and high
viscosities are easily handled with the
pulsed signal technology utilized in the
961/962 switches. Pulsed signal circuitry
drives the transmit piezoelectric crystal
to produce powerful pulses of high-frequency ultrasonic
energy. These ultrasonic pulses are readily transmitted
through liquids, but are attenuated when the transducer
gap is dry. This technology also provides excellent immu-
nity from electromagnetic noise interference.
Pulsed signal technology makes the 961/962 units more
reliable than older continuous wave ultrasonic switches.
Unlike many tuning forks, pulsed signal ultrasonic
switches do not need to be configured for different media
densities, making these units the most universally applied
level switches on the market today.
SAFETY INTEGRITY LEVEL (SIL)
Critical process control applications demand reliable instru-
ments that provide high levels of safety. The introduction of
standards such as IEC 61508, IEC 61511 and ISA 84, have
generated interest in using Safety Instrumented Systems
(SIS) as a means to define the reliability
of instruments in a control loop. All
versions of the Model 961/962 switches undergo a detailed
performance evaluation called a Failure Modes, Effects and
Diagnostics Analysis (FMEDA). Three key components of
the FMEDA are the Safety Integrity Level (SIL), Safe Failure
Fraction (SFF) and the average Probability of Failure on
Demand (PFDavg).
SIL
SFF
PFDavg
961 Relay
2 as 1oo1
92.0%
1.77E-04
961 Current Shift
2 as 1oo1
91.4%
1.61E-04
962 Relay
2 as 1oo1
91.5%
2.31E-04
962 Current Shift
2 as 1oo1
91.8%
1.87E-04
2