Campbell Scientific SR50A Distance Sensor - Manual (Page 8)

SR50A Sonic Ranging Sensor
2. Introduction
The SR50A Sonic Ranging Sensor measures the distance from the sensor to a
target. The most common applications are measuring snow depths and water
levels. The sensor is based on a 50 kHz (Ultrasonic) electrostatic transducer.
The SR50A determines the distance to a target by sending out ultrasonic pulses
and listening for the returning echoes that are reflected from the target. The
time from transmissions to return of an echo is the basis for obtaining the
distance measurement.
Since the speed of sound in air varies with temperature, an independent
temperature measurement is required to compensate the distance reading for
the SR50A. A simple calculation is applied to initial readings for this purpose.
The SR50A is capable of picking up small targets or targets that are highly
absorptive to sound, such as low density snow. The SR50A makes use of a
unique echo processing algorithm to help ensure measurement reliability. If
desired, the SR50A can also output a data value indicative of measurement
quality.
The SR50A was designed to meet the stringent requirements of snow depth
measurement which makes it well suited for a variety of other applications.
The rugged aluminium housing is built to withstand harsh environments and
offers several mounting options.
3. Operation
The SR50A has several output formats: SDI-12, RS-232 and RS-485. The
SR50A is shipped from the factory configured as an SDI-12 sensor (address 0).
By moving a set of three jumpers inside the SR50A the output type can be
optionally set from SDI-12 to RS-232 or RS-485. Refer to Section 6 for details
on opening the SR50A and Figure 13 for jumper settings.
The SR50A performs multiple echo processing regardless of output formats.
The SR50A bases every measurement on several readings and applies an
algorithm to improve measurement reliability.
The distance to target readings that are obtained from the sensor are referenced
from the metal mesh on the face of the transducer. The SR50A projects an
ultrasonic beam that can pick up objects in its field of view that is 30° or less.
The closest object to the sensor will be detected if it is within this field of view.
Unwanted objects must be outside the field of view. If a target is in motion,
the SR50A may reject a reading if the target distance changes at a rate of 4
centimetres per second or more.
The SR50A will complete a measurement and output the data typically in 1
second. In RS-232 and RS-485 Serial modes the data is completed within one
second for BAUD rates of 9600 and above. The total time for an SDI-12
Measurements can exceed 1 second due to the long communications times
associated with the 1200 BAUD data rate.
If the SR50A rejects a reading or does not detect a target, zero will be output
for distance to target or –999 for depth values.
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