Campbell Scientific SR50-L CSC Ultrasonic Distance Sensor - Manual (Page 9)

SR50 Sonic Ranging Sensor
3
3. Operation
The SR50 has several output formats: SDI-12, Pulse Train, and serial ASCII
output. A group of four jumpers inside the SR50 allow setting an address for
SDI-12 operation or as an option setting for Pulse Train and ASCII outputs.
The SR50 performs multiple echo processing regardless of output formats.
The SR50 bases every measurement on several readings and applies an
algorithm to improve measurement reliability. When using SDI-12 or ASCII
output, up to 3 targets can be detected with the SR50 provided the difference
between their height is > 0.4 meters.
The distance to target readings that are obtained from the sensor are referenced
from the metal mesh on the face of the transducer. The SR50 has a field of
view of approximately 22
°. The closest object to the sensor will be detected if
it is within this field of view. If a target is in motion, the SR50 may reject a
reading if the target distance changes at a rate of 4 centimeters per second or
more.
Under most circumstances the SR50 will complete a measurement within 1
second and begin outputting the data. If the SR50 rejects a reading or does not
detect a target, it will retry. The maximum time limit to remain in
measurement state for retries is 3 seconds. If the reading is not obtained during
this time limit, the SDI-12 and ASCII options will output zeroes for distance to
target and measurement quality numbers (see below for description of quality
numbers). The Pulse Train option will output a single pulse.
Measurement quality numbers are also available with the SDI-12 and the
ASCII output formats; these give an indication of the measurement certainty.
The quality numbers have no units of measure but can vary from 162 to 600.
Quality numbers are multiplied by the SDI-12 instruction
multiplier so will often be reported as a negative or some
multiple of the actual value.
Numbers lower than 210 are considered to be measurements of good quality.
A value of zero however, indicates a reading was not obtained. Numbers
greater than 300 indicate that there is a degree of uncertainty in the
measurement. Causes of high numbers include:
• the sensor is not perpendicular to the target surface
• the target is small and reflects little sound
• the target surface is rough or uneven
• the target surface is a poor reflector of sound (low density snow)
It is not necessary to make use of the quality numbers but they can provide
additional information such as an indication of surface density in snow
monitoring applications.
The SR50 does not include a temperature sensor to compensate for variations
in air temperature. Campbell Scientific recommends the Model 107 air
temperature probe for this purpose. A radiation shield may be required.
Temperature compensation must be applied to the sensor output using the
following formula:
NOTE