Campbell Scientific SR50A Distance Sensor - Manual (Page 9)

SR50A Sonic Ranging Sensor
3.1 Quality Numbers
Measurement quality numbers are also available with output data; these give an
indication of the measurement certainty. The quality numbers have no units of
measure but can vary from 152 to 600. When the Quality numbers are lower
than 210, the measurements are considered to be 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:
• sensor is not perpendicular to the target surface
• target is small and reflects little sound
• target surface is rough or uneven
• target surface is a poor reflector of sound (extremely low density snow)
Quality Number Range Quality Range Description
0 Not able to read distance
152-210 Good measurement quality numbers
210-300 Reduced echo signal strength
300 to 600 High measurement uncertainty
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 quality numbers will increase during snowfall
events consisting of low-density snow.
3.2 Temperature Compensation
The SR50A does not include a temperature sensor to compensate for the speed-
of-sound variations in air temperature. Temperature corrections for the speed
of sound will need to be applied to the readings. Use a reliable and accurate
probe, such as the 107-L or HMP45C-L to measure air temperature. A
radiation shield is also required when the temperature probe will be exposed to
solar radiation. Temperature compensation must be applied to the sensor
output using the following formula:
273.15
KELVIN T
READING = DISTANCE
SR50A
°
FORMULA 1. Temperature Compensation
The SR50A calculates a distance reading using the speed
of sound at 0°C (331.4 m/s). If the temperature
compensation formula is not applied, the distance values
will not be accurate for temperatures other than 0°C. CAUTION
3