Banner U-GAGE QT50U Ultrasonic Sensors - Manual (Page 2)

Principles of Operation
Ultrasonic sensors emit one or multiple pulses of ultrasonic energy, which travel through the air at the speed of sound. A
portion of this energy reflects off the target and travels back to the sensor. The sensor measures the total time required
for the energy to reach the target and return to the sensor. The distance to the object is then calculated using the
following formula: D = ct ÷ 2
D = distance from the sensor to the target
c = speed of sound in air
t = transit time for the ultrasonic pulse
To improve accuracy, an ultrasonic sensor may average the results of several pulses before outputting a new value.
Temperature Effects
The speed of sound is dependent upon the composition, pressure and temperature of the gas in which it is traveling. For
most ultrasonic applications, the composition and pressure of the gas are relatively fixed, while the temperature may
fluctuate.
In air, the speed of sound varies with temperature according to the following approximation:
In metric
units:
C
m/s
= 20 √273 + T
C
In English units:
ft/s
= 49 √460 + T
F
C
C
m/s
= speed of sound in meters per second C
ft/s
= speed of sound in feet per second
T
C
= temperature in °C T
F
= temperature in °F
Temperature Compensation
The speed of sound changes roughly 1% per 6° C (10° F). QT50U series ultrasonic sensors have temperature
compensation available; temperature compensation will reduce the error due to temperature by about 90%.
Changes in air temperature affect the speed of sound, which in turn affects the distance reading measured by the sensor.
An increase in air temperature shifts both sensing window limits farther away from the sensor. Conversely, a decrease in
air temperature shifts both limits closer to the sensor. This shift is approximately 3.5% of the limit distance for a 20° C
change in temperature. With temperature compensation enabled, the sensor will maintain the window limits to within
1.8% over the entire -20° to +70° C (−4° to +158° F) range.
NOTE:
• If temperature compensation is enabled, exposure to direct sunlight can affect the sensor’s
ability to accurately compensate for changes in temperature..
• If the sensor is measuring across a temperature gradient, the compensation will be less
effective.
• With temperature compensation enabled, the temperature warmup drift upon power-up is less
than 1.0% of the sensing distance. After 30 minutes, the apparent switchpoint will be within
0.5% of the actual position. After 60 minutes, the apparent switchpoint will be within 0.3% of
the actual position.
Configuration
The sensor can be configured for one of three output response times and to enable or disable temperature compensation.
Both are accomplished using the sensor’s Speed push button, using the procedures described below. A button click is
defined as: 0.04 ≤ Click ≤ 0.8 sec.
1. Select the output response time.
Action Result
Click the Speed button until the desired output response time is
selected.
Response LED cycles throughSolid Red, Solid Amber, and OFF
to indicate selected Output Response Time.
• Solid Red — Slow Response (1600 ms)
• Solid Amber — Medium Response (400 ms) (factory
default)
• OFF — Fast Response (100 ms)
U-GAGE QT50UVR Series Sensors
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