Ultrasonic flow meters are commonly used to measure flow through a flume or across a weir. Often, the temperature effects on ultrasonic sensors are overlooked in the design and installation process. This type of flow meter utilizes an ultrasonic sensor which is mounted above the flow stream. The sensor transmits a sound wave, or pulse, which is reflected off the surface of the water and returns to the sensor as an echo. The water level is determined by the time elapse between sending the pulse and receiving its return echo (ref: Isco Open Channel Handbook, contact us for a copy). Since the speed of sound is affected by the temperature of the air that it is traveling through, most ultrasonic sensors have a built-in temperature probe to compensate for changes in the ambient air temperature.
On most sensors, the temperature probe is encased in the body of the ultrasonic sensor. Because of its “built-in” construction, the temperature probe is actually sensing the temperature of the ultrasonic sensor body which should reflect the temperature of the air around it. However, if the ultrasonic sensor body is exposed to direct sunlight, then its temperature could be warmer than the ambient air temperature, especially on hot summer days. This can lead to incorrect temperature compensation which in turn can result inaccurate level readings and, subsequently, inaccurate flow and totalizer readings.
The best defense against this is to keep the ultrasonic sensor shaded from direct sunlight. This can be done in may ways, from erecting a structure over the metering site to a strategically placed piece of plywood that provides shade. But, the most common, and professional, way is to use a sensor sunshade that attaches directly to the sensor. These are readily available from most meter manufacturers or equipment suppliers. The best time to install a sun shade is before you have installed the sensor and run the cable through conduit, however, sunshades can be modified for a retrofit into an existing installation if necessary.
Preventing exposure to direct sunlight not only preserves the integrity of the sensors performance but could also extend the life of your ultrasonic sensor. Please contact us if we can be of assistance with an evaluation of your specific application. Let us know what we can do for you.