In ultrasonic level instrumentation , a controller measures the time-of-flight of the sound wave produced by an ultrasonic transducer for the round trip between the transducer and the target (material level). The speed of sound is the distance travelled per unit time by a sound wave. In dry air at 68°F, the speed of sound is 1,129 ft/s.
Factors that contribute to the calculation of sound velocity are temperature and the medium it is traveling through such as air or a gas. To measure distance using time-of-flight, you have to know two things accurately:
So why is temperature compensation an important factor with ultrasonic level technology?
First, we must understand what occurs to the sound velocity with temperature. The hotter the air is, the faster the sound velocity becomes. The colder the air is, the slower the sound velocity.
Modern ultrasonic transducers have built in temperature sensors . You might also need an external temperature sensor as well. So, how do you choose whether to go with an internal or external temperature sensor? Let’s make one thing clear: whether it is internal or external sensor, they both compensate the speed of sound as the temperature changes.
Years ago, I had a conversation with someone who was using ultrasonic level transmitters to measure the level of corn, potato flakes and flour in a silo. His issue was that the units were not measuring accurately and the transmitters were reporting a lower level. He stated that he encountered these issues every year during the summer months. After visiting the site, we found that the transducers were on top of metal silos and exposed to the heat of the sun. As the day progressed, the temperature registered by the transducer increased more than the actual temperature in the vessel. This was a result of the transducer being in direct contact with sunlight.
It is a good practice to utilize a sun shield whenever instruments are in direct sunlight, and more so when the external heat source can influence a level measurement. The integral temperature sensor in the transducer is surrounded by the transducer housing and a potting compound. Due to this, it takes more time to heat or cool the integral temperature sensor to meet the actual temperature inside a vessel. In this case, the solution was to bypass the integral temperature sensor and use an auxiliary temperature sensor inside the vessel to detect the air temperature in the process.
This issue could have been resolved with a sun shield and thus, the integral temperature readings would have not been influenced by external factors.
Alternatively, you can use an auxiliary temperature sensor. The auxiliary temperature sensor is not an expensive solution, but it is also not as practical or economical as using a sun shield. An auxiliary temperature sensor requires a cable run from the controller to an available port on the vessel. The latter is not always ready available, which can lead to additional costs if a retrofit is needed.
If you have an application where faster temperature response is required, than an auxiliary temperature sensor is the preferred recommendation. The critical nature of the application may or may not require temperature compensation beyond the ultrasonic transducer capabilities.
Have you encountered inexplicable discrepancies in level monitoring that made you question what you are using to measure?