In the last blog post in our series on sensors in lift stations, we discussed the benefits and challenges of using a submersible pressure transducer for continuous level measurement. As a reminder, continuous level measurement provides large economic benefits in energy savings when using pumps with variable speed motors.
Ultrasonic sensors are another common pick for continuous level measurement in lift stations. They function by sending pulses of high frequency sound waves. When the sound waves bounce off the target object & return to the sensor, the sensor is able to calculate the distance to the object based on the time it takes for the pulse of sound to be sent & received.
Today’s ultrasonic sensors are more accurate than ever before. They are a popular choice for lift stations primarily because of their non-contact approach to measurement. As we discussed in the previous post about submersible pressure sensors, anything that comes into contact with wastewater must be durable enough to handle the turbulence & debris. Ultrasonic sensors avoid these problems because of their non-contact approach. Therefore, installing & maintaining an ultrasonic is quite a bit cleaner, in more ways than one.
The biggest challenge to using ultrasonic technology in a lift station is false measurements. In an ideal world, the sensor is mounted in such a way that no object but the desired target ever returns an echo to the sensor. Unfortunately, this isn’t always easy. The “beam” of an ultrasonic sensor reaches out in much the same way a flashlight does. As the target moves away from the sensor, the area that the sensor can read grows larger. It is not uncommon for ultrasonic sensors in lift stations to have challenges with getting false readings from other stuff in the lift station.
Some ultrasonic sensor manufacturers provide a great deal of programming flexibility, which allows the user to “tune” the sensor for maximum effectiveness. For example, an APG ultrasonic has several programmable features that can help:
- Blanking distance – ignores objects within a certain distance from the transducer face
- Sensitivity – lowering the sensitivity effectively narrows the ultrasonic “beam” by blocking out weaker signals returning from objects on the side of the well
- Pulses – adjusting the power of the transducer pulses allows you to have a lower sensitivity and thus a narrow beam
- Averaging – increasing this effect will smooth out readings during turbulence
- Window – specifies a window for measurement, and ignores objects outside of that window
Mounting is another way to ensure better performance from your ultrasonic sensor. A simple approach, that will have a similar affect as some of the programming options, is to mount the sensor lower in the well, below as many obstructions as possible. This, however, is limited by the blanking distance of the sensor, which demands that the sensor be mounted about a foot above the highest point of measurement. Standpipes help raise the sensor if needed, and a stilling well captures a column of liquid inside a full-length tube. This is a great mounting method when practical.
In some lift stations, large amounts of foam can build up on the surface of the water. Ultrasonic sensors have trouble cutting through excessive foam. This only becomes an issue when the amount of foam becomes critical. For example, one inch of foam may not be a problem, but 18 inches of foam could potentially cause major problems. When the amount of foam could be cause for concern, it’s time to reconsider a submersible pressure transducer.
Ultrasonic sensors are a great way to avoid contact with a rather caustic liquid media. They are very accurate, easy to maintain, and offer a lot of flexibility.
What experience have you had with ultrasonic sensors in lift stations? We'd love to hear about how ultrasonics have have been successful, and ways that they haven't been so successful. You can tell us about it over on our Facebook page or get in touch with out Measurement Experts.