- Successful operation of an ultrasonic sensor inside a pipe has strict pipe requirements
- Ultrasonic sensor performance is limited by the principles of physics
- It is possible to get accurate results in a pipe, however, many factors must be accounted for to achieve success
Many customers have requested the option to mount an ultrasonic sensor in a pipe. During the testing and development cycle, we discovered a number of considerations and requirements that must be met for the application to be successful. When all of these are met, a user may be able to achieve the desired level of success for measuring the liquid level inside of a pipe.
A common request is for a sensor that operates in a two to four-inch diameter pipe. MaxBotix Inc., completed testing to verify such operation. Our testing found that the sensors may work when placed in a seamless pipe. The testing was conducted in an eight foot, schedule 40 PVC pipe.
Illustration 1 shows a smooth wall seamless pipe which yields the best results.
We then added a two-foot section of pipe with the coupling. The results were that the sensor ranged to the joint, however, not beyond the joint. The reason is that when a joint is added in the pipe, typically, the joint is not smooth and leaves a gap in between the two sections of pipe which create a place where sound is reflected back to the sensor resulting in ranging the distance to the joint.
Illustration 2 below shows a section of a pipe at the joint.
Illustration 3 shows a pipe that is only slotted at the bottom of the pipe which may work for some applications. It is unlikely that the sensor will range beyond the slot.
Some pipes by design are slotted the entire length of the pipe and typically are not able to range beyond the slots in the pipe which is shown in Illustration 4.
Another important consideration is the condensation of liquid droplets on the inside of the pipe, the sensor will often report the range to these droplets rather than the liquid level. This is because the droplets reflect the sound back to the sensor. Illustration 5 shows this concept.
Using a sensor in a still well is common. A still well is a pipe placed inside of a larger tank to ensure the liquid level is “still” for measurements. It also serves the purpose of preventing outside objects or debris from interfering with the sensor’s range readings. There are two conditions that should be met to achieve the best results.
- The first condition that must be met is there cannot be water droplets on the inside of the pipe. This can happen from liquid flowing down from the top of the pipe or condensation as previously shown in Illustration 5.
- The second condition is that any interior build-up of debris or sludge must be cleaned out. This can commonly happen at the levels where the liquid typically sits. This is easily corrected with a periodic cleaning of the pipe. Sometimes this is not a feasible requirement for users.
Illustration 6 shows a buildup that would look like a target.
Additional considerations of pipe operation
Now that there is an understanding of the type of pipes that can be used as shown in Illustrations 1 and 3, there are a few more items that one must be aware of when operating a sensor in a pipe.
- Some customers have found that our sensors have worked within pipe diameters of 6 inches or more. MaxBotix recommends that users consider a pipe diameter greater than 8 inches. This allows the surface of the liquid to become a much larger target relative to many of the considerations for pipe operation.
- When operating inside of a pipe with imperfections, dents, dings, and breaks, the imperfections may give the largest ultrasonic reflection resulting in the range being reported to the imperfection. There is limited engineering around these types of issues while still providing usable range data.
- Multiple path reflections (e.g. the path directly to the surface and the other paths that bounce off the sides of the pipe and then hits the surface and bounces back to the sensor). These reflections can cause the target to appear to walk closer and farther as the temperature in the pipe changes. In some cases, this can be between 5-10 cm however it is common to see 1-2 cm.
- Phase cancellation can happen which causes the acoustic reflection from the liquid level to disappear. This can happen at certain distances or temperatures in a pipe. When this happens, the sensor doesn’t see the liquid level. Our MB7052 or MB7092 have some special filtering to help reduce this issue when it occurs for short periods of time.
- A common question is if you can range targets outside of the pipe? An acoustic lens is created at the end of a pipe when it is pointed into the air. This typically creates a target at the end of the pipe which results in the sensor only ranging to the end of the pipe and not beyond the pipe.
Pipe Operation is a limited warranty application by MaxBotix
After reviewing the requirements for successful use inside of a pipe, MaxBotix Inc. has determined this application carries a limited warranty. We warranty our sensor from a manufacturing defect; however, we are not able to extend this warranty into the use inside of a pipe because of the many requirements for success. We believe that this use can work for some users because of their ability to control the environment however through our experience, these requirements cannot be met by a majority of users.
Some users have experienced success using sensors in a pipe. The sensors of choice have been our MB7052, MB7092, MB7369, MB7389, MB7569, and MB7589. Users are encouraged to complete their R&D phase to ensure the sensor will meet the performance requirements.
If you have any questions about a sensor’s specifications, contact our technical support team. We will be more than happy to help.
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