Using an Ultrasonic Sensor Inside of a Pipe

Key Takeaways

  • 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.

Pipe requirements

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.

ultrasonic sensor inside pipe
We then added 2 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 because 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 creates 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.

ultrasonic sensor inside pipe

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.

Typical pipe slotted at the bottom

Some pipes by design are slotted the entire length of the pipe and typically are not able to range 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.

ultrasonic sensor inside pipe water condensationUsing 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.

  1. 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.
  2. 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.

Buildup inside of still well pipe

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.

  1. 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.
  2. 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.
  3. Multiple path reflection (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.
  4. 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.
  5. 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 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.

Recommend Sensor

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.

How Salt Water Affects Electronics

Key Takeaways

  • Know the affects of saltwater on electronics
  • See how saltwater damages electronics
  • Learn how to clean off saltwater residue

Pure distilled water does not conduct electricity and will not damage most electrical equipment that is clean and free of debris. Tap water is not pure water because it mixes with polarized minerals as it makes its way through your pipe. These polarized minerals are what makes water conduct electricity. This is how water, causes most electronics to short.

Salt water, on the other hand can be dangerous to electronics. This is due to the NaCI (Sodium chloride) creating a chemical bond with many surfaces. These bonds happen immediately upon wetting — resulting in a salt residue remaining long after the water is gone. Just one second of saltwater exposure can have the same effects as all day salt water exposure. Over a period of weeks, months or even years, the salt left behind continues to corrode any susceptible, affected surface. The corrosion process continues until the salt residue is exhausted or the corroded surface is consumed.

What Does this Have to Do with Sensors?

While our WR sensors are IP67 rated, saltwater can still cause problems when it comes to ranging. Once a sensor is splashed with saltwater, a residue forms on the transducer. It may not affect ranging immediately, but as the sensor gets repeatedly splashed, this residue continues to build. Eventually, the sensor may begin to report erratic distances or none at all. If the pin out of a WR sensor is not properly weather-proofed during installation, possible salt contamination may erode the connection causing sensor failure.

Figure 1. MB1000 & MB7066 before saltwater exposure

a sensor before saltwater a horn before salt water

Figure 2. MB1000 & MB7066 after saltwater exposure

a sensor after saltwater a horn after salt water

As you can see in Figure 2, the WR sensor has salt deposits in the horn and on transducer. If the residue is not neutralized it continues to build over the transducer which hinders or slows the sound wave and makes ranging difficult. The MB1000 sensor has salt deposits around the integrated circuits and all metal components. Until this residue is neutralized it continues to eat away at this board until the components are rusted and destroyed. This test was done with the indoor sensor to emphasize pin out damage, showing what salt can do to exposed circuitry and non-weatherproofed PCB material.

Figure 3. shows the PCB before (left) & after (right) saltwater exposure

a pcb board before saltwater a pcb board after saltwater

How to Neutralize Salt Residue

You can scrub for hours and the salt residue will remain, but you must neutralize the salt. There are products you can buy to accomplish neutralization, but you can also remove the residue by using isopropyl alcohol 90% or stronger.

Steps to Neutralize Salt Residue

  1. Remove from power source, then remove all wires, ribbon cables, anything that is attached.
  2. Take a soft bristled brush dip in isopropyl alcohol and gently scrub. Scrubbing too hard may cause more damage to the already fragile components.
  3. You may repeat this process until the residue is gone.
  4. Once board is clean, set to the side to dry.
  5. You need to clean every wire and cable that was removed. Pay particular attention to the connectors and ends of the ribbon cables to prevent corrosion of their contact surfaces.
  6. After the board and all parts are clean, use de-ionized water to rinse off the isopropyl alcohol residue.
  7. Do not reassemble until all components are dry and free of residue.

Contact our technical support team if you need any additional help or have any questions about sensor selection or technical support. We are here to help you succeed.

 

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A Guide to Understanding Range Readings

Key Takeaways

  • Resolution indicates the smallest reading or change in readings that can be reported
  • Precision indicates the smallest reading that can be taken repeatedly and reliably
  • Accuracy indicates how close the reading is to the true distance

When it comes down to it, you purchase a rangefinder for the range readings. The success of an application may hinge upon knowing the exact location of a target. However, a sensor may report one meter even if the target is not exactly one meter away from the sensor. Sensor specifications, such as resolution, precision, and accuracy, help us understand what wiggle room and error will be present in a reading.

Resolution

Image The resolution of a sensor indicates the smallest measurement or change in measurement that the sensor can report. Figure 1 shows two targets to help you conceptualize different resolutions by thickness and number of bands on a set of targets. A sensor with ten cm (100 mm) resolution would report one meter if it sees the target sitting anywhere between 950 mm and 1050 mm.
This means that one meter will be reported as long as the target is seen as sitting anywhere in that ten cm (100 mm) band.
Instead, if the sensor has one millimeter resolution the target would be seen as sitting between 999.5 mm and 1000.5 mm. Selecting a sensor with sufficient resolution is important for applications where fine and near exact range readings are required.

Electrical noise is the greatest limiting factor to the resolution of a part. Every electrical component will pick up and create electrical noise. This electrical noise appears as a wiggle in range readings. More electrical noise means more random drift in readings. MaxBotix Inc., limits sensor resolution to help our sensors ignore the random error created by typical levels of electrical noise.

Precision

Precision, which is also known as reading-to-reading stability, ties in closely to resolution. Where resolution explains how fine of a reading the sensor takes, precision explains the reliability and repeatability of these readings. Let us look at a sensor with mm resolution. Imagine taking three readings to the same fixed target: 974 mm, 995 mm, and 1005 mm. Even though the part reports distances down to the millimeter, the precision limits the reliability of these readings. In cases with limited precision, it may be best to treat the readings as if they had more limited resolution to restore precision. The readings from the previous example become a very stable 100 cm, 100 cm, and 100 cm if we treat the readings as if they had 10 cm resolution rather than millimeter resolution. MaxBotix Inc., limits resolution to provide high precision as found in the bottom two targets in Figure 2. Some manufacturers offer high resolution without high precision.

Accuracy

ImageIt is important to understand that neither resolution nor precision is accuracy. Resolution and precision give you an idea of the finest measurements a sensor can reliably report; accuracy compares the true distance to the distance reported by the sensor. A sensor with high accuracy will report a distance close to the true distance. Even if a sensor has very high resolution it may not be very accurate. However, if a sensor has high precision, you may be able to correct the readings to make them accurate. Consider the bottom left target in Figure 2 if we shift the Xs up and to the right we can make them both precise and accurate as seen in the bottom right target. Similar transformations may be applied to range readings. If a sensor has high precision, you can calculate the average offset between the reported reading and the true range and add that to the reported readings to yield higher accuracy.

Comparing the needs of your application to the specifications of a sensor is one of the steps involved in sensor selection. MaxBotix Inc., offers a range of products with diverse specifications to help you as the customer select only what you need for your application. This helps you to pay for only the features you need. 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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MaxBotix® Inc., Wonderful Innovation!

ImageWow! We have come out of a wonderful spring and summer of innovation.We just conducted an engineering test of the accuracy of our lowest-cost line of high-resolution sensors, the HRLV-MaxSonar-EZ. The two graphs show 1000 distance measurements each to a bar target at 500mm and 1000mm respectively.

The test demonstrates how accurately our sensors measure distance, and takes into account the part-to-part consistency, the full operating voltage range, and the reading-to-reading stability. The measured accuracy results speak for themselves. As CEO of MaxBotix Inc., I could not be more pleased with these test results.

Graph Generation

Chart 500mm    Chart 1000mm

A random sampling of 250 rangefinders were selected, (25 of each part number). For this test, the sensors measured the range to a 1” width 3′ long bar when powered at: 2.5V, 3.3V, 3.9V, and 5V. This resulted in 1000 range measurements for each distance. The mode of each range reading was recorded. The standard deviation of the reading-to-reading stability was 0.3mm to 0.7mm typically, and was always less than 1mm. (What this means in layman’s terms is that the sensor’s output never drifted more than three millimeters from the center reading, and typically was within one or two millimeters.) The modes were 500mm (exactly correct) and 1000mm (exactly correct). The means were 499.3mm (0.7mm low) and 1000.5mm (0.5mm high). The standard deviations were 2.3mm and 2.6mm respectively.

What Customers Say

In addition, our customers are noticing just how much better our new products are. Let me use two examples.

One company uses our product to continuously measure the distance to the ground on trucks moving at full highway speeds. While our 1cm resolution product, the MB7060, worked wonderfully, sometimes road noise was high enough to cause an occasional incorrect distance reading. Swapping out the MB7060 with our MB7360, eliminated all incorrect readings. Even though the millimeter resolution and temperature compensation was a plus, the greatly improved noise handling of this sensor made them a happy customer!

The next example is one system integrator testing our HRXL-MaxSonar-WR products for accuracy over temperature. They pleasantly discovered that in their setup, our sensor to sensor variation was generally less than two millimeters, and the reading-to-reading stability was better than plus or minus two millimeters. Only during fast temperature sweeps did they notice (and they expected this) that the thermal lag caused an offset of three to four millimeters, until the sensor and air stabilized at the new temperature. They recommend the evaluation of our HRXL-MaxSonar-WR to their full customer base.

Additionally, for the unmanned aerial vehicle (UAV) crowd, we introduced the I2CXL-MaxSonar-EZ lineup, allowing easy digital communication to multiple sensors. And for the users that desire side by side kiosk use (such as for HIPAA requirements), we have our ProxSonar line, where multiple sensors can operate together, sensing people reliably, without issue.

Our complete company profile can be viewed here.

 

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Tank Sensor Test of the HRXL‑MaxSonar®‑WR

Image
Tank sensors are an important part of industrial automation. MaxBotix Inc., recently performed a liquid level measurement test at our facility. This test ran from July 13, 2012 until July 16, 2012. The test was conducted by filling a bin with water and recording readings from the ultrasonic sensor using the serial connection while the liquid was allowed to drain.

The graph below shows the range reported by the sensor in millimeters as the tank was drained. The plotted graph is the first 100,000 readings that were reported.

Readings were very stable, rarely changing by more than 1-mm (except where secondary reflections from the sides of the tank caused acoustic phase cancellation and readings from the tank sensor were
+/- 2mm). The raw data collected during the test is available to the public here: Tank Sensor Draining Test.

HRXL-WR Draining Test

The HRXL-MaxSonar-WRM sensors are the recommended ultrasonic sensors for tank level measurement. The MB7369 and MB7389 sensors ignore smaller targets and only reports the range to target with the largest acoustic return. The Most-Likely filter is designed to report the distance to the largest acoustic return while ignoring smaller targets. When targets are of similar amplitude reflections, preference is given to the closest target.

Pictures of our test set up can be seen below.

Trash Bin Set Up Top View Trash Bin Set Up Inside View Trash Bin Set Up Inside Lid View

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