This guide suggests a method of developing a reliable approach of reporting the correct level in cases where the sensor could be under water in a level control application. The data depicted in this guide shows that if the sensor is under water, there is still sound that can be detected for an unreliable range report. This will be solved by mechanical means so that air is trapped in a fitting and sound in air will properly capture an overfill condition by reporting the sensors minimum range. Refer to the next page for waveforms and explanation.
Picture of MassaSonic® PulStar® sensor & different fittings used for testing
Picture shown on page 1 includes the PulStar 95 and 3 2” to 1” NPT adapter fittings. All fittings when mounted will create an air pocket allowing for the sound to bounce within and would allow proper reporting of the sensors minimum range. Here is the description of each fitting:
- Fitting #1 is an off the shelf 2” to 1” NPT fitting by Spears Manufacturing. As you can see in the photo, the sensor is inserted as far as possible and it shows the exposed 1” threads of the fitting. This will not work due to the reflections by the threads and is not recommended for use.
- Fitting #2 is a modified 2” to 1” NPT fitting that is machined so that the threads are removed and a slope is developed so no sound is reflected back. This is Massa p/n 200758-1. This is the fitting that will work in the application
- Fitting #3 is another fitting found that has minimal threads that would need to be machined off. It is a Lasco version. This has yet to be evaluated with having the threads removed up to the transducer. It does pose as a future option.
Below are waveform plots for the evaluation. All plots long ping.
Reflection comparison plots within fitting
Reference plot without any fitting
Mounted in Spears fitting #1, reflections seen
Mounted in Lasco fitting #3, reflections seen but not as much as #1. Further review needed for modification
Mounted in Massa fitting #2, no reflections
Sensor without fitting placed in Massa Test Pond. Reflection from various sources. This will not properly report an overfill condition but 26” (0.66 m)
(short ping waveform)
Sensor with #2 fitting placed in Massa Test Pond. Sound bounces within fitting air pocket providing proper range report of 7.8”
(short ping waveform)
Application Considerations When Employing an Air Trap
Ultrasonic sensors are commonly used in level control and distance measurement applications such as industrial water & wastewater treatment operations, municipal sewer surge and flood overflow protection, power generation, and general tank fluid level. In most of these applications, the level sensor is utilized for fill level control or detection of maximum level to alarm an out-of-range event that requires action. If there is a possibility of sensor submersion, the ability of the sensor to function below its designed deadband may become important to an effective set-up and system response management. Although the prior description of proper utilization of an air trap can be effective in most conditions, good system engineering practices suggest confirming its effectiveness for the specific set-up. Other considerations may include:
- The cleanliness of the fluid and if it can leave deposits on the sensor face or become trapped either of which can cause interference once the fluid level recedes
- The viscosity properties of a fluid and related surface tension effects may affect system recovery time when the air trap drains
- If the fluid is aqueous based, in such cases the air trap can be effective because the air gap acts to prevent fluid from entering the space around the sensor as the air is not displaced. This approach is effective even in cases of small amounts of fluid sloshing.
- Is the sensor designed and rated for submersion events? While IP68 is advantageous, a minimum acceptable housing rating of IP67 is required for the sensor to reliably function where submersion is possible
- The performance characteristics of the sensor may affect the ease and effectiveness with which an air trap may be employed. The capability and programmability features of the sensor response algorithms to function under these conditions and respond to an event and then impeccably recover should also be considered.
MassaSonic PulStar Sensor Compatibility with an Air Trap
A diverse range of IP68 capable sensors are offered by Massa Products Corporation. The more sophisticated PulStar Plus variants come standard with a range of programmability features, which include drop down menus to automatically change detection settings to avoid false echoes induced by stationary objects in the sound path, such as mounting tubes that may also serve as an air trap. The sensors also feature a high-power setting for extended range detection advantageous in set-ups where headspace vapors can be present. Massa Products offers the PulStar Plus variants in general use PVC or high chemical resistant PVDF - both of them are IP68 certified and can be used in applications where submersion is possible. The PulStar is ideally suited for a range of mounting situations due to its small footprint and plus models are capable of covering a distance range of 4” (100 mm) to 20’ (6 m) with outputs available 0-10 V, 4-20 ma, RS485 and Modbus compatible.
MassaSonic sensors are field proven reliable, Detect the Difference!
This information has been sourced, reviewed and adapted from materials provided by Massa Products Corp.
For more information on this source, please visit Massa Products Corp.