Field Survival Guide
The Field Survival Guide is a series of articles that show you how to overcome common problems you run into out in the wild. It’s what MacGyver would do if he were in your shoes.
There’s no sense in hiding the truth - ultrasonic sensors can be difficult to mount.
Every now and then, you run into common problems that make it seem like mounting an ultrasonic level sensor is impossible. A few of the more common issues we hear about are:
- Sensor blanking distance is too long – need to measure liquid to the top of your tank
- Sensor picks up false readings from objects inside the tank (like a ladder)
- Sensor won’t function well in the heat
All of these seem like the sensor just won’t work well for the application. That may be, but before you throw in the towel, here is a tip that just might overcome the impossible.
The trick is simple – build standpipe, or a wave-guide. This is a plastic pipe that raises the mount of the sensor as far as you need it to. At the same time, the pipe will block out unwanted echoes from objects inside the tank, and it can lower the temperature at the sensor face quite a bit.
This bit of plastic pipe will raise the sensor up to effectively eliminate the blanking distance of the sensor. So you can measure your liquid all the way up to the top of the tank. Of course, you’ll need to make sure you work in some control to keep the tank from overfilling.
Always make sure the face of your ultrasonic transducer is farther away from the highest liquid level than the blanking distance is long. If not, you’ll lose the ability to measure your liquid once it passes inside the blanking distance.
You may think that the pipe would focus the profile of the sound waves. While the pipe does narrow the sound waves slightly, it actually serves to block any echoes that aren’t perpendicular to the sensor.
In other words, only the echoes directly beneath the sensor are able to make it back up the tube to provide a measurement.
Surprisingly, a standpipe will also serve to lower the temperature at the face of the ultrasonic sensor, so it can measure in tanks hotter than its operational range. We’ve seen a drop in temperature of 20° F on a 4 inch standpipe.
How To Build One
Perhaps the best part about this trick is that it’s so easy. There are a few critical steps that must be done right, but none of it is difficult.
You’ll start with this shopping list:
For two 3 inch standpipes
- One (1) 6” threaded 2” PVC nipple (male threads on both ends)
- Two (2) 2” slip couplers with 2” NPT female threads
For a longer standpipe
- One (1) PVC DWV cleanout adapter with 2” NPT female threads
- One (1) 2” slip coupler (slip coupler on both sides)
- One (1) 2” PVC Nipple (as long as you need)
All parts are standard, and can be found wherever PVC pipe fittings are commonly sold.
If you’re making two 3” standpipes, simply:
- Cut the 6” pipe in half and debur the edge
- Glue the cut edge into the 2” slip coupler
- Finally, debur the inside edge of the pipe with a sharp knife at a °45 anlge – just behind the threads
Make sure it has a smooth finish. Any cuts, shavings, or rough edges will cause a false echo. So will the pipe itself if the inside edge of the pipe, behind the threads, is left as a 90° edge. The 45° cut allows the sound waves to pass the end of the pipe without creating a standing echo.
If you’re making a longer standpipe, simply:
- Debur all the edges of the pipe just like you would for a 3” standpipe
- Glue the cut edge of the pipe into the 2” slip coupler
- Glue the cleanout adapter into the other end of the 2” slip coupler
Again, all surfaces and edges need to be smooth, and the inside end of the pipe must have a 45° angle finish.
These standpipe instructions are meant for an ultrasonic level sensor with a 2” NPT mount. You would simply change the size of your pipe fittings for a sensor with larger or smaller NPT threads.
If your tank doesn’t have female NPT threads, then you can fashion brackets or straps to hold your standpipe and ultrasonic sensor in place. Whatever you do, make sure the sensor face is perpendicular to the surface of the liquid.
Good luck out there.
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