If you need to measure the level of a liquid with a differential pressure (DP) transmitter, you might want to consider the bubbler system. While uncommon, you’ll still see it occasionally, especially with containers that are buried or otherwise inaccessible.
I learned to use it for grounded tanks, where you can’t access the bottom easily. However, non-contact solutions have started to edge out bubblers. You should still know about this type of level measurement, though.
How to set up and use a bubbler system
The bubbler qualifies more as a system than a device. You’ll need a few items to set it up:
- A dip tube
- An air supply
- A pressure transmitter
- A differential pressure regulator
It may sound complex, but it’s way simpler than it looks. I’ll break down what each item does on an air bubbler system.
First, we have the dip tube, where the air coming from the air supply will flow. Usually, the open end goes inside the tank, somewhere near the base of the vessel.
Obviously the air supply comes next, connecting to the dip tube. It supplies a constant airflow to create the bubbles.
Usually, you’ll use a DP device for the pressure transmitter here. You need to connect the dip tube line to the high-pressure cell of your DP transmitter. The low-pressure cell will depend on your application. For open tanks, you let it vent to the atmosphere. For closed tanks, you should connect it to the top of your vessel. In both cases, you should install the transmitter above the tank.
The last item in our list of devices for the air bubbler system is the pressure regulator or constant differential relay. This device is the key to answering the question “How does a bubbler work?” It makes sure that the airflow through the dip tube remains constant. With a constant flow, when the level of the liquid changes, it also changes its hydrostatic pressure, also known as backpressure. Using this backpressure and the liquid’s density, the transmitter can calculate the level of the tank.
A low level will have little resistance on the gas, so the transmitter should read zero. As the level rises, the resistance does too, and the device detects the rising backpressure. When the level reaches 100 percent, the dip tube will contain all the air for the transmitter to measure.
Here are the calculations you will need:
P100% = high pressure side – low pressure side
P100% = [(liquid column height x product density) + (tank height at 100% x product density)] – (tank height at 0% x product density)
If you want to learn more about level measurement, click here!
This video will teach you more about air bubbler systems: