Bubble Sensors – Bubble Size Calibration
Do I need a calibrated bubble sensor?
Many applications only require knowing whether the tube is full of air or water, i.e. gross air detection; the size of the minimum bubble detected is often irrelevant to the end user. In these cases, calibrating the sensor to a particular bubble size is not required. However, final tests may still be performed prior to shipping sensors to ensure that sensors are compatible with the tubing that will be used for the application.
On the other hand, other applications can be sensitive to bubble size and calibration for certain size bubbles is required. Calibration is performed by injecting bubbles of known sizes using our proprietary bubble generation system into customer-provided tubing and changing the sensitivity of the bubble sensor until the desired sensor performance is obtained.
Generally, calibration is completed with a large bubble which must be detected by the bubble sensor and a small bubble (aka “nuisance bubble”) that must be ignored by the bubble sensor. Bubbles with a size between the large bubble and small bubble may or may not be detected and will vary between sensors in a batch. Often, bubble sizes chosen for final test are tighter then what the application demands to account for performance non-repeatability of the viscoelastic tubing used.
What size bubble can I detect in my tubing?
The bubble size that can be detected is dependent on several factors such as sensor design, sensor calibration, tubing properties, and sensor mounting orientation. Generally, a bubble that fills the entire tube diameter will always be detected and with calibration, we can usually detect a bubble that is as small as 50% of the inside diameter of the tube.
Mounting orientation of the sensor with respect to gravity can have a significant effect on the size bubble that can be detected. The ultrasonic beam of the sensor travels through the center of a tube with small ultrasonic “blind spots” above and below the acoustic path, see Figure 1. Buoyant forces will tend to cause the bubble to travel to the “top” of the tube with respect to gravity and if the sensor is mounted as shown in Figure 1, much of the bubble can be located in the blind spot, making it difficult to detect.
In order to detect bubbles that are as small as possible, it’s generally recommend to position a sensor as shown in Figure 2 to ensure a bubble will block as much of the ultrasonic beam as possible.
This can be especially important in large diameter tubes, where surface tension prevents bubbles form ever filling the entire diameter of the tube. A bubble in a large diameter tube may just get longer as it grows in size and won’t fill any more of the tube.
