Features

Fig. 1 – U-shaped flow path.
One of the more common purposes of an ultrasonic flow meter is to measure the velocity of a fluid in order to calculate the volumetric flow rate of a medium through a tube. This can be done through the use of a basic principle involving the transit-time of crystal transducer signals across the medium. Transit-time is a common flow meter measurement method that gives extremely accurate and repeatable results. Since this type of flow meter can work with a variety of fluids at a range of different temperatures, typically they will be calibrated for those specific variables.

Ultrasonic flow meters are designed in many different ways. There are two main types of ultrasonic flow meters: intrusive and non-intrusive. Intrusive is where the transducers are inserted directly into the medium through the tubing. Non-intrusive is where the transducers are located outside of the medium, attached to the outside of the tubing. Both types of flow meters exemplify how they can be designed for a variety of applications for highly accurate and repeatable flow measurements.

Components

Fig. 2 – Reflected signal path.
Ultrasonic flow meters require a couple of components in order to function properly. Crystal transducers are made of piezoceramics and are potted in a housing typically made out of plastic or metal. They must be potted flat against the housing and the housing flat against the tubing, sometimes with a good acoustic coupling material such as rubber, in order to optimize the signal. These transducers are then either inserted invasively into the tubing and medium or attached to the outside of the tubing and medium noninvasively. The tubing is usually made out of a type of hard or soft plastic with the option of many different diameters. The medium can vary in type and range of temperature depending on the application as well.

How It Functions

Fig. 3 – Straight signal path.
The main principle behind the ultrasonic flow meter is called the transit-time ultrasound principle. It begins with the crystal transducers that work in pairs but usually only two or four are needed. They can be positioned both non-intrusively or intrusively in relation to the tube. Non-intrusively, they are typically placed outside the flow meter, positioned so that the signal can be sent and received by the crystals. Some examples of crystal placement are positioning the crystals across from each other so their faces are parallel across the tube. (See Figure 1)

Another method is to place them on the same side of the tube so the signal is sent through the tube and reflects off of the top of the tube back to the second transducer. (See Figure 2)

Intrusively they are typically inserted directly across from each other inside of the tube mounted at both ends. (See Figure 3) These transducers are aligned so that when they are excited with a small voltage impulse, they begin to vibrate causing an ultrasonic signal to be transmitted from one to the other. Typically one crystal acts as the sender and one crystal acts as the receiver, converting electrical energy to acoustical energy and back again. As the signal travels from one transducer to the next, it will travel through the crystal housing, the tubing wall, and the medium. So all three of these materials, along with the positioning and distance of the crystals need to be considered to optimize the strength of the signal path. The flow meter can also be calibrated for specific ranges and applications, depending on the medium and temperature, because the speed of sound will change with these variables.

Fig. 4 – Angled signal path.
After all of the design and calculation variables and parameters are determined and the flow meter is set up, the transit time can be measured. The transit- time is acquired by measuring the time it takes for the transmitted signal to travel from one crystal transducer to the other. As the medium flows through the tube, the signal will move faster with the flow (downstream) and slower against the flow (upstream). This time difference will then be used to find the volumetric flow rate of the medium through the tube. (See Figure 4)

The volumetric flow rate can then be calculated from the known velocity by multiplying it with the cross sectional area of the tube.

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