PRINCIPLES OF OPERATION

Two technologies are available from Central Station Steam Company® which incorporate the use of ultrasonic energy to measure flow. Transit Time is based on time of flight, and Doppler measures frequency shift of the generated signal. The proper application of each technology is dependant on fluid conditions, and is covered below in Meter Selection.

Both flow technologies operate by generating ultrasonic energy via piezoelectric crystals embedded in two separate resin blocks, called transducers. The transducers are attached to the outside pipe wall using a sonic coupling compound (typically a silicon based grease) and retaining system (typically pipe straps), allowing the ultrasonic energy from the transducer to be transferred through the pipe wall and into the moving fluid. From this point on, the two technologies differ in their mode of operation and will be addressed separately.

Transit Time is used on clean (low suspended solids) non aerated fluids by measuring the difference in time (DT) it takes the pulsed ultrasonic energy to move from the upstream transducer and vice versa.

The difference in time of flight is directly proportional to the velocity of the moving liquid. Thus the higher velocity the greater the (DT) which is a conversion made by the flow meters' digital signal processing (DSP) from the phase shift (DØ) between the up and downstream pulsed sonic signals.

Volumetric flow rate is then calculated by taking the measured velocity and multiplying it by the effective internal cross sectional area of the process piping, which must be programmed into the meter prior to transducer installation.

Doppler is used on dirty (high suspended solids) or aerated fluids by measuring the frequency shift of the ultrasound as it reflects or bounces off particles or bubbles in the flow stream.

One transducer acts as the source or frequency generator and the other as the frequency receiver. As the sound waves bounce off objects in the flow stream it causes a shift in frequency which is dependant on particle velocity.

The greater the velocity the more frequency shift experienced. Caution should be exercised when applying Doppler since this is essentially an inferred measurement and is made on the premise that the particles or aeration in the flow steam are moving at the same velocity as the liquid. In certain applications this premise cannot be made and velocities measured can actually be unrepresentative of the actual liquid flow stream (typically reading low in liquids with high solids or slurries). Please consult Central Station Steam Company® techincal support for application review in these cases.