## Ultrasound beam, resolution, window function

#### What is the smallest distance of measurement from transducer?

It takes several microseconds to UVP Monitor to switch from transmitting to receiving mode. This time solely due to electronic switching makes the smallest measurable distance approximately 5mm from transducer face.

#### What is ultrasonic beam divergence?

The beam divergence is determined by the ultrasound wave length and the initial beam size. For most working frequencies and standard transducers, beam divergence is approximately ±5°.

#### Is it possible to measure flow in channels smaller than 90mm?

Yes, without any problems. UVP Monitor always stores a 128-point profile. As the measuring volume length is 0.74 mm for a standard 4 cycles pulse of 4 MHz the smallest distance between these sampling volume is 0.74 mm for a non-overlapping configuration, and thus the smallest measurable full profile length is in fact 90 mm.
But not all measurement points have to be used, and in case of shorter measured length the rest of 'unused' points are simply not shown on screen and in data files. For example, when measurement on 20 mm channel is made, the number of 'used' points is only 20 : 0,74 = 27 points. For practical purposes this is more than sufficient.
Moreover a minimum of 2 cycles per pulse can be used in case of good echo conditions, or a higher frequency of 8 MHz proposed by Met-Flow implying shorter wavelength (4 cycles per 8MHz pulse is equivalent to 0.37 mm sampling volume) can be selected.

#### What is measurement resolution and accuracy for low velocity flows?

A data length for the velocity value is 8 bits. The first bit is used for the sign, which represents flow direction. Velocity values are stored in the remaining 7 bits. This means that the velocity resolution is 1/127 of the maximum detectable velocity which can be selected by the UVP. The best accuracy is thus theoretically 0.8%.
When the maximum detectable length is selected as 748mm for water, the maximum detectable velocity is 8.77 cm/s which gives the minimum velocity resolution as well as the threshold velocity as 0.7 mm/sec. Adding to this, there is an effect of beam size, particle motion, etc. UVP accuracy has been investigated using a rigid body motion of water in a rotating cylinder, giving overall velocity error better than 5%, and overall axial position error better than 1%.

#### What typical spatial resolution can be expected from UVP method?

Each single-point measurement of velocity out of the 128 points profile is made in a finite cylindrical volume which depends on the ultrasound beam size. Its longitudinal resolution corresponds to the burst length (0.74mm for 4 cycles of a 4 MHz US wave) while its lateral resolution corresponds to the beam diameter and divergence of a respective transducer.
For high spatial resolution one should then choose a high US frequency.

#### What typical time resolution can we expect from UVP method?

Measuring speed of UVP Monitor depends of concrete set-up of measurement parameters. Generally, 30 to 200 full profiles can be measured and saved every second, corresponding to a sampling time lying between 4 and 30 ms roughly. Obviously, it is possible to slow down sampling rate to a desired value.