Frequently Asked Questions on DOP features

What about the spatial resolution?

Two types of resolution have to be considered. The first one concerns the dimensions of the sampling volume which is the volume from which particles contribute to the measurement of a single velocity value. Its lateral dimension (perpendicular to the ultrasonic field axis) is determined by the shape of the ultrasonic field. Typical values are from few millimeters to few centimeters. The axial dimension of the sampling volume is fixed by the duration of the emitted burst and the bandwidth of the receiver. Typical values are in the order of tenth of millimeters to few millimeters.

The other resolution concerns the minimum distance between two adjacent gates. This distance is determined by the sampling rate of the incoming echoes. Three different situations may appear:

 
  • The distance between gates is lower than the thickness of the sampling volume.

    In such a case the sampling volumes overlapped each other.
    •
 
  • The distance between gates is equal to the thickness of the sampling volume.

    In such a case the sampling volumes are contiguous
 
  • The distance between gates is greater than the thickness of the sampling volume.

    In such a case a dead zone exists between each gate, zone from which no information is captured and collected by the velocimeter
(to get some numbers, see DOP2000 specifications)

What about the velocity range and resolution?

The velocity range is defined by the Doppler equation, which involved three parameters, the emitting frequency, the pulse repetition frequency and the sound velocity in the liquid. By playing with the first two parameters it is possible to cover a wide range of velocities, from less than 1 mm/s to few m/s. Both positive and negative velocities can be measured simultaneously. Moreover, an original technique allows to distribute unequally the range of positive and negative velocities.

All velocity values are given in a signed byte format, which fixes the velocity resolution to 1/128 of the maximum scale. In order to increase the velocity resolution, the maximum velocity scale can be reduced to 1/2, 1/4 or 1/8 of the maximum velocity corresponding to the pulse repetition frequency.

(to get some numbers, see DOP2000 specifications)

Which velocity component is measured?

The velocity component measured by the velocimeter is always the component in the direction of the ultrasonic beam (Vus). When the direction of the real velocity is known, the velocimeter can automatically compute the real velocity value(Vreal) by using the value of the Doppler angle(q). In such a case the depth values displayed by the velocimeter are the depths perpendicular to the velocity direction (Preal).

How to choose an emitting frequency ?

The emitting frequency is directly linked to the resolution. So, in most situation, it is advantageous to select the highest emitting frequency as possible. Unfortunately, two factors limit the available choice. The maximum velocity that should be measured (see the Doppler equation) and the attenuation of the ultrasonic waves when they travel through the liquid and the wall material that have to be crossed.

The attenuation of the ultrasonic waves depends a lot on the emitting frequency and on the type of liquid. High ultrasonic frequencies are much more attenuated than low frequencies.

How is the flow rate computed?

The velocimeter can compute automatically the flow rate when a measuring section is defined. The flow rate is computed by integrating the velocity profile between two user's limits, placed on the velocity profile. These two limits define a section, which is assumed to be circular. The flow rate can be displayed in real time on the screen.

The integration of the velocity profile is realized by adding the contribution of all small subsections, as represented in the figure above. Each subsection has a thickness (e) equal to the gate width and in each subsection the velocity is assumed to be constant and uniform.

How fast is a velocity profile measured?

Velocity profiles can be measured up to a rate of 300 Hz. But this is not always the case. The acquisition time of a complete profile depends on three parameters:
 
  • The sound velocity in the liquid
 
  • The maximum depth at which measurements have to be done

    Once has to wait until an emitted ultrasonic burst travel to desired depth and comes back to the transducer. The maximum allowed time is fixed by the pulsed repetition frequency (PRF).
 
  • The number of ultrasonic emissions used to estimate the velocities in the gates

    The variance of the measured values is influenced by this number. As velocities are nothing else than first moments order of statistical processes, having more realizations of these processes reduce the variance of the estimated values.
    •

Is it possible to synchronize the velocimeter?

Yes, the velocimeter can be synchronized to an external event by using its trigger input. The trigger signal can be a low or a high logic level (0 or 5 Volt) on the external trigger input or a keyboard action. Moreover after a trigger signal has been accepted, the velocimeter can wait a user's defined lapse of time and then start the acquisition of data profiles. The delay between the trigger signal and the acknowledge of the trigger signal can be as low as few microseconds. ("Waiting for" mode).

The smart trigger interface allows to define complex acquisition sequences, with automatic record procedures.

How close to the transducer a measurement can be realized?

Normally the same transducer is used to emit the ultrasonic burst and to receive echoes. This implies that during the emission it is not possible to receive any echoes. Moreover, just after the emission, the transducer has to dissipate the amount of energy that hasn't been send into the liquid. Only after this dissipation it will be able to sense the very small level of the ultrasonic echoes.

The position of the first measurable gate depends therefore on the emitting frequency, the burst length and the size of the active element that generates the ultrasonic waves. For instance, at 8 MHz, the first measuring gate can be placed at around 3mm from the surface of the transducer, which value should be considered as the minimum value.

As the DOP2000 can emit and receive on separate transducers, it is possible to overcome these limitations by using two different transducers.

How are measured data recorded?

All displayed data profiles can be recorded to a file in a binary or ASCII format. This means for instance than when both the velocity profile and the Doppler energy profile are measured and displayed, both data profiles will be recorded. In order to offer the maximum flexibility, both format can be selected at the same time, which produce two files. Each recorded file contains a user's reserved area for the introduction of comments.

The binary format does not record only measured data but also record all the values of the functioning parameters. This allows the execution of any kind of post-processing methods on the original data, and also allows to replay directly on the instrument or on an external PC any recorded data file.

An accurate time stamp (micro second resolution), the flow rate associated to the velocity profile, an identification byte for the connected channel and the trigger sequence are attached to all recorded data profile.