Ultrasonic Level Measurement

Ultrasonic level measurement works on a "time of flight" principle, a high frequency sound wave being generated, bouncing from an object and returning. The time taken is related to the speed of sound and a distance is derived. If we know other parameters of the applications, such as the overall height and shape of the vessel in which the measurement is being made, then we can calculate volumes, weights etc and provide alarms.

Ultrasonic level measurement devices consist of a transducer and a control unit. In some units these are incorporated into a single box, where in the majority of applications a transducer will be connected by a cable to the control unit. Within the transducer is a piezo-electric crystal, which is "excited" when a voltage is applied across it. This in turn generates a powerful sound wave that bounces off the"target". A proportion of the returning echo will re-excite the crystal, generating a voltage which is analysed by the control unit.

Typically, several hundred volts will be applied across the crystal, and a return voltage of the order of millivolts is available for analysis. Anything in the beam path will provide an echo, and so the echo from the true level has to be discriminated from a whole host of competing signals. Purely in measurement terms, the difference between the very best ultrasonic equipment and the also-rans lies both in the way the transducers are put together and in the quality of the software that analyses the incoming signal.

All transducers have a beam angle and operate at a sound frequency. They also have a blanking distance.

In general, the higher the frequency at which the transducer operates, the higher the resolution of the measurement and hence the transducer can discriminate smaller changes in level. The higher the frequency, however, the shorter the measurement range. Therefore a balance is drawn between range and resolution. Within the Pulsar range, the longest range trasnducer, the dB40, has a frequency of 30kHz and a 40 metre range, while the dB3 has a frequency of 125kHz to give incredible accuracy up to it's maximum range of 3m.

With the advent of digital signal processing, the beam angle is less of an issue than perhaps it once was. That said, a good transducer has a beam angle as narrow as possible to minimise any competing echoes and maximse the strength of the signal. Pulsar's long range transducers have beam angles as small as 5 degrees.

The echo that returns form the target level is analysed by echo processing software. Pulsar's software is called DATEM (Digital Adaptive Tracking of Echo Movement) which has been developed and refined over several years to provide excellent results. DATEM allows echoes to be tracked and competing echoes to be ignored, even in very hostile and difficult environments such as sewage wet wells.

 

 

Pulsar Process Measurement Limited | Cardinal Building | Enigma Commercial Centre | Sandy's Road | Malvern | Worcestershire WR14 1JJ Tel: +44 (0) 1684 891371 | Fax: +44 (0) 1684 575985 | e-mail us ©2007 Pulsar Process Measurement