Multiple path transit time method
Measuring the flow using ultrasonic multiple path transit time methode has been proven since years. At first, the medial flow velocity of the medium is measured. The flow can be calculated accuratly if the cross section of the flume is known. The calculation model is standardized in the ISO6416 (open channels) and the ISO60041 / ASME PTC 18 (filled conduits). The gallery below leads you through the basics and advantages of these established measurement method by means of the application in an open channel. The principle is exactly the same for filled channels or pipes. Please do not hesitate to contact us for further information.
Generating ultrasonic signals
The ultrasonic transducers convert electric signal to ultrasonic signals following piezo’s law. The ultrasonic signal is coupled to the medium via an optimized membrane. An acoustic coating in the transducer’s enclosure ensures a optimized signal exploitation.
Measuring the flow velocity
How can the transducers used for flow measurement?
In the simplest case two transducers are mounted on the opposite banks at an angle of 15..75°. The transducers send ultrasonic signals to each other. Two transducers make on so called acoustic path and work alternately as sender and recipient. The plan view should make the measurement principle clear.
Transit time method - Basics
Like crossing a river by boat in the direction with the flow is faster than the other way round, the same effect is shown with the ultrasonic signals. An ultrasonic signal sent upstream travels at a slower speed than an ultrasonic pulse sent downstream. By measuring separately the transit times of signals sent in the two directions, the average velocity (path velocity) of the fluid crossing the path of the signal is determined vectorially. deltawave makes up to 120 measurement cycles each second to establish an average and to minimize random error.
Multiple path measurement - defy unknown flow profiles
The average flow velocity over the whole cross-section is required for the flow calculation. In principle it is possible to determine this average velocity out of one single path velocity using calibration factors. Problem: The calibration factors are only valid if there is a known and stable flow profile. In most of all cases you unfortunately will not have these ideal flow conditions. The occurence of back waters, waves and side friction are only some of the possible sources of interferences in practice. Therefore you will get a huge measurement error using only one paths. This is by the way a general problem of single point measurements.
It is recommended to install multiple paths to get an accurate measurement even in case of disturbed flow profiles. The picture below shows a 4-paths installation. The measurement of the average flow velocity is determined in the ISO6416 (partially filled flumes) respectivley the ISO60041 / ASME PTC 18 (filled flumes). It is essential that a higher number of paths leads to a higher measurement accuracy. An accuracy up to 1% is possible in open channels – in filled pipes the accuracy is even 0.5%.
The adjoining picuture shows you a possible arrangement of 4 acoustic pathes in an open channel. You have various mounting possibilities. You will get an accuracy of 1% of reading with that installation. The transducers are mounted directly to the channel wall in the easiest case. But you also can use mounting rails to ensure mounting and service workings without interrupting your process. deltawave is not only used for rectangular channels but also for trapezoidal or oval one’s. Please ask us: We have a mounting solution for the most of all applications.
Please see applications for possible and realized mounting solutions.
High Accuracy in case of short inlet ways
To ensure high measurment accuracies even in case of short inlet ways (<10D) deltawave systems always provide the option of utilizing crossed-path configurations. Thus, the systematic error due to effects of transverse flow – caused e.g. by bends – is reduced.
You see a possible 4 path crossed arrangement in the picture above. systec Controls has utilized this 4×4 crossed-path configuration at severals of critical measurement sites in order to maintain flowrate measurement accuracy. This is the code-specified configuration in IEC-600041 and ASME-PTC-18 flowmeasurement specifications, used to verify performance of critical turbo-machinery – where maximum flowrate measurement accuracy is required.
Multiple path transit time method - Overview
- Maintenance-free, calibration-free
- High measurement accuracy up to 0.5%
- High accuracy even in case of disturbed flow profiles
- Standardized flow-calculation (ISO6416, ISO60041, ASME PTC 18)
- High signal exploitation because of the optimized transducers
|Number of acoustic paths||4 / 8 / 12 / 16 (later upgrade possible)|
|Number of pipes or channels||1 to 4 (later upgrade possible)|
|Measuring method||Multiple-path transit-time (time-of-flight)|
|Communication interfaces||RS232, LAN (Ethernet), USB|
|I/O||4..20mA, Relays, Transistors|
|Certificates required / Acceptances||CE, Ex, ISO 9001 / ISO60041, ISO6416, ASME PTC 18|