FAQ Ultrasonic for Pipe Extrusion

What is the pulse echo principle
Similar to the depth sounder, a short ultrasonic pulse is generated in the transducer head by a piezo crystal and transferred into the body to be measured. The coupling medium between the transducer and the measured body is water.. When an ultrasonic signal is lead through areas of different density (plastic material-water -air), there are always reflections in the areas where different materials meet. These reflected pulses return to the transducer where they are recorded. The transmission time of the pulse counted from when it enters the measured body to when it returns through the same spot equals two times the wall thickness of the measured body. Multiplication of this transit time by the material sound speed properties, gives the value of the wall thickness. In respect to flaw detection, echo amplitude is the common method. [top]

When are: Direct-Coupling, Waterpath-Coupling ore Immersion-Coupling suitable
Direct-Coupling, is applied to large pipe dimension. Here a good sound throughput is achieved. This method is applied where the other coupling methods fail to succeed. Waterpath-Coupling, is the most applied method, it is suitable in a wide dimension range. Immersion-Coupling,is generally recommended with small pipe dimensions, thus non contact measurement and measurement without force are necessary. [top]

What kind of Ultrasonic transducers are recommended and what about the frequency
Transducers as in the range of 1 - 30 MHz are applied, the common frequencies are 1, 2, 5 and 10 MHz. Measurement of smaller wall thickness requires a high frequency in order to permit pulse separation between the interface echo and the backwall echo for evaluation. For a wall thickness of 0,5 mm, 10 MHz transducers are used. For a large wall thickness a lower frequency (1 MHz) is required as the sound attenuation in plastic material is very high when high frequencies are applied. Pulse separation for greater thicknesses is not the most important factor as the distance between the interface and the backwall echo is very large. In respect to the large range of applications a transducer is necessary, but usually one per Extrusion is enough. [top]

For what reasons are focused transducers used
Mostly plane transducers are used. Transducers focused lengthwise are used for pipe diameter of less then 6 mm in order to concentrate the sound energy to the surface line of the pipe. Transducers with point focusing are of importance only with flaw detection when flaws smaller than 1 mm size have to be detected. [top]

Why is a coupling medium required
As transmission of ultrasonic energy by air is not possible at this frequency range, a coupling medium is required. A suitable medium for ultrasonic transmission between the transducer and the measured body is water. [top]

Must an Ultrasonic Wall Thickness System have an echo signal display
For service it is a precondition to refer Ultrasonic echo signals on a screen, it is common to have a scope extension. It is importand to point out that a display on the computer screen is preferable. Today systems have already begun to integrate the echo signal display onto the PC's screen. For some cases this is a pre-condition for the operator to carry out adjustment work. [top]

What's about the statements regarding measurements accuracy
The accuracy statements of In-Line ultrasonic measurement systems can only be made when considering the calibration method, and even then it's often so that individual practical conditions are not available. The practically gained accuracy covers a range of 0,02 mm - 0,10 mm. The resolution of wall thickness measurement should be 1/100 mm and is confirmed by most suppliers. [top]

What are the different calibration methods
1. Manual
At the cooled pipe the wall thickness is determined (with mechanical measurements), further a correction factor needs to be calculate and the original ultrasonic measurement result to be recalculated. The manual correction has to be done frequently since the pipe temperature has changed and the sound velocity is different.
2. Temperature equation
During the pipe cooling process there is a temperature profile in pipe axis direction. This temperature curve can be, to the measurement location, determined by mathematics. The mathematics are based on physics models for the cooling procedure. This method is rarely applied, since the mass temperature is not known exactly as a starting point and the cooling model is not always matched with the real cooling devices.
3. Pipe In-Line weight/meter measurements
The safest and simplest method is the calibration with the help of the mass throughput value. A mass throughput measurement unit is a precondition for this function. With the measured "ultrasound time of flight" a relative pipe cross section is determined. Is now the absolute cross - wall thickness by the mass throughput unit known, the real sound velocity can be determined and the real wall thickness is calculated. The preconditions for that calculation are, that the outer diameter and the material density are known. This calibration procedure is automatically by the system done and continuously. [top]

What is the advantages of the Outer and Inner Diameter Measurement
With the combination of wall thickness an economically complete solution is achieved. Thus a separate diameter measurement unit is not required and even have the benefits of a central geometry data record, simple operation with no complicated interface solution. A further advantage is that inside diameter result is gained automatically. Wall thickness and diameter are measured practically simultaneously by means of a rapid electronic multiplex process, all results are from the same pipe section. Thus this is a disadvantage because of the use of a separate outer diameter gage. [top]

What Test Mechanism is the best
There are the various mechanisms available, all have their own advantages and disadvantages. A general classification can be made, these are, fixed mounting and scanning transducers.
Fixed transducers 4/6/8...pieces
Measurement chamber for 2-50 / 10-110 mm pipes
Flanged Measurement ring for larger 110 mm pipes
Flanged ring on the calibration sleeve.
Transducer holder in the calibration sleeve
Reversible mechanics, Scanner
Swivel arm, stands alone between the cooling baths
Immersion (Flanged) scanner at the vacuum bath
Immersion (Flanged) scanner at the pressure calibration [top]

When are reversible scanners or fixed transducers recommended
The user has to decide how many Measurements (4/6/8...32) on the pipe circumference, are necessary and if it needs to be done continuously at the circumference, , since the reliability of the fixed transducer mechanism is preferred. [top]

What is the best measurement position in the extrusion Line
There are the various positions available, all have there advantages and disadvantages. The most recommended position is behind the vacuum bath. For large dimensions it is an advantage to measure as close as possible to the die, thus maximum material saving is gained. If a very early measurement is demanded, for instant in connection to a thermal die centering device, special short (2 meter) vacuum baths are designed. In that case the second bath is installed as a vacuum bath too. In general, the fixtures for fixed transducers are adapted in all situations, e.g. measurement chambers especially for small dimensions, or the transducers mounted into the sizing sleeve. [top]

What Measurements should the system display
Measurement values around the circumferences
Thickness by circumferences for scanners
Max-Min values
Eccentricity
Mean value
Trend in pipe length axis
Trend over the circumferences(scanners)
Alarm display
Outer and Inner- Diameter, mean value, max/min, ovals [top]

Is a Thickness Measurement of Coextrusion Layers possible
There is a wide range of Coex Pipes, with extreme differences, from thin automotive hose up to thick PVC foam pipe. In general there is no statement available wether, ultrasonics can measure the total or the layer thickness. It is depend upon the layer thickness and the kind of materials, whether it is possible to do an evaluation. Each individual case has to be investigated. [top]

Which factors influence the Ultrasonic wall thickness measurement i.e. the sound velocity It's well known that the ultrasonic wall thickness measurement is dependant upon the calibration of the sound velocity, thus is temperature reflected. It is fist essential that the conditions for the initial calibration stay constant. Influences on the temperature can rise by process changes such as, haul off speed, cooling temperature, mass temperature and row material. All influences stay more or less constant. [top]

For which plastic pipe dimensions is ultrasonics suitable
The ultrasonic is applied for plastic pipe dimensions of 0.15 - 75 mm wall thickness and 1.0 - 1200 mm diameter. There are exceptions for special materials and also sometimes for certain measurement points in the extrusion line. [top]

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Rolf Diederichs 18.Dec. 1995, info@ndt.net

/DB:Article /AU:Diederichs_R /IN:NDTnet /CN:DE /CT:UT /CT:thickness /CT:process /CT:plastic /ED:1996-01

FAQ - Frequently asked Questions Ultrasonic for Pipe Extrusion

FAQ - Frequently asked Questions
Ultrasonic for Pipe Extrusion