All these questions were asked by the originators of the development and construction of an automatic pycnometer: Prof. Dieter Schwechten, working until 2020 at the Konstanz University of Applied Sciences, now retired, and Jens Corell, of Siebtechnik GmbH. Jens Corell has, since 2000, been the head of department in the technical sector of sampling, comminution and laboratory equipment at Siebtechnik GmbH.
Statement of particle density is an elementary physical property for the characterisation of particles and bulk materials and is frequently needed in industrial processes. Measurement of true or particle density using gas pycnometers is thus a daily requirement in particle technology. Manual measurement, however, is complex, time-consuming and extremely sensitive to operating errors. A single measurement takes around 15 minutes, and the time needed then accumulates very significantly for systematic series of tests or measurements for process monitoring. For this reason, the idea was evolved in Prof. Schwechten’s process-engineering department at the Konstanz University of Applied Sciences of developing a fully automatic gas pycnometer with an integrated scale for routine measurements. “Measurement of density was, actually, only an initial output variable, which students with me were to perform. Further properties, such as mass flows and balances, were then to be calculated using this. If the pycnometer measurements themselves were already incorrect, however, no useable results could, of course, be obtained for the mass balances. This, of course, greatly frustrated the students! And I thought to myself that an automated density measurement system would be exactly the right remedy,” Prof. Schwechten recounts on the origin of his idea.
Suitable industrial partners to make reality of such an idea were lacking, however. In such cases, the best solution is to turn to long-established, tried-and-proven contacts. Jens Corell and Siebtechnik GmbH were consulted at a POWTECH tradefair. “The chemistry was right, and we quickly reached agreement,” Corell recalls, concerning the genesis of BULKINSPECTOR. “BULKINSPECTOR fitted well into our range, since we already had many years of experience in developing sampling systems and downstream systems for analysis.”
Manual particle density measurement of bulk materials using pycnometers encounters a number of disadvantages. One is sampling. You need a homogeneous and representative random sample of the total quantity. Weighing-in must be performed extremely precisely. Non-conditioned samples and/or measuring receptacles can cause significant deviations in measurement of density, since a pycnometer is used to determine volume, which is, of course, dependent on temperature. And, in addition to these frequent sources of error, there is also a whole series of others that can affect the measurement. An automatic mechanical measurement system eliminates these sources of error and significantly improves the precision and scatter of the measured data. A further benefit is the enormous time-savings, and with automatic sampling and feeding of the measuring instrument, for example, complete series of samples can be measured overnight with no personnel present.
After a development period of around two years, BULKINSPECTOR was launched on the market in January 2022. This was preceded by an intensive interchange between Prof. Schwechten and Siebtechnik GmbH. Students from Konstanz University of Applied Sciences were given the opportunity at the Siebtechnik technology centre in Mülheim to perform series of statistical measurements in order thus to optimise the functioning of the pycnometer.
The PYC 130-A BULKINSPECTOR is a fully automatic mensurational system for measurement of particle or “true” density. The specimen material is inserted from the exterior to the instrument, ideally using a sample magazine. The sample is then inserted into the measuring cell using a handling device and the volume of the sample determined by means of multiple measurement. The handling device then conveys the specimen to the scale, in order to establish the mass of the specimen. Density is then calculated and outputted. Finally, the handling device discharges the measured specimen of material into a used-material receptacle. The measuring cup is cleaned using compressed air and is then ready to accept the next sample. One of the cup positions can be fitted with a calibration volume for automatic calibration, with the result that recalibration can be performed. Temperature in the interior of the insulated measuring system is maintained constant by means of a Peltier element.
The instrument is operated using a tablet supplied with it. The modern software which comes with the analysis system also originates from Siebtechnik and has been specially developed for this instrument.
A modern, user-friendly man-machine interface is available to the user thanks to an app installed on the tablet. Both measuring regime and measuring records can be stored on the tablet. The developers attached importance to sample traceability and test-inspection monitoring. This data can be viewed at any time and is also not lost if a power failure occurs. The operator also has access to the operating manual and can enquire directly with Siebtechnik GmbH for replacement and wear parts.
Possible applications are extremely varied and diverse (see information box). Jens Corell describes a fictional practical application of the BULKINSPECTOR as an example: “A measurement of density is to be performed using BULKINSPECTOR from a flow of bulk material of 100 t/h with a particle size of 0.3 mm. For this purpose, a downpipe swivel sampler is installed in a 300 mm diameter downpipe. This takes an individual sample of 1.4 kg using a rotating slotted vessel. Sampling should take place every 2 min., in order to assure adequate sampling frequency. Since the individual sample quantities would be too large for the BULKINSPECTOR, they must previously be split. For this purpose, a collective sample of 4.2 kg is generated from three individual samples and is then reduced in a turnstile divider to an analytical quantity of approx. 100 g. The splitting ratio can be variably set, for other services and other bulk densities. Screw and piston samplers can also be used in industrial applications. There are limitations on the sample-material side with respect to consistency – the samples must consist of pourable, non-glutinous material. The upper limit of the particle-size range currently measurable is around 20 mm."
Numerous other applications, such as the determination of bulk-material compaction, examination of cavities in metallurgy and in 3D production would also be possible using the BULKINSPECTOR. The two developers find especially exciting the potential of on-line integration of process monitoring and on-line Quality Control, which has up to now not been possible using off-line determination in the field of density measurement. In addition, batch processing of samples using the automatic sample magazine, which can accommodate up to fifty individual samples, would be possible overnight.
Questioned about the future of this new automatic density-measurement instrument, Schwechten and Corell are convinced that this up to now unique system will be open to a broad range of applications, thanks to its precision and its automatic, unmanned operation. Siebtechnik GmbH currently offers test measurements using the BULKINSPECTOR at its Test Centre in Mülheim an der Ruhr. Production for worldwide marketing has already started.
Potential applications for BULKINSPECTOR
• Powder metallurgy: Measurement of sinter and casting densities for examination for cavities
• PET coke: Determination of porosity
• 3D printing, additive production: Component characterisation, including determination of solids content
• Pharmaceuticals: Determination of tablet compaction and of porous inclusions
• Cosmetics: Determination of porous inclusion in lipsticks
• Roller compaction, bulk-material compaction
• Determination of properties/features: Product purity, chemical conversion by reaction, water content, thermal coefficient of expansion, …
• Plastics/composites: Determination of filler content
• Construction industry: Determination of material density/porosity
• Geology: Measurement of the porosity of drill cores
BULKINSPECTOR PYC 130-A: Technical Data
Model: PYC 130-A
Measuring cup volume: 130/65/10 cm3
Number of positions for measuring cup: 4
Sample gas: Helium
Measuring pressure: 0 to 140 kPa
Scale: 0 to 510 g ± 0.0001 g
Measuring temperature: 15 to 35° C
Calibration method: Automatic calibration using calibration sphere
Measuring uncertainty: ± 0.02%
Data interface: WLAN
Dimensions (width x depth x height): 1100 x 675 x 855 mm
Weight: 190 kg
Power supply: AC 110 V/16 A/60 Hz,
AC 230 V/10 A/50 Hz
Dr. Petra Strunk, Editor in Chief of AT MINERAL PROCESSING
Bauverlag BV GmbH, Gütersloh