Two (2) primary methods are utilized for flow rate determination. The first method is known as Direct Rate Measurement, which deals with either “volumetric or gravimetric” measurement of the collected flow. The second method is called Indirect Rate Measurement, which involves “sensing or measuring” the process flow stream in a pipe or tube and correlating that measured value to an actual flow rate based upon a given set of known variables.
There have been more than 20 indirect methods for measuring process flows since the late 1980s. Some of these methods include:
- Rotary vane
- Pilot tube
- Variable area
- Venturi tube
- Reciprocating piston
Of these available methods, only a limited number are in fact ideal for flow rate measurement of biopharmaceutical process streams and even far fewer of these are perfect technologies for implementation into single-use process systems. This article focuses on one of these later technologies, ultrasonic flow measurement or to be more specific, transit-time ultrasonic flow measurement.
To demonstrate that a contact-free from media, compact, clamp-on ultrasonic transducer is capable of achieving the overall performance accuracy and repeatability needed for typical process control applications used by the biopharmaceutical industry and is considered to be a viable alternative to the standard gravimetric methods employed for single-use applications.
Essentially, the em-tec GmbH BioProTT™ flow measurement system employs what is known as a transit-time (TT) ultrasonic method capable of measuring precise flow values in flexible tube and piping systems. The ultrasonic converters (piezoceramics) in the flow sensors have the potential to transmit high-frequency acoustic signals via and against the flow direction. The time differential between these signals is proportional to the volumetric flow.
Figure 1 illustrates the basic principle.
Figure 1. Principle of ultrasonic flow measurement (simplified using only two ceramics)
One piezoceramic (A) is excited by a set of high-frequency vibrations and transmits ultrasonic waves to a second opposing piezoceramic (B) that behaves as a receiver. The piezoceramics are arranged at a particular angle a in relation to the flowing medium. The medium influences the transit time. The measured transit time differential can be used for calculating the flow velocity of the medium. The flow rate value in liters per minute is calculated with the help of the known cross-sectional area of the tubing being scanned by ultrasonic transmission.
Four ultrasound converters configured in a crosswise arrangement help in transmitting high-frequency sound signals in an alternative manner and against the direction of the flow. This is followed by measuring the transit time for each impulse; the transit-time difference between the downstream and upstream movement of the impulses is proportional to the volumetric flow.
An em-tec GmbH BCT 3/4“ X 3/16” clamp-on style transducer has been calibrated for platinum-cured silicone tubing and setup on the retentate line of an SU TFF System and configured in-series with a NIST calibrated coriolis flow meter.
An em-tec GmbH BCT 3/4” X 3/16” clamp-on style transducer was calibrated for platinum-cured silicone tubing and setup on the elution line of an SU Isocratic Chromatography System and then configured in-series with a NIST calibrated coriolis flow meter.
Two em-tec GmbH BCT 3/4” X 3/16” clamp-on style transducers have been calibrated for platinum-cured silicone tubing. A transducer was setup on the outlet line of each of the two feed pumps on a SU Gradient Chromatography System and then configured in-series with a NIST calibrated coriolis flow meter.
Each test setup was carried out using 0.2 µm filtered deionized water at 20-22 °C and allowed to recirculate for NLT 15 min. at an average flow rate of 8-10 lpm for equilibration purposes.
BCT 3/4” x 3/16” Flow Rate
Accuracy Test Data
The test data obtained from this study clearly illustrates that with proper material calibration and unit setup it is possible to attain flow rate measurement accuracies of less than +/- 2% of reading.
- Tubing Material - ASTR750-A65 (supplied by FlowSmart) and LSR60 (supplied by BlueStar Silicones)
- SU TFF System – ABS0475-SYS-001 (supplied by AlphaBio)
- SU Isocratic Chromatography System – ABS0474-SYS-001 (supplied by AlphaBio)
- SU Gradient Chromatography System – ABS0474-SYS-201 (supplied by AlphaBio)
- Reference Flow Meter – 80F15AFTSACAABAAA (supplied by Endress+Hauser)
- PLEASE NOTE: Each datum point illustrated on the above-referenced charts represents the average value of three consecutive flow rate measurement values collected during the test phase of this study.
This information has been sourced, reviewed and adapted from materials provided by em-tec GmbH.
For more information on this source, please visit em-tec GmbH.