Two (2) primary methods are used to determine flow rate. The first method, called Direct Rate Measurement, deals with either "gravimetric or volumetric" measurement of the collected flow. The second method, known as Indirect Rate Measurement, involves “measuring or sensing” the process flow stream in a tube or pipe and associating that measured value with an actual flow rate based upon a given set of known variables.
Since the late 1980s, there have been over 20 indirect methods for measuring process flows. Some of these methods include:
- Rotary vane
- Pilot tube
- Variable area
- Venturi tube
- Reciprocating piston
Of these available methods, only some are actually suited for flow rate measurement of biopharmaceutical process streams and even far fewer of these are ideal technologies for implementation into single-use process systems. This article focuses on one of these later technologies – ultrasonic flow measurement, or to be more precise, transit-time ultrasonic flow measurement.
To demonstrate that a contact-free from media, compact, clamp-on ultrasonic transducer can achieve the overall accuracy, performance¸ and repeatability required for typical process control applications used by the biopharmaceutical industry and is believed to be a viable alternative to the standard gravimetric methods employed for single-use applications.
Basically, the em-tec GmbH BioProTT™ flow measurement system makes use of the so-called transit-time (TT) ultrasonic method that can measure precise flow values in flexible tube and piping systems. The ultrasonic converters (piezoceramics) in the flow sensors are capable of transmitting high-frequency acoustic signals through and against the flow direction. The time differential between these signals is directly proportional to the volumetric flow. The basic principle is depicted in Figure 1.
Figure 1. Principle of ultrasonic flow measurement (simplified using only two ceramics).
On being excited by a set of high-frequency vibrations, one piezoceramic (A) transmits ultrasonic waves to a second opposing piezoceramic (B) that acts as a receiver. The piezoceramics are arranged at a certain angle in relation to the flowing medium. The transit time is influenced by the medium. The flow velocity of the medium can be calculated using the measured transit time differential. The flow rate value in liters per minute is determined 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 assist in the transmission of high-frequency sound signals in an alternative way and in the opposite direction of the flow. Subsequently, the transit time for each impulse is measured; the transit-time difference between the upstream and downstream movement of the impulses is proportional to the volumetric flow.
An em-tec GmbH BCT 3/4“ X 3/16” clamp-on style transducer was first calibrated for platinum-cured silicone tubing, then setup on the retentate line of an SU TFF System, and finally configured in-series with a NIST calibrated coriolis flow meter.
After an em-tec GmbH BCT 3/4” X 3/16” clamp-on style transducer was calibrated for platinum-cured silicone tubing, it was setup on the elution line of an SU Isocratic Chromatography System and 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. Following this, a transducer was setup on the outlet line of each of the two feed pumps on an SU Gradient Chromatography System and then configured in-series with a NIST calibrated coriolis flow meter.
Each test setup was performed using 0.2 µm filtered deionized water at 20-22 °C and allowed to recirculate for NLT 15 minutes at an average flow rate of 8-10 lpm for equilibration purposes.
BCT 3/4” x 3/16” Flow Rate Accuracy Test Data
Through the test data obtained from this study, it is clearly illustrated that flow rate measurement accuracies of less than +/- 2% of reading can be achieved with proper material calibration and unit setup.
- 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.