3 Volumetric Flowmeters
limitation in the applicability to liquid chromatography systems, where volume flow rate is common.
Devices based on the Coriolis principle are known to be accurate and highly reproducible. However, they are also known to be very slow devices, unable to detect changes in fl ow rate in the range of a few seconds. These facts combined limits the applicability of Coriolis type fl owmeters for use in liquid chromatography applications.
Thermal Flowmeters
A volumetric Flowmeter is fundamentally the automated version of the classical ‘Stopwatch and graduated cylinder’ method. A fl ow meter of this type consists of a tube and two optical level sensors placed at a known distance to each other. The time the solvent front takes to travel from one light trap to the next is a function of the volume of the tube, which is constant, and the fl ow rate. After each measurement Is completed, the tube has to be automatically voided so that the measurement can be repeated. As such, a volumetric fl owmeter is not capable of continuous measurements.
For analytical purposes - volumetric fl owmeters must be positioned perfectly vertically and at the end of a liquid chromatography system. As a result, their use is limited to confi rmation of performance of the pump, no real diagnostics is possible as each value reported is not related to the previous one.
Further to this, the result of each measured fl ow rate is the integral over the measurement volume, which makes it impossible to determine pulsations or variations in fl ow with a duty-cycle shorter than the time required to fi ll the tube. However, this technique has been proven to be very reliable particularly in single solvent (often water) applications. The different surface tensions of other solvents don’t allow volumetric tubes to be voided with the same effi cacy as with water.
Volumetric fl ow meters are commonly used in conjunction with HPLC systems as validation tools. It should however be noted, that because this technology is based on the measurement of time required to fi ll a known constant volume, that the accuracy of results decreases markedly at higher fl ow rates.
Coriolis Flowmeters
Thermal fl owmeters were one of the fi rst techniques used for the measurement of fl ow rate in liquid chromatography. Initially their use was limited because only analogue techniques were available. Developments in microelectronics and microcontrollers, however, have enabled thermal devices to become a very useful method of monitoring liquid chromatography fl ow rates.
The method is fundamentally based on the measurement of the difference in temperature between two temperature sensors, one placed upstream of a heating element and the second placed downstream of it. For applications ranging from a few microliters per minute up to several millilitres per minute, thermal fl owmeters typically employ a quartz fl ow measurement tube. As all components necessary to fl ow measurement are located on the outer wall of the quartz tube, and have no contact with the liquid, the technique is non-invasive.
Being non-invasive means that thermal fl owmeters have widespread applicability in terms of solvent used and also guarantees the unperturbed operation of the whole liquid chromatography system. Measurement with thermal fl owmeters is continuous, allowing use of these devices for real-time monitoring of pump performance. Using modern thermal fl owmeters, such as are available from Testa Analytical, it is now possible to interface them directly with chromatography data system allowing storage of fl ow data along with the chromatograms. This technological advance opens a whole new chapter to the concept of total quality assurance, as each and every chromatogram may now be evaluated under the light of the fl ow rate delivered by the pump during that one chromatogram.
Conclusion
Although fl ow rate is a fundamental parameter in any liquid chromatography application, it is one of the most underrated and underestimated measures of the total quality of results obtained with any system. A range of fl owmeters are available to the interested user, all of them offering solid answers within the limits of the utilised technology. Ultrasonic fl owmeters, although appealing from the viewpoint of ease-of-use, have no real place in chromatography, due to the limited accuracy and resolution of the method. Volumetric fl owmeters are a proven solution for aqueous applications where a ‘validation’ is required and there is no necessity for tracking performance over a longer period of time. Coriolis fl owmeters are by nature continuous monitors, their output is however mass fl ow rate, which is of minor importance to liquid chromatography, where all calculations are based on volumetric fl ow rate. The necessity of transformation from mass fl ow to volume fl ow using density also expresses the practical limitations of this technology, as accurate knowledge of the solvent density at the exact temperature the measurement was done at is required.
Coriolis fl owmeters have found wide use in process applications, mainly supplying information about mass fl ow of individual reagents. The Coriolis measurement principle is based on detection of the variation in the oscillation frequency of a U-shaped tube through which the solvent travels. It can be demonstrated that the measured change in oscillation frequency can be related to the mass fl ow of the solvent.
As such, Coriolis flowmeters are by nature mass flowmeters, their data output will be therefore in weight units per time units. This of course represents a strong
Modern thermal fl owmeters are proven to provide continuous non-invasive monitoring of liquid chromatography fl ow rate. When an appropriate standard operating procedure (SOP) is used, thermal fl owmeters can be used to deliver representative validation parameters of any liquid chromatography pump.
Available in formats to measure from nanolitres per minute right up to 650 millilitres per minute – thermal fl owmeters can supply reliable and decisive information about the confi dence level of any chromatogram and help minimise the downtime of a system when used as a fault diagnostic tool.
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