5
However, if we consider the fl ow rate from a peristaltic pump the situation is different. In most cases, the absolute fl ow rate is secondary. The relation of fl ow rates from different peristaltic pumps used simultaneously in the same system is by far more important. The fl uctuation of the fl ow delivered by a peristaltic pump is by the nature of the technological mechanism very high. Therefore a representative average must be selected and put into comparison with averages delivered by the other pumps present in the system.
Similarly, to the above HPLC application example, the calculated fl ow average for a peristaltic pump will be affected by the frequency of data collection of the fl ow measurement. This makes ‘real time monitoring’ of fl ow values even more important as it might directly impact any pump fl ow rate adjustment required to optimise the process the peristaltic pump is servicing.
Gas or Air bubbles - a common enemy
Just to make things slightly more realistic, we should also evaluate the impact of air or gas bubbles in combination with pumping of a liquid. The amount of gas that a liquid can absorb depends on factors including pressure and temperature gradients, the nature and type of liquid and the gas. Unfortunately, gas bubbles are produced in a liquid when there is more available gas than the liquid can absorb. We must however differentiate the effects created by dissolved gas and bubbles on the pump technologies described above, as the effects are very different.
Dissolved gas has an impact on reciprocating piston pumps as it infl uences (increases) the compressibility of the eluent resulting in higher fl uctuation (pulsations) of the fl ow. This causes elution times reproducibility to worsen and increased baseline instability for the connected detectors.
A proven remedy for the described problem is liquid stream degassers placed on the inlet line of the pump. A degasser will effectively reduce the amount of dissolved gas present in the eluent thus preventing most of the problems arising from dissolved gas.
Air or gas bubbles, however, are an even bigger problem for reciprocating piston pumps, as they may cause a system to suddenly stop operating. For instance, if a bubble is suffi ciently large, it might cause a pump to stop delivering solvent altogether.
Peristaltic pumps are less sensitive to dissolved gas compared to reciprocating piston pumps, therefore extraction of dissolved gas is usually not necessary, though I would recommend it. While bubbles rarely lead to a stop in liquid delivery by a peristaltic pump, they are a well-recognised source of large fl uctuations and apparent spikes in fl ow rate. A peristaltic pump can therefore be considered even more sensitive to bubbles, as it will produce erroneous results without any indication of a problem with the pumping system.
The only remedy to air or gas bubbles, no matter what pump is used, is constant real-time monitoring of the status of the tubing connecting the pump to the solvent reservoir, and a strategic handling of alarm conditions detected by such a monitor.
Conclusions
Although the terms Flow and Flow Rates are commonly understood to defi ne the volume streamed within a time, they do require more specifi c interpretation relating to the pump technology and application being undertaken. Moreover, fl ow accuracy and precision are not easily defi ned in real world applications, as the determination of these parameters is easily affected by parameters of the measurement itself.
Real time monitoring of fl ow using suitable non-invasive fl owmeters, however, offers a powerful tool and a safe harbour for all those applications in which fl ow, and fl ow rate, are a critical parameter for achieving the set aim. Combined with such fl owmeters, vacuum degassers, and powerful bubble detectors, are the way forward to create a reliable fl uidic path, reducing operational downtime and increasing the reliability and productivity of any liquid chromatography or fl ow chemistry system.
Author details
The author: Carlo Dessy is Managing Director of Testa Analytical Solutions eK, a respected international OEM developer and supplier of instruments, innovative detectors and software for liquid fl ow applications.
Figure 4: The impact of air bubbles and an empty solvent reservoir on peristaltic pump fl ow rate.
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