Achieving accurate temperature-independent flow measurement

Accurate measurement of flow is a critical parameter in liquid chromatography systems where retention times and peak shape are often directly affected by flow rate. In such systems, even slight variations in flow can degrade reproducibility and data integrity. In HPLC and GPC applications, temperature variations are inevitable either due to heated column compartments, pump motors, or pre-heated solvent. This raises the question of how solvent temperature may influence the accuracy of flow rate measurements. To investigate this, a study employing flow measurements was performed under controlled conditions using pre-heated solvent at two different temperatures.

In this study, a TESTA Analytical liquid chromatography flowmeter was used for flow rate measurements at 27 °C and 45 °C to assess the impact of thermal variation on flow measurement´s accuracy and reproducibility. HPLC-grade water was pre-heated using a water bath and supplied to the pump inlet. The flow path included a solvent degasser, a pulse dampener, and restrictor tubing to reproduce the backpressure typically generated by a chromatography column. After the restrictor tubing, a 100 cm relaxation tube with a larger inner diameter was installed to avoid Jetstream effects. The outlet of the relaxation tube was then connected to the flowmeter. Flow rates of 0.25, 1.0, 1.5, 2.0 and 4.0 mL/min were used for the flow measurements. Each measurement was performed over a 10-minute period and the average values obtained were compared to the nominal flow rates in Figure 1 below.

As shown in Figure 1, using different temperature conditions commonly found in laboratory applications, did not to impact the accuracy and reproducibility of the flow measurements performed.

Figure 1 Comparison of flow rate measurements at 27°C and 45°C

This study highlights the benefits of using a liquid chromatography flowmeter in real-world laboratory scenarios, where systems often operate at varying temperatures. The temperature independent flow measurement capabilities of the Liquid Chromatography flowmeter offer laboratory professionals access to reliable, repeatable flow data throughout experiments and system validations, without the need to constantly adjust for temperature variations.

While traditional flowmeters and gravimetric measurements may struggle with temperature-dependent changes in fluid properties, thermal flowmeters can maintain stable and accurate readings even when chromatography system temperature fluctuates.

The TESTA Analytical thermal flowmeter design encompasses a central heater and temperature sensors that measure the temperature difference upstream and downstream of the heater. The resulting temperature difference is then converted into absolute flow rate units using a proprietary algorithm developed for this flowmeter. Because the measurement is based on dynamic heat transfer, rather than static fluid temperature, it is inherently capable of precisely measuring solvent flow rate under fluctuating thermal conditions, which results in accurate and stable readings.

Figure 2 Internal view of a thermal flow sensor. Traceable tracking of the system performance.

Furthermore, the non-invasive measurements provided by this thermal flowmeter enables simple and easy integration with liquid chromatography systems, as it provides direct flow readings with minimal equipment and no moving parts. They are placed in-line with the system, allowing real-time monitoring and traceable tracking of the system performance. They are also compact, cost-effective, and require less maintenance than many Volumetric or Coriolis flow measurement systems, providing improved uptime and efficiency benefits to the user.

Conclusion

Modern thermal flowmeters offer an easy-to-implement, non-invasive flow monitoring solution for liquid chromatography systems. The liquid chromatography flowmeter has been demonstrated to provide reliable performance even with varying operational temperatures making it an invaluable tool for real world HPLC and GPC applications, increasing confidence in the accuracy and precision of your measured data.