Critical standard ambient temperature

For a standard of flow rate metering, traceability is the top priority the measurement uncertainty should he sufficiently small, and a system that features single operation, low measurement uncertainty, and high repeatability is desirable. Therefore, approaches based on weighing are, above all others, favorable candidates for such a purpose. As shown in Fig. 6, a gravimetric primary metering standard has been set up to achieve these requirements [9]. Liquid water is driven by a pneumatic pressure control mechanism to obtain flow rates down to less than a microliter per minute. The measurement capabUity depends on the weighing of the collection beaker, the stability of the pressure difference, the time interval, and the variation of the ambient temperature. Buoyancy variation and liquid evaporation are critical concerns for measurements associated with such a small quantity of liquid. 

Until recently, standard HPLC methods for polymers, e.g., gradient chromatography or chromatography at critical conditions (LCCC), were limited to ambient or slightly elevated temperatures [134, 135]. The majority of published HPLC separations were conducted at operating temperatures of a maximum of 80°C. These temperatures are too low for the dissolution of polyolefins, which require at least 120°C for dissolution due to their mostly semicrystalline nature. It was, therefore, a challenge to develop HPLC methods for the separation of polyolefins that operate at temperatures of 120°C and higher. 

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