Purge Flowmeters

As inlet pressure rises, the orifice element is pushed in against the spring, thereby exposing fewer openings to the flow, and as a result, increasing the pressure drop so as to maintain the flow relatively constant. (Courtesy of Griswold Controls.) 

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Measurement and control of low-flow rates are a requirement in such applications as fuel cells, purging, bioreactors, leak testing, and controlling the reference gas flow in chromatographs or in plasma-emission spectrometers. The most traditional and least expensive low-flow sensor is the variable-area flowmeter. It has a high rangeability (10 1) and requires little pressure drop. Due to its relatively low accuracy, it is limited to purge and leak-detection applications. 

Soap bubble flowmeters are commonly used to find gas velocities, but again are known to be one source of error that accumulates. Flow rates are usually measured from the column end using the soap bubble flowmeter modified with an inverted U-tube on top. The U-tube allows the flowmeter to be purged with the carrier... 

The ratio of the volumetric flowrate out of the purge vent to the volumetric flowrate in the capillary column is termed the split ratio and provides an estimate of and control over the actual volume of sample entering the column. Care should be taken when using the split ratio to estimate actual injected sample volume, or when using it in comparisons between methods on different instruments. There are subtle differences between instruments and measurement techniques that may affect the measured flows. For example, the column volumetric flowrate measured by injecting a nonretained substance is the average column flowrate, not the flowrate at the inlet, while a flowmeter connected to the split purge vent measures the volumetric flowrate at the vent, not in the inlet. With newer, electronically controlled systems, the flows are measured directly at the inlet, or are calculated from the entered inlet conditions and column dimensions. 

When using a split inlet, there are several flows that provide the exact injected sample amount. The most important of these is the split ratio, which is the ratio of the volumetric flowrate at the split purge vent to the volumetric flowrate in the GC column. Classically, this was measured manually, using a flowmeter to obtain the purge vent flow and by injecting a nonretained substance to obtain the column flowrate. With electronically controlled systems, these values are... 

A technique widely used in the HRE-2 for metering purge flows is a "heat balance flowmeter in which a known amount of heat is added or extracted from the process stream and the temperature change noted. 

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