GC036 Flow Measurement

There are two ways of measuring the flows independently. The first one is an inexpensive soap-bubble flowmeter which consists of a calibrated tube (usually a modified pipet or buret) through which the carrier gas flows. With a rubber bulb we can create a bubble, which is raised into the path of the gas. After that, the ascension of a particular bubble to a defined volume is measured with a stopwatch. The carrier gas in mL/min is easily obtained from this measurement. There are also available electronic soap film flowmeters at a cost around $50. The second alternative is the use of a sophisticated device, composed of a solid-state sensor and a microprocessor to accurately flow measurements without using soap bubbles. Silicone-on-ceramic sensor can be used to measure flow rates of 0.1-500 mL/min for air, nitrogen, oxygen, helium, hydrogen and 5% argon in methane. The cost for this device is around $700. Very small flow rates, like the ones found in tubular columns, cannot be measured reliably. The average linear flow velocity in OT columns is calculated with:

Linear velocity = L / tM

Where L is the length of the column (cm) and tM is the retention time (seconds) for a non-retained peak such as air or methane. Conversion of the linear velocity in cm/s to flow rate, Fc, in mL/min requires the multiplication by the cross-sectional area of the column:

Fc = pi * rc^2 * L /tM

Where rc is the radius of the column.