Precision pressure regulator

Figure 3.15. Schematic diagram of a dual oven two-dimensional gas chromatograph with live switching between two capillary columns. DR = precision pressure regulators NV = needle valves P = pressure gauges Dr = flow restrictors and D = detectors.

Implementation of SFC has initially been hampered by instrumental problems, such as back-pressure regulation, need for syringe pumps, consistent flow-rates, pressure and density gradient control, modifier gradient elution, small volume injection (nL), poor reproducibility of injection, and miniaturised detection. These difficulties, which limited sensitivity, precision or reproducibility in industrial applications, were eventually overcome. Because instrumentation for SFC is quite complex and expensive, the technique is still not widely accepted. At the present time few SFC instrument manufacturers are active. Berger and Wilson [239] have described packed SFC instrumentation equipped with FID, UV/VIS and NPD, which can also be employed for open-tubular SFC in a pressure-control mode. Column technology has been largely borrowed from GC (for the open-tubular format) or from HPLC (for the packed format). Open-tubular coated capillaries (50-100 irn i.d.), packed capillaries (100-500 p,m i.d.), and packed columns (1 -4.6 mm i.d.) have been used for SFC (Table 4.27). 

Even the temperature coefficient of the pressure regulator on the gas supply can affect the flow, for high-precision measurement. 

Adequate gas control for fhune ionisation d ector gases is usually achieved by pressure regulators supplying gas via constrictors. Some flame ionisation detectors reqiure very precise control of hydrogen flow, best achieved by mass flow control. 

A disadvantage of the nozzle method is that the nozzles are not variable in their cross-sectional area, that is, the concentrations and total flow rate are fixed and cannot be varied continuously. The built-in nozzles are chosen and fixed at the time of conception of the system. To enable the flow rate to vary, precise, reproducibly steerable pressure regulators for the inlet pressure would be necessary, which is not economically feasible. Orifices can be manufactured with various cross-sectional areas, but there is a technical lower limit, so that the minimum standard volumetric flow rate achievable lies at approximately 1 SCCM. 

The liquid-nitrogen flow rate was measured with a turbine flowmeter. The total pressure of the jet exhaust Ptj was measured at an orifice in the model plenum chamber by a Precision Pressure Balance transducer. The total temperature of the CO2 exhaust was measured by a thermocouple in the line leading into the vacuum chamber. This thermocouple was downstream of the pressure-regulating valve and therefore measured the total temperature of the CO2 after the expansion to near the jet total pressure. The total temperature and the total pressure of the jet were used in a one-dimensional isentropic flow equation for choked nozzles to calculate the mass flow of CO2. The specific heat ratio used was that corresponding to the total temperature and pressure of the jet. 

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