Electronic mass flow controller

For sensors that are truly mass sensitive and for which the mass flow of sample through the sensing element is held constant as a function of pressure (for example, by use of electronic mass-flow controllers), instrument response is proportional to the mixing ratio independent of the pressure. For concentration-sensitive detectors, such as simple spectrophotometric instruments measuring absorbance or fluorescence, instrument response is a function of the absolute concentration, and the response will decrease for a constant mixing ratio as the pressure decreases. For example, the response of a pulsed fluorescence SO instrument sampling air containing a fixed... 

By a continuous air current streaming in from outside, a defined air exchange rate is adjusted in the test chamber. The inlet air can be regulated most accurately by electronic mass flow controls. As inlet air, ultrapure synthetic air from pressure vessels or purified air from the surroundings can be used. The requirements of the purity of the inlet air follow from the permissible background concentration. 

The experiments were carried out in an all stainless steel microreactor system with four gas lines which was operated at pressures up to 100 bar. The gases were supplied by Linde with the following purities He 99.9999 %, N2 99.9999 %, H2 99.9999 %, the mixture of 25% N2 in H2 used as synthesis feed gas 99.9996 %. The feed gas was further purified by means of a purification unit described elsewhere [4]. The flows were regulated by electronic mass flow controllers. The reactor consisted of a glass-lined U-tube similar to the one described in ref. [13]. It was not possible to detect the desorption of N2, H2 or NH3 from the empty tube within the limits of detection. The U-tube was placed in a copper block to ensure isothermal operation. Gas analysis was performed using a mass spectrometer (Balzers GAM 445) which was calibrated for He, H2, N2 and NH3 by using a reference gas mixture. The calibration for H2O was carried out using a He stream saturated with H2O at room temperature. 

Most important for the short- and long-time stability of a DCP source is the use of electronic mass flow controllers and meters, resulting in a stable observation zone which is much more smaller than in an ICP. 

For mixed gas experiments, the flow rates and composition of the feed gas were controlled using electronic mass flow controllers. The total feed pressure was kept steady at 227.5 cm Hg (303 kPa) while the permeate pressure and temperature were maintained at ambient conditions (62 cm Hg and room temperature). Permeate and retentate streams, with helium as the sweep gas (on permeate side), were analyzed for gas compositions using a gas chromatograph (SRI Instruments, model 8610 C). The total steady state permeate flow rate and the gas composition of the permeate stream were used to calculate the analyte gas flux. From the flux, and the pressure differential across the membrane, the gas permeances and separation factor (ratio of permeance) can be calculated. 

FIGURE 15.26 Mixing manifold nsed to blend VOC working standards in air for TO-14 calibration metered blending of calibration stock standard and zero air accomplished through electronic mass flow controllers (FC-1 and FC-2) (adapted from Reference 121). 

In these experiments we used a well-stirred continuous-flow reactor in which the test specimen was mounted. A sapphire window on the top of the reactor permitted the passage of infrared radiation to the thermal imager. The contents were mechanically stirred with an impeller whose shaft entered the reactor through a gas-tight seal. Hydrogen and oxygen entered the reactor at a rate controlled by electronic mass-flow controllers, and the exit stream was continuously analyzed for product concentration (water vapor in this case) which was recorded on a strip chart. 

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