Posted to News on 16th Dec 2008, 09:54
Mass flow meter measures flow of wet and dirty gases
Based on a simple and robust through-flow design, the Mass-Stream Series from Bronkhorst UK presents no obstructions to the flow path, has no moving parts and features very low to ultra-low pressure drop across the instrument. These features make the Mass-Stream suitable for the mass flow measurement of dirty gases, wet gases and even a combination of the two. Furthermore, the minimal pressure drop prevents harmful liquid drop-out from condensing gases, thereby eliminating internal corrosion and increasing instrument life.
Typical flow measuring applications for the Mass-Stream series include biogas from land-fill sites and digesters, WF6, SiCl4 and other particulating or condensing gases, dirty exhaust gases from combustion processes, carrier gases for particle analysers and humidified powder carrier gases for pharmaceutical research. In addition, the straightforward design can be applied to such cost-sensitive applications as consumption metering for accounting or internal billing purposes, medical gas monitoring, burner ratio control, air flow control in paint spray lines, multiple gas mixing and gas generators.
Suitable for use with a wide range of gases and with standard pipe fittings between 1/8inch OD and 20mm, the Mass-Stream D-62XX series can be calibrated between flows of 0.25 and 6000ln/min air-equivalent; it has a standard turn-down of 20:1, although this can be higher on application. When configured as a mass flow controller, a direct-acting proportional solenoid valve is integrated on the base-block for regulating flows up to 500ln/min. Above this flow rate, a compact external valve is close-coupled to the meter body, taking flow control up to the 6000ln/min limit.
Integral display
Options include an integrated flow display on all models, a range of readout systems for analogue instruments, and digital versions providing fieldbus interfaces. One possible configuration is the combination of an external potentiometer, on-board PID control and integral flow rate indication; while still providing an output signal for data logging, this is a standalone instrument that does not need an external set-point signal.
As part of the Bronkhorst BV Group and the manufacturer of the Mass-Stream brand, M+W Instruments was said to be the first company to introduce direct flow measurement using constant temperature anemometry (CTA), the principle behind through-flow metering. Each meter consists of a metal base-block with a straight through-flow path and two stainless steel encased sensors protruding into the bore. One acts as a heater and the other a temperature probe, with measurements being taken directly from the main gas flowpath.
A constant difference in temperature is created between the two probes and the energy required to maintain this, according to King's Law, determines the mass flow rate. The pressure drop across the instrument is almost comparable with a straight run of pipe and is therefore negligible, although since a number of mesh screens are introduced to condition the gas flow profile, these can create a small pressure drop. Where a very low pressure drop is required, a special version can be built for pressure differentials down to 0.1mbar.
Digital outputs
In addition to analogue models, Mass-Stream flow meters and controllers are available in a digital format and, while still retaining the voltage or current output, these also offer RS232 and bus protocols, such as Profibus-DP, DeviceNet and Flow-Bus. The digital series is characterised by a high degree of configurability, high signal integrity and a host of additional on-board features, including the pre-loading of up to eight calibration curves for different gases.
Mass-Stream instruments are configured with factory settings for a smooth response characteristic, with a nominal two-second valve response to avoid overshoot of the actual controlled flow rate. If required, however, a combination of the digital board and freeware available from Bronkhorst UK allows the user to directly modify the PID and control settings to tune the instrument specifically for a faster set-point response, for preset valve-opening limits or, indeed, for scripted time/set-point menus.
