Posted to News on 9th Oct 2008, 18:42
Mass-View flowmeter aims to supplant variable area flowmeters
Variable area (VA) flowmeters or rotameters are relatively low cost and simple to install, easy to read and maintain, require little or no energy and offer reasonable levels of accuracy and repeatability - which is why the technology is so widely used and trusted for gas and air flow measurement. Yet according to Cambridge-based Bronkhorst UK, for many applications, particularly in the life sciences and medicine, where precision, rangeability, accountability and safety are critical, this centuries-old flowmeter principle no longer measures up. That is why Bronkhorst has developed a high-technology replacement for the venerable VA meter, namely the Mass-View, which offers virtually the same configuration and characteristics, but with the accuracy and integrity of direct thermal mass flow measurement, a digital bar graph of actual gas flow visible from all angles, an onboard menu of pre-installed gas conversions, and an electronic signal output to analogue and fieldbus interfaces.
Aimed particularly at hospitals, research laboratories, industrial cleanrooms and the biotechnology and pharmaceutical sectors, where accuracy of gas metering is central to safe treatment, proper diagnosis, repeatability, accountability and quality, Bronkhorst's Mass-View series consists of three gas flowmeters, with flow ranges of
Bronkhorst UK's Managing Director, Andrew Mangell, says: "VA meters, which measure volumetric flow, have been the uncomplicated choice for many years, particularly as people are familiar with the technology and their operation is simple to understand. But their relatively low accuracy and susceptibility to pressure and temperature variations means they are not really viable for many medical and life sciences applications, as they no longer represent 'best available technology.'
"Price-wise, variable area meters are very competitive, of course, but we have costed our Mass-View flowmeters so they are comparable to a plastic or glass VA plus signal transmitter, or around the price of a metal tube rotameter. Besides, buying on installed price alone is not always valid, since improved accuracy and repeatability can translate into less wastage, more consistent processes, and better research and diagnosis. The credibility of data is also enhanced for research purposes, legal requirements and accountability records."
No need to recalibrate
An acknowledged shortcoming of VA technology is that, during operation, flowmeter accuracy can quickly degrade once temperature and pressure deviate from standard calibration references, requiring regular recalibration and constant monitoring of T and P conditions, which, in practice, is rarely carried out consistently. In contrast, a thermal mass flowmeter, such as Mass-View, measures the actual mass of gas and does not require correction for changes in gas temperature, pressure or density, leading to improved accuracy, repeatability and compatibility of data; any applications that require precise monitoring and control of relatively low-flow gases would therefore benefit from this measuring technique. Moreover, Mass-View's rangeability, or turndown ratio between minimum and maximum flow rates, of up to 1:100 compares most favourably with a typical VA rating of just 1:10 or 1:12.
A potential safety hazard with many VA flowmeters occurs where a glass metering tube breaks, leading to the possibility of potentially dangerous gases leaking into the environment. There is also the more routine problem of parallax error when taking readings off the float from different viewpoints. The Mass-View design overcomes these drawbacks by removing fragile components from the flow path and employing an OLED screen, as used in mobile phones and MP3 players, to display an easily interpreted bar graph and actual flow value. Giving a far brighter readout than LCDs, the display is clearly visible from all angles and in all light conditions, at the same time facilitating easy navigation through an onboard menu of pre-installed gases, flow ranges, alarm functions and other settings, using a four-way push button.
Easy retrofitting
To enable straightforward replacement of existing VA meters, the Mass-View is also installed vertically and can be fitted with adapter sets for most common process connections. A plug-in power supply with universal 100-240V input and 2m cable is supplied, which conforms with current EN safety specifications, while there is also a choice of interconnection cables and free software tools for interfacing with Windows-based systems.
Andrew Mangell concludes: "VA meters fluctuate according to atmospheric conditions and most processes cannot cope with that level of variation. So we have taken what is good about VA instruments, notably their size, straightforward reading and affordability, then improved the accuracy, rangeability and safety, as well as adding the benefits of an output signal for data logging. We have also introduced multiple gas conversion capabilities to reduce inventory and recalibration costs. Mass-View's modern aesthetics and digital technology bring flow measurement of this type into the 21st century."
