Bartington Associates

F L O S C - AN ALL ELECTRONIC LOW VELOCITY GAS FLOW SENSOR

THIS PATENTED TECHNOLOGY REPRESENTS A SIGNIFICANT NEW DEVELOPMENT IN LOW AND ULTRA - LOW VELOCITY GAS FLOW MONITORING. KEY FEATURES OF THIS ADVANCE ARE HIGHLIGHTED BELOW.

PUBLICITY FLYER: F L O S C - AN ALL ELECTRONIC LOW VELOCITY GAS FLOW SENSOR

DESIGNED FOR MILLIMETERS PER SECOND VELOCITIES: Particularly suitable for biomedical and critical environmental monitoring aplications.

VERY LOW OPERATING POWER: designed for use with explosive gases and ideal for battery operation.

NEGLIGIBLE FLOW RESTRICTION: very small components ensure that flow resistance is virtually insignificant - no kinetic energy is extracted from the media.

UNAFFECTED BY PROPERTIES OF GAS: accuracy independent of viscosity, density, specific heat, temperature and presssure.

DIRECT ELECTRONIC OUTPUT: stand alone instrument with direct readout or many units hosted by a pc. Telemetry - rs232/ pulse / frequency/ 4-20 ma.

INHERENTLY DRIFT FREE: output is derived from time and distance not amplitude - can be programmed for any engineering units of velocity or volume.

FREE PROBE TYPE SENSOR OR PIPE MOUNTED: Sensor is ideal for retro - fitting to existing pipework or for manufacture as a complete calibrated flowmeter.

BI - DIRECTIONAL OPERATION EASILY IMPLEMENTED: essential in some circumstances for determining true net flow.

INEXPENSIVE DESIGN: simple to fabricate with very few specialised components or skills required.

DESCRIPTION: F L O S C is an abbreviation of flow- oscillator- the velocity of a fluid between a thermal source and detector determines the frequency of an amplitude stabilised electronic oscillator. An exact mathematical relationship has been established between frequency and fluid velocity.

MARKETS: This technology is intended to impact on markets presently dominated by variable area, pressure differential and some mass thermal devices.

APPLICATIONS: Environmental monitoring - biomedical - semiconductor fabrication - anemometry - hvac - gc calibration.

BENEFITS: An inexpensive and reliable replacement for conventional methods of continuously monitoring gas flows. One device can safely measure a variety of media and deliver a conditioned electronic output over several kilometres for monitoring recording or control.

COMMERCIAL OPPORTUNITIES: A rigorous five year research programme has resulted in a pre – production prototype for which a comprehensive know – how package is in place. The technology has patent protection. A business partner is now sought to share in the production and marketing of a commercial instrument.

For further information on FLOSC commercialisation check with:

Simon Jones
B.D.R.O.
ESSEX UNIVERSITY
http://www.essex.ac.uk/bdro

TECHNICAL PAPER: F L O S C - AN ALL ELECTRONIC LOW VELOCITY GAS FLOW SENSOR

BACKGROUND

FLOSC (1) was developed to address shortcomings in existing instruments, such as Variable Area Flowmeters, when applied to the continuous monitoring of landfill gas. These instruments utilise a float with a specific gravity greater than the fluid being metered, mounted in a relatively close fitting internally tapered vertical tube. Flow rate is visually indicated when the upward force of the fluid equals the downward force of gravity.

APPLICATIONS

FLOSC will find wide application where low velocity flows are encountered and the media is difficult to monitor with conventional instrumentation. An example of this is landfill gas studies. Regulatory reforms, in particular, The Environmental Protection Act 1990, specify criteria for assessing landfill completion. DoE Waste Management Paper No. 27 (1991), recommends guideline flows of less than 15 l/hr for CH⁴ and less than 22 l/hr for CO². Flow is influenced by barometric pressure and any restriction must be avoided. There is experimental evidence that the gas, which can be corrosive, of varying constituents and contain saturated vapour, should be continuously monitored, making an electronic output particularly desirable. Other applications include anemometry, gas chromatography, biological sciences, and process control.

PRINCIPLE OF OPERATION

Figure 1 shows a fine wire heat source A and detector B, located in a gas stream. Typical separation is 5-20 mm. The output of the detector passes through a Zener barrier D, a low noise DC restoring amplifier E and an AGC amplifier F. This amplifier amplitude modulates a drive circuit G, which provides impedance matching to the heat source and transformer isolation from the preceding circuitry. Timing circuitry H, measures frequency, which is linearised by an on-board processor I, in the case of a stand-alone module or data is transmitted to a host computer for multi-station applications. An internal Dc - Dc converter J, provides isolation from the external supply.

When gas flows, thermal energy is conveyed from the heater to the detector. The detected signal, which arises from the differential temperature between the gas and the heat impressed upon it by the heater, is detected, amplified and then modulates the energy supplied to the heater. A feedback loop is established, the circuit oscillates and the frequency of oscillation is a function of fluid velocity.

Variable area instruments are inexpensive but do not perform well in the presence of condensing vapours since the float can stick. Furthermore, calibration is influenced by gas characteristics such as viscosity and density. Similar limitations apply to hot wire anemometers and other thermal transfer devices, which monitor the rate at which heat is lost from a source. FLOSC has no small clearances and is therefore ideal for saturated vapours. Also, since velocity is determined from units of distance and time and not inferred, characteristics of the media are considerably less important. For example, Figure 2 shows identical experimental results for propane or air and Table 1 compares their characteristics.

DRAFT PERFORMANCE SPECIFICATION

Velocity............ (10mm separation)……………...4 to 80 mm/sec.
Volume flow example.... (10mm sep/8mm pipes)....0.72 – 16.2 l/hr. / .0.2 - 4.5cm3/sec.
Oscillation frequency range...…….....Typically 0.8 to 20 Hz
Accuracy.….…………………………..….....Better than 1%.
Flow restriction…….Less than 3% of c.s.a for 12mm pipe.
Display average time.…….....Less than one operating cycle.
Power - on settling time.….………..…Less than 20 Seconds.
Display units...………………………...l/s, l/m,l/h,m3/hr. e.t.c.

DRAFT ENGINEERING SPECIFICATION

Materials in contact: Critical components for all applications are typically stainless steel and platinum.
Media compatibility: Ceramics, glass, metals and suitable polymers, selected according to application.
Pressure: Untested, but no known technical limit apart from limitations of construction materials.
Temperature: Untested, but no known technical limit apart from limitations of construction materials.
Enclosure: To suit application, remote and free-wired to stud mounted sensor or pipe mounted. Typical -IP67.
Telemetry: To suit application. Typically, radio, opto isolated pulse, phantom line, RS232, 4-20 mA, frequency.
DC supply: Unregulated isolated line 5 - 18 Volts 20mA .
Safety: Typical heat source power levels are less than 15mW and practical consideration has been given to stored energy and surface temperature limits for hazardous area operation within IEC 79-1 0/CENELEC classification for Zone 0 applications.

Notes:
(1) Inventors: Dr. B.J.S.Barnard. Mr. J.K.Bartington.