Bronkhorst MASS-VIEW Series Manual de usuario descargar pdf (Pagina 7)

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BRONKHORSTHIGH‐TECH

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9.17.051 MASS‐VIEW®series Page7

1.8 Sensor principle

The MASS‐VIEW

 operates on the principle of direct thermal mass flow

measurement. The thru‐flow design sensor consists of a heater resistor and a

temperature sensing resistor. Both resistors are made of temperature sensitive

resistivematerialthatiscoveredwithastainlesssteeltube.Thenecessaryheating

power to keep the temperature

difference between the heater resistor and the

sensing resistor at a constant level depends on the mass flow. A different and

unique heater current is produced for each value of the flow. The measurement

principledescribediscalledConstantTemperatureAnemometry(CTA).

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Thetransferfunctionbetweenmassflowandoutput

signalcanbedescribedbythe

equation:

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signal

 =outputsignal

K

 =constantfactor

 

(includesλ–heatconductivity,C

–specificheat,μ–dynamicviscosityandρ–densityofthegas)





 =massflow

1.9 Needle valve principle

Theneedlevalvehasanorificewithalong,taperedconicalseat.Aneedle‐

shapedplunger,ontheendofascrew,exactlyfitsthisseat.Asthescrew

is turned and the plunger retracted, flow between the seat and the

plungerincreases.Sinceittakesmanyturnsofthefine

‐threadedscrewto

retracttheplunger,preciseregulationoftheflowrateispossible.

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1.10 Constant-flow controller principle

Flowcontrolwithaneedlevalveatconstantpressureresultsinaconstant

flowrate.Butwhenpressureconditionschange,theflowwillalsochange.

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Withaconstant‐flowcontroller withvaryingupstreampressuretheflow

willalmostbeconstant.

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Bykeepingthepressuredropacrossaneedlevalve

constanttheflowratealsoisconstant.Deviationsof

thepressuredroparecompensatedbyamembraneoperatedvalve.Thismembraneisactuatedbya

spring.Thespringforceclosesthevalve.

Theequilibriumbetweenclosing(valvespring)andopening(membrane)keepstheflowconstant.

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Schematicflowdiagramconstant‐flowcontroller

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msignal

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