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T

HEPRINCIPLEOFOPERATIONOF

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HERMAL

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ASS

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LOW

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ONTROLFORGASES

To measure mass flow, various methods can be applied,but below we outline the thermal measuring principle ,as it is used by Bronkhorst High-Tech B.V.The user need not necessarily know these details tosuccessfully apply Hi-Tec instruments, but it helps tounderstand things better with regard to time- andcontrol characteristics of the instruments.
T 2 0 - Flow TT v m TEMPERATURE 1 FLOW FIGURE B

P

RINCIPLEOFOPERATION As shown in figure A a part of the gas flows throughthe Sensor, and is warmed up by heater R ∆ T = k.C
p . � . φ
v ∆ T= T
2 -T
1 in Kelvin
H .Consequently the measured temperatures T ο rC
p = specific heat
1 and T
2 ∆ T = k.C
p . φ
m � = density drift apart, as shown in figure B. The formulas for ∆ Tdemonstrate that the temperature difference is directlyproportional to mass flow. F φ
v = volume flow φ
m = mass flowElectrically, temperatures T
1 and T
2 are in facttemperature dependent resistors R
T1 and R
T2 . In figureA it is shown how the signals measured in the sensorare amplified to electric signals. All common outputsignals are available and one can be selected.In the case of mass flow control, the output signal iscontinuously compared with a setpoint signal from avoltage source. Any deviations between setpoint signaland measured signal are translated into a control valveadjustment until the two signals are identical.This is only a basic explanation of the principle, as it isapplied by Bronkhorst High-Tech, but be confident that more than 300.000 instruments in the field proveit to be reliable. More details are given in chapter “
Outputcircuit SUPPLYVOLTAGE Voltageregulator Laminar flow element Amplifier +linearisation SensorbridgeTo power supply OUTPUTSETPOINTTo controlvalve Temperaturecompensation Controller R T1 R T2 R T1 R H R T2 The special Bronkhorst High-Tech achievements ”(page 16 and 17).
Sensor Flow Turbulence filter IGURE A

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