1.2. Explanation of the thermal (anemometric) measuring method

I
S

.

m R
T R
S I

The measured value pick-ups for thethermal measuring method are electri-cal resistors and part of a measure-ment bridge circuit (Figure 2). They may be in the actual flow channel (inline instrument) or wound aroundthe flow channel (bypass instrument).The controller in Figure 2 sets thecurrent I so that the temperature dif-ferential between the heating resistor R

R
K Controller R
2 R
1 Figure 2: Simplified electrical diagram of the measurement bridge circuit(resistors placed directly in the flow channel)

Sensor electronicsFlow conditioning and upstream filterResistor R

and the measuring resistor R

iskept constant at all times. Since R

is very high ohm compared to R

, thecurrent I

is almost identical to currentI. The resistor R

is always heated tosuch a degree that there is always acertain overtemperature to the fluidtemperature, measured with R

(platinumlayer measuring resistor in thin-layer technology)Supporting elementResistor R

. If gas flows past R

, heat is dissipatedmore or less effectively depending onthe gas. The heating current that is required to maintain the overtempera- ture is a function of the gas flow pass- ing through the channel and repre-sents the primary measured variable.The method is known as the CTA, Constant Temperature Anemometer,and is a variant of the thermal measu-ring method. Mass flow controllers/ metersare designed as main flow orbypass flow instruments.Figure 3 shows the measuring ele-ment of an inline instrument. The flow conditioning produces a uniform flow through the channel cross-sec- tion on the inline instrument. Inputsections to smooth the flow are there-fore not necessary.

(platinumlayer measuring resistor in thin-layer technology)

Figure 3: Cross section of a main flow sensor block FlowTemperatureNo flowFlowL/2Pipe length
T R
S Bypass capillaryLaminar flow elementMain channelR T
âˆ† T0LR T
1 T R
S Figure 4: Sketch of a bypass sensor block