Bronkhorst IN-FLOW User Manual download pdf (Page 12)

BRONKHORST HIGH-TECH B.V.

page 12

9.17.022

1.2 Sensor principles

1.2.1 Gas flow sensors (by-pass measurement)

The majority of gas flow sensors operate according to the by-pass measurement principle. These types of

instruments operate on a principle of heat transfer by sensing the delta-T along a heated section of a capillary

tube. Part of the total flow is forced through the capillary by means of a laminar flow device in the main

stream generating a delta-p.

The design of the laminar flow device is such that flow conditions in both the capillary and laminar flow device

are comparable, thereby resulting in proportional flow rates through the meter. The delta-T sensed by the

upstream and downstream temperature sensors on the capillary depends on the amount of heat absorbed by

the gas flow.

The transfer function between gas mass flow and signal can be described by the equation:

signal

= output signal

= specific heat

K c

signal

p m

= ⋅

⋅

K = constant factor

= mass flow

The temperature sensors are part of a bridge circuit and the inbalance is linearised and amplified to the

desired signal level.

1.2.2 Gas flow sensors (direct mass flow measurement, CTA based)

The IN-FLOW CTA models operate 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 heating power

required to keep the temperature difference between the heater resistor and the sensing resistor at a

constant level is proportional to 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).

The transfer function between mass flow and output signal can be described by the equation:

msignal

KSS Φ⋅+≅

signal

= output signal

= offset (zero flow) signal

K = constant factor (includes λ – heat conductivity, C

– specific heat, μ – dynamic viscosity and ρ –

density of the gas)

= mass flow

n = dimensionless constant (typically of order 0.5)

1.2.3 Liquid flow sensors

Two digital-liquid flow measurements and two sensor arrangements can be distinguished. They have in

common that there is no bypass system involved, which means that they are of the type: “thru flow”. The

following sensor arrangements can be distinguished:

1) The µ-FLOW model for flowrates up to 2 g/h.

Basically this is a small capillary tube with two sensing elements placed on the tube. The two elements both

serve as heater as well as temperature sensing elements. The delta-T sensed by the upstream and

downstream temperature sensors on the capillary depends on the amount of heat absorbed by the mass of

the liquid. The temperature sensors are part of a bridge circuit and the unbalance is amplified to the desired

signal level. The transfer function between liquid mass flow and signal can be described by the equation:

signal

= output signal

= specific heat

V K c

signal p m

= ⋅ ⋅ Φ

K = constant factor

= mass flow

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