Bronkhorst Mass Flow - Manual (Page 8)

BRONKHORST HIGH-TECH B.V.
1.3 Valves
Two solenoid housing models can be distinguished. The basic mechanical design of both models is the
same. Valves are used as separate units, or integrated with a meter to form a complete control unit. The
solenoid versions are:
1.3.1 Laboratory style
The solenoids of these valves have an IP-50 ingress protection class.
This means that the valves are suited for indoor (dry) use.
1.3.2 Industrial style
The solenoids of these valves have an IP-65 ingress protection class. This
means that they are suited for light industrial (outdoor) use.
1.4 Sensor principles
1.4.1 Gas flow sensors
All gasflow sensors operate according to the same principle. They are operating 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:
V K c
signal p m
= ⋅ ⋅ Φ
V
signal
= output signal
c
p
= specific heat
K = constant factor
Φ
m
= mass flow
The temperature sensors are part of a bridge circuit and the imbalance is linearised and amplified to the
desired signal level.
1.4.2 Liquid flow sensor
- The LIQUI-FLOW mass flow meter for flow rates up to about 1000 g/h is basically a tube of stainless
steel without any built-in obstructions, internal diameter approx 1 mm. This tube is part of a completely
casted aluminium housing. An important part of the instrument is formed by two legs of tubing;
an upstream section and a downstream section.
On these two legs the heater/sensor arrangement of patented design is placed. The sensor measures the
page 8 9.17.001