| MFC
PRINCIPLES
Mass Flow Controllers (MFCs) are used wherever
accurate measurement and control of a mass flow
of gas is required independent of flow pressure
change and temperature change in a given range.
Mass Flow Meters (MFMs) are used wherever accurate
measurement of gas is required without control
of the flow, which is done by another device.
To help understand how an MFC works, it can
be separated into 4 main components: a bypass,
a sensor, an electronics board and a regulating
valve:
The bypass, the sensor, and one part of the
electronics board are the measurement side of
the mass-flow controller and make a Mass Flow
Meter. The regulating valve and the other part
of the electronics board are the controlling
side of the mass-flow controller and exist only
on a Mass-Flow Controller. So every Mass-Flow
Controller includes a Mass-Flow Meter.
MEASUREMENT PRINCIPLES
The flow is divided between a heated sensing
tube (the sensor), where the mass flow is actually
measured, and a flow restrictor or bypass, where
the majority of flow passes. The bypass is designed
in a way that flow thru the sensor and the bypass
is always proportional to the flow range for
which the mass-flow controller is built. The
sensor is designed to deliver an output voltage
almost proportional to the gas flow circulating
thru it, which is due to the bypass design being
proportional to the total flow circulating thru
the mass-flow meter or controller. The electronics
board amplifies and linearizes the sensor signal
so the output of the+ electronics board, named
?readout?, gives a signal proportional to the
total flow circulating thru the mass-flow meter
or controller. Most of the time this signal
is a 0-5 V signal; 0 volts indicating ?no flow?,
5 volts indicating full scale flow of the device.
Full scale is the maximum flow for which the
mass-flow controller is designed and calibrated
to work with at high accuracy. This information
is always written on stickers, which are on
the top of the cover and the side of the mass-flow
stainless steel base. Also written on the sticker
is the gas for which the mass-flow controller
is calibrated.
Why use a bypass? Because the sensor element
can only measure small flow (typically 5 sccm),
the bypass/restrictor allows the controller
to control and measure greater amounts of flow.
On a 5 sccm full scale mass-flow controller,
there is no bypass, all of the gas would flow
thru the sensor. On a 100 sccm full scale mass-flow
controller, the bypass is adjusted so that when
100 sccm flows thru the controller, 5 sccm will
flow thru the sensor tube and 95 sccm will flow
thru the bypass.
SENSORS PRINCIPLES
Basically, the sensor uses the thermal properties
of a gas to directly measure the mass flow rate.
The sensor uses the basic principle that each
gas molecule has a specific ability to pick
up heat. This property, called the "specific
heat" (Cp), directly relates to the mass
and physical structure of the molecule and can
be determined experimentally. The specific heat
is well known for many gases and is generally
insensitive to changes in temperature or pressure.
By adding heat to a gas and monitoring the
change in temperature, the mass flow rate can
be determined. To illustrate this concept, take
the case of cool gas flowing through a heated
tube. Mathematically, the heat loss can be described
by the First Law of Thermodynamics,
It is important to realize that both the specific
heat and the flow rate determine the amplitude
of the heat flux. As the mass and physical structure
of molecules vary widely from gas to gas, so
does the specific heat Cp. For the same molar
flow rate, the heat flux can differ significantly
for different gases. If this heat flux is monitored,
the amplitude can be converted into an electrical
signal. Given that the specific heat is known
for the gas, the mass flow rate can then be
determined directly from the electrical signal.
The MFC sensor includes a capillary tube wound
with two heated resistance thermometers, measuring
the change in temperature distribution created
by the gas flowing inside this tube:
Sensor schematic
For zero flow, the upstream and downstream
temperature will be equal. The windings are
heated electrically to 80°C above ambient
temperature. When the gas is flowing, the upstream
region cools down whereas the downstream region
heats up causing a temperature gradient along
the length of the tube (see the sensor temperature
profile figure).
Sensor temperature profile
The coils of the heating resistances are made
with a thermal sensitive wire so that the temperature
differences due to the flow are directly converted
into resistance changes. These resistance changes
are converted to a voltage by a simple Wheatstone
bridge.
sensor response
For flow under 5 sccm the measurement is proportional
to the flow with a coefficient which depends
on :
p : Volumic mass of the gas
Cp : specific heat for a constant pressure,
N : "spin factor" Constant which depends
on the molecular structure of the gas and compensates
for the temperature dependence of Cp.
Value of N :
Monoatomic gas 1.04
Diatomic gas 1.00
Triatomic gas .94
Polyatomic gas .88
For flows higher than 5 sccm, the sensor is
at first non-linear. Then the measurement starts
to decrease with flow because the gas flow is
too fast and cools the 2 wound resistance wires
instead of cooling the first one and heating
the second one. This is the reason why a bypass
is necessary for higher full scale than 5 sccm.
Also the fact that the coefficients N and Cp
are different from one gas to another explains
why mass-flow CANNOT be changed from one gas
to another without using a special coefficient
to convert the measurement or recalibrate the
mass-flow.
Because of sensor saturation, if flow is ten
times the full scale, output will be almost
?no flow?! This will never happen on a mass-flow
controller as the valve acts as a restrictor
and will not allow the gas to flow at ten times
the full scale. But it can easily happen on
a massflow meter; if there is no restriction
in the gas line there is nothing in the mass-flow
meter to limit the gas flow.
BYPASS PRINCIPLES
Acting as a restrictive element, the bypass
is composed of a series of capillary tubes (or
bypass washers that also come in different slot
sizes for different flow ranges) held in a special
bypass ring. The ring fits around the body and
may hold up to 24 tubes. The number of tubes
and their diameter will depend on the customer's
specifications of gas type and flow range. For
higher flow rates, the bypass tubes are replaced
by a screen.
Bypass washers (equivalent to several thin
tubes) The bypass principles are based on the
laminar flow theory: When flow is laminar, the
flow is proportional to the differential pressure
between the inlet and outlet of the tube:
So when a sensor tube (radius Rs, length ls)
and a bypass tube are in parallel (radius Rb,
length lb), the flow in the sensor tube is proportional
to the flow in the bypass:
However, this is true only if the flow is
laminar and if the tubes are small enough. This
is why the bypass is made of several thin tubes
instead of only one tube. It is important to
note that the flow measured thru the sensor
of a mass-flow meter or controller is not the
total flow, but only one part of the flow split
by the bypass according to last equation. In
this equation, the radius of the sensor tube
and bypass tube is at power 4. Consequently,
any deposition in one of the tubes, changing
the diameter, will change the accuracy of the
measurement. Because of the need to have laminar
flow, mass-flow meters and controllers must
be used with clean, filtered gases to avoid
clogging in the bypass tubes and sensor tube.
CONTROL PRINCIPLES
The electronics board compares the amplified
mass flow rate value (measured by the sensor)
to the desired set point. This comparison generates
an error signal that "feeds" the regulating
valve. The difference is used to drive the control
valve. The control valve will proportionally
open or close until the output is equal to the
setpoint. Note that valve can be normally open
or normally closed. This is the position that
the valve will be in when the mass-flow controller
is not connected to a power supply.
The valve can be actuated by a magnetic solenoid,
which can be normally open or normally closed.
The response time of the valve itself is almost
instantaneous. In practise, however, the response
time of the mass-flow controller is limited
by the response time of the sensor. As a sensor
is based on thermal exchange, it takes 1 to
5 sec. for the sensor to measure a gas change.
Several techniques allow us to increase this
response time and allow us to get the best mass-flow
response time below 5 sec.
The valve can be also made by a heating wire
which heats a small tube, then dilation will
move a ball at the end of the tube. This thermal
type of valve can be only normally open and
is quite slow. Mass-flow controllers using this
valve will have response times around 5 to 6
sec. for flow below 5 slm and up to 10 sec.
for flow up to 5 slm!! However this technology
is simple and reliable and is recommended for
many low cost applications when response time
is not critical.
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