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The Advantages of Mass Flow Meters & Controllers in Fuel Cell Test Standards
By Jan Christensen
Choice of MFCs for test stands must ensure accuracy at different flow rates
and consider turndown and
Fuel cell engineers use test stands to simulate the performance of fuel cells.
The challenge is to mimic reality and determine the optimum fuel mixture and
temperature, while also examining failure mechanisms. Another objective is
to test the performance of anode/ cathode materials, catalysts, and membranes.
Fuel cell test stands consist of systems for mixing, delivering, and humidifying
gases, all of which depend on reliable flow measurement and control. A typical
test stand uses several thermal mass flow controllers (TMFCs) of various flow rates to deliver the precise amount of blended gases to the fuel cell. Since
the test stand must imitate normal operating conditions, these flow controllers
must react to
process signals quickly—much like a car accelerator—to provide a
true test of the desired performance. In addition, the TMFCs must have a broad
turndown to mimic low and
high fuel consumption rates. Many fuel cell test stands use
TMFCs that do not have the performance capabilities to
adequately test fuel cells. (See Figure 1)
For research, development or manufacturing of fuel
cells, engineers need test tools to evaluate and validate
fuel cell technology. Test systems must provide fl exible
data acquisition, monitoring and control to precisely
handle the fl uid for fuel cell operation and experiments.
As research engineers constantly incorporate new
measurements into their tests they need reliable, accurate
and flexible test systems to help shorten development
cycles, increase quality, and lower the cost for all
stages of fuel cell development, from research and design
validation to manufacturing.
One of the most important and complex tasks is the
control of gases to the fuel cell and the selection of
the best measurement and control
equipment. Gas measurement and
control can be done with a variety of
technologies ranging from a simple
variable area meter (VA), commonly
know as a rotameter, to a thermal
mass flow controller. A thermal mass
flow controller is the preferred choice
by most fuel cell developers due to its
response time, turndown capability,
Variable Area Meters
Measurement and control using
a variable area (VA) meter is accomplished
with a glass tube and a float
(ball) and a needle valve as shown
in Figure 2. The measurement itself
is volumetric and readings can be
fairly accurate as long as conditions
are close to normal pressure and temperature;
however, wide variations in
temperature or pressure will affect
the measurement, often significantly.
Nominal accuracy is dependent on
the device selected, with a range of 2-10% of full scale. To approximate
mass using a VA meter, one must
assume that the temperature is 68° F and pressure is sea level, and stay
constant throughout the measurement range. In the early days of fuel cell testing,
the VA meter was a commonly used device; it was simple and inexpensive; however
too many times when results were compared with those using a mass balance,
it was apparent that the VA meter’s accuracy was unacceptably inconsistent
due to wide fluctuations in temperature and pressure.
Mass Flow Controllers
The MFC is the preferred choice for controlling gas flows in today’s
test stations. Two main measurement technologies are used for doing the measurement,
thermal, and pressure/ temperature. Both technologies measure inferred mass.
The mass flow measurement using pressure and temperature is based on the ideal
gas law. The drawback with this technology is that it assumes that everything
stays constant. Unfortunately, this is not the case. Accuracy is in the range
of 2% of full scale, with a repeatability of 0.5% of full scale, still not
a perfect choice for an accurate test and mass balance.
The most common measurement
and control instrument is the TMFC. (See Figure 3) It is available in a variety
of configurations and with a very wide performance range. In this case
the common adage that you get what
pay for is very true. While a TMFC is
a more expensive technology than
the VA or pressure/temperature mass
flow types, the performance benefits
significantly outweigh the costs.. The
measurement is based on the heat
characteristics of a specific gas. A
small part of the flow through the
TMFC is diverted into a sensor tube
where a temperature measurement
is made. From this sample, mass flow
is automatically calculated.
It is very important to understand
the accuracy and turndown limitations.
(See Figure 4) A TMFC with an
accuracy of 1% of full scale controlling
at 40% of calibrated flow rate
will have an accuracy of ± 2.5%, but at 20% of calibrated flow rate
its accuracy changes to ± 5%. These accuracy ranges make the use of
a mass balance almost impossible. Thermal mass flow controllers with an accuracy
of 1% of rate stay as a fl at line over the entire calibrated range.
TMFC is calibrated is also very important. The thermal characteristics
of some gases can be very complex. Mainly with H2 and CO2,
the characteristics become three dimensional, such that the gas factor changes
flow rate, temperature
and pressure. Many MFC manufacturers use surrogate gases for calibration and then apply a factor
to correct for the actual gas. When choosing an MFC for use on
H2 or CO2 it is very important to select
a manufacturer who calibrates on the
actual gas, that is, a CO2 MFC should be
calibrated on CO2.
Digital vs. Analog MFCs
The internal operation of an MFC has changed greatly over the last 10 years,
with advances of microprocessor-based MFCs. The zero and span adjustments have
been replaced with computer interfaces. Specific gas thermal characteristics
are defined as an ‘S’ curve; the linearization of the ‘S’ curve
is directly proportional to the turndown and accuracy of the MFC. Older analog
MFCs have a very limited circuitry for adjusting or “linearizing” the ‘S’ curve,
typically incorporating only two or three points. CO2 has a very complex ‘S’ curve.
A high performance microprocessor-based MFC can use up to 25 points for linearization
of the ‘S’ curve, plus it will also use 4th order polynomials in
the calibration. (See Figure 5)
A digital MFC will also allow for multiple
calibration curves to be stored in the MFC. These are real calibrations as
opposed to gas factors, which are not substitutes for real calibration curves.
In reading most manufacturers’ instruction manuals, the conclusion is
inescapable that applying surrogate gas calibrations or using gas factors will
result in ± 5% full scale accuracy, typically inadequate for fuel cell
testing and production applications.
In addition to using an MFC calibrated
to the actual gas, microprocessor- based electronics will also greatly improve
the valve control of the MFC. This is very important for fast response and
When selecting digital mass flow controllers in gas blending/ delivery systems,
look for response times of less than 1 second. While turndown (when measured
as control range staying within 2% of rate accuracy) can range between 2:1
and 12:1 for many devices, 30:1 is achievable.
JAN CHRISTENSEN, SENIOR FLOW SPECIALIST, BROOKS INSTRUMENT. CHRISTENSEN
IS A GRADUATE ELECTRONICS TECHNOLOGIST WITH MORE THAN 24 YEARS OF FLOW EXPERIENCE
AND 12 OF THOSE YEARS AS A FIELD SERVICE APPLICATION SPECIALIST. HE HAS WORKED
WITH MOST OF THE WORLD’S FUEL CELL COMPANIES OVER THE LAST FEW DECADES,
ASSISTING THEM IN DEVELOPING GAS DELIVERY SYSTEMS.
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