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ADVANCED MATERIALS & PROCESSES •
SEPTEMBER 2014
58
HTPRO
ters, heat input and coefficient of spe-
cific heat, are constant.
Another type of mass flowmeter uses one
flow channel with a temperature sensor
located in the path of the flow. The tech-
nology is simpler, but often less accurate,
and is limited to higher flow rates.
Accuracy and repeatability
Thermal-mass flowmeters are gas-spe-
cific devices, and they must be cali-
brated using either the actual gas or a
reference gas. This inconvenience led to
the development of many “fixes,” and
drives the development of smarter de-
vices. However, primary calibration
using the actual gas or a gas of similar
molecular characteristics is currently
the only way to ensure accuracy.
Two factors that determine the accuracy
of mass flowmeters and mass flow con-
trollers are flow calibration and repeata-
bility. Proper instrument calibration
ensures starting point accuracy. Re-
peatability is the measure of continuous
performance-to-specification over the
lifetime of the device. Most mass
flowmeters and mass flow controllers
have an accuracy of ±1% of full scale and
a repeatability of ±0.25% of full scale.
Several factors affect repeatability.
Highly stable materials and electronic
components, as well as precise internal
voltage and current regulation are used
to compensate. Sensor and bypass de-
sign also play a major role in preventing
errors caused by contamination and
clogging. For example, U-type sensor
tubes exhibit residual stresses from
bending, which can cause long-term
strains and unraveling of sensor coils.
These sensors are also more likely to de-
velop drift due to contaminant deposits.
Consideration should also be given to the
bypass element. Accuracy is degraded by
changes in temperature if the bypass is an
orifice (or venturi), as opposed to a pure
laminar-flow element. With an orifice by-
pass, the pressure drop is proportional to
the square of the bypass flow. In this case,
the ratio of bypass flow to sensed flow is
not a constant, but instead is a complex
nonlinear function having temperature-
dependent terms such as gas viscosity.
Both the nonlinearity and temperature
dependence of the orifice bypass can se-
riously degrade the accuracy of a mass
flow controller.
Mass flowmeters
for use with vacuum furnaces
A common heat processing application
of thermal mass flowmeters and mass
flow controllers is maintaining a specified
gas flow rate into a vacuum chamber
when the process requires a partial pres-
sure of additive gas. Typically, a throttle
valve or an orifice-limiting device is used
to control the output of a vacuum pump.
This is an extremely pressure-sensitive
method and can result in inefficient gas
delivery and poor product quality. Mass
flow controllers automatically compen-
sate for changes in system pressure
caused by vacuum pump fluctuations
and deliver a precisely controlled gas flow
rate to the chamber.
HTPRO
For more information:
Daniel H. Herring
(The Heat Treat Doctor) is president, The
Herring Group Inc. P.O. Box 884,
Elmhurst, IL 60126-0884, 630.834.3017,
email:
dherring@heat-treat-doctor.com,
heat-treat-doctor.com.
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