Thermal flow measurement summary
History
The thermal flow measurement principle bases on the research of Louis Vessot King. He also developed
the first hot-wire anemometer which is something like the elder of modern thermal flowmeters.
Measuring principle
The t-mass of Endress + Hauser is based on the principle of “thermal dispersion”.
Thermal dispersion type meters can operate in either of two ways. In both cases, however, the heated
and unheated sensor elements are connected in the form of a Wheatstone Bridge arrangement. If the
current through the heated element, e.g. a wire, is kept constant, there is a direct relationship between
temperature (resistance) and flow. Alternatively, the resistance can be maintained constant, in which
case power varies with increasing or decreasing in flow.
Two sensors are installed in the gas stream. One sensor monitors the actual process temperature whilst
the second sensor is heated to a defined differential temperature. Even with no flow, the gas extracts
some energy from the heated sensor due to convection. As the gas flow increases, more energy is
drawn from the heated sensor. To maintain the differential temperature, the power to the heated
sensor must be increased. The electrical power needed to maintain the differential temperature of the
two sensors is in relation to the mass flow in the pipe. The relation is not linear, therefore a thermal
mass flow meter must be calibrated to define the characteristics of the unit. The sensitivity of the
principle is best a low flow. At high flow rates less and less increase of heating power is needed to
compensate an increased mass flow. The resolution of the flow signal is therefore reduced.
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Gas groups
Standard gases
Special gases
Other gases
This group of gases can reliably be
measured. Basic knowledge of the
application is needed to select and
size the meter.
Due to the properties of this group of
gases the application might be
compromised. Changing gas
composition of Biogas, condensing
Butane, or the very high specific heat
capacity of Helium can lead to
unacceptable results or even failure of
the measurement. Expert knowledge
and detailed information of the
process conditions and the customer
expectations are needed to select the
best solution.
These gases should not be
attempted without approval
of an experienced expert.
Sometime thermal mass
flowmeters are the only
technical solution to measure
these gases however. All
these gases bear some
additional risks that can lead
to corrosion, unacceptable
errors, zeropoint shifts and so
on.
Example:
• Biogas
• Natural Gas
• Hydrogen
• Helium
• Butane
• Propane
Example:
• Ammonia
• Chlorine
• Always check all other
gases…
Example:
• Air
• Oxygen
• Nitrogen
• Carbon Dioxide
• Argon
• Methane
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Basic meter construction
A thermal massflow meter consists of the following main components:
Transmitter
Amplifies and filters signals, computes relevant data (totalizer, flowrate), generates interface signals
(Hart, Profibus etc). Is the human machine interface (HMI) and display. It can be mounted integrally or
remote, depending on the installation situation. Is available in aluminum (non-Ex and Ex versions).
Sensor
Cointains all parts and functions of the device mounted in the pipe and connected to the process.
Process connection
It provides connection to customer process piping. Choose from many options.
Obstacle (3 pin sensor form)
The obstacle prevents the green heater “decider” from being cooled as
much as the red heater if the flow goes reverse. At forward flow both
heaters will be cooled with the same rate.
Decider (3 pin sensor form)
The green temperature sensor is heated via electrical energy, so that a
predefined temperature difference is maintained, even if the sensor gets
cooled by flow. If the flow goes in forward direction, the sensor will get
cooled in the same way as the red heater.
If the flow goes reverse, the obstacle prevents the green heater from being cooled as much as the red
one. The job of this green heater “decider” is to detect if the flow goes in forward or backward direction.
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Heater (in every sensor form)
The red heater is heated via electrical energy so that a predefined
temperature difference is maintained, even if the sensor gets cooled by
flow. The flow measuring itself relies on the red heater and the blue
ambient sensor.
Ambient (in every sensor form)
This temperature sensors measures the actual temperature of the gas as a reference.
Advantages
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Direct mass measurement
No external pressure or temperature compensation necessary
Negligible pressure drop
Very high turndown ratio (typically 100:1, sometimes 1000:1)
Measures at low velocities and pressures
Capable of detecting extremely low flows (leak detection)
High repeatability
Cost-effective solution for large line sizes
Multivariable output: Flow and temperature
Adaptability of insertion version for circular pipes and rectangular ducts
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Limitations
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Gas type and composition must be known
Requires stable gas mixtures for best performance
Generally dry and clean gases only
Installation is important to ensure fully developed flow profile:
• Sufficient inlet and outlet run lengths
• Selecting correct pipe diameter in setup
• Other invasive measuring probes installed preferably downstream to avoid disturbances
• No intruding gaskets in the gas flow
Regular recalibration intervals are recommended
Process influences:
Pulsating flow can result in an unstable measurement signal. In some cases, pulsating flow may
lead to reverse flow, which for some measurement technologies either remains undetected or is
regarded as positive (forwards) flow.
• Moisture/wet gas can create the impression of increased flow rate with thermal mass meters. Due
to its high thermal conductivity, water strips the heated sensor of additional heat energy, which
appears as increased flow in the device. This problem can be alleviated by installing the devices at a
radial angle of 135° to ensure that condensation flows to the base of the sensor and away from the
sensor tips.
• Dirt/coating on the sensors of a thermal mass meter can also have an influence on the
measurement. Dirt can function as an insulation to the sensors restricting heat flow from the heated
sensor to the measurement fluid. This can give the impression of lower flow.
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Application examples
Chemical
e.g. Nitrogen blanketing
Water & Wastewater
e.g. Air measurement in aeration basin
Food&Beverage
e.g. CO2 measurement
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Power&Energy
e.g. Biogas consumption
Primaries&Metal
e.g. Oxygen gas measurement
Utilities
e.g. Compressed air measurement
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