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nita
ICE Expert
Joined: June 30, 2006
Posts: 164
Location: Hiroshima,Japan
Status: Offline
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 Posted:
Feb 24, 2007 - 12:04 PM |
 
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| Post subject: Thermal mass flow measurement |
Thermal mass flow measurement techniques- from http://www.bronkhorst.com/en/products/theory/thermal_mass_flow_measure ment/
for gases
As shown in figure A a part of the gas flows through the sensor, and is warmed up by heater RH. Consequently the measured temperatures T1 and T2 drift apart, as shown in figure B. The formulas for dT demonstrate that the temperature difference is directly proportional to mass flow. Electrically, temperatures T1 and T2 are in fact temperature dependent resistors RT1 and RT2.
figure A
figure B
In figure A it is shown how the signals measured in the sensor are amplified to electric signals. The sensor is mounted as a by-pass to the main channel, where a patented flow resistance splitter takes care of proportional flow division, also under varying process conditions. This laminar flow element consists of a stack of stainless steel disc with high-precision etched flow channels, having similar characteristics as the flow sensor.
for liquids
Liquid mass flow meters of the LIQUI-FLOW® series are built around a stainless steel tube without any moving parts or obstructions. The heater/sensor assembly is arranged around the tube and, by following the anemometric principle, a constant difference in temperature (dT) is created and the energy required to maintain the dT is dependent on the mass flow rate. Due to the benefits of the unique patented sensor, the fluid will be warmed to a maximum of 5°C, thereby making the LIQUI-FLOW® Series suitable for fluids with low boiling points. |
Last edited by nita on Feb 24, 2007 - 12:29 PM; edited 1 time in total |
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nita
ICE Expert
Joined: June 30, 2006
Posts: 164
Location: Hiroshima,Japan
Status: Offline
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| Posted:
Feb 24, 2007 - 12:27 PM |
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How Thermal Flowmeters Work
Thermal flowmeters use the thermal properties of the fluid to measure the flow of a fluid flowing in a pipe or duct. In a typical thermal flowmeter, a measured amount of heat is applied to the heater of the sensor. Some of this heat is lost to the flowing fluid. As flow increases, more heat is lost. The amount of heat lost is sensed using temperature measurement(s) in the sensor. The electronic transmitter uses the heat input and temperature measurements to determine fluid flow.
How to Use Thermal Flowmeters
Thermal flowmeters are most commonly used to measure the mass flow of clean gases, such as air, nitrogen, hydrogen, helium, ammonia, argon, and other industrial gases. Mixtures, such as flue stack flow, can be measured when their composition is known. An advantage of this technology is its dependence upon thermal properties that are almost independent of gas density. Be careful when using thermal flowmeters to measure the flow of gases with unknown and/or varying composition, such as hydrogen-bearing off-gases and other mixtures.
Thermal flowmeters can be applied to clean, sanitary, and corrosive gases where the thermal properties of the fluid are known. Thermal flowmeters are most commonly applied to measure pure gases, such as would be used for laboratory experiments, and in semi-conductor production. They can also used in chemical and petrochemical plants when the thermal properties of the gas are known. With proper attention to materials of construction, the flow of corrosive gases, such as hydrogen chloride and hydrogen sulfide can be measured.
Application Cautions for Thermal Flowmeters
Thermal flowmeters should not be applied to abrasive fluids because they can damage the sensor. Fluids that coat the sensor can render the sensor inoperable unless the sensor is routinely cleaned. This can increase maintenance associated with these flowmeters. Varying the percentage of certain components that have different thermal properties from calibrated values can cause thermal flowmeters to become highly inaccurate.
Aerosols and gases with droplets can cause thermal flowmeters to become erratic and/or measure full scale flow. This is because the large amount of thermal energy used to heat the liquid/droplet is interpreted as a high flow signal. Operating a thermal flowmeter above its maximum flow rate can cause measurement error because its calibration curve can become unpredictable. |
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instruite
ICE Expert
Joined: May 29, 2006
Posts: 321
Location: Japan/India
Status: Offline
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Feb 25, 2007 - 06:44 PM |
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nice short descriptive tutorial |
_________________
Instruite 
======================
Ideas that work,
Concepts that sell.
====================== |
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fundu_instru
ICE Expert
Joined: June 20, 2006
Posts: 180
Location: Singapore
Status: Offline
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Mar 19, 2007 - 02:25 PM |
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The Coriolis mass flow meter measurement principle is based on Newton's Second Law of Motion: Force = Mass x Acceleration (F = M x A). Inside the meter, flow is diverted into two parallel tubes that vibrate at high frequency via an electromagnetic drive coil. The tube loops have varied shapes, sometimes forcing the fluid through several 90° turns before rejoining the flow line. Upward and downward forces are exerted when the fluid goes through the turns, causing a tube deflection known as the Coriolis Effect. This tube deflection is directly proportional to mass flow. Velocity detectors measure the time difference between the upward and downward forces of the flow and the tube, to indicate the mass flow rate.
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nita
ICE Expert
Joined: June 30, 2006
Posts: 164
Location: Hiroshima,Japan
Status: Offline
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| Posted:
Apr 30, 2007 - 03:54 PM |
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nita
ICE Expert
Joined: June 30, 2006
Posts: 164
Location: Hiroshima,Japan
Status: Offline
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| Posted:
May 29, 2007 - 11:09 AM |
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| Post subject: Basics of Thermal Mass flow Control |
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