In-Situ Thermal Mass Flow Meter Calibration Validation
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In-situ calibration validation. Industrial thermal mass flow meters comprise
a family of instruments for precision measurement of total mass flow rate of gases in the pipes and ducts. So when evaluating thermal mass flow meters for in-situ calibration validation
capability, be aware that sensor drift will create false-positives, reducing the reliability
of the validation. When innovators like, Dr. John George Olin, founder of Sierra instruments, saw the rapidly growing need in industry for a truly industrial,
thermal hot wire anemometer. To make a thermal flow sensor truly industrial strength, Dr. Olin pioneered the innovation of an all metal clad thermal sensor in the early 1980s. Wet thermal sensor design, because of the
organic potting cement used, would shift and crack overtime, causing unwanted sensor drift,
resulting in gradual degredation of flow measurement accuracy. Today, wet thermal sensors are used
by all industrial thermal sensor manufacturers on the market. With one exception, and that
is Sierra. Sierra never stopped improving the technology, knowing that wet sensor drift
problems must be solved. After all, by its very principle of operation, a thermal sensor
is heated and cooled over time, expanding and contracting the cement inside the sensor. Making it crack, settle, and shift from its original state. Dr. Olin believed there had to be a better
way. He needed to get rid of all cements to make the most stable thermal sensor in the
world. After many years of experiments and testing, the answer finally revealed itself.
On March 9, 1999, Dr Olin patented a breakthrough thermal sensor design. Like other thermal
mass flow sensors, it was made of platinum resistance temperature detector wire, wound
around a ceramic mandrel, but this is where the similarities ended. Through a proprietary highly controlled manufacturing
process, a metal sheath is tightly formed on the mandrel and wire assembly. To be in
such intimate contact that no air gaps exist and no organic filler cements are needed.
This new sensor was literally all metal. In contrast to wet sensor design this innovative
cement-free sensor is know as a dry sensor. It is patented by Sierra, and is brand named
Dry Sense. Because the all metal dry sense sensor is
so rugged, reliable and stable, Sierra has backed quality claims to be the only company in the industry to offer a lifetime warranty on their thermal sensor. With wet sensor versus dry sensor design in
mind, lets briefly discuss in-situ calibration validation. Quality systems, like ISO 9000
may require annual or even semi-annual calibration and validation of all measurement instruments
in the facility. One of the nice features of thermal mass flow meters is that the calibration
can be easily validated in the field at a very low cost. In-situ, or field calibration validation of
thermal meters is a highly marketed feature because it saves money on calibration and
downtime. Sierra customers have enjoyed this advantage for many years. Sierra’s motto has always been don’t re-calibrate, validate. But be careful, there’s a lot more going on
behind this statement than you may first think. There is a lot of smoke and mirrors by thermal manufacturers about in-situ calibration. All thermal meter manufacturers have generally the same method of In-Situ calibration validation to confirm the original factory calibration,
while checking for drift and assuring accuracy. The basic process makes use of the electrical
characteristics of the sensor, either by checking that the electrical resistance of the velocity
sensor’s platinum resistance temperature detector wire has not changed over time. Or by assuring
that the power sent to the velocity sensor at zero flow is the same as when it was first
calibrated at the factory. The problem with both methodsd is that they
only measure the characteristics of the internal sensor windings, and do not address sensor
drift. The big question is, how can you validate a sensor that will drift out of spec due to
the very nature of its mechanical design? The resistance of the platinum wire shouldn’t
change, and the current running through the sensor might be the same, both of which would
validate okay. This is a false-positive, as the drift inherent in a wet sensor design
would remain unseen. Sierra’s DrySense all-metal no-drift sensor
has a big advantage during in-situ calibration validation. With Sierra’s all-metal DrySense
velocity sensor, you have the mechanical design confidence that there will be no drift, and
that in-situ calibration validation is indeed actually valid. Sierra offers a free in-situ calibration validation
software package we call ValidCal diagnostics. Unlike some other validation routines, Sierra’s
ValidCal Diagnostics program provides a complete check of all the meter’s components. Including,
the velocity and temperature sensors, sensor drive circuitry, accuracy of the pressure transducer, and all digital and analog outputs at alarm relays. When complete a detailed calibration certificate
and diagnostics report is created. All of this can be accomplished without even removing
the meter from the processed piping. For accurate and repeatable gas mass flow
measurement down to the last molecule, choose thermal mass flow technology. Experience our
passion for flow. For more information go to: www.sierrainstruments.com. The global leader in flow instruments.

One thought on “In-Situ Thermal Mass Flow Meter Calibration Validation

  1. Watch our video to find out how to get reliable thermal flow meter in-situ calibration results that will save your company money! http://bit.ly/1yOrmx4

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