| What
is this thing called calorimetric?
In the $3 billion dollar world market of flow
metering, a little known jewel is gradually
finding its way to acclaim…the calorimetric
flow sensor. The use of the calorimetric principle
in flow is now a standard in the industry. Significant
advances over the past decade have enabled the
calorimetric sensor to assume a very active
and respected role in flow switching and flow
metering of both liquids and gaseous mediums.
The most common use of these sensors is in
flow no-flow applications for detecting the
movement of air or liquids. Popular for their
solid state construction with no moving parts,
the calorimetric flow sensor is impervious to
particulate matter that spells doom for mechanical
type devices.
Historically, the calorimetric switching devices
have been costly in comparison to mechanical
switches, but with new technological breakthroughs
in both electronic and mechanical construction,
there are, now available, units that compete
directly with their primitive counterparts.
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| Traditional
Paddle Switch |
Modern Calorimetric
sensor |
Flow metering has also become a substantive part
of the calorimetric sensor’s function. The sensing
technology, having no moving parts, allows measurement
of flows so low as to be below the inertia threshold
of turbine or paddlewheel sensors. This has made
them very popular for chemical dosing, leak detection,
etc. Also, extended range calorimetric sensors
are now available that can enable measurements
of much higher flow rates (up to 60 feet per second
for some liquids) than previously thought possible
with calorimetric technology. The traditional
calorimetric technology utilizes two PTC Thermistors,
one of which is heated a predetermined amount
above the other, which monitors the medium temperature.
Flow of the medium conducts heat away from the
sensor probe and the corresponding change in
the heated Thermistor’s resistance value is
then amplified, linearized and converted into
an output (switching or analog depending upon
sensor type) proportional to the rate of flow.
The fact that calorimetric sensors do not require
the medium to be electrically conductive (as
would a mag meter for example) makes measurements
of polymers, oil, grease, and numerous other
mediums possible. The technology also is not
based upon suspended particles for measurement
accuracy (as would be the case with doppler
type meters). The addition of optional specialty
metals for use in the calorimetric construction
(such as Hastelloy, Monel, Titanium etc.) allows
measurements of innumerable aggressive medium
such as chemicals and acids.
The calorimetric sensor’s minimal power requirements
have enabled the development of battery operated
flow sensors for remote locations. These can
be paired with wireless transmission devices
to enable internet based monitoring or dial
up fault alarming for such applications as loss
of flow to pumps, leaking of pipes, and loss
of lubrication oil flow or coolant flow in transportation.
Inherent in the design is the continual monitoring
of the medium temperature, which enables the
calorimetric sensor to also function as a temperature
monitor. This provides the user with two sensors
in one. This has been a popular feature with
chiller manufacturers, as the same device can
alarm if flow is lost, or temperature exceeds
predetermined limits.
Microflow technology in calorimetric sensors
has also enabled here-to-fore unrealizable low
flow metering of less than 2 milliliters per
hour of liquid flow.
In summary, the calorimetric flow sensor is
the rising star of the flow sensing industry
and offers capabilities unmatched by other flow
measurement technologies.
Larry Flanagan
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