Thermal Mass
Thermal Mass meters utilize a
heated sensing element isolated
from the fluid flow path. The
flow stream conducts heat from
the sensing element, which is
directly proportional to the mass flow rate.
The meter’s electronics package includes
the flow analyser, temperature compensator,
and a signal conditioner providing a linear
output directly proportional to mass flow.
Thermal mass meters carry a relatively low
purchase price. They are designed to work
with clean gases of known heat capacity, as
well as some low-pressure gases not dense
enough for Coriolis meters to measure. The
main disadvantage of thermal technology is
low-to- medium accuracy, although suppliers
have improved the capabilities of these
meters in recent years.
Turbine
Turbine meters contain a freely suspended
rotor, and the flow against its
vanes causes the device to
rotate at a rate proportional
to flow velocity. A sensor/
transmitter is used to detect
the rotational rate of the rotor;
when the fluid moves faster, more pulses
are generated. The transmitter processes
the pulse signal to determine the flow of the
fluid in either forward or reverse direction.
Turbine meters incorporate a time-tested
measuring principle, and are known for high-
accuracy, wide turndown and repeatable
measurements.
They produce a high-resolution pulse rate
output signal proportional to fluid velocity,
and hence, to volumetric flow rate Turbine
meters are limited to use with clean fluids
only. Bearing wear, a common concern with
this type of device, was largely addressed by
the development of ceramic journal bearings.
As a mechanical meter, turbines require
periodic recalibration and service.
Impeller
Impeller meters are
frequently used in large
diameter water distribution
systems. The device
consists of a paddle wheel
inserted perpendicularly
into a process stream. The number of
rotations of the paddlewheel is directly
proportional to the velocity of the process.
Impeller meter attributes include: direct
volumetric flow measurement (often with
visual indication), universal mounting,
fast response with good repeatability, and
relatively low cost.
Note their performance suffers in applications
with low fluid velocity. The meters are also
sensitive to flow profile. They can only be
used in clean, low-viscosity media.
Variable Area
Variable Area meters are
inferential measurement
devices consisting of two main
components: a tapered metering
tube and a float that rides within
the tube. The float position — a
balance of upward flow and float
weight — is a linear function
of flow rate. Operators can take
direct readings based on the float position
with transparent glass and plastic tubes.
Simple, inexpensive and reliable,
Variable Area meters provide practical
flow measurement solutions for many
applications. Be advised most of these meters
must be mounted perfectly vertical. They
also need to be calibrated for viscous liquids
and compressed Gases. Furthermore, their
turndown is limited and accuracy relatively
low.
Ultrasonic
There are two types
of ultrasonic meters:
transit time and Doppler. Both designs will
detect and measure bi-directional flow rates
without invading the flow stream. Ultrasonic
meters are ideal for troubleshooting,
diagnostics and leak detection. They can be
used with all types of corrosive fluids, as well
as gases, and are insensitive to changes in
temperature, viscosity, density or pressure.
Ultrasonic meters have no moving or wetted
parts, suffer no pressure loss, offer a large
turndown ratio, and provide maintenance-
free operation—important advantages over
conventional mechanical meters. Conversely,
the precision of these meters becomes much
less dependable at low flow rates Unknown
internal piping variables can shift the flow
signal and create inaccuracies.
Vortex
Vortex meters make use of a principle
called the von Kármán effect, whereby
flow will alternately generate vortices
when passing by a bluff body. A bluff
body is a piece of material with a broad, flat
front that extends vertically into the flow
stream.
Flow velocity is proportional to the frequency
of the vortices. Flow rate is calculated by
multiplying the area of the pipe times the
velocity of the flow. Vortex meters have no
moving parts that are subject to wear, and
thus regular maintenance is not necessary.
Only clean liquids can be measured with this
type of instrument. They are particularly well
suited for measurement of gas emissions
produced by wastewater. Vortex meters may
introduce pressure drop due to obstructions
in the flow path.
Oval Gear
Oval Gear meters utilize a positive
displacement meter design,
whereby fluid enters the inlet
port and then passes through the
metering chamber. Inside the chamber, fluid
forces the internal gears to rotate before
exiting through the outlet port.
Each rotation of the gears displaces a specific
volume of fluid. As the gears rotate, a magnet
on each end of the gear passes a reed switch,
which send pulses to the microprocessor
in the register to change the LED display
segments. The latest breed of Oval Gear
meters directly measures actual volume. It
features a wide flow range, minimal pressure
drop and extended viscosity range. This
design offers easy installation and high
accuracy, and measures high temperature,
viscous and caustic liquids with simple
calibration.
Nutating Disc
Nutating Disc meters are most
commonly used in water-
metering applications. A disc
attached to a sphere is mounted
inside a spherical chamber. As
fluid flows through the chamber,
the disc and sphere unit wobble or “nutate
”. This effect causes a pin, mounted on the
sphere perpendicular to the disc, to rock.
Each revolution of the pin indicates a fixed
volume of liquid has passed. Nutating Disc
meters have a reputation for high accuracy
and repeatability, but viscosities below
their designated threshold adversely affect
performance. Meters made with aluminium
or bronze discs can be used to meter hot oil
and chemicals.
Issue 40 PECM
133