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 Cost of accuracy
Those of us in the instrumentation industry know
there is a simple rule; if you want higher accuracy, it will usually cost more money. Paddlewheel
Flowmeters come very close to dispelling that rule. Due to modern technology, advanced plastics and
metals, solid-state circuitry, and good old capitalism the high cost of accuracy has been challenged. With
the use of computer-enabled testing facilities, paddle sensors are individually bench tested and are
accurate to within 1% of the full scale range reading. The sensor's flow parameters are then printed and
packed with that particular Flowmeter. The key is the
use of technologies that were simply not available some years back. It is important to note that
although the paddle sensor is tested, and accuracy confirmed, installation variables can affect the
meter's ultimate performance. But that would be the case with most types of Flowmeters. Dollar for
dollar, the paddlewheel Flowmeter simply gives you hands down, more accuracy for your dollar.
Ease of installation
Comparatively speaking, paddlewheel Flowmeters have distinct advantages over many kinds of
Flowmeters. Paddlewheel style Flowmeters can be installed either horizontally or vertically, with flow
in either direction, without compromising overall meter performance. Paddlewheel Flowmeters can be
purchased complete with a sensor, digital display and in-line meter body (pipe fitting) which can be
easily installed into a piping system. You can also purchase the meters with saddle mount type pipe
fittings which are very convenient for installation on existing, larger pipe sizes where in-line fittings are prohibitive.
Battery or AC operated
Today's Paddlewheel Flowmeters use advanced solid-state circuitry which can operate on common
batteries eliminating the need for electrical connections. The technology has advanced to the point
where batteries last for a minimum of one year. When the batteries are replaced, no harm is done to
the factory settings. Paddlewheel Flowmeters have considerable capabilities such as batch process
control, analog output signals and remote readouts. These full featured meters require a power
source and are typically supplied with an AC/DC plug-in style transformer. Keep in mind the flexibility this meter offers the user.
Versatile meter
Paddlewheel Flowmeters are deserving of the attention they are receiving. While most applications
are with water, paddlewheel meters also work remarkably well with viscous fluids, provided a fully
developed turbulent flow profile exists. If the fluid's Reynolds number is greater than 4000, the fluid
we be a fully developed turbulent flow regardless of viscosity changes.
REYNOLDS NUMBER EQUATION
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7741.92 x ID x V
Cst
Where:
ID = the pipe inside diameter in inches
V = the flow velocity in feet per second
Cst = the fluid viscosity in
centistokes
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To maintain accuracy, the Reynolds number must be above 4000 over the entire flow range. Unlike variable area meters, paddlewheel meters are not affected by changes in the fluid's
specific gravity. Special calibrations are not necessary.
Another compelling reason people are turning to paddlewheel style Flowmeters is their ability to have
a remote readout. Paddlewheel Flowmeters easily permit the panel mount readout to be installed in
the most desirous area, while the actual sensor is installed in an obscure area of the pipe system.
For example, in any application, the paddle sensor can be installed in the proper location and the
digital display can be remotely located in a more desirable location in the system such as on a centralized control panel.
Maintenance
The meter's paddle and axle are in direct contact with the fluid. Since the paddle will spin at a
velocity that is directly proportional to the rate of flow, these components will wear over time. Meters
which are operated at the high end of their calibrated flow range will tend to wear more than units
operated at the low end. Because every fluid has different characteristics, it is difficult to estimate the
life expectancy of these components. Some water (DI water), may be very aggressive while some
water may be exceedingly hard or full of abrasives. The resistance of the components to the chemical
being measured should also be considered. Axles and paddles are easily replaceable. A neglected
paddlewheel Flowmeter will in time have degraded accuracy.
The maintenance factor should not in any way deter using paddlewheel meters, just
remember, turbine meters have the similar problems; rotameters also have to be maintained. This is
a case where the benefits far outweigh the cost.
Maximize paddlewheel performance
If your flow falls below one foot per second
you should expect some accuracy challenges. Avoid
using paddlewheel Flowmeters for measuring very dirty fluid, or liquids with rocks or pebbles that
could break or damage the paddle or axle. Follow the manufacturer's installation recommendations
regarding straight lengths of pipe. If you cut corners, do not expect optimum performance from your meter.
In summary
Paddlewheel Flowmeters should be considered by anyone who requires above average metering
accuracy where cost is a consideration. These meters are also recommended because they are
easier than most meters to install, can measure flow in either direction and serve the intended purpose of measuring flow. Considering the overall cost of ownership, it is very difficult not to consider
the paddlewheel Flowmeter.
Following are some terms you might find useful. The more you understand how and why a Flowmeter
works, the greater success you will have with your selection.
K-Factor –
The number of signal pulses generated per unit volume of flow. Example: 225 pulses per gallon.
Reynolds Number –
A dimensionless number that combines the effects of viscosity, density, and
flow velocity which is used to identify either a turbulent or laminar flow profile.
Specific gravity-
(relative density) The ratio of the density of a fluid, at it's temperature, to the density of water at a specified reference temperature.
Feet per second of flow –
The velocity of fluid flow in a pipe expressed as the number of linear feet of flowing fluid passing a given point in a pipe, per one second of time.
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