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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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