Glossary
Variable
Area Rotameter – The principle of operation
for most of Key Instruments flow meters. The design consists
of a “float” in a vertical, tapered tube. As flow
enters the bottom of the tube, it raises the float proportionally
to the flow rate. As the flow rate is increased, the float
moves upward increasing the area for passage of fluid. When
the upward force of the fluid is equal to downward force of
float it will reach an equilibrium position.
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Laminar
Flow – fluid flow (air & water & gases)
flowing in a smooth, uniform motion. The flow is considered
to remain in its “layers”.
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Turbulent
Flow - fluid flow (air & water & gases) flowing
in random, mixing motion. This flow will not remain in layers;
rather it flows within adjacent layers.
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Hysterisis
– The lagging of a reaction from the initial cause.
For example, with flow meter valves, there is a slight Hysterisis.
When a valve is opened, the increase in flow may lag slightly
behind the actual movement of the knob.
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Turndown
– Is the effective measurement range of the flow meter;
or the ratio of maximum flow value to the minimum flow value.
For example, if the maximum stated flow rate on a meter is
100 LPM, a 10:1 turndown will allow the minimum flow rate
to be 10 LPM. If the turndown were 100:1, the minimum flow
rate would be 1 LPM.
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Resolution
– The ability to read a flow rate, not specifically
called out on a flow meter. For example the ability to see
1 LPM mark on a flow meter with 100 LPM as a maximum, is much
more difficult than if the maximum flow rate were only 10
LPM. Resolution could also could be defined as the value of
each intermediate marking on a flow meter. With an increasing
maximum flow rate, each intermediate flow rate has an increasing
value, therefore decreasing the resolution of the intermediate
points.
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Repeatability
– The ability of the flow meter to read a given flow
rate the same every time when a common set of parameters are
applied.
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PSIA
– Pounds per square inch absolute. The sum of Atmospheric
pressure (14.696 PSI) and any additional pressure forces.
Atmospheric pressure is the amount of force we experience
in everyday life. Although we do not feel this force, there
is a constant force of 14.696 pounds per square in acting
on us.
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PSIG
– Pounds per square inch gage. The amount of force applied
to any object. Uses atmospheric pressure as a relative zero
baseline. Because most everything in the world is subject
to Atmospheric pressure, measurements sometimes ignore this
value and only measure the additional forces applied to objects.
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STP
– Standard Temperature and Pressure. A base value used
for pressure and temperature in calculations that are dependent
upon these variables. Because these values can vary depending
on conditions, an assumption of 68° F (20°C) and 14.696
PSI (1 Bar) is used for calculations. An “S” at
the beginning of any unit of measure (i.e. SLPM, SCFH) indicates
that the flow meter was made assuming STP, and will only read
accurately when used at STP as well.
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“Normal”
– Similar to STP, only the assumption uses standard
temperature as 32°F (0°C) and 14.696 PSI (1 Bar).
Any unit of measure beginning with an “N” indicates
that flow meter needs to operate at Normal conditions to be
accurate.
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FAQs
NIST
Traceable – National Institute of Standards
and Technology Traceable.
This means that all gages and equipment used in the manufacturing,
assembly, and testing of Key Instrument products have been
compared and calibrated against a national standard.
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FDA GMP
– Food and Drug Administration “Good Manufacturing
Practices”. This serves to indicate that Key Instruments
actively maintains a quality control system for the design,
manufacture, packaging, labeling, storage, installation, and
servicing of finished medical devices intended for commercial
distribution in the United States. This is a set of requirements
set forth by the FDA, and shows our commitment to provide
the highest quality product possible.
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C.O.C.
– Certificate of Conformance. This is a document that
can be supplied with any product that will verify the product
you are buying has been manufactured, assembled, and tested
to all agreed upon parameters as stated in the Purchase Order,
technical drawings, and engineering related materials. It’s
your reassurance that what you ask for is what you are receiving.
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Primary
Standard - Primary standards are directly traceable
to the National Institute of Standards and Technology (NIST)
Key Instruments offer a 10-point calibration with NIST traceable
certificate.
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Secondary
Standard –Secondary standards are calibrated
by reference standards traceable to NIST.
Products are flow checked using secondary standards before
they are shipped.
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Volumetric
flow – The rate at which a volume of a fluid
is flowing or has flowed
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Mass flow
– The rate at which a mass of a fluid is flowing or
has flowed
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Volumetric
Flow vs. Mass Flow –
1. The volume that a gas occupies is affected by atmospheric
pressure.
• In the illustration
below, the balloon occupies a larger volume while in the air.
If the balloon were taken underwater
(where the
pressure is higher) the balloon would shrink.
• Although
the balloon shrinks and occupies less volume, the mass contained
inside the balloon remains the same as
the balloon
above the water.
2. Exhausting the balloon
• Suppose
it took the same time for each balloon to completely empty
°
The mass flow rate of the two balloons would be equal because
each balloon contains the same number of molecules
°
However, the volumetric flow rate of the balloon in the air
would be greater because it “empties” a larger
volume
in the same amount of time.
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Definitions
of Units:
• CCM – Cubic Centimeters per Minute
• LPM – Liters Per Minute
• LPH – Liters Per Hour
• CFM – Cubic Feet per Minute
• CFH – Cubic Feet per Hour
• GPM – Gallons Per Minute
• GPH – Gallons Per Hour
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How to
Read a Float? – The float in any Key Instruments
Flow meter is read at the lowest and largest diameter on the
float. See examples below. For a spherical float, the flow
rate is read at the center of the ball. For cone shaped floats,
a key is printed on all meters to show where to measure. In
general this point is the lowest and largest diameter on the
float.
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Does
a meter have to be mounted vertically? Yes, and No.
All standard products have been designed to work when mounted
vertically. To maintain stated tolerances, flow meters need
to be mounted vertically. However, Key Instruments can design
a flow meter that can be mounted at any desired angle!
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Pressure
Drop – Is the loss of pressure between two
points. This is associated with the resistance a gas or liquid
experiences while flowing through a flow meter. In most flow
meter applications, if an inlet pressure of 50 PSI is used
for example, the pressure at the outlet of the flow meter
will not be exactly 50 PSI. There are many things that can
affect the flow meters pressure drop. Piping that is too small
to handle the amount of volume of liquid or gas through a
flow meter, piping that is too long, and too many curves and
bends in piping can all cause excessive pressure drop.
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Percent
of Full Scale – Accuracy tolerance that is
determined by taking a percentage of the maximum flow rate
(full scale value)
– For example, if you are reading 5 LPM on a 10 LPM
meter with an accuracy of ± 5% full scale:
± 5% full scale = 0.5 LPM
Making the reading of 5.0 LPM to be in the range of 4.5 LPM
to 5.5 LPM.
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Viscosity
– The resistance of a liquid to flow. For example, Molasses
has a much higher viscosity than water and will not flow out
of a container as easily as water.
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Specific
Gravity – The ratio of a mass of liquid to
an equal mass of water. Also, the ratio of a volume of gas,
to an equal volume of Air. This comparison needs to be done
at the same prescribed temperature and pressure. The ratio
is used to design flow meters for gases other than air; and
liquids other than water.
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Parallax
Line – A second line added to a flow meter
to prevent an incorrect flow measurement made based on a line
of sight that is not perpendicular to the flow meter. In order
to use the parallax a user will line up the desired measurement
line with that lines corresponding line on the rear of the
meter. This will ensure a measurement is accurate and repeatable.
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