Flowmeter FAQs
- How does a differential pressure flowmeter work?
- Do I need a filter?
- Can a differential pressure flowmeter handle turbulent flow?
- My gas is not at STP/ or changes—will this work?
- What are the advantages of using a differential pressure flowmeter?
- What are the limitations of using a differential pressure flowmeter?
- How does a doppler flowmeter work?
- Can I use a doppler flowmeter with particulates?
- Some flowmeters measure in velocity (ft./sec). How can I convert the readings to volume/time?
- What if my fluid is not water?
- Will pipe insulation/thickness affect my reading?
- Must a doppler flowmeter be permanently installed?
- Does a doppler flowmeter require a minimum upstream straight pipe length?
- What are the advantages of using a doppler flowmeter?
- What are the limitations of using a doppler flowmeter?
- How does a mass flowmeter work?
- Can a mass flowmeter give a total accumulation of gas?
- Can I calibrate a mass flowmeter for my own gas mixture?
- Do I need a filter?
- What are the advantages of using a mass flowmeter?
- What are the limitations of using a mass flowmeter?
- How does a paddle-wheel flowmeter work?
- What if my liquid is foamy or turbulent?
- How long of a straight section of pipe do I need?
- What do I need for a paddle-wheel system?
- My meter reads in GPM—the flow sensors are in ft/sec. How do I know which one is appropriate for my flow?
- What do I need to know about my system when ordering?
- What are the advantages of using a paddle-wheel flowmeter?
- What are the limitations of using a paddle-wheel flowmeter?
- How does a turbine flowmeter work?
- Can I use a turbine flowmeter with small particles?
- Do I need a minimum straight distance before the sensor?
- What if I have air in my liquid?
- What are the advantages of using a turbine flowmeter?
- What are the limitations of using a turbine flowmeter?
- How does a rotameter work?
- Where do I take the reading?
- What is the difference between correlated and direct reading rotameters?
- What if I use a gas or liquid other than water or air? What if I use distilled water?
- Can I use a rotameter in a vacuum application or with back pressure?
- Can I use one flowmeter to measure different flow rates?
- What are the differences between a 150-mm vs. a 65-mm flowmeter?
- Must a rotameter be mounted vertically?
- Which float do I have?
- What are the advantages of using a variable area flowmeter?
- What are the limitations of using a variable area flowmeter?
A pressure drop is created as water or gas enters through the meter's inlet.
The fluid is forced to form thin laminar streams that flow in parallel paths between internal
plates separated or capillary tubes. The pressure differential created by the fluid drag is
measured by a differential pressure sensor connected to the top plate. The differential
pressure from one end of the laminar flow plates to the other end is linear and proportional
to the flow rate of the liquid or gas.
Go to Top
A 50-μm filter is recommended to prevent impurities from clogging the laminar
element.
Go to Top
Yes; though meters are unidirectional a straight run of tubing or pipe is not required.
Go to Top
Some non-thermal mass flowmeter versions are available for fluctuating stream
temperature or pressure. These meters will automatically correct to STP.
Go to Top
—can handle low flow gases and liquids
—has an output signal for totalizing
—switch selectable for different gases
Go to Top
—for use with clean liquids only
—maximum liquid viscosity of 5 cps
Go to Top
A high frequency signal is projected through the wall of the pipe and into
the liquid. The signal is reflected off impurities in the liquid such as air bubbles
or particles, and sent back to the receiver. The frequency difference between the
transmitted and received signal is directly proportional to the fluid's flow velocity.
Go to Top
Yes. In order to use a doppler flowmeter, the liquid must have particulates or bubbles. Most require a minimum size of 25 ppm or 30 μm; check with each doppler flowmeter for specific particle size requirements.
Go to Top
GPM= 2.45 * (ID in inches)² * (VELOCITY in ft/sec)
GPM= gallons per minute
ID = inside diameter of the pipe in inches.
This formula is for water—it does not consider viscosity, temperature, or pressure. However, temperature, viscosity, and pressure will not effect a doppler flow reading.
Go to Top
The speed of sound through water is approximately 1470 ft/sec. Most instruments are calibrated for that rate. Other fluids may be used, but your instrument should then be recalibrated.
Go to Top
Yes. Insulation should be removed before mounting the sensor.
Go to Top
No. Because doppler flowmeters measure flow externally, most can be easily
removed and moved from site to site.
Go to Top
Yes. Doppler flowmeters require ten pipe diameters from any valve, tee, bend, etc. Doppler flowmeters also require a full pipe flow.
Go to Top
—non-invasive
—good for slurries, aerated liquids
—portable
Go to Top
—not suitable for clean liquids
—requires straight upstream piping
Go to Top
A volume of gas has a known mass at standard conditions. As pressure and temperature are applied, the volume will change, but the mass remains constant. Mass flowmeters measure flow based on the molecular mass of the gas; this measurement is independent of temperature and pressure.
One technique to measure mass flow is to send a part of the flow through a sensor tube. In the tube, the gas is heated in a coil and then measured downstream. The temperature differential is directly related to the mass flow.
Go to Top
Yes, most mass flowmeters have outputs of either 0-5 VDC or 4-20 mA. To monitor total accumulation, connect a totalizer/monitor with a matching input ( 0-5 VDC or 4-20 mA).
Go to Top
This is possible as long as the mixture is not too complicated. Contact our applications Department for pricing and availability of gas mixture calibrations.
Go to Top
Mass flowmeters require clean gases; generally any particles larger than 50 μm require a filter upstream of the meter. Check each meter for specific requirements.
Go to Top
—measure mass directly
—can handle applications whose stream temperature and line pressures fluctuate.
Go to Top
—calibrated to a specific gas type
Go to Top
Magnets are installed on each paddle of the sensor, which is inserted into the liquid. As the paddle turns, an electrical frequency output proportional to the flow velocity is generated.
Go to Top
Because these sensors use laminar flow characteristics, foamy or turbulent
liquids will not be read accurately. The sensors must also be installed in a full
flowing, straight section of pipe.
Go to Top
For systems with no bends or restrictions, allow a minimum of 15 pipe diameters
upstream and 5 pipe diameters downstream.
Go to Top
a. flow sensor
b. pipe fitting
c. meter or controller to read the signals from the sensor and indicate them in GPM or LPM
Go to Top
To convert from velocity to flow, use:
GPM= ft/sec x (ID)2 x 2.45
GPM= gallons per minute
ID = inside diameter of the pipe
This formula is for water—it does not consider viscosity, temperature, or pressure.
Go to Top
In order to correctly calibrate your flowmeter, we need to know:
a. Type of fluid
b. Expected flow rate
c. Max. fluid temp and system pressure
d. % suspended particles by volume
e. Pipe size (ID), material, and wall thickness (schedule)
Go to Top
—good repeatability
—low pressure drop
—easy maintenance
Go to Top
—minimum upstream/downstream piping requirements
—full pipe require.
Go to Top
As liquid or gas flows through the turbine, it turns an impeller blade that is
sensed by infrared beams, photo-electric sensors, or magnets. An electrical pulse is
then generated and converted to a frequency output proportional to the flow rate.
Go to Top
No. Turbine flowmeters are best used with clean, low-viscosity liquids.
Go to Top
To maintain an even cross-sectional flow, it is recommended that there be a
straight pipe length of at least 10x the meter's inner diameter upstream and at least 5x
the meter's inner diameter downstream of the sensor. Check each flowmeter for specific
requirements.
Go to Top
Some turbine flowmeters can be used with air. However, if there are air bubbles or vapor
pockets in the liquid, the reading will be inaccurate. There should be a laminar (stable)
flow through the cross-section of the pipe.
Go to Top
—good accuracy with liquids
—easy to install and maintain
—signal output for totalizing
—low flow rates available
Go to Top
—sensitive to viscosity changes
—straight pipe line required
—clean liquids and gases only
Go to Top
Rotameters, or variable area flowmeters, operate on the principle that the
variation in area of flow stream required to produce a constant pressure differential
is proportional to the flow rate. The flowing fluid enters the bottom of the meter,
passes upward through a metering tube, and around the float, exiting at the top.
The flow rate is read by noting the position of the float against the calibrated scale
etched on the glass
Go to Top
With the Cole-Parmer flowmeters, the reading is taken at the center of the
float. It is recommended that the float be at eye level to minimize reading errors.
Go to Top
A direct reading flowmeter indicates the flow rate on its scale in specific
engineering units (e.g. ml/min or scfh). Direct reading scales are designed for a
specific gas or liquid at a given temperature and pressure. While it is more convenient
than a correlated flowmeter, a direct reading flowmeter is less accurate and limited in
its applications.
A correlated flowmeter is scaled along either a 65mm or a 150mm length, from which a
reading is taken. The reading is then compared to a correlation table for a specific
gas or liquid. This will give the actual flow in engineering units. One correlated
flowmeter can be used with a variety of fluids or gases.
Go to Top
If you have a correlated flowmeter, give us the tube number and type of float, and we can fax you a correlation chart for the gases advertised in our catalog. We have a limited number of unadvertised gas correlations as well.
For distilled water, use the correlation chart for water.
Go to Top
Yes, but if you have a valve, it must be placed at the outlet (top of the
flowmeter). This is done by inverting the tube inside the frame, and then turning over
the frame. At this position, the tube should read correctly from the original perspective
and the valve should be at the outlet, or top of the flowmeter. This allows for proper
control of the vacuum.
Go to Top
Yes. If a correlated flow tube is used, different flow rates can be attained
by using different floats, i.e. carboloy, stainless steel, glass, or sapphire.
Go to Top
A 150-mm flowmeter has a 150 mm scale length and is graduated accordingly.
It provides better resolution than the more economical 65-mm flowmeter.
Go to Top
Generally, rotameters must be mounted vertically, because the float must
center itself in the fluid stream. At high flow rates, the float assumes a position
towards the tip of the metering tube and at low flow rates positions itself lower in
the tube. Some of our rotameters have spring loaded floats and therefore may be mounted
in any orientation.
Go to Top
Glass floats are black, while the sapphire floats are red. Carboloy and
stainless steel floats both look metallic, but the Carboloy floats are magnetic.
Go to Top
—inexpensive
—somewhat self-cleaning
—no power required
—available in different materials for chemical compatibility
Go to Top
—no output for data transmission
—sensitive to differing gas types and changes in temperature and pressure
Go to Top
Complete Selection of Flowmeters
Flow Meters and Flowmeter Control
Flowmeter Applications
How Volumetric Flowmeters Work
Installing Your Paddle-Wheel Flow Sensor
Selecting the Right Flowmeter—Part 1
Selecting the Right Flowmeter—Part 2
|
|
Products & Services
