Flowmeter FAQs



Single-Use Flowmeters
  1. How do the Masterflex® flowmeters work?

  2. Which single-use sensor type makes the most sense for my application?

  3. What accuracy level can I expect to achieve with each sensor type?

  4. Can these sensors be used with opaque tubing and working fluids?

  5. How do these sensors communicate with my system?



Differential Pressure Flowmeters
  1. How does a differential pressure flowmeter work?

  2. Do I need a filter?

  3. Can a differential pressure flowmeter handle turbulent flow?

  4. My gas is not at STP/ or changes—will this work?

  5. What are the advantages of using a differential pressure flowmeter?

  6. What are the limitations of using a differential pressure flowmeter?


Doppler Flowmeters
  1. How does a doppler flowmeter work?

  2. Can I use a doppler flowmeter with particulates?

  3. Some flowmeters measure in velocity (ft./sec). How can I convert the readings to volume/time?

  4. What if my fluid is not water?

  5. Will pipe insulation/thickness affect my reading?

  6. Must a doppler flowmeter be permanently installed?

  7. Does a doppler flowmeter require a minimum upstream straight pipe length?

  8. What are the advantages of using a doppler flowmeter?

  9. What are the limitations of using a doppler flowmeter?



Mass Flowmeters
  1. How does a mass flowmeter work?

  2. Can a mass flowmeter give a total accumulation of gas?

  3. Can I calibrate a mass flowmeter for my own gas mixture?

  4. Do I need a filter?

  5. What are the advantages of using a mass flowmeter?

  6. What are the limitations of using a mass flowmeter?



Paddle-Wheel Flowmeters
  1. How does a paddle-wheel flowmeter work?

  2. What if my liquid is foamy or turbulent?

  3. How long of a straight section of pipe do I need?

  4. What do I need for a paddle-wheel system?

  5. My meter reads in GPM—the flow sensors are in ft/sec. How do I know which one is appropriate for my flow?

  6. What do I need to know about my system when ordering?

  7. What are the advantages of using a paddle-wheel flowmeter?

  8. What are the limitations of using a paddle-wheel flowmeter?



Turbine Flowmeters
  1. How does a turbine flowmeter work?

  2. Can I use a turbine flowmeter with small particles?

  3. Do I need a minimum straight distance before the sensor?

  4. What if I have air in my liquid?

  5. What are the advantages of using a turbine flowmeter?

  6. What are the limitations of using a turbine flowmeter?



Variable Area Flowmeters/Rotameters
  1. How does a rotameter work?

  2. Where do I take the reading?

  3. What is the difference between correlated and direct reading rotameters?

  4. What if I use a gas or liquid other than water or air? What if I use distilled water?

  5. Can I use a rotameter in a vacuum application or with back pressure?

  6. Can I use one flowmeter to measure different flow rates?

  7. What are the differences between 150 mm and a 65 mm flowmeter?

  8. Must a rotameter be mounted vertically?

  9. Which float do I have?

  10. What are the advantages of using a variable area flowmeter?

  11. What are the limitations of using a variable area flowmeter?



Single-use flowmeters


1. How do Masterflex® flowmeters work?

In the ultrasonic flow sensor, two sets of piezo sensors are positioned in an “X” configuration across the tube carrying the working fluid which send ultrasonic signals in the direction off and in the opposing direction to the flow. Electronics inside of the sensor converts the piezo signals into a flow rate signal output. The single-use sensor contains a frictionless turbine wheel that is extremely responsive to changes in fluid flow moving through it. A continuous IR beam reflects off of the turbine blades as the rotate creating a pulsed IR signal that is proportional to the flow rate of the medium. Each sensor is calibrated to give an accurate flow rate to 1% or better.

2. . Which single-use sensor type makes the most sense for my application?

If you are looking for a very high accuracy flow sensor that has a very low operating cost and will work with fluids under 20 cp, then the Masterflex single-use flow sensors with it’s 1% accuracy is an excellent choice. If high accuracy is required and you wish to reuse the sensors without the need for CIP or you have a viscus working fluid, then the Masterflex ultrasonic sensor with its 2% accuracy and wide viscosity range will be a better fit.

3. What accuracy level can I expect to achieve with each sensor type?

The ultrasonic sensors are accurate to 2% down to flow rates of 30 ml/min and are calibrated to Masterflex platinum cured silicone. The sensor will work with other tubing types even if opaque, but best results are achieved when Masterflex platinum cured silicone is used.

The single-use sensors are accurate to 1% down to flow rates of 20 ml/min. The light weight turbine blade with ruby bearing is extremely responsive to flow as long as viscosity is below 20 cp. Higher viscosity fluids detract from their accuracy so other options are necessary at higher viscosities.

4. Can these sensors be used with opaque tubing and working fluids?

Both the Masterflex® ultrasonic and single-use sensors can be used with opaque tubing and working fluids. The ultrasonic sensor’s calibration should be checked and adjusted if necessary, for the working fluid if it deviates from platinum cured silicone, and it is important that the working fluid has a viscosity below 20 cp to work with the single-use sensor.

5. How do these sensors communicate with my system?

The ultrasonic sensor has RS485, 4-20mA, 0-20kHz, PNP-NPN-push pull output options that are created inside each sensor, so no additional signal conditioning elements are necessary to connect the sensor to a control input.

The single-use sensors output a frequency pulse that is proportional to its flowrate. If plugged into the scanner optional accessory, the calibration data for the sensor is automatically captured and the scanner output automatically generates the correct output to your control input.


Differential Pressure Flowmeters


1. How do Masterflex® differential pressure flowmeter work?

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.

2. Do I need a filter?

A 50-μm filter is recommended to prevent impurities from clogging the laminar element.

3. Can a differential pressure flowmeter handle turbulent flow?

Yes; though meters are unidirectional a straight run of tubing or pipe is not required.

4. My gas is not at STP/ or changes—will this work?

Some non-thermal mass flowmeter versions are available for fluctuating stream temperature or pressure. These meters will automatically correct to STP.

5. What are the advantages of a using a differential flowmeter?

—can handle low flow gases and liquids
—has an output signal for totalizing
—switch selectable for different gases

6. What are the limitations of using a differential flowmeter?

—for use with clean liquids only
—maximum liquid viscosity of 5 cps

Doppler Flowmeters


1. How does a doppler flowmeter work?

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.

2. Can I use a doppler flowmeter with particulates?

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.



3. Some flowmeters measure in velocity (ft/sec). How can I convert the readings to volume/time?

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.

4. What if my fluid is not water?

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.

5. Will pipe insulation/thickness affect my reading?

Yes. Insulation should be removed before mounting the sensor.

6. Must a doppler flowmeter be permanently installed?

No. Because doppler flowmeters measure flow externally, most can be easily removed and moved from site to site.

7. Does a doppler flowmeter require a minimum upstream straight pipe length?

Yes. Doppler flowmeters require ten pipe diameters from any valve, tee, bend, etc. Doppler flowmeters also require a full pipe flow.

8. What are the advantages of using a doppler flowmeter?

—non-invasive
—good for slurries, aerated liquids
—portable

9. What are the limitations of using a doppler flowmeter?

—not suitable for clean liquids
—requires straight upstream piping

Mass Flowmeters


1. How does a mass flowmeter work?

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.

2. Can a mass flowmeter give a total accumulation of gas?

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).

3. Can I calibrate a mass flowmeter for my own gas mixture?

This is possible as long as the mixture is not too complicated. Contact our applications Department for pricing and availability of gas mixture calibrations.

4. Do I need a filter?

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.

5. What are the advantages of using a mass flowmeter?

—measure mass directly
—can handle applications whose stream temperature and line pressures fluctuate.

6. What are the limitations of using a mass flowmeter?

—calibrated to a specific gas type

Paddle-Wheel Flowmeters


1. How does a paddle-wheel flowmeter work?

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.

2. What if my liquid is foamy or turbulent?

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.

3. How long of a straight section of pipe do I need?

For systems with no bends or restrictions, allow a minimum of 15 pipe diameters upstream and 5 pipe diameters downstream.

4. What do I need for a paddle-wheel system?

a. flow sensor
b. pipe fitting
c. meter or controller to read the signals from the sensor and indicate them in GPM or LPM

5. My meter reads in GPM—the flow sensors are in ft/sec. How do I know which one is appropriate for my flow?


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.

6. What do I need to know about my system when ordering?

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)

7. What are the advantages of using a paddle-wheel flowmeter?

—good repeatability
—low pressure drop
—easy maintenance

8. What are the limitations of using a paddle-wheel flowmeter?

—minimum upstream/downstream piping requirements
—full pipe require.

Turbine Flowmeters


1. How does a turbine flowmeter work?

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.

2. Can I use a turbine flowmeter with small particles?

No. Turbine flowmeters are best used with clean, low-viscosity liquids.

3. Do I need a minimum straight distance before the sensor?

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.

4. What if I have air in my liquid?

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.

5. What are the advantages of using a turbine flowmeter?

—good accuracy with liquids
—easy to install and maintain
—signal output for totalizing
—low flow rates available

6. What are the limitations of using a turbine flowmeter?

—sensitive to viscosity changes
—straight pipe line required
—clean liquids and gases only

Variable Area Flowmeters/Rotameters


1. How does a rotameter work?

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

2. Where do I take the reading?

With the 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.

3. What is the difference between correlated and direct reading rotameters?

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.

4. What if I use a gas or liquid other than water or air? What if I use distilled water?

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.


5. Can I use a rotameter in a vacuum application or with back pressure?

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.

6. Can I use one flowmeter to measure different flow rates?

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.

7. What are the differences between 150 mm and a 65 mm flowmeter?

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.

8. Must a rotameter be mounted vertically?

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.

9. Which float do I have?

Glass floats are black, while the sapphire floats are red. Carboloy and stainless steel floats both look metallic, but the Carboloy floats are magnetic.

10. What are the advantages of using a variable area flowmeter?

—inexpensive
—somewhat self-cleaning
—no power required
—available in different materials for chemical compatibility

11. What are the limitations of using a variable area flowmeter?

—no output for data transmission
—sensitive to differing gas types and changes in temperature and pressure