Quick Guide to Buying Flow Meters

Buying a flow meter takes a considerable amount of design and planning, so keep in mind these factors when you are in the market for one.

In this article, we discuss:


What is a flow meter?

A flow meter is basically a device used to monitor and measure the quantity – or more specifically, the mass flow rate or volumetric flow rate -- of the amount of vapour, liquid or gas passing through a conduit or pipe.

Some flow meters are designed to monitor the amount of fluid flowing through them over a given period (of time). 

How they measure amount and period varies.  

Here are some examples: 

  • Cubic feet per second 
  • Cubic feet per minute 
  • Cubic meters per second 
  • Cubic meters per minute 
  • Litres per minute 
  • Gallons per minute 

Others are designed to monitor the total amount of fluid that has passed through them (eg. 500 litres) without taking the period (of time) into consideration.


Factors to consider when buying flow meters

  • Fluid characteristics: Is it going to measure fluid in liquid or gas? What about its viscosity and corrosiveness?
  • Flow rate: What are the expected minimum, maximum and typical flow rates?
  • Accuracy and repeatability: How accurate / precise do you need the readings to be? Is this accounted for in a mission-critical system?

    Remember that straight pipe runs are crucial here because they directly impact measurement accuracy, since most flow meters rely on a stable and predictable flow profile to function correctly. Any disturbances upstream or downstream of the meter can disrupt this profile, leading to inaccurate readings.
  • Pipe size and orientation: Are they compatible with the existing plumbing diameters you have? Do you have the flanges and adapters that are up to spec with the rest of the system?
  • Operating pressure and temperature: Is it able to withstand deviations and extraordinary conditions?
  • Installation, maintenance and calibration: Does it have to be easily accessible to perform maintenance tasks on? How often do yo need to calibrate them to stay complaint with regulations or specific applications?
  • Output and communication: Do you need meters that process and display data in analog or digital format? 
  • Budget: How do you strike a good balancing between cost, performance and quality? Remember to choose the right kind, as you have a handful options.  


Different kinds of flow meters

There are a wide variety of flow meter designs and configurations available in the market. 

Here are some of the popular designs: 

  • Coriolis flow meters measure mass flow rate through inertia (loosely related to the Coriolis Principle). It uses an open-flow design that measures mass flow rate over a wide range of temperatures. They are known to require very minimal maintenance and are commonly used for measuring viscous fluids and in “custody transfers”. They are regarded to be highly accurate and good for a wide range of fluids, but all that comes with a big price tag.
  • Differential Pressure (DP) flow meters are very popular because of their simplicity. They have no moving parts, although the unit partially obstructs the flow in the pipe to create a static pressure resulting from the upstream and downstream movement of the fluid. The difference in the pressure is used to determine the  flow rate. 
  • Electromagnetic flow meters, sometimes called magnetic meters or simply mag meters, are best for measuring conductive fluids such as slurry. Similar to DPs, they have no moving parts and are very non-invasive, so they do not obstruct the flow of fluids. 
  • Impeller flow meters are commonly employed in water distribution systems. They measure direct volumetric flow measurement that’s best used in high fluid velocity applications. 
  • Ultrasonic flow meters measure the fluid velocity using ultrasonic sound pulses emitted by transducers. They typically have high turndown ratios and are commonly used in heavy-duty applications, usually in the oil & gas industries. Some clamp-on types are even used for diagnosing flow problems. 
  • Vortex flow meters are mostly used to measure low viscosity fluids. They may sometimes cause obstruction to the fluid flow and a considerable amount of pressure drop in the system. 
  • Positive Displacement (PD) flow meters are known for their very high turndown ratios. They are generally designed to give precise readings for low flow rates and viscous fluids. Although there’s a small chamber containing the measuring rotor (usually gears) that creates minimal flow disruption, they make up for this by returning consistently accurate readings. 

Although electromagnetic and ultrasonic flow meters are becoming popular, PD flow meters are still what our customers commonly look for, so we discussed them in greater detail below. 

Shop for Macnaught PD flow meters made in Australia.


Benefits of PD flow meters

Here are the advantages specifically of the Macnaught M-Series PD flow meters:  

  • High accuracy and repeatability  
  • Suitable for viscous fluids  
  • Exceptional turndown ratio / rangeability  
  • Easy installation and maintenance  
  • Can be easily installed in modules  
  • No pipe conditioning required  
  • Extension adapters are available 
  • Available in analog or digital LCD display

buy flow meters





Safety precautions with PD flow meters

Make sure you have these things taken care of: 

  • Ensure the fluid is compatible with the meter. 
  • PD meters hate dirty fluids. 
  • Install a clean, quality strainer (at least 100µm) before the meter to avoid internal corrosion in the system. 
  • Factor in that the more viscous the liquid, the more pressure drop may occur. 
  • The viscosity of the calibrated and operating fluid should be the same, or it could lead to measurement errors. 
  • Flush the entire system to make sure it’s free of debris and particles prior to installing the meter. 
  • Slowly refill the system to prevent damage caused by air purge. 
  • Use an air eliminator when sealing off the meter. 


Questions to ask when buying flow meters

We may ask you these to help you choose the right PD flow meter: 

  • What is the type of liquid /fluid that you will use (eg. water, diesel etc)? 
  • What is your flow rate  (eg. how many liters per minute)? 
  • What is the line size (eg. 1”, 50 mm)? 
  • What is the process pressure (eg. 5 bar, 100 psi)? 
  • What is the expected system temperature (eg. ambient 35°C)? 
  • What is the process connection (eg. BSP, NPT, ANSI flanges)?  
  • What is the viscosity of the liquid (eg. 15 centipoise)? 
  • Do you have a strainer and/or air eliminator in the system? 
  • Do you need a mechanical or digital display? If digital, what is your required output (eg. 20 mA, scaled pulses)? 
  • Is it intended for indoor or outdoor use? 
  • Is it to be installed in an explosive environment? 

(If you’ve already bought a Macnaught flow meter, here are some installation and operating guidelines.) 

Macnaught makes it easy for you to choose your flow meter with this selection tool, but we make it easier by walking you through it. 

Let’s have a chat or send an email to sales@aimsindustrial.com.au.

You can also call us at 02 9773 0122 to enquire.