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What is a Flow Meter?
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A flow meter (often referred to as a flow sensor) is a device to measure flow rates of a gas or liquid in many types of applications, for example, liquid passing through a conduit. This can be either via the mass (linear or nonlinear ) or volumetric flow.
Despite many flow meters working differently, they all achieve the same result. Flow meters give you the most precise flow rate report based on the application you are using.
Before choosing a flow meter, there are a few things to consider first such as:
- Familiarity of plant personnel
- Understanding calibration and maintenance
- Spare parts availability
- Meantime failure history
- Finally, the cost of installation
In plants that make or utilize liquid, it is vital to get the flow rate correct. If the data is incorrect, it could mean a loss, not a profit for the company. That is why it is extremely important to buy a good quality flow meter, trust me, it is worth the investment! Choosing an inexpensive model could become the most costly installation.
How Do Flow Meters Work?
Flow meters utilize a variety of technologies to measure water flow. These include mechanical gears and electromagnetic and ultrasonic technology. The advantage of each approach depends on the level of accuracy, maintenance required, and the environment that the flow meter will operate.
There are two ways flow meters measure flow:
- Measuring the amount of liquid that gradually passes through the flow meter
- Measuring the entire amount of fluid that passes through the flow meter
But all flow meters either measure the volumetric flow or mass flow.
In a volumetric flow meter, we can use the fluids velocity equation:
Q = A * v
Q = The flow (equivalent to the cross-sectional area of the pipe)
A = Area of the pipe
v = Velocity
And in mass flow, the flow rate is calculated with the following equation:
m = Q *ρ
m = Mass flow
Q = Volumetric flow rate
ρ (rho) = Fluid density
Choosing the Right Flow Meter
With so many flow meters on the market, it can be very overwhelming to decide which is best for you. There is not a flow meter out there that is suitable for every application. We recommend starting by making some notes on flow specifications that will cover the use of the meter. Once you know these specifications, then you can start looking into the different types.
Your first step would be to determine if the flow rate will be continuous or totalized and if this information is required locally or remotely.
Reading the signal from the flow meter sounds easy until you actually try to do it…Any misreads can lead to massive inaccuracies over very little time. For accurate readings and to totalize the flow rate, a subsystem such as a totalizer must also be used.
Fluid & Flow Characteristics
Fluids are described by:
- Density (or specific gravity)
- Allowable pressure drop
- Vapor pressure
- How the above varies and/or interact with each other
Pressure & Temperature Ranges
It is vital to understand temperature values. When selecting a flow meter, you will want to know: the minimum and maximum pressures, if sudden temperature changes are expected, if the flow can reverse or not always fill the pipe, if slug flow can develop, knowing if pulsating and aeration are to be expected and if there are any safety measures when cleaning or carrying out maintenance.
Piping & Installation Area
When installing the flow meter, it is important to consider the following:
- The direction
- The material passing through the flow meter
- The flange-pressure rating
- Accessibility of the meter
- Whether the flow meter has upstream or downstream turns,
- The valves
- If straight-pipe run lengths are available
- The engineer must know if vibration or magnetic fields are present/possible
- Availability of electric or pneumatic power
- If the area is known for explosions or potential hazards
- Are there any special requirements in that area, for example, sanitary or CIP (clean-in-place) regulations
Different Types of Flow Meters
Accurate liquid flow monitoring requires two key components, a precision flow meter, and a well-designed totalizer (subsystem). The most accurate flow meter in the world is useless if the totalizer misses pulses or doesn’t compute flow rates correctly.
Flow meters usually attach directly to piping and measure the amount of water that flows through the pipe per time. Most commonly this unit will be measured in liters or meters cubed per minute (L/min = m3/min), or gallons per minute (gal/min) for those that are still “old-school”. This can then be converted into PSI, and most pieces of equipment will perform this conversion for you.
Water flow meters come in different designs and types, but they all achieve the same result, and that is to measure the amount of moving water, either directly or indirectly. Below are a few different types of flow meters and what they are used for.
Mechanical/Positive Displacement Flow Meters
Mechanical flow meters are used when there is no straight pipe available and you want to reduce the amount of turbulence that occurs when using a turbine flow meter and paddlewheel sensor. Mechanical flor meters are also commonly used for viscous liquids.
They work by using a physical wheel that causes it to spin from the water flow. This means that mechanical water meters have a proportional water flow rate to the wheel spinning inside the flow meter. This water flow can be converted into the volumetric flow – which is what a flow meter is used for.
They are popular as they are simple and easy to understand. However, if the water is dirty and/or contains contaminants, the mechanical flow meter will start to degrade, possibly even breaking. So, they should be used for clean water applications only.
Vortex Flow Meters
Cortex flow meters are great for low sensitivity to variations and their low initial and maintenance costs, making them popular for many users. They do however require sizing by flow engineers.
They work by using the water flow with an obstruction inside made of flexible material that vibrates creating a frequency proportional to the volumetric flow rate.
They are less sensitive to dirty water, so can be used in a variety of water environments.
Magnetic Flow Meters
These flow meters are used to generate magnetic fields on flowing water, measuring how fast the water flows. They are pretty impressive, removing suspended parts in the water, however, this technology comes at a hefty price! That is why they are most commonly used in industrial settings, as they can afford them.
Another disadvantage with magnetic flow meters is, they only function on conductive water/liquids as the piping (metallic) will interfere with the signal causing the volumetric flow rate readings to be incorrect.
Magnetic flow meters are most commonly used in industrial settings, as they can afford them.
Ultrasonic Flow Meters
Ultrasonic flow meters are mainly used for wastewater and other dirty applications. They measure the velocity of water/liquid with ultrasound to calculate the volumetric flow. It does this by bouncing sound waves both downstream and upstream which measures the time offset giving you the volumetric flow rate.
They are popular due to their very low maintenance cost and ability to secure them on the outside of the piping, making them much easier to install.
Turbine Flow Meters
Turbine flow meters are our “go-to” flow meters, as they are extremely accurate and can be used for both clean and viscous water/liquids.
They are also simple to use and set up, cheaper than many other flow meters, highly accurate and precise, suitable for both gases and liquids, and can be used for a wider range of applications than the ones listed above.
They can cause errors due to high viscosities in samples, but at Atlas Scientific, we equip our flow meters with filters to better maintain the lifetime of the devices.
All our flow meters also come as kits that include both a flow meter totalizer circuit and a flow connector board.
More information on our products can be found in our datasheets under the “Documents & Downloads” section on our flow meter products. This includes information on pre-filter requirements, lamina flow, liquid exiting the flow meter, and data output.
How Much Maintenance Does A Flow Meter Require?
As with any equipment, maintenance will be required, whether we like it or not…
A flow meter’s maintenance and life expectancy are influenced by several factors. The most obvious is matching the right flow meter with the right application. Failure to do this will cause problems (breakages & device damages) early on.
NOTE: A flow meter with no moveable parts usually requires less maintenance and attention than ones that consist of moving parts. However, it is important to note that all flow meters will require some maintenance from time to time.
Summing Up Flow Meters, Equipment & Advice
Flow meters are sophisticated pieces of equipment designed to quantify the flow rate or volume of a moving fluid (gas or liquid) in a conduit. Despite many flow meters working differently, they all have one end goal – to give you the most accurate flow rate report based on the application.
If you are unsure which flow meter will fit your specific application, reach out to us at Atlas Scientific so we can understand more about the piping system and the requirements you seek.
If you would also like to learn more about other water quality measurements, characteristics, or applications for water flow meters and regulators, do not hesitate to reach out to the world-class team at Atlas Scientific.
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