Magnetic vs. Ultrasonic Flowmeters

Based on your query, my assumption is that you will be using clean water for landscape irrigation.

Magnetic flowmeter is one of the best solutions for this application. However, for the large HDPE pipe sizes, you have some sizing issues to consider. Large HDPE pipes normally have thick walls, so you will need to choose a mag-flowmeter a few sizes smaller than the HDPE pipe size so that the inside diameters match each other. That means you will have to provide adapter spools. Adapter spools may have to be made of internal-coated carbon steel. You also need to provide isolating and bypass valves so you can remove the magflow tube for lab calibration if required.

If you use Ultrasonic flowmeter, consider using Clamp-on Transit-Time Flowmeter if you have clean bubble-free water. Clamp-on configuration eliminates the need for adapter spools and valves that mag-flowmeters need. The bigger the pipe size, the cheaper Ultrasonic becomes, compared to magnetic. For sensor probe selection, you have to check with the vendor the maximum thickness the Ultrasonic probe can handle and make sure it is within the thickness of your HDPE Pipe.

Magnetic Flowmeter product technology is more mature than Ultrasonic, and is generally more accurate at +/- 0.3% or better. Ultrasonic Accuracy is typically around +/-0.5% to 1.0%. If accuracy is not an issue, Clamp-on Ultrasonic flowmeter is well worth considering.

Magnetic and ultrasonic flowmeters are velocity flowmeters and can meet your request.

For the request, you need to consider the cost, and output, and power supply. Ultrasonic flowmeters have the clamp-on features which don't need to cut pipes, and almost low maintanance.

You can visit the website www.ultrasonic-flow.com for various kinds of ultrasonic flowmeters.

Hai,

I want to share my experience. it may help to you.
Ultra sonic flow meters are easy to use and are portable in nature. Like your projects these are suitable. But we must take care during measurement. It requires skill. Placing the measuring probes in tact and using the manufacturer recommended jelly/grease. enter the pipe dia. material of construction details,pipe wall thickness...enough it displays the flow rate.

Magnetic flow element as the size increases the minimum flow factor influences our measurement. Installation requires that the flow tube must be always in filled condition. It is not portable. If at all you are going with mag flow meters better buy an integrated model. It requires only power, both amplifier and flow element are integrated. This avoids signal cable related troubles. Hope this information helps you to some extent. Good luck

Hi,

I was watching the comparison. The electromagnetic flow meter requires high conductive process fluid to detect the change in flow and they are generally applicable for low temperature cases. We were preparing the specs for an non-intrusive flow measurement where the ultrasonic and em flow meter were competing. We opted ultrasonic since our process water was low conductive and process condition is high pressure and temperature.

Santanu

Hi,

First you need to check the budget of your project. As price remains same/similar for Ultrasonic Flow Meters for any line size. Whereas, EMFM it starts increasing significantly over the higher line sizes. Which further increases issues for handling and installations.

Advantages Ultrasonic over MAGs:

1. Ultrasonics (Clamp-on NOT Inline Ultrasonic) are easier to install.

2. No need to cut the pipe, so installation is possible on running process line.

3. Most of the Ultrasonic Meters comes with %age of Aeration monitoring in it. You can configure alarms based on these Aeration levels.

4. Easy to Handle as its size similar for almost all the line sizes.

5. Best to check whether there is leakage in the region due to quick installation and packing.

Disadvantages of Ultrasonic over MAGs:

1. Standard Accuracy starts from 0.5% to 1%. Whereas MAGs having 0.25%, 0.2% or 0.5% based on selection of each OEM.

2. Alignment of Transducer should be perfect else who will verify the reading? 1cm difference in transducer location can cause 1-2.5%loss of Ultrasonic Waves, this will lead to another deviation in its accuracy.

3. For Line sizes below 24inches it is going to be expensive technology as compared to Mags. from 24inches to 32 inches prices are comparable to Magflow meters. These Ultrasonics are best for bigger line sizes costwise.

As your application is for Normal Irrigation Water,hence you can proceed with ElectroMagnetic Flow Meters. It will give you better accuracies & prices for the line sizes mentioned by you.

Regards,
Davinder Singh

Thank you Davinder Sing,
vijay

100 - 800mm

In the larger sizes there is no question ultrasonic wins hands down.
32" flanged mag meters don't come cheap and even wafer style require pipe flanges.

I have used U/S time of flight meters on HDPE pipe with a wall thickness of nearly 2" and had good corelation to an adjacent mag.

The same Ultrasonic meter could be applied to the 100mm and the 800mm pipe.

I mentioned propeller type earlier for irrigation purposes because you can have a simple meter with totalizer not dependent on any power source. 4-20mA or pulse outputs to drive remote display.

Saddle type propeller meters for large pipe are a cost effective.

Cheers
Roy

Thanks, Roy. I was going to say the same thing. I caution about using transit time ultrasonics on HDPE. Unless it is thick enough to get rigid, under pressure the pipe sometimes ovals, changing the inside diameter and sometimes even loosening the mounting hardware and breaking signal.

A weld saddle propeller meter is the least expensive way to go, and will work extremely well.

Walt

Walt Boyes, FInstMC, Chartered Measurement and Control Technologist
Life Fellow, International Society of Automation
Editor in Chief, Control and ControlGlobal.com
[email protected]
office: +1-630-625-1132
cell: +1-630-903-7991

Hello,

Thank you guys for all the help and replies. I was able to generate the report.

Susan,

Thanks for the offer. May I have a copy of the manual?

Again, Thank you all.
please send this information in english language on my e-mail: [email protected]

Hi there!

There are a lot of points to compare when it comes to Magnetic and Ultrasonic Flow Meters. I can't explain them myself as I have only worked with them briefly, but a great site to take a look at is http://www.micronicsflowmeters.com. They can definitely provide you with some very useful information on Magnetic and Ultrasonic Flow Meters. Take a look, you won't be disappointed!

I CAN explain the difference.

About the only similarity is that they both work on round pipes, full of fluid, and they measure volumetric flow.

MAGMETERS
A magnetic flow meter uses Faraday¹s law to generate a small current (pico amperes) from the electrodes in the primary element that is proportional to the velocity of the fluid (the moving conductor, you knowŠ). The converter amplifies, linearizes, and applies a correction factor (called a meter factor) to the current signal that produces a linear signal proportional to velocity, and characterized for the diameter of pipe the primary element in in.

A magnetic flow meter is the most accurate volumetric flow device in the world. The reason for this is that it directly measures the average velocity in the line, not a point velocity, or a partial average. Magnetic flow meters are durable and long lasting.

ULTRASONIC FLOW METERS
Not counting ultrasonic open channel meters, which are a subspecies of level transmitters, there are two most common types of ultrasonic flow meter.

The Doppler ultrasonic meter uses the principle of the Doppler shift to measure a partial average velocity in the line by measuring the apparent speed of entrained solids or bubbles in the line. Doppler meters do not work on clean fluids. The operating range of a doppler meter is from an entrained solids level of approximately 100 mg/l of 4.00 g/cc solids with a diameter of approximately 10 microns or larger‹ to a maximum entrained solids level of approximately 20% of the same type and size solids by volume.

The other type of ultrasonic flow meter is based on the transit time principle. The sensor sends a sound beam into the fluid to a receiving sensor. The difference between the time the signal needs at zero flow and the time the signal needs when there is flow is proportional to velocity. Wetted sensors in spool pieces are more accurate than clamp-on sensors, because the inside diameter is known.

Transit time ultrasonic meters cannot be used with solids or entrained air in the fluid.

You can find more information about flow meters in my book, ³The Instrumentation Reference Book, 3rd Edition², and in David W. Spitzer¹s book, ³Industrial Flow Measurement.² These type of references are better than vendor sites, because they are vendor agnostic and not trying to sell you anything.

Best,

Walt Boyes, Life Fellow, ISA; FInsMC, Chartered M&C Technologist
Spitzer and Boyes LLC
Editor and Publisher <i>The Industrial Automation and Process Control INSIDER</i>