For those of you familiar with the data acquisition phase of modeling, I have a question. I have been using Marsh-McBirney Flo-Totes, (260 cs and 264s), for about four years now, and have had reasonable success with these instruments. Other than the long turn around time for repairs, I am happy with MM as a whole. I am considering the purchase of four new 260 IIs. But before I do I would like to see if there are any opinions out there about:
Which flow monitors are the best?
Which company provides the best tech support and instrument maintenance?
I am aware that ADS, MGD and SIGMA are competitors in the market.
To answer your first question, I don't believe that MM meters are nearly as good as the company professes. The technology used for these meters differs from most of the other models on the market. The MM meters use a magnetic flux technology in their sensor. There are two problems with this technology. First, it only measures the flow (by cutting the magnet flux field) within an inch or two above the sensor. A profiling technique then has to used to "adjust" the sensed velocity to an average velocity over the depth of flow measured. This presents the second problem. If you use these meters for CSO or stormwater, how do profile flow when the pipe is empty? The accuracy of flow measured without the profiling adjustment can range tremendously. Even if a good profile is measured when the meters are installed, they still only represent the velocity profile at that flow and depth. The real velocity profile will change as the flow changes. Therefore, the true accuracy of these meters - that is the accuracy of the flow being measured in the field (not just a constant flow repeatability test in the lab) has always been a question in my mind. I used this type of meter on a sanitary sewer project that had over 100 metering locations. The flows were then modeled in SWMM and it was virtually impossible to match the measured flow in an upstream direction (many downstream meters measured less than upstream meters, and there were no overflows in between). When both meters were working properly which one do you adjust for your modeling???
I have also used the other type of meters that work with Doppler technology. The doppler technology gets about as close as you can get to an average flow in a pipe by reflecting a dispersed beam off of the approaching particles. ADS uses their own custom meters. MGD usually uses SIGMA's (but will work with you if you have other meters - at least in CA). I have also used ISCO's and the old Montedoro Whitneys (before MM sued them and put them out of business). I've worked closely with SIGMA in the past and found them to have an excellent product and good support (but that was several years ago). ISCO's products are also very good. Both companies sampling equipment is also good.
The only problem I've had with these type of meters is that with water that has very low TSS (ie. some stormwater), the doppler would not pick up a consistent velocity. Another limitation of all of these meters is that they should not be used for pipes larger than 42 inches in diameter (36" is a good rule, however depending on the installation, you may be able to get accurate readings up to 48"). Regular maintenance (cleaning the probes) is also a necessary process to get good results (at least every 2 weeks in the field). Above these diameters, other flow meters should be used (ie. Accusonics), but the price usually doubles. Another thing, all flows measured by these meters should be considered unreliable until adequate QA/QC is performed on the data (checking depth vs. velocity vs. flow).
Sorry (Bob James) if this was a long-winded discussion on products on this listserver, but as you know, accurate modeling begins with accurate monitoring. There also appears to little hands-on information on these meters (other than manufacturer info.) out there.
I think that Shawn has hit upon a good point here when discussing how different meters measure velocity. My company strictly uses Sigma flow meters, unless the flow monitoring was done prior to our involvement. The primary reasoning is WHAT velocity is being measured.
It is my understanding that Sigma meters actually measure velocity over the entire cross-sectional area of flow and calculate a true average while most other meters (ADS and MM, that I know of) measure velocity at ONE point and calculate a "typical" average by using some fixed factor.
We have been using depth/velocity meters made by Rocky Mountain Instruments with good results. They provide a wireless link that allows data to be downloaded through the internet. This provides a means to monitor the gauges and direct field crews to clean the probes if they become ragged. They operate a web site called www.flowrat.com.
If anyone has had any experience with in-service (back-to-back) comparison of different gauges I would apreciate a note. Thanks.
In Michigan we've used ISCO bubbler water level recorders for open channel measurements. They've been very reliable and we've had good service from the company. However, if you've had good luck with your meters, I recommend you not mess with success!
Ralph Reznick, P.E.
MI Dept. of Environmental Quality
Also in the market are samplers and flow meters from ISCO.
When I first purchased instruments from ISCO in 1993 they seemed to be most comparable to SIGMA. the 6 units (3 bubbler flow meters and 3 samplers) are still inservce today. I have had good results when needing tech support or repair. For one simpler repair, they sent me the part and talked me through replacement over the phone.
I now have some area-velocity flow meters and newer model samplers and have been very pleased with them as well.
ISCO does recognize the weakness of the doppler units at low turbidities. The units need something in the water to bounce the signal off. To overcome this, they recommend installing something that will create bubbles upstream of the meter probe. An aerator unit for a fish tank works wonderfully.
Another problem with the MM meters is that their sensors will not read velocity if there is any sort of debris on the sensor, hence they require frequent cleaning. However, the Sigma's will can sense velocity even through a fair amount of sediment and debris. In addition, their meters now use a very high doppler frequency, and hence will function in virtually clear water. Unfortunately, any submerged sensor is prone to failure.
MM has come out with a new meter called "Flow-Dar" that is actually installed above the flow and bounces the signal off the surface. The limitations with this technology are that it only measures the velocity at the surface so it still requires a velocity profile calibration, and it will not work under surcharge conditions. It is also rather expensive. I would be interested to hear if anyone has had any experience with these meters.
After working with MM, ISCO, SIGMA, Detectronics and ADS we have been using ADS. Right now we have more than 500 locations/month experience with ADS in pipe diameters ranging from 10" to 7 feet. Although we find ADS meters reliable, we still calibrate them by dye-dilution tests or installing them with primary devices (weirs and flumes). Like in modelling, your data is as good as your calibration.
I particularly like the ADS zero-dead band ultrasonic sensor (outside the flow regime) to measure flow depths and the minimal velocity sensor which is practically debris-free.
Last year we tried the new SIGMA with excellent results in two locations (12" and 30" diam). A third meter installed in a 6 feet diam pipe had to be replaced by an ADS due to unreliable data. In each case we used ultrasonic sensors for measuring flow depths
I would be interested in knowing a little bit more on your use of dye dilution for calibrating your flow meters. do you have any documentation on your procedures or approach?
I am currently involved with a project where we are using two global transducers behind two weirs in a grass channel/bioretention zone. So far they have worked fairly well, one had to be sent back but the company seems responsive. So I guess after one year our success rate is 2 out of 3. They may need to be calibrated to local altitude, here in Boulder at nearly 5400' the factory set intercept was way off. The transducers measure the differences in pressure between atmospheric and submerged, our atmospheric is light compared to sea level. The dry readings have been a bit flaky however, you have to watch out for that. This is not top of the line equipment, but it may be adequate if your budget is tight, as is the project I am involved with. We have been very happy with the global raingauge we are using.
Note: Transducers only measure depth. The Marsh McBirney, Isco and others that were discussed on this thread measure depth AND velocity. The various pros and cons discussed in this thread were centered on the velocity measurement. They all use a pressure transducer to measure depth. ISCO also sells other depth sensors besides pressure transducers, like the bubbler. I believe that ISCO also sells a pressure transducer without the velocity probe.
The transducer should be used with a primary device such as a weir or flume, otherwise you only have a depth measurement (then you are stuck with either creating a rating curve with a hand held velocity meter or trying to get something meaningful out of mannings equation - not recommended). For the pipe sizes you are working with I would spend the $$ and try to get something that measures velocity as well as depth.
Revisiting the flow monitoring thread, does anyone have any experience with Global Water's Water Logger (www.globalw.com)?
Our application will be in storm sewers (no CSOs) from 36" to 84" dia. Their promo lit says it works via a submerged pressure transducer to detect changes in head. They have a very compact sewer flow option and claims to be accurate to 0.2%. While the ISCO, SIGMA & RMI monitors look good to me, our limited funds may go farther with this transducer type. This is my first foray into this bewildering world. Would appreciate any Words of Wisdom from those more experienced.
Just a general reply on the subject of flow monitors. I have 14 years experience of installing and maintaining flow monitors as well as analysing and reporting on the data. During this time I have used many different types of flow and depth monitoring equipment and comments on some of these are as follows:
Detectronic (Buhler Montec) 3510 and Zytec monitors are the most reliable and easy to use piece of kit I have come across and provide good data for most sewer flow monitoring applications. Unfortunately they are no longer in production. However, they have been taken over by Hach-Lange who supply the American Sigma range of flow monitors which I have also found to be very good although harder to maintain (in a workshop sense) than the Detectronics. Both of these monitors have user friendly, functional software.
I have tested some of the Isco bubbler monitors in the past with very poor results however I would highly recommend their automatic samplers.
ADS monitors seem to provide very good data but (now come on all you brainwashed ADS users, admit it) they have to be about the most user unfriendly piece of equipment known to man. The Quadrascan software is unbelieveable! A windows version has been promised but has to date not materialised for anyone other than ADS themselves. The Sigma 940 seems to do the same job, works and is easy to use. I have also been testing the Sigma 1000 GSM telemtry unit which is excellent although unfortunately at present in non IS.
I have also tested Rocky Mountain equipment but the test monitor failed to work so confidence was dented immediately and that was the end of that. I have heard that they are good however.
In the dark recesses of my memory I remember using Prolec Water rats and even Golden River flow monitors. Anyone remember them?
I am about to take delivery of a couple of ADFM monitors from MGD to use in a large storm monitoring project in Hong Kong. Not used them before but hope that they will live up to the marketing!
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