Sewer Meters

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Between electromagnetic and ultrasonic meters, which one is the best for measuring raw sewer? Advice on any other device that can be used.

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9 Answers

  1. The thought that electromagnetic flow meters also have a non-contact working way,or  magnetic flow meters have a lower request to straight pipe installing section. is wrong. Actually,  ultrasonic flow meters can get the accuracy of 5% and there are two basic types of ultrasonic meters - transit-time and Doppler meters. Transit-time meters send ultrasound signals into the flow using transducers that are either clamped onto or inserted into the pipe at two locations.  Ultrasonic meters use the frequency of 1 MHz, which is not even close to the frequency for acoustic noise. When mag meters measure very small flows it is done on the basis of very little voltage. When you measure very little voltage, you will be more vulnerable to electric noise.  As a result, these types of meters are somewhat nonlinear, especially at low flow rates, which necessitate the use of flow corrections. For small dimensions, ultrasonic meters only measure the middle part of the flow, which is where the velocity is the highest. It is, therefore, necessary to correct for the part of the flow that is not covered.

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  2. Since I work primarily in the aerospace flow metrology industry, where we typically try to measure with +/- 0.1% uncertainties or better, I was not aware of the details of the Parshall flume.  Ian Pearson's comment spurred me to take a closer look at the theoretical background of said device as a +/- 2%  "instrument".  Take a look at the Wikipedia article on Parshall flumes and at the ASTM and ISO standards listed therein.  Good reading, if +/- 2% is all that you need.

  3. It depends on the raw sewer water analysis because each meter of the two has different specifications.

  4. Conventionally a Parshall flume (open channel) is constructed just after the inlet screens and de-gritter.  This provides a direct relationship between depth of flow and flow velocity, and hence flow volume.  The parshall flume results in a change of flow from sub-critical to super-critical, with critical flow being the measuring point.  An ultrasonic meter is mounted above the flume just upstream of the critical flow point which measures depth.  This has proved fairly reliable as long as the meter is serviced, calibrated and maintained.

  5. Due to sewage's potentially varying composition (affecting instrument calibration) and varying depth of flow / void fraction , it will probably be best to utilize some sort of non-fouling velocimeter and (laser?) level gauging device. calculating the instantaneous wetted cross sectional area and multiplying by instantaneous flow velocity to  approximate instantaneous volumetric flow.  Primitive, yes, but one wouldn't have to worry about composition affecting instrument calibration, etc.

    If one attempts to restrict the flow to conform to a full pipe flow there may be problems of sewage backup should anomalously high flow occurs -- as could be seen on multiple occasions recently during "100 year" storm events.

  6. Hi Caroline, what are you trying to measure within the sewer specifically?

  7. Hi Caroline. The first consideration isn't the type of meter but whether or not the pipe flows full bore or not. Usually sewers are not full bore which means neither meter types will work unless especially manufactured (i.e. mag flow) to work in part full pipes. ABB manufactured a version some years ago. A pipeline section can be designed and modified to ensure that a short section remains full at all times, then either will work and a mag low can easily be installed during this work. Ultrasonics can be retrofitted externally providing that the bore (as in working condition diameter) is known and the pipe material is made from a homogenous material. These considerations are OK for most sewers but if yours is an exceptionally large diameter then ultrasonics may be the better option however the pipe stills needs to be flowing full bore. Alternatively if not full bore have you considered depth measurement, e.g. capacitance tubes, to provide data for calculation into a flow estimate?