Key points
There are many ways of effectively measuring liquid flow. From the ubiquitous – MagFlowmeters, turbine meters, to the highly specialised – Coriolis meters, for example.
This article looks at developments in ultrasonic non-contacting measurement and acoustic non-invasive flow measurement for both open channel flow, including MCERTS applications, and also in closed pipes, based on Digital Signal Processing (DSP) techniques developed specifically for the task.
By far the most popular automatic, continuous (full measurement range) open channel flow (OCM) measurement approach worldwide remains the use of non-contacting ultrasonic technology which, for those who aren’t familiar with it, consists of a compact transducer, usually piezo-electric, that emits a high-frequency sound pulse, inaudible to humans, which then reflects from the liquid surface and returns, the time of flight for the echo being converted to a distance. The transducer can stand alone or form a system with a control unit.
MCERTS Measurement
The basic principle is a simple one, depending on the piezo-electric crystal being re- energised by the returning signal to produce a small voltage spike, a measurement being derived from the speed of sound, and the technique has been around in one form or another for decades.
In practice, however, the signal processing required to accurately discriminate the signal has developed enormously, moving from analogue to digital, and then has followed the same improvements in speed and accuracy as every other modern digital technique.
High-frequency transducers are used nowadays, maximising the resolution of the measurement in these short-range applications.
Depending on the manufacturer of the equipment, processing of the signal is then undertaken in the transducer itself and/or in a wall- or panel- mounted controller which provides processing and communications. All manufacturers provide a choice of controller, or, indeed, in some instances no controller, depending on the complexity of the application.
A modern, high-frequency ultrasonic system provides the high accuracy demanded by accreditation protocols such as MCERTS, the Environment Agency’s emissions monitoring scheme while being easy to install and maintain and very cost-effective.
Ultrasonic systems continue to represent the most accurate MCERTS approved technology on the market, for example Pulsar offer three measurement devices approved to MCERTS Class 1, depending on application.
Ultrasonic measurement uses the speed of sound in the calculation, and all OCM systems compensate for changes in speed of sound due to temperature changes. The latest MCERTS standard also includes a requirement for solar radiation shields to mitigate the effect of direct sunlight on the transducer.
Other Open Channel Liquid Flow Measurement
For high-accuracy, MCERTS type applications, there is another part of the equation, which is the need for a well-engineered and installed Primary Measurement Device (PMD) such as a calibrated flume or a weir.
A PMD works by constricting the flow in such a way that a change in flow rate results in a calculable change in level or ‘head’, which is what we actually measure. The flume manufacturer will also supply a head to flow curve of a minimum 21 points, entered into the controller for greater accuracy.
There are many other situations, for example in a canal or in an inaccessible sump, where there is no room for a PMD to be fitted. In those cases we have two options: we can derive a ‘curve’ of flow against level by measuring the flow rate using other means at various levels, and electronically recording a set point for each level within the controller.
A Pulsar system will typically allow around 32 points of calibration. Alternatively, the velocity of the flow can be measured, which may be enough in some applications where all that is required is to know that water is flowing in a certain situation.
New flow velocity measurement technology
Traditionally, flow velocity has been measured using contacting devices, typically turbines or Doppler immersion probes, plus some other time of flight techniques. These have been highly successful but occasionally problematic, obviously potentially prone to fouling and, particularly the Doppler sensor, reliant on various conditions within the flow.
New approaches such as Pulsar’s MicroFlow and MicroFlow-I offer direct non-contacting measurement of water velocity to be made. Using microwave (RADAR) technology, along with completely novel digital signal processing, a compact transducer is positioned above the flow and makes a non-contacting, time-of- flight measurement of flow velocity. A simple bracket can be used to mount the transducer at the optimum angle and multiple sensors can be used for wide channels (over 1.5m).
Going back to flow measurement, of course once a flow velocity has been determined, it can be combined with level measurement to produce a flow measurement using a velocity x area calculation.
This is a powerful, relatively low cost approach that does not require the civil engineering that is so often required when measuring using a flume or weir. More often used for process control than MCERTS type applications, it can nevertheless yield good results and some systems have MCERTS approval.
Communications and Calculations
Once the core measurement has been made, then all other operations derive from it. Ultrasonic systems have been developed around the needs of the water/wastewater industry.
Over time, the number of routines and applications built into these devices as standard has multiplied, so that now it is possible to perform pretty much any typical control function – penstock control, pump control, valve management etc., directly out of the box.
For communications purposes, industrial Ethernet such as Profibus and Modbus, plus other digital protocols are available along with the more usual 4-20mA and volt-free relay closures are included, depending on model.
If you look at the top of the range devices, such as Pulsar’s Ultimate Controller, then more exotic options are available, such as a camera so you can literally keep an eye on the process!
Non-invasive liquid flow measurement in closed pipes
For many years, the usual approach for measurement of flow in closed pipes has been the reliable and generally accurate magnetic flow (magflow) meter, and it is unlikely that will change anytime soon. However, there are drawbacks.
First, if a magflow can be installed from day 1 in a new installation, all well and good. If, on the other hand, flow monitoring needs to be introduced later, then there can be a significant interruption to the process and the costs associated with the design and installation of the meter.
Large sizes of magflow meters can be very expensive too. And, if it is critical that an accurate measurement is made, then that may well be worth it.
If, on the other hand, what you really need is a reliable and repeatable method of monitoring the process, then the latest non-invasive methods, exemplified by Pulsar’s Flow Pulse, could well be the answer.
Small, clamp-on acoustic sensors including a high-output ceramic crystal, literally fixed using just a screwdriver, and operating on pipe materials including rigid plastic, stainless or mild steel or even cast iron, are producing good results where flow rates are between 0.3m/s and 10m/s.
Wide-bore pipes of 1 metre diameter are possible and there is no intrusion into the process whatsoever, and Pulsar’s system will work with slurries through to liquids with as little as 200ppm entrained particles, the equivalent of hard water. Ultrasound is ‘fired’ through the pipe wall at 90º to the flow.
The novel spread spectrum analysis technique employed by Pulsar is called RSSA (Refracted Spread Spectrum Analysis), which analyses and integrates data over a wide frequency range to derive a flow rate, which can then be output to SCADA or telemetry.
The compact system is so easy to use, even mobile operations are possible with hand-held, better powered versions of these devices.
With relatively low cost per installation, this means that they can be more generally deployed within a process, so process managers can have real clarity of flow profiles within their operations.
