What’s the issue?
There are two types of flow measurement with which the EM has extensive experience: gas for anaesthesia systems; water in pipes and mains. Commercial and domestic meters must sometimes be developed to measure these flows. This article examines the rules and challenges encountered in developing such systems.
How is flow measured? Which method is best?
Four methods are common place.
- Pressure drop: if fluid or gas flows in a pipe that has a resistance to flow, measuring the pressure at two points over which the hydraulic/pneumatic resistance is known allows flow to be calculated from an Ohms law analogy. If the flow over the resistance is turbulent the Pressure drop is approximately the flow squared times the resistance. If the flow is laminar the pressure drop equals the flow times the resistance. This method is very effective but the following must be considered:
- To measure net flow must be restricted to allow sufficient drop for sensors to detect the pressure drop
- the system will likely need to be forced into a totally laminar or turbulent flow system. If the flow is a mixture of laminar and turbulent flow computation become complex.
- Hydraulic systems don’t lend themselves to having just one pressure sensor because fluids from two sides of a differential pressure sensor tend to interfere with the control electronics. It is usually necessary to have 2 pressure sensors – increase in cost
- Hydraulic systems may operate at a significant base pressure, say 10 bar, and then require measurements in milli or even micro bar. This can be challenging and require expensive pressure sensors.
- Mechanical: mechanical flow sensors have existed for centuries, e.g., the anemometer. The biggest advantage is the lack of a need for a power source. Obviously mechanical blockage/wear or similar is a limitation. The mechanical method is a ‘contact’ method and is being replaced by modern non-contact methods more and more. This article will not examine them further.
- Ultrasonic: ultrasonic flowmeters use the properties of sound to compute flow. An example is water flow with which propagation properties of sound through the water are measured. The method is effective but the following must be considered:
- Air or detritus in flow path is very disruptive to measurements because the transmission properties for sound in air or solids are dramatically different from those in water. This issue can cause complete failure to make measurements or occasional erroneous measurements.
- Turbulent flow in flow paths that have step changes in dimension can cause cavitation. Cavitation is literally tearing the water apart, allowing air/oxygen that was trapped in the water to create bubbles or cavities. An everyday occurrence of this is in the region of boat propellers. This is catastrophic to ultrasonic flow measurement because the transmission properties of sound in air and water are very different.
- Ultrasonic methods involve transmission of beams of sound through the cross-section of the flow and are therefore only sampling the flow. If the sample is the same as the whole that is fine, but it often isn’t.
- Unlike the pressure drop method, ultrasonics can’t determine if flow is turbulent or laminar. The nature of the flow determines the calibration of a flowmeter. Ultrasonic systems have to calibrated in such a way that they are either suitable for laminar or turbulent flow.
- Electromagnetic: The dominant feature of electromagnetic methods is they can energy harvest if the flow medium is conductive, e.g., impure water, metal. Electromagnetic flowmeters are complex but relatively resilient to impurities in the flow, and, of course, non-contact.
The EM can go into much more depth on this topic if you require. Please don’t hesitate to CONTACT US if this is the case.