Greetings from SoundWater Technologies! Today we discuss Measurement Accuracy.
Clamp on style ultrasonic flowmeters (like SoundWater’s Orcas) are extremely convenient tools that install on the outside of pipe to measure the flow rate inside – and they accommodate to various pipe diameters and materials. However, with convenience comes freedom to alter program settings and hardware configurations that may affect accuracy. It is important to know what parameters are most important – so you get the most of your flowmeter.
At SoundWater, we perform a standard evaluation of the measurement process to best understand the significance of each parameter involved. The method is called an Uncertainty Analysis, or Error Propagation Analysis and is a well established engineering method for determining critical variables. The results have a clear message to users: the most critical parameters are the pipe dimensions, both the external diameter and wall thickness. Why? Because ultrasonic flowmeters measure velocity and use the specified pipe dimensions to convert velocity to a volume flow rate. For large pipe, the error in pipe dimensions may NOT significantly affect the flow rate measurement. However, as the pipe diameter gets smaller, the small variances in wall thickness and diameter become very important.
How do you control this in practice? The Orcas App provides a very convenient list of pipes for quick selection (you can see the easy menu selections in our quick start guide). This works well for most cases and will provide you with good measurements. For best accuracy, it is best to edit pipe dimensions yourself after selecting the defaults. The pipe external diameter may be measured with calipers or ‘OD tape’, but the wall thickness is a bit more difficult and in many cases the default values may be the only option. If you are installing the pipe yourself, take a quick measurement with calipers before it is installed. Or, even better, purchase an acoustic wall thickness gauge for a quick and easy wall thickness measurement – from the outside of the pipe.
These second greatest parameter for good measurement is straight pipe. This is the length of pipe upstream from the flowmeter, and is critical to stabilizing and conditioning the flow before it passes the flowmeter. This is necessary because as fluid turns a bend, exists a valve, or passes through a pump it may become unsteady and irregular and not ideal for making good flow measurements (more on straight pipe later). The straight length of pipe upstream of the flowmeter simply allows the fluid sufficient time to become steady and regular (developed).
So, next time you install your Orcas Flowmeter, be sure to (1) confirm you have accurate pipe dimensions, and (2) make sure you have plenty of straight pipe.
Happy flow metering!
