Flowmeters are well known to be sensitive to effects like pulsating flow, which can cause errors in flow measurement. In a pulsating flow system, the velocity profile of the flow undergoes a dramatic change during each cycle of pulsation compared to even flow conditions.
Figure 1. Image Credit: Titan Enterprises Ltd.
Therefore, the key source of potential error in electronically sampling flow meters, like ultrasonic devices, is the sampling rate and how it will sense the real flow that passes through it.
How can Errors Generated by the Effects of Pulsating Flow be Minimized During Sampling?
Titan Enterprises provides a wide array of ultrasonic small bore flow meters that ensure highly accurate flow measurement for various applications. But similar to all flow metering devices, they are sensitive to installation effects that cause variations in the velocity profile of the flow.
Customers who install Titan’s flow meters in their pulsating flow system are advised to dampen any pulsation to the best possible extent before performing measurement.
Following Nyquist Theorem to Increase Accuracy of Flow Measurement
According to the Nyquist Theorem, to sufficiently reproduce a signal (the liquid flow in this case), it must be sampled periodically at a rate at least twice as fast as its highest frequency component being recorded.
Thus, in the case of a pulsating flow scenario, the electronic flow meter’s sampling rate must be at least double the frequency rate of the pulsations themselves. In case the pulsation rate is not considered, aliasing of the flow signal will be observed, leading to erroneous measurement from the flowmeter.
Aliasing in Flow Metering
Aliasing can be exemplified in film making using the wagon wheel effect in which the rate of sampling of the camera is less than two times the rotations of the spokes of the wheel. The visual effect produced is the reversing of the direction of the wheel, although the cart continues along its original path.
Aliasing can be graphically demonstrated with flow measurement. For instance, if sampling is considerably lower compared to the pulsations, a random scatter of the flow will be seen, apparently without following any pattern.
Figure 2. Image Credit: Titan Enterprises Ltd.
In theory, when sampling is performed at the same frequency as pulsation, a constant yet inaccurate flow measurement is the result. In reality, this scenario is rare and the kind of aliasing depicted in Figure 2 is more probable and follows a cyclic pattern within the troughs and peaks of the pulse.
Figure 3. Image Credit: Titan Enterprises Ltd.
Titan’s Ultrasonic Flow Meters
Time-of-flight acoustic flow measurement method is used by all ultrasonic flow meters from Titan Enterprises. The velocity, and hence the flow can be precisely measured by quantifying an acoustic signal upstream and downstream in the liquid measurement tube.
For instance, the Standard Atrato range will measure to ±1% of reading from 2 mL/minute to 20 L/minute.
The Atrato® and Metraflow® series of ultrasonic flowmeters from Titan Enterprises use advanced electronics and thus can resolve the ultrasonic signals to about 50 picoseconds. The flowmeters achieve this through repeat measurements to ensure measurement stability.
The measured flow is sampled by the software every 3 ms. However, with the downstream processing, the reported flow is efficiently filtered to about 50 ms or 20 Hz.
The ultrasonic devices from Titan Enterprises follow the Nyquist theory and thus perform accurate measurement of the pulsating flow that is less than 10 Hz. Pulsation above this frequency will probably result in aliasing. Inline pulse dampening is always suggested for systems that use piston, diaphragm, or peristaltic pumps.
Titan Enterprises Ltd draws upon more than four decades of flowmeter innovation and is a manufacturer of high performance solutions such as the Oval Gear flowmeters, Atrato ultrasonic flowmeter, a flow instrument range and low flow Turbine flow meters.
The company’s philosophy of “pushing the envelope by trying to do things a little different and better” has led to sales of more than 500,000 products in 50 countries across the world and a repeat purchase percentage of 95%.
All flowmeters manufactured by Titan Enterprises are designed and produced to ISO9001 standard and calibrated to an uncertainty of ±0.25%.
This information has been sourced, reviewed and adapted from materials provided by Titan Enterprises Ltd.
For more information on this source, please visit Titan Enterprises Ltd.