Flowmeters are often chosen according to the specifications as listed within the documents supplied by the manufacturer. It is only later, when the instrument is being used under actual operating conditions, that an engineer can assess the validity of those specifications.
At this point it is important to state that the efficacy of a manufacturers specifications at calibration conditions is not being questioned. However, the unknown variables can result in a "highly accurate device at calibration conditions" not meeting the requirements of a "highly accurate device in my particular application".
This test is supplied by a major calibration laboratory. Calibrations are performed with DAkkS certified test benches according to the ISO 17025 norms. Two turbine flowmeters were selected; once being the DM-16 turbine flowmeter from TrigasDM and the other being a competitive product at the same flow rate and within the same price bracket. For this discussion we can call this turbine Flowmeter 2.
The tests below were conducted within the viscosity range of 10 mm2/s and 50 mm2/s. At a low viscosity of 10 mm2/s flowmeter 2 stays within the 2% accuracy limit at above 90% of the range. But at the low flowrates deviations increases up to 30%. This may be acceptable if the customer is not running the device in the lower 10% range. Performance deteriorates as the viscosity increases to 50 mm2/s. Below 75% of the range the deviations rise above 2% and up to 34%. Clearly, the higher the viscosity, the worse the deviation.
The raw frequency output from the DM-16 also showed similar deviations at low flowrates, but a better performance at higher rates. Still this is not an ideal solution for an oil application. For turbines in oil applications of varying viscosity, TrigasDM applies a UVC calibration. Multiple calibration curves are generated within the viscosity range that the customer expects during operation. The data is then combined into a Universal Viscosity Curve (UVC) which is used by the electronics to compensate in real time for changing viscosities.
The UVC calibration is only one of the features that speaks in favour of this device. The highly precise construction of the DM-turbine makes it more responsive to viscosity shifts and consequently more predictable when UVC principles are applied. Additionally, because of its proprietary blade design, it also generates a surprisingly low pressure loss.
Engineers who are looking for flowmeters for oil applications should talk to their suppliers in detail about the accuracy at changing viscosities. As seen however, advanced modern turbines, which are specifically designed for applications with changing viscosities, meet many all of the requirements of high technology flowmeters in relation to a long-life, low pressure drop, high accuracy and proven reliability.