All Vortex Flow Meters are systematically calibrated and adjusted, which means compare to a reference and adjust to reach the designed accuracy.
To prove the uncertainty of the measurement, we need to prove the quality of the calibration. Calibration of all Vortex flow meter sizes from DN15 to 300 on the rig in the factory from ABB, Endress+Hauser, Yokogawa, etc.
Those rigs are accredited as per ISO/IEC 17025 by any Accreditation office, with mutual recognition with other European Metrological Institute (like Cofrac, PTB, NMI…) and other certified bodies.
The vortex meter is a linear based principle and is compared to a reference on several points within a range. A standard calibration is carried out in 3 points within the measuring range and maybe calibrated on 5 pts in option!
As most of you know, the calibration is done with water. During the procedure, the K factor of each device is defined. The use of water has several advantages due to excellent knowledge of water properties, a very accurate and outstanding calibration may be achieved. Therefore the accuracy reached as per the accreditation is better than +/-0,05%. Furthermore, the media used (water) enables to optimize the dynamic flow conditions to regulate a perfect flow profile.
If the device is intended for gas application, the calibration with water shows, without a doubt, the validity of the universal K factor defined during the calibration process.
In other words, the K factor, defined during calibration, is simply a picture of the corresponding volume of one vortex shedding. The bluff body/Shedder Bar linearity (Strouhal number) means that the K factor depends on each sensor’s geometry. And this geometry is not influenced by the fluid type (gas or liquid) or process conditions like pressure or density. Therefore the defined K factor with water aid can be realized for all fluids, including GAS.
The fluid temperature has a minor impact on the K factor, for example, for Stainless steel. The occurred uncertainty is about 0,5% of mV every 100K.