Colorado Engineering Experiment Station (CEESI) in Nunn, Colorado, USA has constructed a multi-viscosity flow calibration loop with which they can calibrate liquid flow meters. Calibrations can be performed using a diversity of liquids that have different fluid properties in order to simulate field conditions. This new facility is now open for the calibration of flow meters up to flows of 3000 m3/h and with a maximum internal diameter of 16 inches. An accreditation of 0.024% for direct proving and 0.05% for master metering by NVLAP according to standard ISO/IEC17025 will provide a guarantee to customers that calibrations are done with great care.
Calibrations need to be traceable to internationally accepted measurement standards with specified measurement uncertainties. The standard for this facility is known as a bi-directional pipe prover. The volume at reference conditions needs to be established by calibration to guarantee measurement traceability to SI and USC measurement units. VSL and Certified Calibrations Inc. (CCI) were hired to ensure that the calibration is executed with great care and with low measurement uncertainty.
How is the calibration performed?
The calibration method used to establish the reference volume is the water draw method according to standard API MPMS 4.9.2. This method utilises test measures as the reference standard. CCI brought in their water draw trailer with specially dedicated test measures for this job. As this job was performed in the USA, the calibration of the test measures is performed and traceable to our colleague institute in the USA, NIST. After placing the trailer close to the pipe prover, the water draw unit was connected and the system was filled with water. In the pipe prover, a sphere is moved from a launch chamber to a receiving chamber and back again, flowing through a pipe section. Two detectors are placed in this pipe section and these give an electrical signal when the sphere passes the detector. The volume between the two detectors is collected in the selected test measures and the volume can be compared to the certificates that are issued for these test measures. This cannot be done by just comparing volume to volume, as corrections need to be made for temperature and pressure difference in the system. VSL calculated the reference volume and the measurement uncertainty for the calibration executed. All uncertainty sources were identified and include the equations utilised and the operations. Due to the highly skilled operator of the water draw unit, excellent repeatability in measurements was obtained, leading to a low measurement uncertainty of just 0.02% of what is referred to as the round trip volume.
VSL certificate internationally accepted
VSL is presently still the only National Metrology Institute which is allowed to use the CIPM MRA logo for calibration of all kinds of provers. Along with this, VSL holds an ISO/IEC17025 accreditation for this type of calibration. The certificate issued by VSL gives CEESI direct traceability to internationally accepted measurement standards and a solid basis for this new calibration facility.