05.03.2009
As to the flow measurement of gaseous media, the European metrology organisation EURAMET has set itself the target to reduce the on-site measurement uncertainty by factor 5. With a novel fundamental apparatus, PTB has now significantly approached this target.
For the calibration of gas meters, PTB is developing fundamental apparatuses, which are on top of the calibration chains. To be able to accurately present medium volume flows, a novel bell-piston prover device with a spindle stepper motor drive has been developed and has already been successfully tested. The piston device allows flow rates with a very good repeatability to be generated and also permits the calibration of test objects, which generate a continuous flow rate themselves, like critically operated nozzles, which are often also used as transfer standards.
The bell-piston prover, which is moved via a stepper motor and a ball screw spindle including a ball screw nut at the top end (see Figure 1), is located in a pressure-proof container. At its outside and inside at the guide bars, the bell body is sealed by a double seal. A potential leakage can be detected by means of a pressure measurement between the seals. The system consists of an upper and a lower chamber which are interconnected by a pipeline with an integrated valve V1 and a flow indicator.
In comparison to a conventional piston device, this construction has the advantage that the traceability to the unit of length is provided via outside diameter values. The latter can be measured with considerably lower uncertainties than inside diameter values. With a coordinate measuring device of PTB, the diameter values relevant to the realization have been determined with a relative uncertainty of less than Urel = 8·10-5 (k = 2).
With an outside bell diameter of approx. 380 mm and a lift of 720 mm the maximum displaced volume amounts to approx. 80 litres. The range of realization is between QV = 40 l/h and 4000 l/h, the measuring time is at least 1 min.
To achieve a temperature adjustment, the test air flows through the whole system for a certain period of time prior to the measurement, i.e. through the upper chamber to the lower chamber via the connecting pipeline, past the standing piston and finally through the test object. Depending on its principle, the test object can also generate the flow rate, like, for example, for a critical nozzle.
During the actual measurement of a critical nozzle, the bell-piston prover is slowly moved downwards with increasing velocity starting from the pole position. The flow rate led through the chambers is increasingly carried out by the bell-piston prover, i.e. the flow rate value indicated by the flow indicator in the connecting pipeline decreases. As soon as the flow indicator has reached the value 0, the bell-piston prover has generated exactly the flow rate QV,nozzle specified by the nozzle, which can be traced back via the diameter values dpiston and dguiding, the stepper motor frequency f and the spindle pitch s according to the following equation:
The presented measuring principle for the first time realizes a piston device, which can be operated as flow comparator. Due to its design, the device allows for the first time direct nozzle calibrations for inlet pressures of up to 10 bar with an actively driven piston. Moreover, the operation with different test gases is possible in principle. With the new apparatus, the measurement uncertainty can be reduced as to the traceability of volume flows to Urel = 0.04% (k = 2). Comparison measurements with PTB standards, especially the gas meter bell, show a very good agreement (see Figure 2).
Figure 2: Results of the comparison measurements by means of critically operated nozzles between established reference standards of PTB (CMC entries DE37, DE41 and DE43) and the new bell-piston prover device (zero line in the diagram).
Contact person:
Rainer Kramer, FB 1.4, AG 1.42, E-Mail: rainer.kramer@ptb.de