Essentially, there are three separate calibration methods that Litre Meter have in-house to calibrate a range of flowmeters. As the fluids can vary from water to thick oil and the flows range from a few millilitres per hour to 1,000 litres per minute, there are not only three calibration methods but 10 rigs in total, all traceable to National Standards.
The most common calibration method for Litre Meter is gravimetric with flying start and stop. This method, as the names suggests, records the weight to provide a mass flow. Frequently, this is converted to a volume flow after determination of density. After that, the use of master meters is popular for higher flows, above a few litres per minute. Finally, where the flow rates are outrageously low – i.e. just millilitres per hour – Litre Meter has a volumetric pistonless design – the FlowLabPro.
Gravimetric Flying Start-Stop
Most Litre Meter flowmeters are calibrated using a gravimetric method. A flow rate is established through the flowmeter with the output flow returning to the main reservoir. At the commencement of test, the output flow is diverted into a weigh tank. When sufficient volume of fluid has been collected in respect of that particular flow rate, the output flow is diverted to the main reservoir once more. The time for the volume of fluid to be collected is recorded, together with the number of pulses produced by the transmitter.
The density of the fluid is determined at time of calibration. Volume divided by time equals flow rate. The number of pulses divided by volume equals the pulses per litre Meter Factor. The calibration certificate is prepared from a table of these values. If on-site calibration is required and a known volume of fluid flow can be established, then the same calculations apply to reproduce the calibration certificate.
Significant effort should be spent ensuring that the start and stop timing accurately reflect when flow is being weighed and that the speed of changeover is maximised. With modern, accurate weigh scales and timing the stop/start transitions are where precision improvements are made. Litre Meter have developed a simple program that enables quick monitoring of diverter performance and aids the operator in adjusting the timing.
When pulse rates are very low, such as in the VFF and some other positive displacement flowmeters, the flow rig electronics are altered such that a calibration run commences on a pulse and finish on a pulse. For example, a calibration run is dialled in requiring 60 pulses. The operator starts the test with continuous flow running through the MUT. When the next pulse is sensed the diverter operates and the scales begin weighing the fluid. After 60 pulses pass the diverter switches back and measurements are taken. There will be 60 pulses, a specific weight and a specific time enabling the calculation of flow rate and pulses per mass unit. With an sg reading from a suitably accurate densitometer then the rate can be calculated in volumetric terms and the pulses used to calculate pulses per litre. There is no uncertainty in the number of pulses i.e. whether the next pulse was about to arrive or had just been. This is usually an uncertainty of up to two pulses. There is an unknown time at Start of diversion but this is somewhat equalled out at the Stop side. Good calibration of the start/stop sequence can almost eliminate this unknown.
As the flow rate increases the compactness of the Gravimetric method with its diverter starts to die away and the rigs become very large. At Litre Meter we have a good crossover between the upper end of the gravimetric to the lower end of the master meter which provides some flexibility and confidence. For water the gravimetric rigs are suitable for flows up to 30 l/min and the master meters commence at 1 l/min. For oils it’s 10 and 0.1 respectively. Without the knife-edge start of the gravimetric flying start (above) the master meter technique pivots (sorry for the pun) around dual chronometry.
The Meter Under Test produces a pulse train simultaneously with the Master Meter but at a different frequency. The computer program starts and stops the test period depending on the preset length of test and the number of pulses desired. The preset length is then subtly adjusted so that a precise number of pulses is used. Further analysis of the MUT pulse rate counts the number of pulses, and more importantly, determines the fractional number of pulses at the start and end of test. Finally, the test is repeated at least three times at each flow rate. These are installed in a continuously running system, in a loop, thus enabling multiple tests in quick succession. The pump or valve position is then altered to process all of the desired flow rate range in a suitable number of steps.
Piston-less piston prover
For investigation of very low flows our traditional gravimetric rigs were unable to control and/or develop low flows consistently. For the LF03 the low flows are actually lower than the leak flows through the valves or our then smallest system. A change of method was required. Starting with a series of pumps a method was instigated using a rising column of fluid being monitored by an accurate height sensor. With a variety of column diameters a wide range of flow rates can be generated and measured accurately. Pulse interpolation is used in much the same way as for dual chronometry, above, except that the MUT and the height sensor are the two sources of pulses. All of the components including a choice of fluid reservoirs are together in one enclosure, temperature controlled, to maintain consistency. Up to 4 rotors and chambers can be calibrated together over the required flow rate range enabling calibration to continue overnight, unsupervised. Additional software programs can quickly determine minimum achievable flow which significantly speeds up parts selection for our LF and ULF calibration levels.