search.noResults

search.searching

saml.title
dataCollection.invalidEmail
note.createNoteMessage

search.noResults

search.searching

orderForm.title

orderForm.productCode
orderForm.description
orderForm.quantity
orderForm.itemPrice
orderForm.price
orderForm.totalPrice
orderForm.deliveryDetails.billingAddress
orderForm.deliveryDetails.deliveryAddress
orderForm.noItems
CALIBRATION


CONFIDENCE IN YOUR CALIBRATION Dr Chris Mills,


consultant engineer at TÜV SÜD National Engineering


Laboratory, takes us through the various flow meter calibration methods available, and the factors that determine which one to choose


C


alibration is not an absolute operation, but instead a comparison between the


reading of a device and that of a reference standard. It is therefore necessary to consider what properties are required from a standard. For flow measurement, the standard is a system comprising a measure of quantity and the subsidiary measurements to determine the fluid conditions, properties and influence factors. To provide confidence that the measurement taken by the standard is accurate, all the measurements in the system must demonstrate traceability to higher level measurements and ultimately to national and international standards. To correctly express the ‘performance


accuracy’ of a standard or a calibration, the ‘uncertainty’ must be determined and quoted. For flow measurement the confidence in the result lying within the uncertainty is normally quoted with a ‘coverage factor’ of k=2, which is approximately 95 per cent confidence level. All calibration results should have a stated uncertainty and ‘coverage factor’ on the calibration report or certificate.


Calibration fluid and conditions All flow meters are affected to some degree by fluid properties and velocity profiles. The nature of how flow meters interact with the flowing fluid is affected by the properties of the fluid or the velocity distribution of the fluid passing through the device. When a fluid passes through a pipe, the distribution of velocity across the pipe alters, depending on the pipe’s internal diameter, roughness


14 APRIL 2021 | PROCESS & CONTROL


and fluid Reynolds number. The presence of bends and valves can introduce asymmetry to


the velocity distribution and potentially swirl. All of these effects must also be considered in the calibration. Ideally, the calibration should be


completed using the same fluid and pipework configuration within which the meter will normally operate. In reality, this is seldom possible. Whilst a calibration laboratory will try and match process conditions as much as possible, some degree of disturbance to the meter is inevitable.


Frequency of calibration? Specific industry standards or a third-party (regulator or trading partner) dictate the calibration frequency. In this case the meter is calibrated at specific intervals and it is assumed to be accurate between calibrations. For most applications however, it is the


user who must define the calibration interval and the calibration methodology. The interval is normally chosen to minimise the risk of an incorrect meter reading having a significant impact on the process. For example, high flowrates of oil attract huge tax liabilities and could possibly require weekly in-situ calibrations of the meter. The methodology dictates whether the device is calibrated in-situ or at an accredited


The TÜV SÜD National Engineering


Laboratory is a global centre of excellence for flow measurement and fluid flow systems


laboratory and even whether the reference is mass or volume. Other factors affecting the decision are


the history of the meter and maintenance periods. Whatever the frequency, it is always good practice to keep calibration graphs and control charts of the meter performance as this will assist in determining calibration intervals and demonstrate meter performance over time.


Methods for liquids and gases Common flow calibration standards are usually classified as being ‘bucket and stopwatch’ systems. The ‘bucket’ is a container which is weighed or has a known volume. The ‘stopwatch’ is a method of measuring the time to fill the bucket. The ‘Standing start and stop’ calibrations


of flow meters are the simplest method available and can be used for both high and low accuracy calibrations. The quantity of fluid collected is measured and compared with the meter reading, and combined with the time to fill, to give the flowrate. The ‘flying start and finish’ method is


sometimes called the diverter method, where the flow through the meter is not stopped but continues uninterrupted and is physically diverted between a return path to the liquid supply tank and the collection container. A switch on the diverter mechanism starts and stops a timer and a pulse totaliser. Beyond ‘bucket and stopwatch’ systems,


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64