BS 7965:2009 pdf download – Guide to the selection, installation, operation and calibration of diagonal path transit time ultrasonic flowmeters for industrial gas applications
The four classes represent different measurement specifications commonly applied in UK industry. Depending on the importance of measurement with respect to regulatory or fiscal demands, the total uncertainty budget for the system is different, as outlined within Table 3. To aid the user in meter selection, an approximate uncertainty budget for the meter is suggested in order to meet the system uncertainty budget. A significant part of the total value for the USM uncertainty comes from the uncertainty of the calibration facility where the meter was flow calibrated. Typical uncertainty values from recognized gas flow calibration establishments are in the order of ±0.3%.
NOTE 1 These applications are for guidance only, each application to be considered on its own merits.
NOTE 2 See Annex A for the application offlowcalibration results and Annex B for methods ofcorrecting flow measurement error. The difference between the uncertainty figures of the meter and the system indicates the uncertainty budget permitted in determining additional uncertainties associated with the metering process e.g. secondary instrumentation, derivation of density and flow computers. For example, the composition, pressure and temperature of a gas might result in an uncertainty value for the derivation of density that causes the budget available for the system to be exceeded: for these reasons it is important that this aspect is not neglected. For natural gas, A.G.A. Report No. 8  should be consulted for guidance on the derivation of density and uncertainty. When identifying a meter for a particular purpose, it is important for the user to have access to the information in Table 2 and Table 3.
In respect of Table 2, this information should be obtained from the meter manufacturer and it is recommended that the user reviews actual test data regarding the meter performance when purchasing for the first time or from a new supplier or manufacturer. Other criteria for selection can be determined by regulatory or statutory bodies, such as the Department of Energy and Climate Change (DECC) Energy Group:Energy Development Unit, and also agreements between companies for custody transfer/allocation metering. USMs have a large potential advantage over other forms of flowmeter owing to the availability of diagnostic software. This means that not only is a measurement made, but also information is gathered regarding the measurement process and the fluid itself. This can in turn allow for further criteria for selection. For example the accuracy of measurement in the event of a path failure is important in many custody transfer applications. For most other forms of flowmeter a major failure in the measurement process would lead to a complete shutdown while the meter is replaced. In the event of a path failure with USMs, it is possible for some meters to substitute values in place of the failed path measurements from knowledge of the velocity profile present in the meter prior to the failure. For Class 1 meters, the additional error owing to a path failure should, under stable flowing conditions, be less than 0.1%.
5 Installation recommendations
5.1 General The guidance in this clause has been derived from comparable meter standards, good engineering practice and experience gained to date in the testing of these meters. Installation criteria should be considered in conjunction with specific recommendations from outside agencies such as the meter manufacturers, the statutory authorities and other interested parties.
5.2 Environmental considerations
5.2.1 Temperature Some USMs might be sensitive to differences between ambient and line temperature and adequate insulation, heating and/or cooling should be installed in order to minimize the effects of extreme differences between ambient and line temperature. The meter should be able to operate within specification within an ambient air temperature range of −25 °C to 55 °C. This temperature range applies to the meter body with and without gas flow, field mounted electronics, ultrasonic transducers, cabling, etc. The manufacturers should provide ambient temperature specifications for the USMs if different from −25 °C to 55 °C.