How To Do Uncertainty Assessment and What Needs Attention

In the process of monitoring, reporting and verification of greenhouse gas emissions, perhaps the most important issue is “uncertainty”. Because the emission report that will be prepared together with the annual greenhouse gas emission value should be reliable.

Despite the fact that same measuring device measures the same amount, the result of each measurement may vary. This difference originates from uncertainty of the devices, in other words their sensitivity. In order to keep the quality of uncertainty at a certain level, steps are defined in M&R notification. The step must be determined for each source flow when calculation method is used, and it must be determined for each measurement point when the measurement method is used. There is a threshold of uncertainty required by each of the determined steps. In order to keep the quality of the uncertainty at a certain level, uncertainties arising from the measuring device for each source flow or measurement point must be below the specified uncertainty value in the applied step. Regardless of the monitoring method that was used, uncertainty issue has a critical role during monitoring emissions. Therefore, the verification body is obliged to check the pledged information on the approved monitoring plans during the facility visit. Moreover, it is obliged to check the calibration documents of the measuring devices and ensure that calibrations are performed as often as necessary.

Either continuous measurement devices are used or stock changes are taken in consideration, when determining the activity data in the calculation based method. When determining the activity data, if continuous measurement devices are chosen, a distinction is made a based on whether the measurement system is controlled by the enterprise. If the measurement device is under the control of the enterprise, whether the device is included in national legislation[1] on legal metrological control or not is considered. If the measurement device is included in this legislation; the maximum permissible error in the legislation is taken as the uncertainty value of the device. In this case, the verification body checks whether the measurement device is subject to statutory control during the facility visit. For instance; it controls the official verification certificate of the device along with the features of the device in national legal metrology control legislation. In addition, the verification body must ensure that the measuring device is adequately controlled (calibrated) [2]. If the device is not included in the legislation, the uncertainty value can be used which came from the frequently used calibration. In this case, the verification body checks the calibration certificates. However, it is important to note that different uncertainty values will be obtained for different measurement ranges. Therefore, the verification body should also make sure that whether uncertainty value in the measurement range used is being taken. Moreover, if the Device isn’t included in the legislation, the uncertainty value that comes from the device specification can also be used in the condition that it is installed in accordance with usage instructions of the measuring system.

The fact that the measurement system is not under the control of the enterprise means that this measurement system belongs to the commercial partner. In this case, the point to be considered is whether this measurement system is included in the legislation on metrological control. In case the measurement system is not included in the legislation, the information and documents of the responsible commercial partner are used. If the measurement system is included in the legislation, the maximum permissible error in the legislation can be taken as the uncertainty value. In such cases, as described above, the verification body must check the values of the instrument in the legislation, whether the measuring range is correct and whether calibration is carried out at the appropriate frequency.

In the calculation based method, the activity data can also be determined by taking stock change into consideration. In this case, it is decided whether the uncertainty assessment will be made by looking at the storage capacity. Uncertainty is assessed when the storage unit’s capacity is 5% or more of the amount of fuel or material used annually. In cases where uncertainty assessment is carried out, a number of issues arise that must be checked by the verification body. These are; capacity of the stocking area, the product or raw material quantity in the area at the beginning and end of monitoring period, the amount of raw material or product purchased from the area during the monitoring period, the uncertainty of the storage yard and the uncertainty of the used measuring device.

If continuous measurement is selected as the method of determining activity data, relevant measurement devices should be defined in the system and the uncertainty value of the measurement device should be entered in the system. Apart from this, the official document indicating the uncertainty value of this measurement device should be added to the system. In case the stock change is selected as the method of determining activity data, the measurement device doesn’t have to be identified to the system but the uncertainty value has to be entered in the system.

The uncertainty value must be entered in the system as percentage. However, in some cases, the uncertainty value can be obtained as ± kg. In this case, by using the most cautious method, percentage uncertainty value is obtained by using the range in which the uncertainty value applies.

As a conclusion, uncertainty assessments for the reliability of emissions are one of the most important issues in monitoring, reporting, and verifying of greenhouse gas emissions. Consequently, facilities should be very careful about their uncertainty assessments and calibrations while preparing the monitoring plans and at the same time while preparing their emission reports. In a similar way, during the verification process, the verification body will also check carefully whether the uncertainty assessment has been made correctly by the facility.

 

You can access the presentation about the uncertainty here, which was given by Life Enerji team, at Hilton Hotel/Ankara, in 2014.

 

[1] See http://www.mevzuat.gov.tr/Metin.Aspx?MevzuatKod=7.5.12316&MevzuatIliski=0&sourceXmlSearch=

[2] See http://ec.europa.eu/clima/policies/ets/monitoring/docs/kgn_3_process_analysis_en.pdf, page 17

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