Horndon SMER – Proposed Methodology July 2011

Overview Background Identification of the Error Correction of the Error Data Processing Rules

Background Loose connection of 3-wire RTD. Temperature under read by ~5°C Resulted in lower density and therefore under-registration of flow

Identification of Error Problem started in Sept Prior to this there were also intermittent low temperature readings which will be investigated

Identification of Error Error persisted despite annual validation checks. Problem was discovered and resolved in July 2010 by tightening the connections on the RTD device

Correction of Error Nearby sites investigated for temperature correlation were: Matching Green, Luxborough Lane, Shorne, Yelverton, Roudham Heath, Gt Wilbraham and BarkingPS.

Correction of Error Temperatures can only be compared during flowing periods. Low DP cut-off will be used to indicate if the site was flowing or not Downstream to upstream temperature correction of 0.5 °C/bar used Bias will exist between temperatures because of differences in site installations and locations. The bias will be investigated and the sensitivity of the bias to variation in DP will be quantified. Results were compared for a known good period (July 2010 to Dec 2010) CountBiasStandard deviation Luxborough Lane Matching Green Shorne Yelverton Gt Wilbraham Roudham Heath

Correction of Error The most suitable temperature will be used to calculate the temperature error and the bias will be corrected for. The temperature error will be examined over time to see whether the offset is constant or if it varies with other parameters. If the temperature offset is constant then the sensitivity of the flow error to pressure and differential pressure fluctuations and the sensitivity to the bias will be quantified.

Correction of Error If the error is not sensitive to these fluctuations then it will be applied across the period. If the error is sensitive to these fluctuations or the temperature offset is not constant then it will be necessary to recalculate the flow rates from the raw data. One set of calculations will be made on a four-minutely basis using the corrected temperature and another using the recorded temperature. Daily volumes will be calculated for both sets of data, the difference between them being described as the ‘error’. The ratio of the daily volumes will be used to produce a daily correction factor to be applied to the daily volume total (DVol).

Data Processing Rules Where data is erroneous (such as during periods of validation) the erroneous data will be interpolated before calculation. Where data is missing for up to 3 hours the missing data will be interpolated before calculation. Where data is missing for longer than 3 hours a default daily correction factor will be applied. This will be the average of the factors on surrounding days or the average for the entire error period.

Data Processing Rules Sites will be ranked in order of best temperature correlation. The temperature from the site with the highest rank which is also flowing will be used and the bias for that site will be corrected for. The offset will be calculated on a daily basis and where no suitable flowing data is present the offset for the surrounding days will be applied. The uncorrected calculated daily volume will be compared to the billed daily volume and where the difference is more than 10% a default daily correction factor will be applied.

Thank you for your attention