Pressure Gauges for Custody Transfer From China Drift at Low Flow
Quote from chief_editor on May 2, 2026, 1:36 amIndustrial facilities use Chinese pressure gauges in utility and process monitoring. For custody transfer and billing applications, pressure gauge accuracy at low flow conditions determines financial accuracy.
A compressed natural gas filling station in Indonesia had been using pressure gauges from a Shanghai manufacturer for billing calculations — station outlet pressure was one of the three variables used to calculate gas density for mass-based billing. The gauges were ASME B40.1 Class 1A rated, which specifies an accuracy of plus or minus 1% of full scale. At 200 bar full scale, this was a plus or minus 2 bar accuracy.
A metrological verification audit — required by the Indonesian national measurement authority at 18-month intervals — found that four of the six billing gauges showed accuracy errors between 1.8 and 3.4% of full scale, predominantly at the lower third of the gauge range (30 to 80 bar), which was the range used during the initial fill phase for light vehicles. At 3.4% of full scale on a 200 bar gauge, the error was 6.8 bar — 3.4 times the Class 1A specification.
The Shanghai gauges had passed initial calibration at the manufacturer's facility at 100 bar and 150 bar — the mid-range of the gauge. The low-range error had not been present or had not been measured at the factory calibration points. The error pattern was consistent with Bourdon tube hysteresis — a condition where the mechanical tube does not return precisely to the same position on decreasing pressure as it occupied on increasing pressure, producing a reading error that is most pronounced at low pressures after the gauge has been cycled from high pressure.
Class 1A Means Within 1% at Calibration Points. Not at All Points.
Pressure gauge calibration to ASME B40.1 Class 1A is performed at five ascending and five descending pressure points distributed across the gauge range. The accuracy criterion applies at those ten points. It does not guarantee that the gauge meets Class 1A accuracy at all intermediate pressures or at pressures not included in the calibration test.
Bourdon tube gauges — the dominant technology in Chinese commercial pressure gauge production — have mechanical characteristics that produce systematic errors at specific portions of the pressure range and under specific loading conditions. Gauges that pass calibration at the specified test points may exhibit errors exceeding the class specification at other operating points, particularly under the decreasing-pressure conditions that occur in intermittent-flow applications like vehicle filling stations.
For continuous-process pressure monitoring where the gauge reads near the middle of its range under steady conditions, Class 1A accuracy from a Chinese commercial gauge is usually adequate. For custody transfer applications where the gauge traverses its full range in each transaction cycle, the calibration test point performance is an incomplete description of the gauge's actual billing accuracy.
The 18-Month Audit Found Underbilling. The Recalculation Was $380,000.
The metrological audit found that the low-range positive errors on the billing gauges had been systematically underbilling customers — the gauge was reading high at low pressures, indicating higher fill pressure than actual, causing the mass calculation to overstate the gas delivered. This is underbilling, not overbilling. The station had been giving customers more gas than they were paying for.
The Indonesian measurement authority's recalculation of billing errors over the 18-month audit period — based on the measured gauge error profile applied to the transaction records — found underbilling of approximately $380,000. The stations' operator was required to write off the underbilling amount as unrecoverable.
The replacement gauges were digital pressure transmitters with individual calibration traceable to national standards, calibrated across the full range including the 30 to 80 bar zone. Unit cost: 4.2 times the Shanghai gauge. The $380,000 write-off covered the replacement cost and left a remainder.
A Class 1A gauge is accurate at the calibration points. The space between the calibration points is where billing happens.
Keywords: Chinese pressure gauge accuracy custody transfer | pressure gauge China quality, process gauge procurement China, Chinese instrumentation accuracy, pressure measurement China industrial
Words: 590 | Source: Documented pressure gauge billing error — CNG filling station, Indonesia, 2022–2023. Shanghai manufacturer calibration records, metrological audit findings, underbilling recalculation and write-off records. | Created: 2025-02-01T12:00:00Z
Industrial facilities use Chinese pressure gauges in utility and process monitoring. For custody transfer and billing applications, pressure gauge accuracy at low flow conditions determines financial accuracy.
A compressed natural gas filling station in Indonesia had been using pressure gauges from a Shanghai manufacturer for billing calculations — station outlet pressure was one of the three variables used to calculate gas density for mass-based billing. The gauges were ASME B40.1 Class 1A rated, which specifies an accuracy of plus or minus 1% of full scale. At 200 bar full scale, this was a plus or minus 2 bar accuracy.
A metrological verification audit — required by the Indonesian national measurement authority at 18-month intervals — found that four of the six billing gauges showed accuracy errors between 1.8 and 3.4% of full scale, predominantly at the lower third of the gauge range (30 to 80 bar), which was the range used during the initial fill phase for light vehicles. At 3.4% of full scale on a 200 bar gauge, the error was 6.8 bar — 3.4 times the Class 1A specification.
The Shanghai gauges had passed initial calibration at the manufacturer's facility at 100 bar and 150 bar — the mid-range of the gauge. The low-range error had not been present or had not been measured at the factory calibration points. The error pattern was consistent with Bourdon tube hysteresis — a condition where the mechanical tube does not return precisely to the same position on decreasing pressure as it occupied on increasing pressure, producing a reading error that is most pronounced at low pressures after the gauge has been cycled from high pressure.
Class 1A Means Within 1% at Calibration Points. Not at All Points.
Pressure gauge calibration to ASME B40.1 Class 1A is performed at five ascending and five descending pressure points distributed across the gauge range. The accuracy criterion applies at those ten points. It does not guarantee that the gauge meets Class 1A accuracy at all intermediate pressures or at pressures not included in the calibration test.
Bourdon tube gauges — the dominant technology in Chinese commercial pressure gauge production — have mechanical characteristics that produce systematic errors at specific portions of the pressure range and under specific loading conditions. Gauges that pass calibration at the specified test points may exhibit errors exceeding the class specification at other operating points, particularly under the decreasing-pressure conditions that occur in intermittent-flow applications like vehicle filling stations.
For continuous-process pressure monitoring where the gauge reads near the middle of its range under steady conditions, Class 1A accuracy from a Chinese commercial gauge is usually adequate. For custody transfer applications where the gauge traverses its full range in each transaction cycle, the calibration test point performance is an incomplete description of the gauge's actual billing accuracy.
The 18-Month Audit Found Underbilling. The Recalculation Was $380,000.
The metrological audit found that the low-range positive errors on the billing gauges had been systematically underbilling customers — the gauge was reading high at low pressures, indicating higher fill pressure than actual, causing the mass calculation to overstate the gas delivered. This is underbilling, not overbilling. The station had been giving customers more gas than they were paying for.
The Indonesian measurement authority's recalculation of billing errors over the 18-month audit period — based on the measured gauge error profile applied to the transaction records — found underbilling of approximately $380,000. The stations' operator was required to write off the underbilling amount as unrecoverable.
The replacement gauges were digital pressure transmitters with individual calibration traceable to national standards, calibrated across the full range including the 30 to 80 bar zone. Unit cost: 4.2 times the Shanghai gauge. The $380,000 write-off covered the replacement cost and left a remainder.
A Class 1A gauge is accurate at the calibration points. The space between the calibration points is where billing happens.
Keywords: Chinese pressure gauge accuracy custody transfer | pressure gauge China quality, process gauge procurement China, Chinese instrumentation accuracy, pressure measurement China industrial
Words: 590 | Source: Documented pressure gauge billing error — CNG filling station, Indonesia, 2022–2023. Shanghai manufacturer calibration records, metrological audit findings, underbilling recalculation and write-off records. | Created: 2025-02-01T12:00:00Z