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Closed Loop Electronic Calibration (CLEC) Technology eliminates cost associated to conventional fuel dispenser meter calibration

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Technology corner
Posted / Last update: 23-12-2014

Publication: PetrolPlaza Technology Corner
Issued: December 2014
Author: Lee Krause, Cantest Solutions Inc.

Originally storage tank integrity testing services were ordered due to inventory reconciliation problems but precision testing results would seldom provide the answers to the inventory reconciliation discrepancies.



Figure 1: Positive Displacement Pump Meter

1 2 3 4


In 1997, Cantest launched the Alarm Management Program (AMP); this service investigated inventory reconciliation problems by studying and correcting tank charts, inventory procedures, and compensating for tank deformation and tank tilt as well as evaluating product purchase vs. average fuel dispensing temperatures. The significant majority of the Alarm management report findings indicated poor meter calibration as the source of the problem resulting in meter calibration services being requested.

Often recalibration of the meters, using traditional calibration services, would not solve the problems and further investigation was required.

This further investigation resulted in highlighting the following facts:

  • Inventory reconciliation has a regulatory standard of 0.5% of fuel purchases. Fuel dispenser meter calibration standards are also at 0.5%. The problem is if you consider vapor loss associated to product storage and dispensing as well as the repeatability factor associated to fuel metering systems the actual meter calibration must be tighter than 0.25% (1/2 of the regulatory standard) just to balance fuel purchases to sales at a 0.5% standard (inventory reconciliation objective). Meter calibration at a more accurate standard is necessary.
  • The North American Weights and Measures Regulatory Standard of 0.5% was originally deemed appropriate due to the perceived repeatability of  fuel dispensers and the variables associated to using the open neck prover to calibrate meters in field conditions.  Twenty years ago fuel dispenser manufacturers advertised the ability to repeat measurement to a 0.3% Standard.  This perpetuated a belief that even with a good calibration you may still have problems reconciling to a 0.5% standard.
  • Open neck provers (traditional calibration device) require significant effort, expertise and very strict procedures to detect meter wear, pulser inaccuracies, software programing errors and general meter failure.  Finding meter failures responsible for inventory problems with traditional calibration services may require multiple trips to site in a trial and error scenario. 

Recent Weights and Measures Studies have illustrated the following:

  • Vapor loss during the open neck prover use may be as high as 0.3% (Measurement Canada Study – Product Loss During Retail Motor Fuel Dispenser Inspection – April 10, 2007)
  • Open neck prover use without adequate fuel temperature stabilization may create an inaccuracy greater than the total allowable 0.5% standard.  (Nebraska Weights and Measures Study - Nebraska RMFD Test Results –Variables That Effect the Accuracy of Tests – April 2006)
  • Please see the following summary of measurable errors for further recognized variables associated to conventional prover use.

Summary of Measurable Errors

Volume (in mL)

Percentage

Adhesion differences due to drip time (at 15°C)

8

.04%

Initial calibration of Open Neck Prover

10

.05%

Vapor loss during Prover filling

20-60

0.1 to 0.3 %

Non-stabilized temperature effects

100

0.5 %

Human error including meniscus reading

20

.1%

Open Neck Prover shell expansion and contraction (per 1°C)

1

.005%

Table 1: All of these errors take place each time an Open Neck Prover is used. Due to differing temperature effects some of these errors may be either a plus or minus to the final calibration. Vapor loss is always in favor of the consumer and at the cost of the dealer. (Source-http://tokheim.com/dispenser/automatic-temperature-compensation/)

Why Meters Require Calibration

The diagram in Figure 1 illustrates the setup of a positive displacement meter, with all of its components included. The chambers each have a piston that moves in and out, while the cam rotates along the piston rollers. This action forces the piston to release the fuel. When the dispenser level is activated, the submersible pump pressurizes the product line with gasoline which then loads the pistons with fuel that allow the camshaft to rotate. Above the meters, pulsers are mounted in the dispenser cabinet. At the camshaft, the meter rotates the pulser and counts the number of rotations. Next, a computer program can be utilized to calculate the volume of fuel dispensed in this process.

See figure 1: Positive Displacement Pump Meter

When the electronic dispensers are calibrated, the pulse count is being compared to a measured volume.

Meter cylinder walls and other components wear over time, the meter accuracy drifts from the initial calibration resulting in the dispenser giving away product until the meters are recalibrated again to correct the volume of the meter output.

Meter Drift is not consistent and will vary from site to site and even from dispenser to dispenser on the same site. The following factors affect the rate of Meter Drift:

  • Volume Dispensed
  • Type of Product (Viscosity and Lubricity features)
  • Ethanol and Bio diesel content
  • Range of Operating Temperatures
  • Consistency of use.
  • Operating pressures.

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