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ATG Calibration Methodology GeneralEvery new or repaired tank, or new or repaired ATG, requires a field calibration of the ATG to set its level to an accurate manual gauge. The American Petroleum Institute (API) has set out specific requirements for performing a field calibration of an ATG. These requirements are quite detailed and involve a lot of procedures to follow with a number of mathematical tests or limits that determine whether a calibration has passed or failed. All of the API requirements are not detailed in this document, only those items relative to using PGL in the calibration are addressed here. Please refer to the API document, Manual of Petroleum Measurement Standards Chapter 3 - Tank Gauging, Section 3.1B.4 "Field Calibration of Automatic Tank Gauges" for more information. Since you are "calibrating" the ATG to a manual gauge using a linear measuring tape it is critical to have a "calibrated" tape when doing the ATG calibration. It would not be appropriate or fair to a good ATG system to have a less accurate caliper decide what its reading should be. By using an NIST (National Institute of Standards and Technology) calibrated tape, whose parameters have been entered into the Tape No. configuration file, PGL can instantaneously provide the "corrected" readings for the manual gauge. Calibration Stages There are three (3) consecutive stages to an ATG calibration:
Every new calibration must start at stage 1, Preliminary. The Preliminary stage calibration is performed when the tank is approximately half full (Middle Third on the PGL). In one sense, this is the easiest of the 3 stages in that readings of the ATG do not have to be logged and tested, because this is the calibration that sets the ATG to its initial value, but then that is what makes the Preliminary the most crucial because the Final testing is contingent on how well the Preliminary calibration was done. The average of the manual readings for the Preliminary is what is used to set the initial value of the ATG. The Final calibration stage is where the essence of the testing is performed. The Final calibration involves three (3) separate calibrations of its own at three different random points within the tank called Test Levels:
The first two calibration stages are conducted within as close a time frame of each other as is practical and are what bring the ATG on-line for facility use. The Verification calibration is conducted at a much longer time span later and is performed multiple times. In the case of a Custody Transfer, a monthly verification would be performed until "operating experience confirms stable performance within the calibration tolerance". Afterwards the verification schedule can be extended to once per quarter. For Inventory Control the Facility can determine what verification schedule meets their requirements. The Verification stage is also simplified for the Custody Transfer in that only 3 readings have to be taken instead of the 5 required for a Preliminary and Final calibration. Accuracy Category Whenever a new calibration cycle is begun for an ATG on a tank, the level of accuracy for that calibration cycle must be decided. The accuracy levels breakdown into two categories:
IC CT
*only 3 for Verification |
Calibration Error Bands Each of the three Calibration Stages has its error band tests that are applied to the ATG gaugings and the manual gaugings. There are two tests that are run for a calibration:
The Average Error Band test consists of checking that the arithmetic average of the ATG readings and the arithmetic average of Manual Level readings are within a stated error band. For example, for an Inventory Control, the average error band is 1-inch. On the PGL, each of the tests is run automatically any time a data entry is made. Both a visual display in color (green = pass, red = fail) and in words as well as an audible alert provide an indication of the test result, which is either a Pass or Fail. For clarity, the table listing the error bands and the required number of readings per Accuracy Category is repeated here (IC = Inventory Control, CT = Custody Transfer). IC CT
*only 3 for Verification  An example of where the average ATG & the Manual level match exactly but still the manual gauging readings were not within the defined error bands! Therefore the calibration failed and must be repeated. Tape Number In order to have the most accurate manual gauging possible it is necessary to apply some "corrections" to the tape measurements that are used in performing the gauge, whether innage or outage. There are four (4) corrections, not one, that need to be applied to any manual tape gauging. These are discussed in our white paper "Tape Correction". In order to apply these corrections, specific calibration information is required for each tape used for gauging. PGL has a Tape Configuration file that contains all the pertinent data from a National Institute of Standards and Technology (NIST) calibration for each tape. The number of tapes entered in this file determines how large the PGL Tape No. control is. If there are 3 tapes in the Configuration file, then the control will go from 0 to 3. The zero (0) position on the Tape No. control is reserved as a "No Calibration" setting. If 0 is selected then no corrections of any type are applied to any of the measurements - for example, the Outage and Outage Corrected indicators will always show the same values. Selecting any non-zero number of course applies the appropriate parameters and corrections for that tape to your Reference Point and Cut entries.  If the number that is selected is an Innage tape, the Cut Type button automatically defaults to [-] and is disabled. Selecting another tape number that is an Outage tape or selecting Tape No. 0 will re-enable the Cut Type. On the Innage screen, the Cut Type is always at [-] and is always disabled. Selecting some number that is an Outage tape will give you an error message saying that you cannot use an outage tape to perform an innage and then when the message is acknowledged, the Tape No. control will revert back to whatever was the legal setting prior to the illegal selection. It is very important to always enter the best average temperature for the tape AND to match the physical tape you are using with the correct Tape No. selection on the PGL.
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PGL: The New ATG Level Calibration Compliance Assistance Tool
The salient feature of Pocket PGL is its ability to significantly aid the process of performing an Automatic Tank Gauge (ATG) calibration for any of your facility's tanks.
No other device can provide you with the complete format for all 3 calibration procedures, for both Inventory Control and Custody Transfer, for either an innage or outage gauge, and with the full accuracy required for the necessary tape corrections.
PGL is compliant with all API standards and provides you with the format for all three calibration procedures: This device is unique to the petroleum gauging industry.
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Save $100 and purchase both PGL and PGLR. |
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